Runners and gates

Injection moulding and extrusion may be carried out at temperatures in the range of 300-380°C. The polymer has a high melt viscosity and melt fracture occurs at a lower shear rate (about 10 s ) than with low-density polyethylene (about 10 s ) or nylon 66 (about 10 s ). Extruders should thus be designed to operate at low shear rates whilst large runners and gates are employed in injection moulds. 

Solution Firstly it is necessary to determine the volume flow rate through the runners and gates. The total volume of the mnners, gates and cavities may be calculated as follows ... 

To get the pressure loss in the runners and gates. The flow rate in the long runner is (2/4, hence... 

Since the materials have a very soft plasticity rating, or inches of flow on spiral mold, the materials are all molded in special transfer molding presses and specially designed transfer molds. Because an acid catalyst and glass or mineral fillers are used to make the compound, these materials may have an adverse effect on the hardened steel mold with the result that there may be excessive runner and gate wear. 

The feed system is the flow melt passage in the mold, between the nozzle of the IMM and the mold cavity (Figure 17.1). This feature has a considerable effect on both the quality and economy of the molding process. The feed system must conduct the plastics melt to the cavity via a sprue, runner and gate at the correct temperature/pressure/time period, must not impose an excessive pressure drop or shear input, and should not result in non-uniform conditions at the cavities of multiimpression molds. 

Beaumont, J. Runner and Gating Design Handbook Tools for Successful Injection Molding Hanser, 2004. 

Injection equipment consists essentially of two parts the injection and clamping units. The injection unit melts the polymer and injects it in the mold. The vast majority of modern injection presses use a reciprocating screw. As in an extruder, the screw rotates, melts the polymer and pumps it. Furthermore, in most injection molding units, the whole screw can also axially move forward, acting like a plunger to inject the resin into the mold cavity through runners and gates. 

The two most widely used methods for molding thermoset are compression and transfer molding. Thermosets, you will recall, undergo a permanent set, i.e., become essentially insoluble and infusible under the action of heat. Consequently, techniques for fabricating thermosets must take due cognizance of and make allowance for the fact that sprues, runners, and gates are not reusable and therefore constitute rejects. 

One of the major advantages of compression molding is that it is relatively inexpensive because of its simplicity. Also, since there are no spmes, runners, and gates, material waste is reduced considerably. The consistency of part size is good and the absence of gate and flow marks reduces finishing costs. 

The main limitation of the process is the loss of material in the sprue, runners and gates - as thermosetting materials cure during moulding, this cannot be reused. Moulding tools are usually more complex and therefore more costly than compression-moulding tools. 

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