Processing, thermoplastics runner systems

Hot runner systems are therefore used in the processing of thermoplastics. However, the processing of elastomers involves the use of cold runner systems, which prevent the elastomer compound from cross-linking prematurely in that system (see Figure 4-5). 

Figure B.l Runner system (a) balanced and (b) unbalanced. (Ref Barry, C.M.F., and Orroth, S.A., Processing of Thermoplastics , Modem Plastics Handbook, C.A. Harper, ed., McQraw-HiU, New York, 2000)...

In contrast to hot runner systems, the material to be processed in the CRS is cooled or specifically tempered. Thus, In CRS, material temperatures of 20 to 80 °C are standard, and the mold Is then heated up to 200 °C. By comparison, the (thermoplastic) material In the hot runner is heated to about 180 to 380 °C and then cooled in the mold. 

As an alternative to the conventional injection system, cold runner systems are coming more and more into use in the processing of liquid silicone. These systems may be compared with the hot runner technology found in thermoplastic processing. 

In RIM, as in injection moulding, whether thermoplastic or thermoset, there is a transition zone between the nozzle (mix head) and the mould. In injection moulding, this transition zone includes the sprue/runner/gate system, and, in special cases, can also include a static mixer. The analogy to RIM is very apt. The transition zone for the RIM process also includes a sprue/runner/gate system, as well as an after mixer. One function of the RIM transition zone is to convert the mixing head liquid stream from a turbulent state to one which is in a laminar flow mode. If this does not occur, air traps (i.e. large subsurface voids in the finished part) can become a problem. 

The conventional injection system, that is, the non-temperature controlled, mould-integrated sprue runner, provides the simplest way of achieving this. Injection systems such as those with thermoplastic processing are possible here. 

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