U.S. patent number 3,776,145 [Application Number 05/243,507] was granted by the patent office on 1973-12-04 for slip pallet.
This patent grant is currently assigned to Best Quality Plastics, Inc.. Invention is credited to Raymond F. Anderson, Ronald C. Mackes.
United States Patent |
3,776,145 |
Anderson , et al. |
December 4, 1973 |
**Please see images for:
( Certificate of Correction ) ** |
SLIP PALLET
Abstract
A slip pallet formed as a square or rectangular sheet of a
synthetic resin of selected types of polyolefin. The sheet is
proportioned to fit upon a platen lift and includes tabs at each
edge to be gripped when the platen is thrust underneath the sheet
or the sheet is pulled upon or pushed off from the platen. The slip
pallet will ordinarily carry loads of boxes formed as interlocking
layers and an important feature of the invention resides in
providing a sheet having a greater coefficient of friction at the
upper surface whereon the boxes are placed, than at the
undersurface which is contacted by the platen.
Inventors: |
Anderson; Raymond F.
(Evergreen, CO), Mackes; Ronald C. (Northglenn, CO) |
Assignee: |
Best Quality Plastics, Inc.
(Denver, CO)
|
Family
ID: |
22919019 |
Appl.
No.: |
05/243,507 |
Filed: |
March 27, 1972 |
Current U.S.
Class: |
108/57.16;
108/51.3; 108/901; 428/121 |
Current CPC
Class: |
B65D
19/36 (20130101); B66F 9/195 (20130101); Y10T
428/2419 (20150115); Y10S 108/901 (20130101) |
Current International
Class: |
B65D
19/00 (20060101); B65D 19/36 (20060101); B65d
019/00 () |
Field of
Search: |
;108/51-58 ;206/65R,6A
;214/1.5R ;217/43A ;220/97B |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Gilliam; Paul R.
Claims
We claim:
1. A slip pallet for holding a load of cardboard boxes, or the
like, and carried by a platen which slides underneath the slip
pallet and its box load for loading and unloading the same onto and
off from the platen, said pallet comprising: the platen, said
pallet comprising:
a tough, pliable sheet of single thickness of synthetic
thermoplastic resin having a thickness in the range of
approximately 20 to 125 mils;
a central area whereon the box load is placed;
a tab means alongside at least one edge of this central area folded
upwardly along the edge of the central area to be gripped and held
whenever the platen is moving underneath the pallet from the tab
side; and
a modified upper surface at the central area adapted to enhance the
contact thereof with the cardboard surfaces of the box load upon
the pallet sufficient to increase the static frictional resistance
between the box load and the pallet to a degree which exceeds the
sliding frictional resistance between the pallet and the
platen.
2. The slip pallet defined in claim 1, wherein:
said thermoplastic resin is a polyolefin resin characterized by a
tensile strength in the range of 2,500 to 4,500 psi and a vicat
softening point temperature in excess of 250.degree. F.
3. The slip pallet defined in claim 2, wherein:
the said modified upper surface is a texture having a roughness
such that the coefficient of static friction between the box load
and upper surface exceeds the sliding coefficient of friction
between the platen and the undersurface of the slip pallet by at
least 0.15.
4. The slip pallet defined in claim 1, wherein
said sheet is a laminate consisting of an upper layer and a lower
layer of different synthetic thermoplastic resin materials as a
unitary sheet with the material of the upper layer, forming said
modified surface, having a natural frictional resistance
significantly greater than that of the lower layer.
5. The slip pallet defined in claim 1, wherein:
said sheet is a laminate consisting of an upper layer of fibrous
absorbant material and a lower layer of synthetic thermoplastic
resin material as a unitary sheet.
6. The slip pallet defined in claim 2, wherein:
the upward tab fold includes a permanent crease at the fold.
7. The slip pallet defined in claim 2 wherein the thermoplastic
resin is polypropylene and the upward tab fold includes a permanent
crease at the fold.
8. The slip pallet according to claim 7 wherein the said modified
upper surface is produced by a texture having a roughness not less
than approximately 60 micro-inches, the undersurface of the central
area is modified by texturing the same to produce a toughness of
not over approximately 30 micro-inches, but sufficient to permit
air to be present between the pallet and the platen and the
coefficient of static friction between the box load and upper
surface exceeds the sliding coefficient of friction between the
platen and the undersurface of the slip pallet by at least
0.15.
9. A slip pallet according to claim 1 wherein when in use the
coefficient of static friction between the box load and the upper
surface exceeds the sliding coefficient of friction between the
platen and the under surface of the pallet by at least 0.15.
10. A combination of the slip pallet of claim 1 with its upper
surface supporting a load of cardboard boxes, or the like, and its
lower surface being on a platen, and the coefficient of static
friction between the box load and the upper surface exceeding the
sliding coefficient of friction between the platen and the under
surface of the pallet by at least 0.15.
Description
This invention relates to pallets, and more particularly to
improvements in slip pallets.
The background of the invention concerns the practice of
palletizing merchandise, as in warehouses. Canned goods, bottled
goods, beer and the like are ordinarily packaged in cardboard boxes
which are proportioned to hold several dozen cans to permit a man
to easily handle a single box. However, over the past 20 years, the
practice of handling boxed merchandise stacked on pallets with
forklift trucks has become a standardized warehouse procedure
because of the enhanced economies possible over manually handling
large number of boxes. A common type of pallet is formed as a
flat-board surface supported upon short, spaced rails so they are
about four inches thick. In use, the forks of a forklift truck move
into the pallet underneath the flat surface between the rails to
lift the pallet and the stack of merchandise upon it.
A more sophisticated development in this art involves the use of
slip pallets which are ordinarily made of flat sheets of chip
board, or similar heavy, cardboard-like material in the order of
1/8-inch thick. In use, a slip pallet is placed upon a flat surface
to support a stack of boxes upon it. Slip pallets are desirable
over regular pallets in many installations because they are much
cheaper and because the space savings possible with the thin
pallets is significant in many warehouses. A special type of
lifting surface is required for slip pallets, and the forks of a
forklift truck cannot be used. Instead, a flat, spatula-like
member, commonly called a platen, is mounted upon the lift truck.
In using a platen lift to pick up a load consisting of a stack of
boxes, the platen is elevated so that its extended leading edge is
beneath the slip pallet and it is then pushed underneath the slip
pallet to engage and pick up the slip pallet and its load.
Subsequently, the truck will move the load to where it is to be
relocated, and the load and the slip pallet are pushed off the
platen.
Several problems have been encountered in developing the slip
pallet system. For example, it was found necessary to grip the edge
of the slip pallet and hold the same in tension whenever the platen
was moved underneath it to pick up the load. In unloading the
platen, it was found necessary to use a positive pushing action to
push the slip pallet and the load off the platen. Thus, a lift
truck carrying a platen is provided with a special push-pull
attachment which overlies the platen. This attachment includes a
transverse abutment wall which extends to the leading edge of the
platen and retracts to the opposite back of the platen. A gripper
along the lower edge of the abutment wall will grip a pull tab at
the edge of the slip pallet to assist in holding the pallet and its
load while the abutment wall is retracting and the platen is being
pushed underneath the pallet. The abutment wall will thereafter
extend to push the pallet and its load off the platen.
Chipboard sheets, heretofore found to be the best suited for slip
pallets and almost universally used for the purpose, are usually
damaged every time a load is moved. Often, when the pull tab of the
pallet is being held while the platen is being pushed underneath
it, the pull tab will be torn away and the pallet will buckle
against the pressure of the platen. Also, in pushing the pallet and
its load off the platen, the frictional resistance between the
pallet and the platen will damage the chipboard. As a result, an
average chipboard slip pallet can be used only once or twice before
it must be replaced.
Heretofore, various experiments with several types of materials
have been conducted in an effort to replace chipboard type slip
pallets and such experiments have generally been unsuccessful for
various reasons. Apparently, the slip pallets must have certain
properties of rigidity and toughness over a wide range of
temperatures. Also, the frictional resistance between the surface
of a pallet and the platen and the frictional resistance between
the surface of the pallet and the box materials carried on the
pallet must be in selected ranges in order for the pallets to
function properly. The needed necessary properties have not been
readily apparent nor easily defined.
The present invention was conceived and developed with the
foregoing and other considerations in view and comprises, in
essence, a slip pallet formed as a rectangular sheet of a selected
type of thermoplastic polyolefin resin. The form of the plastic
pallet is similar to that of a conventional chipboard slip pallet.
However, it was found that the plastic sheet had to be specially
constructed and that more than a mere substitution of materials was
required. Not only was it necessary to use plastic sheets having
selected properties, but also, the plastic sheets had to have
differing physical properties at their upper and lower surfaces.
The plastic slip pallet is also formed with gripping pull tabs
about its edges which are angled upwardly by shallow compression
grooves to permit easy gripping by the push-pull attachment above
the platen on the lift truck, without impairing the strength of the
pallet, all as hereinafter set forth in detail.
It follows that the primary object of the present invention is to
provide a novel and improved plastic slip pallet which overcomes
the objections directed towards conventional chipboard pallets.
Another object of the invention is to provide a novel and improved
plastic slip pallet which can be used and re-used many times
without being damaged, and which, when eventually damaged or
discarded, can be recycled to be formed into a new pallet.
Another object of the invention is to provide a novel and improved
plastic slip pallet which is exceptionally tough and strong and
which will withstand repeated rough usage at differing temperature
and humidity environments found in various warehouses.
Another object of the invention is to provide a novel and improved
plastic slip pallet having its surfaces selectively finished to
render the pallet effective in holding loads upon the pallet, while
at the same time permitting a platen to easily slip underneath the
pallet when loading and unloading the pallet upon the platen.
Other objects of the invention are to provide, in a novel and
improved plastic slip pallet, a unit which is simple, versatile,
rugged and tough and which provides a desirable economy and
efficiency in warehousing operations.
With the foregoing and other objects in view, the present invention
comprises certain constructions, combinations and arrangements of
parts and elements as hereinafter described, defined in the
appended claims, and illustrated in preferred embodiment by the
accompanying drawing in which:
FIG. 1 is a small scale sketch showing a side elevational view of a
lift truck carrying a platen and a push-pull attachment above it,
and showing further, two loads of boxes stacked, one upon the other
and separated by a slip pallet, the figure being exemplary of the
manner in which the slip pallets are used and the type of apparatus
used to handle them.
FIG. 2 is a perspective view of a slip pallet per se, constructed
according to the present invention.
FIG. 3 is a plan view of a corner portion of the pallet shown at
FIG. 2, but on an enlarged scale.
FIG. 4 is a fragmentary sectional portion as taken from the
indicated line 4--4 at FIG. 3, but being somewhat exaggerated in
thickness to better illustrate the construction thereof.
FIG. 5 is an isometric view of a portion of the stacked box loads
shown at FIG. 1, showing pull tabs of the slip pallets protruding
from the sides of the stack.
FIG. 6 is a perspective view of the platen of the lift truck with
its push-pull attachment being extended to the end of the platen
and gripping the tab of a slip pallet, the view depicting further,
in dotted lines, the outline of a box load upon the slip pallet and
also, the retracted position of its push-pull attachment.
FIG. 7 is a fragmentary sectional view showing a portion of the
stacked box loads with the boxes being separated by a slip pallet
and with the pallet being gripped by the push-pull attachment of
the platen lift preliminary to picking up the pallet and the box
load upon it.
FIG. 8 is a view similar to FIG. 7 showing the platen as being
thrust underneath the slip pallet as a step in picking up the
pallet and the box load upon it.
FIG. 9 is a diagrammatic view similar to FIG. 8, but showing the
slip pallet and the box load as being carried by the platen.
FIG. 10 is a fragmentary sectional perspective detail as taken from
the substantially indicated line 10--10 at FIG. 2, but on an
enlarged scale.
FIG. 11 is a fragmentary sectional view of the pallet, as taken
from the indicated line 11--11 at FIG. 10, but on a greatly
enlarged and exaggerated scale to better indicate, in a somewhat
diagrammatic manner, the differences in texture between the upper
and lower surfaces of the sheet.
FIG. 12 is a fragmentary sectional view, similar to FIG. 11, but
showing the pallet as being manufactured from a laminate.
FIG. 13 is a fragmentary sectional view similar to FIG. 11, but
showing another modified construction where the material forming
the pallet is a laminate of a plastic sheet and an absorbent,
cloth-like material.
FIG. 14 is a diagram indicating various frictional coefficients
which are significant in considering the behavior of a loaded
pallet when a platen is being thrust underneath it.
FIG. 15 is a fragmentary digrammatic view similar to the showing at
FIG. 8, but on an exaggerated scale to better illustrate the
behavior of a loaded slip pallet whenever a platen is being thrust
underneath it.
FIG. 16 is a diagrammatic view similar to FIG. 15, but illustrating
the behavior of a loaded slip pallet whenever the platen is being
pulled out from underneath it.
Referring more particularly to the drawing, a typical use of a slip
pallet is illustrated at FIG. 1, which is exemplary of the manner
in which box loads B are stacked in a warehouse upon slip pallets
P. A slip pallet per se is illustrated at FIG. 2 as a flat,
rectangular or square sheet having pull tabs as hereinafter
described in detail. First, however, the manner in which slip
pallets are used and the apparatus for using them will be described
to provide a better understanding of the operative problems
encountered in the use of slip pallets and the need for the
improvements disclosed in the present invention.
In the usual storage operation, the length and width of boxes are
proportioned in such a manner as to permit a box load B which is
carried upon a slip pallet P to be made up of a selected number of
layers of boxes with each layer of boxes forming a rectangle or
square of selected proportions which is the same size as the slip
pallet P. The boxes of each layer in the load are arranged in an
interlocking pattern with respect to the boxes of adjacent layers
to maintain the integrity of the load. In the drawing, each load of
boxes B is depicted as having a square base with six boxes in each
layer and being three layers high and with the boxes in the layers
arranged in an interlocking pattern. To complete each box load, a
plywood sheet W is placed upon the top of each box load to form a
protective surface whereon a slip pallet P of the upper load will
rest. Two such box loads are illustrated, with one being stacked
upon the other and with each box load being carried upon a slip
pallet P. Such is illustrative only and the number of layers in a
box load may be considerably more than the three illustrated. Also,
the number of loads forming a stack can vary and such will depend
entirely upon the size and weight of the boxes, and the capacity
and reach of the platen lift L.
The platen lift L, which will pick up a box load upon a slip
pallet, will be carried upon a truck T of a type ordinarily
referred to as a fork lift truck and indicated in a somewhat
diagrammatic manner at FIG. 1. This truck T is provided with
driving wheels 20, a steering wheel 21, a suitable power plant, not
shown, and an operator's section 22 where controls for driving the
truck and for raising and lowering and otherwise operating the
platen lift L are located. The platen lift is carried upon a
normally vertical guide 23 at the front of the truck. Suitable
controls and mechanisms will tip this guide 23 forwardly or
rearwardly from its vertical position and raise and lower the
platen lift along the guide, all in a conventional, well-known
manner. The platen 24 of the lift is a comparatively thin,
spatula-like sheet affixed to an upright backstop 25 which, in
turn, is mounted on the vertical guide 23. The width and depth of
this platen 24 is selected to hold a pallet P and to accommodate a
box load B such as in the stack illustrated at FIG. 1. It is
contemplated that even though several types of boxes are used in a
given warehouse operation, they will all be proportioned in such a
manner as to form box loads which fit upon the pallets P and upon
the platen lift L, all in the interest of economy of handling
operations.
As illustrated at FIGS. 1 and 6, the platen lift L will include a
push-pull attachment A, which is mounted upon the backstop 25 of
the platen. This push-pull attachment includes a shiftable abutment
26 held in a vertical transverse position above the platen by a
horizontal swing arm 27 and secured in a transverse relationship
with respect to the edges of the platen by a comparatively wide
upward fold arm 28 attached to the backstop 25 and to the back face
of the abutment 26. Suitable hydraulic rams 29 coact with this arm
to push the abutment wall 26 forwardly and to the forward leading
edge 24a of the platen, as illustrated, and to pull the abutment 26
rearwardly towards the backstop 25 as indicated in broken lines at
FIG. 6. Suitable controls at the operator's section 22 regulate
this push-pull movement of the abutment 26 as well as the raising
and lowering and tipping of the platen 25. It is thus a simple
matter for the truck operator to and the leading edge 24a of the
platen 24 against the under edge of a pallet P when the truck is
moved against a stack of box loads B.
An elongated gripper 30 is pivotally mounted at the back side of
the abutment 26 and it is adapted to swing against the undersurface
31 of the abutment 26 to grip and hold a pallet tab as will be
described. The gripper is operated by conventional hydraulic
pressure members within the framework of the abutment 26 which are
not shown. The undersurface 31 slopes upwardly and rearwardly from
the bottom edge of the front of the abutment and the gripper 30 is
formed as a hook-like member having a projecting, finger-like edge
which swings into and against this surface 31 without projecting
below the front bottom edge of the abutment. Thus, an upturned
pallet tab can be gripped and held at the bottom of the abutment,
with the bottom of the abutment being closely adjacent to the top
surface of the platen 24, and with the platen thus being positioned
to move underneath the slip pallet P.
The improved slip pallet P is a sheet of a selected blend of
polyolefin resin as hereinafter described. This sheet is
comparatively thin for its size and a practical range of thickness
is from approximately 20-mils to 125-mils, and preferably, in the
range of 1/16-inch. This sheet is rectangular or square and is
proportioned to hold a selected box load B and to set upon the
platen 24.
The sheet forming the slip pallet P will include the rectangular or
square central area 35 for the box load and one or more marginal
pull tabs 36 at the edges of this central area. Preferably, a pull
tab will extend from each of the four edges of the central area as
illustrated. Thus, while only one pull tab 36 will be used at a
time, several advantages are to be gained with a pull tab
projecting from each side of the pallet. In the first place, if one
tab is accidentally torn away, the pallet is still useable. Also,
with four tabs, the pallet may be gripped and loaded onto the
platen from any direction.
To facilitate the gripping of a tab 36 at the undersurface of the
abutment, each pull tab is folded upwardly at an angle of about
thirty degrees from the plane of the central area 35 by creases 37.
Such creases may be pressure and/or heat formed in the surface of
the pallet and may be formed at one, or at both sides of the pallet
as illustrated at FIG. 4. To complete the pallet, each corner 38 of
each tab is cut at an angle of approximately 45.degree. with
respect to the tab edge, so a tab corner will not interfere with an
adjacent corner as when the tabs are folded upwardly.
A better understanding of the problems arising in the use of a slip
pallet, when the platen is moved underneath it, is possible by
referring to FIGS. 7, 8, 9, 15 and 16. In moving the platen 24
underneath a slip pallet supporting a box load, the first steps
are: to adjust the elevation of the platen to the elevation of the
pallet; extend the abutment 26; move the leading edge 24a of the
pallet and abutment against the box load; and, grip and hold the
projecting pallet tab 36 at the underside of the abutment, as shown
at FIG. 7. The platen 24 then commences its movement underneath the
pallet and the thin, spatula-like platen lifts the slip pallet and
its load of boxes a small vertical distance, the thickness of the
platen, as illustrated at FIG. 8. When the platen is completely
underneath the slip pallet, as illustrated at FIG. 9, the box load
B may then be lifted and transported by the truck T. It is to be
noted that the preferred mode of loading a slip pallet upon the
platen is to move the platen itself under the box load by movement
of the truck carrying the platen. At the same time, the abutment 26
moves to its retracted position with respect to the platen, but
remains stationary with respect to the box load. It is also
possible to pull the box load onto the platen by moving the
abutment 26 to its retracted position without moving the truck.
However, this latter mode of loading the platen is not recommended
because a much larger pulling force is required on the tab 36 being
gripped by the abutment and this force could tear the tab from the
central area 35, especially if the material forming the slip pallet
does not have a high tensile strength.
Regardless of whether the platen 24 is thrust underneath the slip
pallet P or the slip pallet is pulled onto the platen, the force
pulling the tab 36 will be substantial. This will be caused by the
friction between the undersurface of the pallet and the platen when
the platen moves under the pallet and also, by a ripple 40 which
forms in the pallet ahead of the leading edge 24a of the moving
platen. This ripple 40 will assume various forms such as that
illustrated at FIG. 15 and can become quite serious if the pallet
is not effectively restrained. This ripple also requires that the
material forming the pallet be tough and flexible, for otherwise,
the leading edge 24a of the platen would move into this ripple to
tear the pallet material. The pallet material cannot be brittle,
otherwise the bending action of the ripple would cause it to
crack.
When the box load B and its pallet P are to be discharged from the
platen, the frictional drag between the pallet and the platen is in
the opposite direction to the pull hereinabove described. In
discharging the box load, the abutment 26 holds the box load while
the platen is retracted from underneath it. To accomplish this, the
truck T carrying the platen moves away from the box load while the
abutment 26 is extended over the platen to push off the box load.
This creates a substantial frictional force tending to pull the
pallet from underneath the box load or buckel the pallet. A
buckling action 41 will occur at the edge of the tab gripped by the
abutment, as shown at FIG. 16. If the buckling 41 at this edge is
severe, it can damage the hinge crease 37 to cause the tab to be
torn away from the pallet during a subsequent use. This buckling 41
is especially serious when using chipboard pallets.
It has heretofore been proposed that sheets of synthetic resin
plastic could be used for slip pallets. In considering the problem,
many types of synthetic resins can be eliminated as having
insufficient strength to withstand the pull against the tabs or as
being too brittle to stand up under the ripple action which occurs
when the platen moves underneath the pallet. The factor of cost
also becomes important. A resin plastic sheet having the thickness
of a quarter of an inch or more, such that the pallet is so rigid
that it will withstand all stresses and not permit rippling or
buckling actions, would function satisfactorily but it would be far
too expensive for consideration. Thin, tough plastic sheets of high
strength material were proposed and have been tried, but they have
not heretofore performed satisfactorily. For example, when loading
a box load onto a platen, the rippling 40 would become exaggerated
and the leading edge 24a of the platen would move into a ripple as
its fold and cut the sheet in half. In other instances in
discharging a load, not only would a severe buckle 41 occur at the
abutment 26, but also, an entire sheet would sometimes underfold
upon itself. The tearing away of tabs was also a problem. In
addition, a major problem was that the box load upon the pallet
would shift off the pallet as the platen moved underneath it to
pick up or to discharge the pallet. Thus, it had been concluded
that if a plastic pallet were thin enough as to be
price-competitive with the conventional, cheap chipboards, it would
not be satisfactory.
Observations led to the conclusion that the rippling and buckling
actions of the plastic pallets and the shifting of the loads upon
them were caused by the frictional behavior between the box loads
and the pallets and between the pallets and the platen. For smooth
operation, the upper surface of a platen must slide freely along
the undersurface of a pallet whenever the platen is being pushed
underneath it or pulled therefrom. At the same time, the box load
must not slide or shift upon the upper surface of the pallet. Thus,
it is essential that the static frictional resistance of the box
load B upon the pallet P be significantly greater than sliding
frictional resistance of the platen underneath the pallet. The
jostling and other small moves of the boxes which occur as the
platen moves underneath the pallet, and the very nature of the
material involved, does not permit such desirable friction effects
to occur when using a plastic pallet formed in an ordinary manner
by standard extruding apparatus.
The present invention requires the upper surface of a slip pallet
be textured or otherwise modified to increase the frictional
resistance between the upper surface and the box load upon it.
Also, it is desirable to modify the undersurface of the pallet in a
manner which will decrease the sliding frictional resistance
between the undersurface of the pallet and the platen. To have a
satisfactory pallet, it was found that the static frictional
resistance between the pallet and the box load, which will be
hereinafter designated as a coefficient of friction, should be at
least 0.15 greater than the sliding coefficient of friction between
the platen and the pallet and preferably, this difference should be
as much as 0.25 or 0.30.
Significant differences between the friction coefficients at the
upper and under surfaces of the plastic pallet sheets were obtained
by providing a textured upper surface which was comparable in
roughness to, or even exceeded the roughness of the surface texture
of the cardboard of the boxes forming a box load. Such roughness
produces an interlocking action between the upper surface of the
pallet and the cardboard surfaces of the boxes to increase the
effective frictional resistance to movement. Simple qualitative
tests obtained by sliding small cardboard tabs upon various types
of textured surfaces demonstrated surprising differences between
the frictional resistance of a smooth plastic surface and of
various surface textures. Such a simple test can be used to obtain
an optimum degree of roughness of any selected texture at the upper
surface of the pallet, for any given type of cardboard from which a
box is to be made, in order to obtain the maximum possible
frictional resistance
While it would be anticipated that a minimum frictional resistance
could be obtained between a smooth-surfaced, plastic sheet and the
smooth, steel surface of a platen, it was found that the frictional
resistance between the undersurface of the pallet and the platen
could be minimized if the plastic sheet were roughened to a slight
degree, apparently to permit air to be present between the plastic
and steel surfaces and prevent an undesirable suction effect.
The textured surface of the plastic pallets can best be obtained
when the web from which the pallets are formed is being
manufactured. A standard manufacturing procedure for producing a
web is by extruding hot thermoplatic material into the web form and
thereafter, finishing the web by moving it about sizing and cooling
rolls. The web will have a width sufficient to form a central
section 35 and tabs 36 at each side of this central section. It
will then be cut to length as the pallets are finished to provide
the central section 35 and tabs 36 at all four edges. In a normal
course of production, the web will pass about finishing and cooling
rolls which will impart the smooth, slick surfaces ordinarily found
in sheets of plastic materials. To provide a pallet according to
the present invention, however, one of the finishing and cooling
rolls about which the sheet moves is roughened to produce a
textured surface which will become the upper face of the pallet. A
suitable method for texturing this finishing and cooling roll is by
sandblasting the surface of the roll, but other modes of roughening
can also be used. A sandblasting operation need not be described in
detail, since it is done conventionally for other purposes. It is
to be noted that a suitable degree of texture roughness can be
measured comparatively with known mold surface standards defined in
micro-inches of depth. To assure a comparatively high coefficient
of friction between the surface of a plastic pallet and the paper
material of an ordinary cardboard box, a depth of approximately 60
micro-inches of surface roughness is satisfactory. A much rougher
texture may be used and the ideal texture for any given box
material can be found by simple tests hereinbefore described.
A suitable material for the improved plastic slip pallet having a
preferred thickness of approximately 1/16-inch as heretofore
described, can be selected from several types of polyolefin
thermoplastic resins or compatible blends of the same. However,
such material must be tough and pliable and capable of withstanding
a substantial amount of abuse. A specification of a suitable
polyolefin material is as follows: a tensile strength in the range
of 2,500 to 4,500 psi as determined by the ASTM designation
D638-67T; a vicat softening point temperature in excess of
250.degree. F. as determined by ASTM designation D1525-65T; a
brittle point temperature of at least below -20.degree. F. as
determined by ASTM specification D746-64T; and a stiffness
measurement in the range of 100,000 to 150,000 psi as determined by
ASTM specification D747-63. Where the pallets are to be used in low
temperature environments, as in refrigerated warehouses, the
selected material should have a lower brittle point temperature,
such as in the range of -40.degree. F.
Materials which can be used for the manufacture of slip pallets
according to the present invention include:
1. Polypropylene WM-110, furnished by the Shell Chemical Company of
Houston, Texas. This is a rubber-modified polypropylene, the exact
components of which are proprietory information of the Shell Oil
Company.
2. Profax 7823, furnished by the Hercules Powder Company of
Wilmington, Delaware. This is a copolymer of polypropylene and one
or more other monomers, the exact components of which are
proprietory information of the Hercules Powder Company.
3. Bestflex 401, furnished by Best Quality Plastics, Inc. of
Denver, Colorado. This is a blend of high impact polypropylene and
ethylene hexene copolymer, the exact components of which are
proprietory information of Best Quality Plastics, Inc.
4. PlasKon PP 60-002, furnished by the Allied Chemical Company of
Morristown, New Jersey. This is a linear polyethylene.
5. PlasKon AB 50-003, furnished by the Allied Chemical Company of
Morristown, New pallet This is a copolymer of ethylene and one or
more monomers, the exact components of which are proprietory
information of Allied Chemical Company.
Sample pallets were tested for each of the specified materials and
production runs of pallets were prepared for the Polypropylene
WM-110, the Bestflex 401 and the PlasKon PP 60-002. These
production pallets were manufactured as hereinbefore described,
with a rough textured upper surface as produced by a sandblasted
finishing and cooling roll. The upper surfaces of the pallets had a
surface roughness of approximately 100 micro-inches in depth. The
undersurface of these pallets was roughened slightly by the use of
a sandblasted sizing roll which contacted the web only as a
momentary line contact to produce a surface roughness of
approximately 30 micro-inches in depth. These pallets were used for
handling box loads of packaged beer cans at the Denver warehouse of
the Coors brewery. Each boxload included 120 cartons and the load
weighed approximately 3,800 pounds. The pallets performed
satisfactorily through a large number of repeated uses, with very
few causing trouble, although the tabs of some of the PlasKon PP
60-002 polyethylene pallets did tear away. This was anticipated
since the tabs were subjected to both tension and hinging actions
when loading and unloading box loads and polyethylene does not
effectively resist repeated bending.
Tests were made to establish the validity of the increased
frictional effects by the use of a textured upper surface. Simple
qualitative tests were made by turning pallets upside down to place
the box load upon a smooth surface pallet and these tests quickly
indicated that such an arrangement was not satisfactory because the
pallets would slip and buckle and the box loads would shift
significantly whenever they were picked up by the platen 24. Such
demonstrated the need for a textured upper surface whereon the
boxload was placed. FIG. 14 indicates diagrammatically,
measurements of coefficients of sliding and static friction for the
three materials used for production runs, the Polypropylene WM-110,
the Bestflex 401 and the PlasKon PP 600-02. Friction measurements
between a cardboard surface and a textured plastic surface of a
pallet were significantly higher than the friction measurements
between a smooth steel surface such as the platen 24 and the
undersurface of the pallet. These differences, between the static
friction of a box load upon the textured upper surface of the
pallet and the sliding friction of the platen underneath the pallet
ranged from 0.22 for polyethylene to 0.30 for Bestflex 401 and
generally, it was observed that pallets manufactured from this
latter material appeared to function better although such was a
matter of degree.
Other frictional tests were made. The effect of texture was
measured for the Best flex 401 blend with the results set forth in
the following table:
TABLE I
Effect of Texture Between Cardboard to Plastic (Bestflex 401)
SURFACE COEFFICIENT OF FRICTION (static) Smooth 0.40 Fine Texture
-- approx. 30 micro-inches 0.45 Coarse Texture -- approx. 100
micro-inches 0.55
A similar test was made to show the effect of texture between steel
and the Bestflex 401.
TABLE II
Effect of Texture Between Steel and Plastic (Bestflex 401)
SURFACE COEFFICIENT OF FRICTION (sliding) Smooth 0.35 Fine Texture
0.25
The obvious and apparant explanation of the increasing coefficient
of friction between the plastic sheet and the cardboard as the
texture of the sheet was roughened lies in the fact that the
textured surface of the cardboard tended to interlock with the
rougher texture of the plastic sheet. On the other hand, the tests
of sliding friction between the steel and the plastic sheet showed
an opposite result. A smooth sheet apparently caused air to be
removed from between the steel plate and the smooth plastic sheet
as evidenced by a suction effect between the surfaces. A slight
roughening of the plastic surface apparently permitted air to
remain between these surfaces, as evidenced by the lack of suction.
This textured surface reduced the frictional resistance. The
coefficient of sliding friction between a smooth polyolefin sheet
and a smooth steel plate does not follow the standard pattern of
that of most materials in that it may be greater than the
coefficient of static friction. It is well known that the
coefficient of friction will increase with an increase of
temperature and apparently, the sliding friction produces heat
between the sheet and plate sufficient to produce such an effect.
The slight texturing of the polyolefin sheet apparently retains air
between the sheet and plate thereby reducing the friction and
apparently dissipating the heat.
The above described studies demonstrate that other arrangements are
possible for the manufacture of slip pallets. For example, the
pallet can be formed of laminated plastic sheets as shown at FIG.
12, providing that the upper surface layer 42 has a greater
frictional resistance than the undersurface layer 43. A sample
sheet using Polypropylene WM-110 as the upper surface 42 and
PlasKon 60-002 as the undersurface 43 demonstrated that a laminated
slip pallet would function satisfactorily.
A further modification of the invention was proposed to protect the
box load B against moisture, as where one or more cans in the box
load would leak. The moixture from such a leak would normally be
absorbed by the cardboard boxes and could cause the boxes to break
apart very easily. To avoid this, it was proposed to provide a
laminate having an absorbant sheet at the upper surface as
indicated at FIG. 13. The absorbant layer 44 could be of wood or
cardboard affixed to a substrate 45 of a material such as PlasKon
PP 60-002. Another material for the upper surface which can be used
is a fibrous polyethylene mat known as TYVEC, a registered
trademark of E. I. DuPont of Wilmington, Delaware.
We have now described our invention in considerable detail setting
forth various embodiments and alternatives to the same. However, it
is obvious that others skilled in the art can devise and build
alternate and equivalent constructions and operate slip pallets in
a manner equivalent to that herein described. Hence, we desire that
our protection be limited, not by the constructions illustrated and
described, but only by the proper scope of the appended claims.
* * * * *