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New presses for thermosets & BMC.

A new line of injection molding machines for processing thermoset and unsaturated polyester bulk molding compound (BMC) and other thermosets is being introduced by HPM Corp., Mount Gilead, Ohio. The line encompasses machines in the 150-500 ton range and is available in both toggle or hydraulic clamp versions. It includes an innovative BMC prefeeder to load the material to the injection unit without having to interrupt the molding process.

BMC PREFEEDER

The BMC prefeeding system permits continuous production through the use of a shuttle system to load a fresh charge of material. When processing dry granular thermosets, the prefeeder may be bypassed, allowing the main feeder to be used as a standard material hopper.

The BMC prefeeding system includes a main feeding hopper of approx. 150-lb capacity, mounted vertically over the injection feed chamber. A prefeeding hopper of equal capacity is mounted on a shuttle slide mechanism atop the main feed hopper. A hydraulic plunger is located above the main feeding hopper and alongside the prefeed hopper on the shuttle.

The shuttle system is sequenced to reload the main hopper automatically. A sensor built into the plunger unit monitors the material remaining in the main hopper. Upon actuation of a low-level switch, the main feeder's plunger retracts and a shuttle cylinder moves the prefeeder over the main feeder as the plunger cylinder moves out of normal position. At the same time, the prefeeding hopper slides into position over the main hopper. A drop cylinder on the prefeeder opens the doors on the bottom of the prefeeder assembly, allowing the material to fall into the main feeder. When the hopper is finished being reloaded, the shuttle cylinder moves the plunger back into position. Throughout this cycle, molding continues uninterrupted.

Approximate time for the sequence is 11 sec. The prefeeding system has its own dedicated hydraulics, allowing the reload sequence to occur during the regular molding cycle. (An exception is that reloading cannot take place during the recovery phase, when the main feeder is sequenced to automatically feed material into the barrel.) Material level switches ensure that the main feeder is replenished before full depletion to avoid short shot.

The complete feeder is designed for ease of cleaning and simple conversion to standard thermoset processing. The plunger mechanism is simply slid to one side of the shuttle table, leaving the prefeeder at the other side and allowing the main feeder to be used as a standard material hopper for conventional thermosets.

SPECIAL FEATURES FOR THERMOSETS

In addition to the prefeeding system, the machines are equipped with a zero-compression, nitrided screw of 12:1 L/D. The screw is said to be suitable for molding both conventional thermosets and BMC, making it economically feasible to mold both products. A smear tip and open nozzle provide minimum material retention and prevent precuring of material in the barrel. However, a special screw with nonreturn valve in lieu of the smear tip is available for BMC processing, reportedly providing excellent shot control without damage to reinforcing fibers.

Water jackets are provided for three zones of barrel cooling, with temperature monitoring through the machine control. Four zones of mold heating control are also provided. Sprue-break circuitry allows the nozzle to retract from the sprue bushing an adjustable amount during single or automatic cycle.

A machine with 300-ton hydraulic clamp is planned to be available in HPM's lab this month. Price of a 300-ton hydraulic machine, including 55-oz BMC injection unit and Command 90 control, is about $210,000.
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Copyright 1992, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Title Annotation:Injection Molding; new line of machines for processing thermoset and unsaturated polyester bulk molding compound
Author:De Gaspari, John
Publication:Plastics Technology
Date:Jul 1, 1992
Words:581
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