U.S. patent application number 14/754942 was filed with the patent office on 2015-10-22 for vacuum drying method and apparatus.
This patent application is currently assigned to MAGUIRE PRODUCTS, INC.. The applicant listed for this patent is Michael E. GERA, JR., Stephen B. MAGUIRE. Invention is credited to Michael E. GERA, JR., Stephen B. MAGUIRE.
Application Number | 20150300737 14/754942 |
Document ID | / |
Family ID | 25383431 |
Filed Date | 2015-10-22 |
United States Patent
Application |
20150300737 |
Kind Code |
A1 |
MAGUIRE; Stephen B. ; et
al. |
October 22, 2015 |
VACUUM DRYING METHOD AND APPARATUS
Abstract
Method and apparatus for drying granular resin material by
drawing vacuum over heating resin material in a vessel, while
periodically purging the vessel with the material therein with dry
air and bathing the vacuum dried material with dry air until
furnished to a processing machine.
Inventors: |
MAGUIRE; Stephen B.; (West
Chester, PA) ; GERA, JR.; Michael E.; (Aston,
PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
MAGUIRE; Stephen B.
GERA, JR.; Michael E. |
West Chester
Aston |
PA
PA |
US
US |
|
|
Assignee: |
MAGUIRE PRODUCTS, INC.
Aston
PA
|
Family ID: |
25383431 |
Appl. No.: |
14/754942 |
Filed: |
June 30, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14272721 |
May 8, 2014 |
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14754942 |
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11402492 |
Apr 11, 2006 |
8776392 |
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14272721 |
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14693951 |
Apr 23, 2015 |
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11402492 |
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Current U.S.
Class: |
34/507 ;
34/92 |
Current CPC
Class: |
B29C 31/061 20130101;
B29B 7/005 20130101; B29B 9/16 20130101; B01F 15/00805 20130101;
F26B 5/042 20130101; F26B 2200/08 20130101; B29B 13/065 20130101;
F26B 17/128 20130101; F26B 5/04 20130101 |
International
Class: |
F26B 7/00 20060101
F26B007/00 |
Claims
1. A method for drying granular plastic resin prior to molding
plastic into finished products comprising sequentially heating and
vacuum drying batch portions of plastic resin material with a next
succeeding batch portion being heated while a current batch portion
is being vacuum dried.
2. The method of claim 1 further comprising performing the heating
and vacuum drying to supply dried plastic resin material at the
rate of material consumption by an associated process machine.
3. Apparatus for drying granular resin material prior to molding or
extrusion processing thereof, comprising: a. a heating chamber; b.
a vacuum chamber below the heating chamber; c. a retention hopper
below the vacuum chamber; d. a blower for supplying heated ambient
air upwardly through the heating chamber; e. a conduit for
introducing dry purge air into the vacuum chamber; and f. a conduit
for introducing blanketing dry air into the retention hopper.
4. Apparatus of claim 3 further comprising: a. a first support
bearing the weight of the heating hopper; and b. a second support
bearing the weight of the vacuum chamber.
5. Apparatus of claim 4 wherein the vacuum chamber is suspended
from the second support.
6. Apparatus of claim 4 wherein the first and second supports are
connected.
7. Apparatus of claim 3 further comprising: a. a heater for heating
ambient air for introduction into the heating hopper, comprising:
i. a hollow housing; and ii. an electrical heating element within
the housing.
8. Apparatus of claim 3 further comprising: a. a first gate between
the heating chamber and the vacuum chamber, movable between an open
and closed positions, for controlling downward resin flow into the
vacuum chamber; b. a second gate between the vacuum chamber and the
retention hopper, movable between open and closed positions, for
controlling downward resin flow from the vacuum chamber into the
retention hopper.
9. Apparatus of claim 8 further comprising: a. a plurality of first
gates between the heating chamber and the vacuum chamber, movable
between an open and closed positions, for controlling downward
resin flow into the vacuum chamber; b. a plurality of second gates
between the vacuum chamber and the retention hopper, movable
between open and closed positions, for controlling downward resin
flow into the retention hopper.
10. Apparatus of claim 9 wherein the plurality of first gates
includes at least one slide gate.
11. Apparatus of claim 9 wherein a first one of each of the two
pluralities of gates are connected to the vacuum chamber.
12. Apparatus of claim 8 wherein the first gate is a slide
gate.
13. Apparatus of claim 3 further comprising a sensor for sensing
weight of the vacuum chamber hopper and any resin material
therein.
14. Apparatus of claim 9 further comprising a sensor for sensing
weight of the retention hopper and any resin material therein.
15. Apparatus of claim 4 further comprising a frame connected to
the first and second supports.
16. Apparatus of claim 3 wherein the sensors are load cells.
17. Apparatus of claim 3 further comprising: a. an adjustable
blower for blowing warm air through the heater and into the heating
chamber; b. a sensor for detecting air temperature at the top of
the heating hopper; and c. a control for adjusting speed of the
blower in response to the detected air temperature.
18. A method for drying granular resin material prior to processing
thereof by molding or extrusion, comprising: a. heating granular
resin material in a heating hopper; b. monitoring air temperature
at the top of the heating hopper; c. regulating heat input to the
heating hopper so that monitored air temperature at the top of the
heating hopper dies not exceed a preselected temperature; d.
releasing heated granular resin material from the heating hopper
for flow downwardly into a vacuum chamber while replenishing the
heating hopper from above with fresh resin material in an amount
substantially equal to that released into the vacuum chamber; e.
drawing vacuum in the vacuum chamber while periodically purging the
vacuum chamber with dry air; f. draining resin material from the
vacuum chamber into a retention hopper; g. blanketing resin
material in the retention chamber with dry air.
19. The method of claim 18 wherein regulating heat input to the
heating hopper further comprises regulating speed of air passing a
heating element.
20. The method of claim 18 wherein heating the granular resin
further comprises introducing heated ambient air into the
hopper.
21. The method of claim 20 wherein heating granular resin material
in the heating hopper further comprises introducing heat into the
hopper at the hopper bottom.
22. The method of claim 18 wherein purging the vacuum chamber with
dry air comprises: a. passing compressed air along a membrane
dryer; b. introducing the compressed air into the vacuum chamber at
the bottom of the chamber.
23. In a method for drying granular resin material prior to
processing thereof by molding or extrusion, the improvement
comprising: a. introducing heated air into a granular resin
material storage hopper at the hopper bottom; and b. monitoring the
temperature of air leaving the hopper at a position above granular
resin material in the hopper and regulating the rate of heated air
introduction into the hopper so that monitored temperature of air
leaving the hopper does not exceed a preselected level.
24. The method of claim 23 further comprising: a. drawing vacuum in
the hopper thereby drawing moisture from the granular resin
material; and b. draining granular resin material from the hopper
as needed for processing.
Description
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS
[0001] This patent application is filed under 35 USC 120 as a
continuation-in-part of co-pending U.S. patent application Ser. No.
14/272,721 entitled "Simultaneous Resin Drying and Molding" filed 8
May 2014 in the name of Stephen B. Maguire, which in turn was a
division and claimed the benefit of the priority of co-pending U.S.
patent application Ser. No. 11/402,492 entitled "Resin Drying
Method and Apparatus" filed 11 Apr. 2006 in the name of Stephen B.
Maguire, which is now U.S. Pat. No. 8,776,392.
[0002] This patent application is further filed as a 35 USC 120
continuation-in-part of co-pending U.S. patent application Ser. No.
14/693,951 entitled "Method and Apparatus for Vacuum Drying
Granular Resin Material" filed 23 Apr. 2015 in the names of Stephen
B. Maguire and Michael Gera.
[0003] The benefit of the priority of the '721, '492, and '951
applications is claimed for this patent application under 35 USC
120.
BACKGROUND OF THE INVENTION
[0004] 1. Field of the Invention
[0005] This invention relates to drying granular or powdery
material, preferably granular resin material, prior to processing
thereof into intermediate or finished plastic products, preferably
by extrusion or molding.
[0006] 2. Description of the Prior Art
[0007] Plastic resins are initially granular materials and are
produced in pellets. These pellets are processed by extrusion or
other means in which the granular resin pellets are heated until
the pellets melt and are then molded or extruded into a desired
shape. Typically granular resins melt at elevated temperatures, for
example from 300-400.degree. F., which is well above the boiling
point of water.
[0008] Many granular resins have affinity for moisture. These
hydroscopic resins absorb moisture and cannot be properly processed
by molding or extrusion until dried. If processed before they are
dry, moisture in the resin boils at or approaching the high plastic
molding or extrusion process temperature, leaving bubbles and
perhaps other imperfections in the finished plastic product. Hence,
hydroscopic granular resins must be dried prior to molding or
extrusion.
[0009] Some granular resin materials are extremely hydroscopic and
become unprocessable by molding or extrusion in ten minutes or less
after exiting a dryer, due to the rapid absorption of moisture by
the granular resin material.
[0010] It is known to dry granular resin material by placing the
granular resin material pellets on large shallow trays to a depth
of one or two inches, and putting those trays into ovens for
several hours. With this approach to granular resin material
drying, drying temperatures of up to 150-180.degree. F. but no
higher can be used since many granular resin materials begin to
soften at 200-210.degree. F.
[0011] During the drying process, the granular resin material
cannot be permitted to soften, since it becomes unmanageable. Once
granular resin material begins to soften, at temperatures above the
boiling point of water, the granular resin material pellets stick
together in lumps or even melt into useless masses of solid
plastic, making it impossible to further process the resin material
into a useful article.
[0012] U.S. Pat. No. 6,154,980 represents a substantial improvement
in dryer technology providing vacuum-based methods and vacuum
drying apparatus which substantially accelerate the drying process,
providing greater throughput of dried granular resin material at
lower cost than known heretofore.
SUMMARY OF THE INVENTION
[0013] In one of its aspects, this invention provides a method for
continuously supplying dried granular resin material for processing
from a supply of material which is excessively moist where the
method preferably includes substantially simultaneously performing
the steps of (i) heating a portion of the moist granular resin
material to a selected temperature at which moisture evaporates
from the granular resin material when the material is exposed to a
preselected level of vacuum; (ii) drawing the preselected level of
vacuum over a second portion of the granular resin material which
has been heated to the selected temperature for time sufficient to
cause the moisture to evaporate therefrom and result in the second
portion of granular resin material being at the preselected dryness
while optionally supplying at least one short burst of heated dry
air, preferably at the conclusion of the drying cycle, to the
drying material to achieve an even greater degree of dryness; and
(iii) supplying to granular resin material processing equipment
from an inventory position granular resin material which was dried
to the preselected dryness by evaporation in the preselected level
of vacuum after having been heated to the selected temperature, and
sequentially and repeatedly replacing each portion by the next
succeeding portion.
[0014] In another of its aspects this invention provides apparatus
for drying granular or powdery material prior to molding or
extrusion where the apparatus includes a first material processing
chamber, a second material processing chamber, manifold means for
furnishing material to be dried selectably to one of said first and
second processing chambers preferably most recently having had
dried material evacuated therefrom, means for heating material in a
selected one of said first and second processing chambers into
which material needing to be dried has been introduced, means for
drawing vacuum over material in a selected one of said first and
second processing chambers preferably having had said material most
recently heated therein and means for withdrawing material from
said chamber preferably having most recently dried material
therein.
[0015] In yet another of its aspects this invention provides a
method for drying granular powdery material prior to molding or
extrusion where the method includes feeding a first portion of
material to a first processing chamber, heating the material in the
first processing chamber to a preselected temperature, drawing
vacuum over the material in the first processing chamber, feeding a
second portion of material to a second processing chamber, heating
the material in the second processing chamber while the first
portion of material has vacuum drawn thereover, withdrawing
material from the first processing chamber when needed for molding
or extrusion, and drawing vacuum over the heated material in the
second processing chamber thereby to evacuate moisture from said
material and prepare such material for molding or extension.
[0016] In yet another of its aspects this invention provides a
method for supply of dried granular resin material for processing
from a supply of material which is excessively moist where the
method includes heating a portion of the moist material to a
temperature at which moisture evaporates at a preselected level of
vacuum, drawing at least the preselected level of vacuum over a
second portion of the material which has been heated to a
temperature and for a time sufficient to cause the moisture to
evaporate therefrom and result in the second portion of material
reaching a preselected dryness while periodically introducing hot
air into the second portion of material under the preselected
vacuum to purge moist air from around such material, and supplying
to granular material processing equipment for molding or extrusion
a third portion of the material which has been dried to the
preselected dryness by moisture evaporation in the preselected
level of vacuum after having been heated.
[0017] This invention uses gravity to move granular plastic resin
material in a vacuum dryer. The granular plastic resin material
preferably is heated in a top heating hopper. The granular plastic
resin material is then preferably dropped into a vacuum chamber.
From the vacuum chamber the granular plastic resin material is
preferably dropped into a retention hopper.
[0018] A plastic product manufacturing process, either molding or
extrusion, can preferably draw dry granular plastic resin material
from the retention chamber as required, while the heating hopper
and the vacuum chamber preferably continuously prepare subsequent
batches of granular plastic resin material. The preferable
essentially straight down processing and drying of granular plastic
resin material results in a much lower cost dry granular plastic
resin material as compared to granular plastic resin material dried
using known vacuum and other types of dryers.
[0019] In the invention, preferably at least one slide gate
selectably allows and blocks granular plastic resin material
downward flow from one part of the dryer to another. Costs are
reduced by about forty percent and drying capacity is actually
higher in the advantageously small footprint dryer embodying this
invention. The small footprint afforded by the vertical, "stacked"
configuration of the dryer is advantageous in that space in a
plastic manufacturing processing plant, whether an extrusion
operation or a molding operation, is often at a premium.
[0020] The vacuum chamber of the granular plastic resin material
dryer is preferably closed by at least one slide gate having a
vacuum tight seal. The slide gate preferably closes and seals
against an o-ring to provide a vacuum tight seal. Use of the slide
gate avoids vacuum leakage that could occur from contamination that
is present everywhere in a plastic molding or extrusion facility.
With the slide gate, plastic dust, flakes, and pellets of granular
plastic resin material do not interfere with the vacuum tight
seal.
[0021] In one of its aspects, the invention introduces dry air into
the vacuum chamber periodically. As moisture is released from the
granular plastic resin material while under vacuum, a vacuum pump
or other source of vacuum preferably continues to pull the
resulting air-water vapor mixture from the vacuum chamber. Over
several minutes, this mixture changes to become a very high
percentage of water vapor relative to the air remaining in the
chamber.
[0022] If the moisture in the form of water vapor is not purged
from the vacuum chamber, when vacuum is released from the vacuum
chamber the resulting "thin" but moisture-laden air would reenter
the pellets of granular plastic resin material resident in the
chamber and would reverse the drying that has occurred. To prevent
this, in one of its aspects the invention preferably purges the
vacuum chamber of moisture several times while vacuum is present.
The invention preferably permits very dry purge air to enter the
vacuum chamber and then draws the resulting mix of the very dry air
and the water vapor-laden air carrying the moisture that has been
drawn out of the resin pellets, out of the chamber.
[0023] When drying polyethyleneterephthalate ("PET"), which is used
conventionally for beverage bottles, it is essential that moist
ambient air not enter the vacuum chamber at the end of a vacuum
cycle. The dry air purge allows effective drying of PET
pellets.
[0024] To supply such dry purge air, in one of its aspects the
invention preferably uses a separate dry air source. Suitable dry
air can be obtained in several ways. Desirably in the practice of
the invention in a preferred manner, the invention utilizes
compressed air, which passes through at least one oil separator
coalescing filter and a compressed air membrane dryer so that the
air exiting the oil separator coalescing filter and the compressed
air membrane dryer is extremely dry. This dry air is desirably
heated to a desired level for introduction into the vacuum chamber.
Since only a relatively small amount of dry air is required for
purging the vacuum chamber, the compressed air membrane dryer can
be very small and of very low capacity.
[0025] In one aspect of the invention, the hopper in which the
granular plastic resin material is initially heated is preferably
designed such that hot air enters the bottom of the hopper, passes
upwardly through the granular plastic resin material resident in
the hopper, and exits the hopper at the top. As the hot air is
passing through the heating hopper, granular plastic resin material
may be dropped from the bottom of the hopper into the vacuum
chamber, while new granular plastic resin material is added at the
top of the hopper. In one aspect of the invention, the heating
hopper preferably holds sufficient granular plastic resin material
to provide from three to five hours of residence time for the
granular plastic resin material before exiting the bottom of the
heating hopper. In this way, the granular plastic resin material is
exposed to hot, dry air for from three to five hours, which is the
time required for the granular plastic resin material to flow
downwardly through the heating hopper.
[0026] The invention in a preferred manifestation does not dry the
granular plastic resin material using "hot" air in the conventional
sense. Hot air is used only to bring the granular plastic resin
material up to a desired temperature. By carefully controlling the
speed of a blower that moves the hot air, air flow is preferably
adjusted so that the invention provides hot air preferably at the
correct rate to heat the granular plastic resin material. Viewed
differently, most of the useful heat, in terms of calories or BTUs,
is removed from the hot or "heating" air before the heating air
arrives at the upper surface of the granular plastic resin material
in the heating chamber and is preferably allowed to escape.
[0027] In the invention, since the invention is not concerned with
heating the granular resin material during the drying stage, the
drying stage, namely the stage during which the pellets are exposed
to vacuum in the vacuum chamber, is as short as possible, and may
be as little as fifteen or twenty minutes, as contrasted to three
to five hours of drying time required when using a conventional
desiccant dryer.
[0028] Preferably, there is no air filter for the heating air in
the invention. The heating air is preferably used only once and is
preferably vented to the atmosphere after it has been used for
heating and has given up most of its heat. Most preferably, the
hearing air is not recirculated.
[0029] The single pass flow of heating air and the elimination of
the need for a filter for the heating air is believed unique to
this invention. Earlier vacuum dryer designs involved recirculation
of air with filtering being required. In its most preferred
manifestation, this invention eliminates the need for a filter by
having the "heating" air pass through the granular plastic resin
material only once. The invention further regulates the speed of
the blower forcing the air through the material to avoid, to the
extent possible, loss of unused, residual heat remaining in the
"heating" air leaving the heating hopper. Blower speed is
preferably adjusted so that only enough heated air, at a desired
temperature for the resin material prior to drying, is fed to the
heating hopper at the bottom so that the bottom potion of resin in
the heating hopper reaches the desired final temperature to meet
the appetite of the process machine, namely a molding machine or
extruder, for dry granular plastic resin material to be molded or
extruded.
[0030] In one of its aspects, this invention provides a method for
drying granular resin material, prior to processing of the granular
resin material by molding or extrusion, which includes heating
granular resin material in a heating hopper, monitoring air
temperature at the top of the heating hopper, and regulating
introduction of heat to the hopper bottom based on monitored air
temperature at the top of the heating hopper.
[0031] The method may further proceed by releasing heated granular
resin material from the heating hopper for flow downwardly into a
vacuum chamber while replenishing the heating hopper from above
with fresh resin material, preferably in an amount substantially
equal to that released into the vacuum chamber. The method
preferably proceeds by drawing vacuum in the vacuum chamber,
periodically purging the vacuum chamber interior with dry air while
the chamber is under vacuum, draining resin material from the
vacuum chamber into a retention hopper, and blanketing dried resin
material in the retention hopper with dry air for so long as the
material is resident therein.
[0032] Heating the granular resin material preferably further
includes introducing dry heating air into the heating hopper at the
heating hopper bottom.
[0033] In still another aspect of the invention, there is provided
an improved method for drying granular resin material prior to
processing thereof by molding or extrusion by loading granular
resin material into a heating hopper from above the hopper,
introducing heated air into the hopper at the hopper bottom,
monitoring the temperature of the air leaving the hopper at a
position above the resin material, and regulating the rate of
heated air introduction into the hopper so that monitored
temperature of air leaving the hopper does not exceed a preselected
level.
[0034] In still another one of its aspects, this invention provides
apparatus for drying granular resin material prior to molding or
extrusion of the material. Desirably the apparatus includes a
heating hopper, a vacuum chamber positioned below the heating
hopper, and a retention hopper positioned below the vacuum chamber.
A blower is provided for providing heating air as an air blanket
for dried resin material in the retention hopper.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1 is a schematic view of a dryer in accordance with the
invention.
[0036] FIG. 2 is a schematic view of a second dryer in accordance
with the invention.
[0037] FIG. 3 is a schematic view of a third dryer in accordance
with the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT AND BEST MODE KNOWN FOR
PRACTICING THE INVENTION
[0038] The dryers of the invention require less floor space than a
conventional desiccant dryer having the same capacity.
Additionally, there is no desiccant maintenance as there is in a
conventional desiccant dryer thereby eliminating lost production
time, which maintenance is required with a conventional desiccant
dryer. Moreover, as desiccant material deteriorates, the quality of
the plastic granular material being dried suffers. However, with
the dryer of the invention in any of its embodiments, since there
is no desiccant material involved there is no risk of deterioration
of product quality from the desiccant material. Performance of
dryers according to the invention remains constant and does not
deteriorate over time.
[0039] Dryers according to the invention shorten drying time
relative to desiccant dryers thereby avoiding prolonged exposure of
the granular resin material to heat. This helps to maintain desired
physical properties of the resin material.
[0040] Dryers according to the invention reduce labor costs in that
clean-out time for hoppers for a color or material change is
minimal. Typically, a dryer according to the invention requires
less than ten minutes of total time to clean whereas a conventional
desiccant dryer can take up to one hour for cleaning.
[0041] Desiccant dryers typically require material feed hoppers to
be at least half full for proper air flow. Hence if material usage
is low for particular molding operation, extended exposure to heat
in a conventional desiccant dryer may degrade the plastic resin
molding material. There is no such requirement for a full canister
for the dryers in accordance with the invention to operate
properly.
[0042] Test data reveals that operating costs of dryers according
to the invention are less than one-half that of a desiccant dryer
having the same capacity. In many cases operating cost is reduced
by as much as 80% over that of a desiccant dryer having the same
capacity. Additionally, startup time using a dryer in accordance
with the invention is under one hour whereas typical desiccant
dryers require four hours or more for startup.
[0043] Use of dryers in accordance with the invention permits
material changeover time to be reduced to zero if the operator
plans about one hour ahead. Color changes in material can be made
"on the fly" with no lost time simply by changing the material.
When dryers in accordance with the invention operate on a 20-minute
cycle, unused inventory of blended material represents at most 40
minutes of dryer time, not four hours as is the case with a
conventional desiccant dryer.
[0044] Dryers in accordance with the invention minimize the need to
expose material to be dried to high heat for extended periods,
dramatically eliminating or minimizing the loss of important
physical properties which some materials experience when exposed to
high heat for extended periods.
[0045] Dryers in accordance with the invention permit drying of
plastics at lower temperatures than known heretofore; PET
heretofore has had to be dried at about 350.degree. F. (180.degree.
C.) but with dryers in accordance with the invention PET can be
dried at 245.degree. F. (120.degree. C.)
[0046] No cooling water is required for the dryers in accordance
with the invention.
[0047] The lower temperature at which the dryers in accordance with
the invention operate allows addition of color concentrates to
resin materials prior to drying rather than afterwards.
[0048] Another problem addressed by the dryers in accordance with
the invention is the separation of preblended materials during
extended residence times in large hoppers. The chambers of dryers
in accordance with the invention are typically lower in volume than
conventional desiccant dryers and may fill and may empty in
distinct and complete batches thereby eliminating the problem of
material separation.
[0049] The dryers in accordance with the invention require less
floor space than a desiccant dryer having similar capacity.
[0050] The dryers of the invention do not require and do not
utilize a dew point meter or a dew point control, both of which are
subject to reliability problems but are necessary with desiccant
dryers.
[0051] Dryers in accordance with the invention preferably operate
using an air supply from 75 to 80 psi. This air, which is typically
from the air supply in the facility in which the dryer is used,
serves to generate the required vacuum as well as operate all of
the air cylinders of the dryer. To conserve usage of facility air,
the venturi vacuum generator in a dryer of the invention is
desirably cycled on and off during operation to maintain a minimum
vacuum of 25 inches. For large installations, a vacuum pump may be
substituted for the venturi vacuum generator.
[0052] The microprocessor controllers of the dryers preferably
include thumbwheel switches or functionally equivalent structure,
which are used to set temperature to which the resin or other
granular material is to be heated prior to drying. Another
thumbwheel switch or functionally equivalent structure is
preferably used to set the minimum acceptable time for a heating
cycle and a drying cycle. Typically 20 minutes is the cycle time
for acrylic, ABS and polycarbonate while 40 minutes is the cycle
time for PET.
[0053] FIG. 1 schematically depicts a vacuum dryer embodying
aspects of the invention where the vacuum dryer is designated
generally 200. A material supply container 202 or equivalent
structure is provided as indicated schematically at the top of FIG.
1; material supply container 202 need not be a part of vacuum dryer
200.
[0054] A preferably tubular material feed line 224 or equivalent
structure leads out of material supply 202, preferably downwardly,
and connects to a material flow control valve or equivalent
structure depicted schematically as 204 in FIG. 1.
[0055] Material flow control valve 204 provides material to either
of two material feed lines 226, 226A, or equivalent structure,
which lead to respective ones of first and second material
processing chambers 210, 212 or equivalent structure, both of which
are illustrated as vertically oriented cylindrical processing
chambers in FIG. 1. Other geometric configurations and shapes may
also be used.
[0056] First and second material processing chambers 210, 212 are
equipped with means for heating granular material, such as plastic
resin, delivered thereinto via material feed lines 226, 226A. The
heating means may be one or more electrical resistance heaters as
illustrated schematically and designed 214, 216 in first and second
material processing chambers 210, 212. Alternately and preferably
hot air is blown through first and second material processing
chambers 210, 212 to effectuate heating of material contained
therewithin.
[0057] Vacuum dryer 200 further includes a vacuum pump or
equivalent structure designated generally 208. Vacuum pump 208
draws vacuum within a selected one of first and second material
processing chambers 210, 212 according to the position of vacuum
control valve 206 or equivalent structure, which is connected to
draw vacuum created by vacuum pump 208 from a selected one of first
and second material processing chambers 210, 212. Vacuum lines 228,
228A or equivalent structure connect first and second material
processing chambers 210, 212 to vacuum pump 208. A further vacuum
line 230 or equivalent structure connects control valve 206 to
vacuum pump 208.
[0058] Preferably leading from the bottoms of first and second
material processing chambers 210, 212 are a pair of outlet lines
232, 232A or equivalent structure, which in turn connect to first
and second dried material flow control valves 218, 220
respectively. First and second dried material flow control valves
218, 220 or equivalent structure control downward flow of dried
granular or powdery resin material from respective processing
chambers 210, 212 into a reservoir 222 in which the dried granular
material is retained until needed by the manufacturing process.
Line 234 carries material released by valve 218 or 220 into
reservoir 222. Material feed line 236 carries dried material as
needed from reservoir 222 to a process machine for fabrication
where the process machine is desirably either a molding press or an
extruder.
[0059] During operation of vacuum dryer 200, moist granular
material requiring drying is initially fed via valve 204 under the
influence of gravity into the first material processing chamber
210. While in processing chamber 210, the granular resin material
is heated, preferably by flow of hot air therethrough, until the
material reaches a temperature at which vacuum is highly effective
to evaporate moisture out of the material.
[0060] Once application of heat ceases, first material processing
chamber is then sealed so vacuum can be drawn therein and vacuum
pump 208 or equivalent structure actuated, with valve 206 or
equivalent structure connecting vacuum pump 208 to first material
processing chamber 210. Vacuum is drawn for sufficient time to
evaporate the required amount of moisture out of the granular resin
material within first material processing chamber 210.
[0061] While vacuum is being drawn over and moisture is being
evaporated from the material in processing chamber 210, second
material processing chamber 212 has preferably been filled with
material and the granular resin material within chamber 212 is
heated to the required temperature for vacuum-based evaporation of
moisture therefrom.
[0062] Once the evaporation operation has been completed with
respect to the material in chamber 210 and the heating has been
completed with respect to the material in chamber 212 by virtue of
that material having reached the required temperature for
evaporation of moisture therefrom, the position of valve 206 may be
switched so that vacuum pump 208 draws a vacuum within chamber 212
through conduits 228A and 230. During this time, dried material
within chamber 210 may be evacuated via lines 232 and 234 by
opening valve 218 so that material may flow downwardly into
reservoir 222 and be stored therein until needed for processing by
the process machine, to which that material may then be carried by
line 236. Once first material processing chamber 210 is empty,
chamber 210 may be refilled using material from supply 202 by
appropriate positioning of valve 204 whereupon material may flow
from supply 202 via conduits 224, 226 into chamber 210 and the
process repeated.
[0063] Because evaporation of moisture under vacuum is temperature
sensitive and increases greatly in rate with increasing
temperature, little is gained by seeking to apply vacuum to the
moist granular material before the material has been raised to the
appropriate temperature. As a result, a "dual" vacuum dryer system,
namely one having two material processing chambers in which one
batch of material can be heated while a second batch of material
(having already been heated to the desired temperature) is having
vacuum drawn thereover and moisture evaporated therefrom, is a more
efficient system in terms of the amount of dried material delivered
per unit time than a system in which vacuum is drawn over the
material as the material is being heated.
[0064] Vacuum dryer 200 illustrated in FIG. 1 is depicted
schematically. First and second material processing chambers 210,
212 are desirably equipped with heated air inlet and outlet hoses,
with vacuum inlet and outlet hoses and with vacuum sealing means of
the type disclosed above respecting the invention.
[0065] Valve 204 functions as a manifold, preferably being
connected to the first and second processing chambers 210, 212 and
preferably selectably furnishes material to be dried to one of the
two first and second processing chambers. Desirably, valve 204 acts
as a manifold to furnish material to a selected one of first and
second chambers 210, 212 most recently having dried material
evacuated therefrom. Furthermore, it is desirable that first and
second processing chambers 210, 212 have separate means for heating
material in each of, or associated with, those two chambers.
[0066] The apparatus illustrated in FIG. 1 may be modified to
utilize only a single material processing chamber, either 210 or
212, for both heating and vacuum drying. While this arrangement may
be less expensive, it is also less efficient in that granular
material to be dried cannot be effectively dried under vacuum until
heating has been completed, as noted above.
[0067] A second embodiment of a vacuum dryer manifesting aspects of
the invention is illustrated schematically in FIG. 2 with the
vacuum dryer being designated generally 300 and including a
material processing chamber designated generally 302.
[0068] A material supply container or equivalent structure is
designated generally 304 and serves as a storage receptacle for
granular or powdery material requiring drying; material supply
container 304 need not be a part of dryer 300.
[0069] Material processing chamber 302 or equivalent structure is
preferably equipped with a preferably sealing lid designated
generally 306 and positioned to close an inlet end 326 of
processing chamber 302 or equivalent structure. Sealing lid 306 is
preferably moved by a preferably pneumatic actuating cylinder 308
connected to sealing lid 306 by a suitable pivoting arm 310. Upon
actuation of cylinder 308, sealing lid 306 moves into position to
seal inlet end 326 of processing chamber 302.
[0070] Granular resin or powdery material requiring drying is
conveyed, preferably by gravity, from material supply 304 or
equivalent structure to inlet end 326 of processing chamber 302 or
equivalent structure via material conveying tube 328.
[0071] Material processing chamber 302 is preferably divided into
two zones, a heating zone or equivalent structure designated
generally 312 and a vacuum drying zone or equivalent structure
designated generally 314. Zones 312, 314 are preferably separated
by a sealing trap door or equivalent structure, such as a slide
gate, which is designated 318 and referred to as a first sealing
trap door within preferably cylindrical material processing chamber
302. Heating zone 312 is preferably adapted to heat granular or
powdery material contained therewithin. An electrical resistance
heater has been designated 316 and is shown schematically as a part
of heating zone 312 to indicate the heating function; heating may
also be and preferably is provided by hot air in the manner
described above.
[0072] A vacuum pump 322 or equivalent structure is preferably
connected to vacuum drying zone 314 of processing chamber 302.
[0073] The lower or exit end of vacuum drying zone 314 is
preferably bounded by and defined by a second sealing trap door or
equivalent structure designated generally 320 in the drawings.
Second sealing trap door 320 preferably leads to a dried material
discharge conduit 332 providing dried granular or powdery material
to a reservoir 324 from which material may be supplied to a molding
machine or extruder as required, preferably via outlet conduit
334.
[0074] During operation of the embodiment of the dryer apparatus
illustrated in FIG. 2, a first portion of granular or powdery
material to be dried is preferably advanced from a supply in
material supply container 304 preferably through material inlet
conveying tube 328 into heating zone 312 of material processing
chamber 302. Once within heating zone 312, that first portion of
material is heated, preferably by forcing or drawing hot air
through the material. Temperature of the material is preferably
regulated substantially in the same manner as described above,
namely by comparing temperature of the air going into the material
and temperature of the air coming out of the material; when those
air temperatures are equal, the material is known to be
substantially heated to the required temperature.
[0075] Once the first portion of heated material is known to be
substantially at the required temperature, that first portion of
material preferably is advanced from heating zone 312 preferably
into vacuum drying zone 314 preferably by opening sealing trap door
318 or equivalent structure, such as a slide gate, separating
heating zone 312 from vacuum zone 314 and allowing the heated
material to fall due to gravity from heating zone 312 into vacuum
drying zone 314.
[0076] Once the first portion of heated material has been evacuated
from heating zone 312 into vacuum drying zone 314, a second portion
of heated material preferably may be advanced from supply 304 via
tube 328 into heating zone 312, whereupon heating of that batch of
material may commence.
[0077] For the first portion of material which is now in vacuum
drying zone 314, vacuum is preferably drawn over that first portion
of material to dry the first portion of material while a second
portion of material, which is now preferably in heating zone 312,
is preferably heated.
[0078] Once drying of the first portion of material is
substantially completed in vacuum drying zone 314, second sealing
trap door 320 or equivalent structure preferably may be opened and
the first portion of material, which is now dried to the required
level, may preferably advance downwardly, preferably due to the
force of gravity, through dried material discharge conduit 332, or
equivalent structure, into reservoir 324 or equivalent structure in
which the dried granular material is preferably stored until needed
by the process machine.
[0079] These steps of advancing portions of granular material from
the supply into the heating zone, heating of material in the
heating zone while the next preceding portion of material is being
dried in the vacuum drying zone, and then advancing the two
portions of material successively from the drying zone into the
reservoir and from the heating zone into the drying zone, may
preferably be repeated until such time as no additional dried
material is required by the process machine to which conduit 334 is
connected or leads.
[0080] Referring to FIG. 3, an air purge granular resin material
vacuum dryer in accordance with the invention is shown
schematically and designated generally 10. Air purge dryer 10
includes a heating hopper 12, a vacuum chamber 14, and a retention
hopper 16, with the heating hopper being positioned above the
vacuum chamber and the vacuum chamber in turn being positioned
above the retention hopper 16, with the heating hopper 12, vacuum
chamber 14, and retention hopper 16 being desirably vertically
substantially aligned, as shown in FIG. 3.
[0081] Heating hopper 12, vacuum chamber 14, and retention hopper
16 are all preferably independently supported by a support frame
designated 20 shown only schematically in FIG. 1. Preferably,
heating hopper 12 does not rest on vacuum chamber 14. To the
contrary, support frame 20 preferably supports heating hopper 12
above vacuum chamber 14 so that none of the weight of heating
hopper 12 or any resin within heating hopper 12 is supported by
vacuum chamber 14. Heating hopper 12 is desirably an insulated
stainless steel hopper and can accommodate drying temperatures of
up to 350.degree. F. The heating temperature is adjusted or set on
a control panel portion of controller 76.
[0082] Similarly, vacuum chamber 14 is preferably independently
supported by support frame 20 so that none of the weight of vacuum
chamber 14 is transferred to or borne by retention hopper 16. While
support frame 20 has been depicted in FIG. 3 in three sections, it
is to be understood that support frame 20 can be a single
structural member so long as support frame 20 provides separate
weight-bearing support for heating hopper 12, vacuum chamber 14,
and retention hopper 16. While support frame 20 has been
illustrated in the drawing as being under vacuum chamber 14, vacuum
chamber 14 may also desirably and most preferably be suspended from
above by a suitable frame member similar to schematic frame 20.
[0083] The vertically aligned "stacked" arrangement of heating
hopper 12, vacuum chamber 14, and retention hopper 16, as depicted
generally in FIG. 3, permits gravity-induced flow of granular
plastic resin from heating hopper 12 downwardly into vacuum chamber
14, and from vacuum chamber 14 downwardly into retention hopper 16.
Desirably, retention hopper 16 is supported by support frame 20 in
a manner that retention hopper 16 is somewhat above floor level in
the facility in which air purge dryer 10 is located. Having
retention hopper 16 above the floor permits dried granular resin
material to be supplied directly out of retention hopper 16 by
gravity flow to a process machine such as a molding press or an
extruder, or to a vacuum-powered or pneumatically powered resin
distribution system within the processing facility. Support frame
20 has been illustrated in schematic form as supporting retention
hopper 16, vacuum chamber 14 and heating hopper 12; desirably in
addition to vacuum chamber 14 being mountable on rails and in a
suspended disposition from support frame 20, heating hopper 12 and
retention hopper 16 may both also be mounted on rails to facilitate
movement and removal of heating hopper 12 and retention hopper 16
as needed for maintenance, etc.
[0084] Air for heating granular plastic resin within heating hopper
12 is preferably supplied by a centrifugal blower 22 that draws in
ambient air and forces that ambient air through an air heating
chamber 23, which preferably includes a heating element 24
positioned within an open ended cylindrical housing 25. The open
ended cylindrical housing 25 is preferably a 6 inch diameter, 6
inch length stainless steel cylinder having suitable insulative
material around the exterior thereof. Voltage applied to heating
element 24 within cylindrical housing 25 causes heating element 24
to rise in temperature. Air passing along heating element 24, as
blown through air heating chamber 23 by centrifugal blower 22, is
heated by heating element 24 and exits air heating chamber 23 at
the top of chamber 23 and travels via a hot air conduit 74 to
heating hopper 12, where the hot air enters heating hopper 12 at
the bottom thereof for upward passage through granular plastic
resin material residing in heating hopper 12. A variable frequency
drive 30 is provided for centrifugal blower 22 to modulate the
speed of blower 22 and thereby control and adjust the amount of
heating air, and therefore the amount of heat introduced into
heating hopper 12.
[0085] Vacuum chamber 14 is mounted on support frame 20 with one or
more load cells 36 between vacuum chamber 14 and support frame 20.
Load cell 36 provides data to controller 76 as to the weight of
vacuum chamber 20 and any granular plastic resin material being
dried therein.
[0086] Similarly, retention hopper 16 is mounted on support frame
20 using one or more load cells 38 to provide data to controller 76
as to the weight of dried granular plastic resin material resident
within retention hopper 38.
[0087] Temperature sensors are provided to monitor air temperature
at the inlet connecting conduit 74 to heating hopper 12 and at the
top of heating hopper 12, where the heated air, having given up
most of its heat, is exhausted. The temperature sensor at the hot
air inlet to heating hopper 12 is designated 44 in the drawings,
while the temperature sensor at the outlet, at the top of heating
hopper 12 where heated ambient air is exhausted, is designated
46.
[0088] A material level sensor 42 is provided in heating hopper 12.
Level sensor 42 provides a signal indicating excessively low level
of material in heating hopper 12. Controller 76 receives a signal
from heating hopper level sensor 42 and in response to a low
material level signal, controller 76 either actuates apparatus to
provide granular resin material for replenishing heating hopper 12
or, if no material is available, controller 76 shuts down the air
purge dryer 10.
[0089] A temperature sensor 56 within retention hopper 16 senses
the temperature of the dry purge air with which dried granular
resin in retention hopper 16 is blanketed. A granular resin
material temperature sensor 58 may be provided at the bottom, close
to the material outlet from retention hopper 16, to sense the
temperature of the resin material being supplied from retention
hopper 16.
[0090] Controller 76 desirably has two display screens. The upper
screen 82, which desirably has a red background, shows actual
temperatures and set point temperatures. The lower screen 84, which
desirably has a blue background, shows various running mode
information, set up information, and dryer configuration
information, as selected by the operator using touch controls that
are a part of controller 76 and are associated with the two
screens.
[0091] One or more oil separator coalescing filters 32 are provided
to remove entrained oil and some moisture from the compressed air
supply. A compressed air membrane dryer 34 further dries the air
and provides very dry purge air for vacuum chamber 14 and a dry air
blanket for maintenance of dry conditions for granular resin
material in retention hopper 16.
[0092] As operation of the air purge dryer begins, material in
heating hopper 12 is brought up to temperature. The time for
preheating is determined by a specified preheat time, which may be
entered by an operator into controller 76, or by an automatic
set-up option in controller 76 which establishes an inlet-to-outlet
temperature difference for the air input to and exhausted by
heating hopper 12, and a minimum preheat time. Once resin material
in heating hopper 12 is up to temperature, as determined by the
inlet-to-outlet temperature difference as measured by temperature
sensors 44 and 46, and the temperature difference is supplied to
controller 76, approximately one-third of the resin material in
heating hopper 12 is dispensed into vacuum chamber 14. Once this
occurs, a first vacuum drying cycle begins. Each vacuum drying
cycle, namely the time a batch of resin material remains in vacuum
chamber 14 under vacuum, has a minimum time that the material is
maintained under vacuum. This time may be set by an operator using
the inputs available on controller 76 or a default time of twenty
(20) minutes may be used.
[0093] During normal operation, vacuum in vacuum chamber 14 is
brought to a level of about 700 mm Hg and held to about a plus or
minus 20 mm Hg differential for the vacuum cycle time. A typical
vacuum cycle lasts from 15 to 20 minutes, depending on the material
being dried.
[0094] As vacuum chamber 14 receives the heated granular resin
material through first conduit 102 via operation of material flow
control gates 60 and 62 and the vacuum cycle begins, a suitable
loader, either human or mechanical, such as the loader that is the
subject of U.S. Pat. No. 8,753,432, loads heating hopper 12 with
new replenishment material, desirably concurrently with the start
of the vacuum cycle. Granular resin material loaded into heating
hopper 12 remains in heating hopper 12 for a minimum of the time
for a vacuum cycle in vacuum chamber 14.
[0095] After completion of a vacuum cycle in vacuum chamber 14,
granular resin material that has been dried in vacuum chamber 14 is
dispensed downwardly through second conduit 104, via operation of
material flow control gates 64 and 66, into retention hopper 16 and
is ready for use. Dried granular resin material residing in
retention hopper 16 and not immediately removed therefrom for
molding or extrusion is blanketed with dry air so long as that
granular resin material remains in retention hopper 16. The dry air
blanketing the dried granular resin material remaining in retention
hopper 16 is maintained under positive pressure and is desirably
slightly heated so as to be warm. The dry air blanketing the dried
granular resin material remaining in retention hopper 16 prevents
that granular resin material from absorbing moisture, which would
render the material unsuitable for subsequent processing by molding
or extrusion.
[0096] The rate of consumption of dried granular resin material
from retention hopper 16 dictates the time granular resin material
will be heated in heating hopper 12 and dried under vacuum in
vacuum chamber 14. For example, if thirty (30) minutes are required
for operation of a process machine to deplete retention hopper 16,
the vacuum cycle in vacuum chamber 14 will run past the normal
twenty (20) minute set point and will last thirty (30) minutes.
This is normal operation and does not in any way degrade the
granular plastic resin that has been dried in vacuum chamber 14.
However, if retention hopper 16 is depleted in fifteen (15) minutes
due to being consumed by operation of a process machine and the
time for a vacuum cycle in vacuum chamber 14 has been set to twenty
(20) minutes, a five (5) minute window will result when no granular
resin material is available for the process machine. This indicates
that the throughput capacity of the dryer has been exceeded for the
particular granular resin material being dried and the particular
process machine being supplied. Upon such occurrence, controller 76
senses that retention hopper 16 is empty, that vacuum chamber 14 is
still drying material, and with no material being available in
retention hopper 16 for the associated process machine, controller
76 sounds an alarm.
[0097] Vacuum chamber load cell(s) 36 and retention hopper load
cell(s) 38 allow controller 76 to always have in memory the current
weight of material in the vacuum chamber and the current weight of
material in the retention hopper. This permits calculation by
controller 76 of throughput of granular resin material in pounds of
resin material per hour.
[0098] Venturi vacuum generator 28 requires an operating air
pressure of about 80 psi. The pressurized air is desirably supplied
by an in-house air system.
[0099] A purge air inlet temperature sensor 56 is provided in
retention hopper 16. A granular resin material outlet temperature
sensor 58 is provided at the bottom of retention hopper 16. Both
sensor 56 and sensor 58 provide temperature data to controller
76.
[0100] The desired temperature of air being outlet from the top of
heating hopper 12 may be set in controller 76 such that once the
temperature of air escaping from the top of heating hopper 12
reaches a desired level, centrifugal blower 22 and heating element
24 will shut down for a predetermined time period specified by an
operator and programmed into controller 76 or until a vacuum cycle,
which is under way, ends, whichever event comes first.
[0101] Fill and the fill rate for vacuum chamber 14 are controlled
and may be adjusted by material flow control gates 60 and 62 above
vacuum chamber 14 as actuated and controlled by controller 76.
Similarly, material dump and material dump rate from vacuum chamber
14 can be controlled and adjusted by material flow control gates 64
and 66 below vacuum chamber 14 as actuated and controlled by
controller 76. These parameters, namely vacuum chamber fill and
fill rate and vacuum chamber dump and dump rate are programmable
into controller 76. Similarly, the timing by which dry purge air is
introduced into vacuum chamber 14 is desirably adjusted and
controlled by controller 76. Typically during a twenty (20) minute
vacuum cycle, purge air will be introduced into vacuum chamber 14
six (6) times.
[0102] Controller 76 controls and allows adjustment of the heat
provided to heating hopper 12.
[0103] While the vacuum dryer of the invention produces dried
material in batches, the dryer is a continuous supplier of suitably
dry material for molding or extrusion. Dry material may be
withdrawn from retention hopper 16 on a continuous basis. Vacuum
chamber 14 preferably processes one batch of material every 20
minutes, which is sufficient to keep retention hopper 16 and any
process machine being fed by retention hopper 16 supplied on a
continuous basis.
[0104] The vacuum dryer of the invention uses fresh air without
recycling any air in the dryer. Air coming into the dryer is used
once and goes out of the dryer; there is no recycling of air in
this embodiment of the invention.
[0105] The load cells, together with controller 76, facilitate
tracking throughput of granular resin material by the vacuum dryer
of the invention, permitting optimization of manufacturing
parameters in the plastic molding or extrusion facility in which
the dryer of the invention is located.
[0106] During operation, vacuum is drawn by Venturi vacuum
generator 28 from vacuum chamber 14 via vacuum drawing conduit
90.
[0107] Incoming compressed air from the plastics molding or
extrusion facility is supplied to pressure regulator 100 as
indicated FIG. 3. This regulated pressurized air, with pressure
regulated to a required level, is then supplied via regulated
pressure air line 106, which splits as illustrated in FIG. 3 with
one portion of line 106 leading to oil separating coalescing filter
32 and the other portion of line 106 leading to Venturi vacuum
generator 28. An exhaust line 92 leads from Venturi vacuum
generator 28 to ambient air.
[0108] Purge air is provided via purge air supply line 94 which
exits compressed air membrane dryer 34 and supplies purge air in
very dry form after exiting dryer 34 to both retention hopper 16
and to vacuum chamber 14. Introduction of purge air to retention
hopper 16 is controlled by valve 96, which in turn is actuated by
controller 76. Introduction of purge air to vacuum chamber 14 is
controlled by vacuum chamber purge air valve 98, which in turn is
also controlled by controller 76. The wiring for connection of
valves 96, 98 and the other components to controller 76 is not
illustrated in the drawing to enhance the drawing clarity.
[0109] Flow of granular plastic resin material downwardly from
heating hopper 12 to vacuum chamber 14 is desirably through a first
conduit 102. Flow of dried granular resin material from vacuum
chamber 14 to retention hopper 16 is desirably through a second
conduit 104. Conduits 102, 104 are respectively mechanically
connected, preferably substantially air tightly, respectively to
heating hopper 12, vacuum chamber 12 and retention hopper 16.
[0110] Gates 60, 62, 64, and 66 have been illustrated in FIG. 3
positioned respectively in the bottom of heating hopper 60, at the
top and at the bottom of vacuum chamber 14, and at the top of
retention hopper 16. These gates may desirably be positioned in
respective first and second conduits 102, 104 according to the
manner of selected construction for the flow through vacuum
dryer.
[0111] It is desirable to have two gates, such as gates 60, 62,
above vacuum chamber 14 to control downward flow of resin from
heating hopper 12, with an upper gate 60 providing gross, course
control and a lower gate 62 providing air tight vacuum sealing of
the vacuum chamber. Use of the two gates, 60, 62, with course
control afforded by upper gate 60, minimizes the possibility of
resin material becoming stuck in gate 62 and thereby precluding
gate 62 from making the vacuum tight seal required for effective
operation of vacuum chamber 14 during the drying phase. Desirably,
gate 62 is a slide gate providing a vacuum tight seal using a
rubber gasket, with the movable slide portion of the gate closing
against the rubber gasket and moving first in a direction laterally
across, with respect to the direction of downward flow of resin,
and then vertically parallel with the direction of downward flow of
resin, with such horizontal and then vertical movement of the gate
effectuated by the shape of the slot in which the slide gate
moves.
[0112] Material gate 64 may similarly be a slide gate or may be a
pivoting gasket-equipped gate actuated by an air cylinder with the
gate pivoting downwardly to effectuate downward flow of dried
plastic resin material out of vacuum chamber 14 upon the conclusion
of the vacuum cycle. Use of a pivoting-type gate at gate 64 reduces
cost over the cost of a slide gate since gravity will carry any
residual granules of plastic resin material downwardly through
second conduit 104 into retention hopper 16. Gates 60 and 66 may be
of any suitable type, desirably actuated by air cylinders
controlled by controller 76.
[0113] All components illustrated in FIG. 3 are controlled by
controller 76. This includes the drive 30 for centrifugal blower
22, heating element 23, the various gates that control the flow of
resin downwardly through the dryer, the load cells that detect
weight thereby allowing the computation of amount of material
flowthrough, and the like. Controller 76 controls all aspects of
the operation of the dryer and once the dryer is started, human
intervention is not necessary. Of course, controls provided on
controller 76 to allow human intervention if desired.
[0114] Conventional industry practice is to dry, then blend, and
then process granular resin material using a desiccant dryer, then
a gravimetric blender and then a molding machine. The dryer of the
invention facilitates reversal of the first two stages of that
process, namely permitting drying to be done after measuring and
blending. This is advantageous because of problems associated with
desiccant dryers including separation of the blend resulting in a
large quantity of resin material being already preblended that
might not be usable in the event of such separation. This is the
reason desiccant dryers are conventionally used prior to or
upstream of gravimetric blenders in the plastics molding industry.
Since the invention facilitates drying of granular material after
the measuring and blending of such material, the invention
eliminates risk involved in storing preblended material, namely
separation of the blend which may render the material unusable.
[0115] With dryers of the invention, removal of moisture is on the
order of 2/10 of 1% of the weight of the material so there is no
adverse effect on the blend or on the proportions of the blend that
have been effectuated by a gravimetric blender positioned upstream
of a dryer in accordance with the invention.
[0116] Dryers in accordance with the invention uniformly and
consistently exhibit a six-fold reduction in drying time over that
experienced using conventional desiccant dryers when drying
granular plastic resin material prior to molding or extrusion. Such
conventional desiccant dryers rely entirely on blowing warm air
over the plastic material and having the warm, dried air absorb
moisture out of the plastic material of interest.
[0117] In the dryers according to the invention, especially the
embodiments shown in FIGS. 1 and 2, the vacuum drawn during the
drying process can be as low from one to three inches of mercury
short of absolute vacuum. Hence, under standard conditions these
dryers preferably develop a vacuum of from 27 to 29 inches of
mercury in the vacuum drying canister.
[0118] Preferably a dryer supplies hot air to heat granular resin
material at fill and heat position 100 at a temperature as high as
260.degree. F. or even as high as 300.degree. F.
[0119] In a typical application where a molding machine may require
100 pounds per hour of processed, dried, ready to mold plastic
resin, a dryer in accordance with the invention can supply about
105 pounds of material per hour, ready to be processed by the
molding machine.
[0120] For a molding machine operating with a conventional
desiccant dryer supplying granular resin material at the same 100
pounds of material throughput per hour, a desiccant dryer having
capacity of 400 pounds would be required in order to provide the
100 pounds per hour of material, due to the four hour desiccant
drying time.
[0121] Dryers in accordance with the invention take up less space
and generally provide a more efficient operation for a molder than
a conventional desiccant dryer.
[0122] A desiccant dryer process requiring 100 pounds throughput of
material per hour requires a four hour lead time since such a
desiccant dryer typically requires four hours to provide the first
batch of material at acceptable dryness. In contrast a dryer in
accordance with the invention only needs 40 minutes or less to
provide the first batch of material at acceptable dryness for
startup of the molding operation.
[0123] A new color may be introduced into the drying procedure
while the preceding color or final batch of plastic resin material
with the preceding colors is being dried and delivered. Hence there
is no interruption in operation of the dryers of the invention in
order to change colors of the granular resin material being dried.
In contrast, a conventional desiccant dryer would require four
hours of down time in order to change the color of the granular
plastic resin being dried.
[0124] Dryers in accordance with the invention make economical the
recycling of nylon scrappage which heretofore has been practical
due to the drying time required for such scrappage. When nylon is
conventionally processed and scrap nylon results as a byproduct of
the process, in some cases it may take up to three days, using
known methods and equipment, to dry the scrap nylon to a sufficient
extent that the nylon can be reground and reprocessed. A dryer in
accordance with the invention has been tested experimentally on
such nylon recyclage and has been found to adequately process the
nylon recyclage in six hours, amounting to a 92% reduction in
drying time over that known heretofore. Hence, use of a dryer in
accordance with the invention may provide a source of continuous
supply of dried reprocessable nylon for recycling which has
heretofore not been practical due to the affinity of nylon for
moisture and the length of time it has taken to dry nylon recyclage
to a sufficient degree to make it processable in a recycling
mode.
[0125] Another important advantage of the invention is that plastic
resin material being dried is exposed to heat for a much shorter
time than with known methods, thereby reducing the risk of plastic
degradation due to exposure to heat. Many molding materials,
especially more expensive molding materials, are highly sensitive
to exposure to heat. These materials, commonly referred to as
"engineering" materials, include nylon, PET and various
polycarbonates.
[0126] The foregoing describes the preferred embodiment and
alternate embodiments of the invention and sets forth the best mode
contemplated for carrying out the invention in such terms as to
facilitate practice of the invention by a person of ordinary skill
in the art. However, it is to be understood that the invention has
many aspects, is not limited to the structure, processes, methods
and embodiments disclosed and/or claimed, and that equivalents to
the disclosed structure, processes, methods, embodiments and claims
are within the scope of the invention as defined by the claims
appended hereto or added subsequently.
[0127] In the claims, "comprising" means "including, but not
limited to", while "consisting of" means "having and no more", with
both definitions being in accordance with conventional patent
application prosecution procedure in the United States Patent and
Trademark Office.
* * * * *