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Liquid Silicone Rubber Injection Molding--Design Considerations


How does the liqiud silicone rubber injection molding process when making silicone parts ? The first step in design is typically material choice; like thermoplastics, the particular grade and manufacturer of LSR have a great impact on final part design.Once a material is chosen for final product performance, many other decisions must be made regarding part shape, gating,material usage, and most importantly cost.The best method for determining these is to ask a series of questions:

How will the product be molded?
Part geometries can have a great deal of impact depending upon the type of molding used. Some work well with automation typical for horizontal LSR molding, while others work better for the manual operation of vertical machine molding. Also, the complexity of the geometry plays a role in governing the size of the mold and thus the size of the molding machine.

How will the part be demolded?
Design for ease of demolding is just as important as design for molding. Silicone has a high green tear strength: meaning that very large undercuts may be designed into a product and the parts can be stretched over them during demolding without permanent deformation. This, however, does not mean that any geometry may be easily removed from the mold. Silicone’s tendency to adhere to the mold steel after molding coupled with
its natural tackiness can make it quite difficult to remove thick walled highly undercut parts. Undercuts and wall thicknesses should be
carefully chosen during the design phase.

How should the mold be heated?
Consistent heating of LSR molds is crucial to producing parts of proper geometry both throughout a single run and between runs.
Options for heating methods include resistance or induction. Both styles may be placed inside the plates of the mold or inside a universal
base which heats the entire mold from the outside only. More precise control is given by placing heaters directly in the mold plates, so
that the thermocouples tied to the PID controller can more effectively change the heater on/off timing. Choice of heating type
depends largely on budget and geometric considerations, since heating the mold directly increases costs and heaters must be placed so
that they avoid mold cavities, cores, and slide actions.