Plastic article or earthenware shaping or treating: apparatus – Female mold and charger to supply fluent stock under... – Molding of thermosetting or cross-linking stock
Reexamination Certificate
1998-05-14
2001-01-30
Heitbrink, Tim (Department: 1722)
Plastic article or earthenware shaping or treating: apparatus
Female mold and charger to supply fluent stock under...
Molding of thermosetting or cross-linking stock
C425S572000
Reexamination Certificate
active
06179603
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to injection molding and more particularly to an improved method and apparatus for the injection molding of rubber thermoset polymers.
BACKGROUND OF THE INVENTION
A wide variety of thermoplastic parts are formed by injection molding. The thermoplastic material is typically heated to an elevated temperature such that the material becomes substantially molten and will flow readily under pressure into the mold. To cure the thermoplastic material, it is cooled in the mold until it is sufficiently solidified.
Thermosets, such as rubber, can also be injection molded. However, these materials have different properties than thermoplastics and must be treated differently in the molding process. For instance, to injection mold rubber it is typically heated to a relatively low initial temperature (on the order of about 150° F. to 200° F.) so that is pliable, although not molten or readily flowable, and can be injected into a mold. Once injected into the mold, the rubber cures as a function of its temperature and the time held at an elevated temperature. For instance, rubber will cure relatively rapidly at higher temperatures and will require longer curing times at lower temperatures. Thus, rubber cures in a very different way than thermoplastics and hence, requires a different injection molding process which has its own parameters and presents problems different from those involved in injection molding thermoplastics.
Prior systems for injection molding of rubber have used a single block of steel with the desired passages or runners formed therein to communicate the main injection nozzle with secondary nozzles through which the rubber is injected into the mold. To form the runners in the single block of steel a plurality of holes must be drilled and portions thereof subsequently plugged to prevent leakage from the block. Careful machining of the block to provide the runners is necessary to avoid sharp comers in the passages or runners which may inhibit the flow of the rubber material and eventually cause blockages of the runners.
Furthermore, the injection block is typically at a sufficiently elevated temperature to insure that the rubber remains pliable and can be injected into the mold. When the molding process is interrupted or stopped for a duration, the rubber within the injection block may begin to cure and block the runners preventing subsequent injection molding operations. Blockage of the runners also occurs throughout use of the injection block and especially around comers or other non-linear portions of the runners wherein the flow of rubber through the runners may be inconsistent or inhibited and some of the rubber begins to cure in the runners eventually causing a blockage. With the injection block formed of a single piece of steel, it is extremely difficult to adequately clear a blockage from a runner and often such blockages must be machined from the block, such as by drilling, which can damage the surface of the runners and may thereby cause subsequent blockages. Thus, with an injection block formed from a single piece of steel it is labor intensive and expensive to initially form the runner system therein and plug the open ends, and it is subsequently difficult, labor intensive and expensive to remove blockages from them and they are thus, costly to maintain.
Attempts to form a suitable injection block for the injection molding of rubber from multiple flat plates secured together by fasteners such as cap screws, such as has been done for the injection molding of thermoplastics, have been unsuccessful because after a period of use the fasteners break and the plates are forced apart and damaged. This problem is not encountered with the injection molding of thermoplastics which tend to flow more uniformly through the runners when in their generally molten condition such that the metal to metal seal between adjacent plates is sufficient to prevent the plastic from leaking or creeping between the plates.
SUMMARY OF THE INVENTION
It has been discovered that when injection molding rubber with an injector block having flat plates defining runner passages, the rubber material creeps between the adjacent plates. This is believed to increase the surface area on which the rubber acts under pressure thereby producing a tremendous force acting on the plates which breaks the fasteners, forces the plates apart and damages them. This force becomes so great that it is impossible as a practical matter to releasably fasten the plates together. It is believed this problem does not occur in injection molding of thermoplastic materials because they do not creep between adjacent plates of the injection block.
According to the present invention, an injection block is provided for the injection molding of thermoset polymers, such as rubber, formed from at least a pair of plates bolted or otherwise releasably held together and defining passages of a runner system between the plates. Generally complimentary grooves are formed in each plate to define, when the plates are mated together, an enclosure constructed to receive a sealing member therein to prevent the thermoset material being molded from leaking or creeping from the passage beyond the sealing member between the plates. By limiting the leaking or creeping of the molding material from the runners, the sealing member limits the surface area acted on by the relatively high pressure under which the thermoset material is molded to limit the force tending to separate the plates. With this limited force, the plates may be held together by a plurality of bolts without failure in use of the injection block. Bolting the plates together is desirable because it facilitates readily separating the blocks to clean the passages or otherwise service and maintain the injection block throughout its life.
Typically, the injection block communicates with a vertically disposed injection nozzle of an injection molding machine and distributes the material received from the nozzle through a plurality of runners to one or more secondary injection nozzles which inject the material into the mold. The runners extend initially generally horizontally or perpendicular to the main injection nozzle and then extend vertically through one plate to the secondary injection nozzles which are typically vertically disposed. To improve the flow from the horizontal to the vertical sections of the runners, the cross sectional area of the runner upstream of the turn from horizontal to vertical is reduced to increase the rate of flow of material through that section of the runners. Further, a pickup portion or upwardly more steeply, tapered lower surface of the runners is provided immediately upstream of the turn to direct the flow of material from that surface and generally towards the upper or radially outer surface of the turn of the runners. This improves the material flow around the turn and substantially prevents the stagnation of material in the passage adjacent to the turn to prevent material from curing over time in the runners.
Objects, features and advantages of this invention include providing an injection block for the injection molding of thermoset polymers such as rubber, which is formed of at least two separate plates, limits the creeping of material between the plates to limit the force tending to separate the plates, enables the plates to be releasably held together by a plurality of releasable fasteners such as bolts, facilitates cleaning of the runners and removal of any cured material or blockage therein, has runners designed to decrease the potential for blockages of the runners, can be used with high injection pressures of 20,000 psi or more, is of relatively simple design and economical manufacture and assembly and has a long and useful life.
REFERENCES:
patent: 3776676 (1973-12-01), Kessler
patent: 4826416 (1989-05-01), Majerus et al.
patent: 5032078 (1991-07-01), Benenati
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