Electrical Heating Bands For A Feeding System

Abstract

Heating bands and cartridge heaters, adapted to be mounted about cylindrical chambers, manifolds and nozzles conveying heated plastic from a feeding system to a molding assembly, are arranged so that spacing between heated elements of the bands can be increased or decreased depending upon whether one or more of the elements is energized. The bands are comprised of dual or multiple wound coils to provide fail-safe operation for the nozzles in the event one of the coils should be inoperative. A useful method to achieve the multiple winding is to first generate at least a dual threaded geometric, and then lay insulated conductors in the groovings between windings so that strands of independent coils are in parallel side-by-side relation throughout.

Description

BACKGROUND OF THE INVENTION

This invention relates to the dispensing of viscous material, such as thermoplastic, and particularly to an apparatus for maintaining the material in a heated condition while being supplied to a molding assembly. In the past, radiant heating has been used to heat flow channels conveying thermoplastic material, the heat being radiated from coils supplied uniformly with three-phase electrical power to all coils. Each coil was independently defined but not independently powered; by that it is meant that each electrical strand is defined as a single coil wound in either a serpentine or helical configuration with no other electrical heating strands interposed between the beginning and ending of such strand. Premature failure of one of the coils would leave large unheated zones. This is a particularly troublesome problem in that the relative value of the heating element itself is small in comparison to the resulting economic detriment caused by the downtime of a press worth considerable in excess of the heating element itself.

One prior art approach to solving the premature failure of a heating element is to stagger the separate heating elements in bands and to divide energization of the plurality of electrical bands in at least two groups, each group having its own separate power supply. The hope being that in the event of failure of one of the power supplies or bands there is another group of bands which can continue to operate. However, it has become apparent through experience that uniform heating of the viscous material by radiant means cannot tolerate very wide gaps between the radiant energy elements without detrimentally effecting the viscous material itself.

Thus many injection molding shops have attempted to compensate for the irregular and discontinuous type of heating resulting from an operational failure in one electrical line, by increasing either pressure or prolonging the time dwell of the material within the flow channels. However, the increase in pressure or the increase in time has lead to many typical types of molding defects as well as excessive wear and premature failure of the molding assembly.

Still another problem is the inflexibility of varying the spacing between strands of the radiant heating elements in order to meet the needs of various stages of the injection molding process. For example, during the initial heat-up period, the demand for thermal energy is very high and the closer spacing of the electrical strands in a specific winding would be desired. Unfortunately, as certain elevated temperature levels are reached, excessive operating temperatures occur at the central region of an independent winding as opposed to the somewhat lower temperatures occurring at the extremeties of the winding. It has been proposed in the art that a plurality of consecutive fixed windings should be arranged to comprise a heating band and at the central region of a band the windings would be more widely separated and spaced so as to alleviate this nonuniform condition. However, this sacrifices uniformity in the rapid-heat up stage.

SUMMARY OF THE INVENTION

The principal object of this invention is to provide a more effective heating sleeve which may be disposed about one or more portions of a heated feeding system, the latter being used for introducing viscous material to the mold of an injection molding device. The heating sleeve has electrical resistance elements wound in a pattern with independently energized elements closely interleaved so that each traverses substantially the same zone as the other elements whereby failure of at least one of the elements does not cripple the operating capability of the entire sleeve.

Another object is to provide a unique disposition of independently energized heating strands coupled with control means for effective staged heating as well as a capability for selective spacing between commonly energized strands.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a heated feeding system adapted for conveying viscous thermoplastic material;

FIG. 2 is a schematic view illustrating a typical power supply arrangement for the heated feeding system of FIG. 1;

FIG. 3 is one embodiment of one heating sleeve or band adapted for placement about a portion of the system of FIG. 1; and

FIG. 4 is another embodiment of a heating band adapted for use similar to the embodiment of FIG. 3.

DETAILED DESCRIPTION

Referring now to the drawings and initially to FIG. 1, a feed system 10 is illustrated useful in acting as a heated conveyance for viscous material, such as thermoplastic, to be communicated between the entrance 9 of a manifold A and the exit portion 8 of the plurality of nozzles B. The manifold A receives thermoplastic material from an injection cylinder and delivers the thermoplastic to six stations at the mouth of a mold cavity of a conventional molding assembly (not shown). The system 10 of FIG. 1 can be characterized as runnerless molding in that conventional runners, usually defined in the molding die block and fed from a single source, are eliminated; instead, a plurality of heated nozzles communicate directly with the gate or mouth of the molding die block. The runnerless molding approach has the advantage of eliminating scrap losses and time delays due to reclaiming the scrap.

The feeding system here particularly comprises the manifold A having an H configuration with the legs 12 and 13 of the H interconnected by a cross member 11. The manifold is formed of a heat-conductive material such as low carbon steel and contains a central channel 14 (see FIG. 2) in each of the legs and member, effective to convey the viscous material from the entrance 9 to the various nozzles B. The legs and cross member each are maintained in a heated condition by means D having resistance elements 28 and 29 insulated at 7 embedded in the walls thereof and independently energized. The elements 28 and 29 are continuous and extend throughout the side walls of the manifold; the elements are sized to impart a considerable quantity of heat.

Each of the nozzles B (here six in number; 5, 15, 16, 17, 18 and 19) comprise a cylindrical body of heat conductive material having a central channel 20 defined therein adapted to communicate with the riser 6 leading from the channel 14. Each nozzle has a flat exit face 21 adapted to mate with the complimentary surface of a molding assembly for maintaining a tight seal therebetween.

The improved heating apparatus herein comprises a plurality of heating sleeves or bands, 22, 23, 24, 25, 26 and 27 adapted to be mounted as a group on one and each of the nozzles; the bands are arranged at spaced locations along the axis of the nozzle for imparting radiant heat to the nozzle and thence to the viscous material to maintain a selected temperature in material. Each sleeve or band (see FIG. 3) has at least two electrically conductive strands 31 and 32 wound to form a single layered configuration having the independent strands 31 and 32 interleaved and in side-by-side relationship throughout the configuration. Circuit means E is connected to the extremeties of the strands 31a, 31b, 32a, 32b so as to selectively and independently energize each of said strands. Each strand extends substantially throughout the entire zone of heating to be affected by each sleeve on each nozzle. It is important that the spacing 33 between adjacent independent strands be no greater than about five times the transverse dimension of a strand.

Circuit means E is here preferably shown to comprise two independent circuits, circuit 40 being commonly connected to certain strands in all of the bands and circuit 41 being commonly connected to other strands in all of the bands. In the event of failure of one of the circuits, the failed circuit can be shut down leaving the other strands operable in each of the heating sleeves to maintain uniform heating unattainable by the prior art.

To enable the heated feeding system to provide a staged heat-up sequence, the circuit means E has a variable voltage control connected in each circuit for selectively allowing for initially ultra-high voltage to obtain a rapid heat input and at controlled points the voltage can be reduced to maintain uniform temperature levels. In addition, each heating sleeve may be arranged with several strands acting in groups, each group commonly energized with strands of each group interleaved in sequence. For example, group one may have strands one and four, group two has strands two and five, and group three has strands three and six. When wound, the strands are arranged in side-by-side relation and in the sequence of one-two-three-four-five-six. Thus spacing between heated strands may be varied. If all groups are energized, a condition may be reached when excessive heat is generated in the central region of the sleeve. To correct this, one of the groups may be deenergized or reduced in current to equal a larger unheated air space between remaining energized groups. Alternatively, one of the groups may be relatively short and located only at the central region of the sleeve; thus deenergization of this one group will have a more direct effect on only the central region.

Various winding arrangements can also be achieved within the framework of this invention. In FIG. 3, the interleaved strands are wound in a helical pattern about the axis 34 of the sleeve. In FIG. 4, the strand group is wound in a toroidal pattern about the axis 34.

A method for fabricating the winding of a sleeve comprises:

a. Providing a double (or multiple) threaded geometric generation on the periphery of a fixture; continuous adjacent grooves will result as provided by the threads;

b. Insert an independent continuous insulated electrical conductor in each groove of the generation;

c. Bind the winding in the configuration thus assumed in the generation by such means as a suitable supporting wrap 35 having one location 36 where the extremeties of the conductors may extend through and beyond for connection to a power source;

d. Collapse the fixture and remove from the winding without disturbing the winding configuration.

As alternative embodiments to the heating bands illustrated in FIGS. 3 and 4, the inventive winding can be utilized in cartridge heaters adapted to be immersed within the viscous plastic rather than surround a chamber, manifold or nozzle as previously disclosed.

Claims

1. In an injection molding machine, having a runnerless molding assembly and a heated feeding system, said feeding system having legs of heat conductive material and channel therein for thermoplastic material and interconnecting an injection means with a plurality of nozzles of heat conductive material communicating with the molding assembly, the improvement comprising a heating apparatus for the nozzles of said feeding system:

a. a plurality of electrical heating strands insulated one from the other and placed in adjacent side-by-side relation and wound to form a single layered annular shape while in said side-by-side relation, said winding surrounding at least one nozzle of said feeding system,

b. electric circuit means for independently energizing said heating strands and selectively effective to energize less than all of said strands at any one time, and

2. The apparatus of claim 1, in which said circuit means is arranged so that said heating strands within a winding are energized in independent groups, said groups being arranged so that strands alternate in the

3. The apparatus as in claim 2, in which each strand extends substantially throughout the zone of effectiveness of said heating apparatus so that in the event of failure of one of the groups of heating strands, at least one

4. An apparatus as in claim 1, in which said heating apparatus comprises strands arranged in a pattern to form a sleeve with independent strands interleaved so that each strand traverses the same zone of the sleeve as the other strands whereby failure of one strand does not destroy the

5. An apparatus as in claim 3, in which the spacing between adjacent strands within the winding is no greater than five times the transverse

6. The apparatus as in claim 1, in which said strands are arranged in groups independently energized and control means effective to add to or subtract from those groups being energized to increase or decrease the spacing between adjacent strands and thereby promote uniform heating

7. The apparatus as in claim 2, in which the strands are wound in a helix

8. The apparatus as in claim 2, in which the strands are wound toroidally

9. The apparatus as in claim 1, in which said circuit means further comprises control means for independently varying the voltage applied to said strands, and switch means for providing a staged heat-up sequence whereby one or more of the strands may be simultaneously energized at different voltage levels to effect a desired heating condition.