Differences in Molding Silicone vs. Plastic
Written By: Matt Bont
The primary difference between silicone and plastic properties drives the differences in mold design. The largest difference is the temperature profile between silicone and plastic. The mold design and construction will also be dependent on the product geometry, size, materials, equipment, quantities, cycle times, undercuts, flash limitations and parting line and gate restrictions.
Silicone processing is a curing reaction between two parts. These parts are mixed and driven through a cool barrel and screw. The mixture is injected into a hot mold where the heat accelerates the curing reaction until the product is solid. Silicone has a significantly lower viscosity than plastic melt and flashes into spaces less than 0.0002" requiring precise parting lines and tight shut offs. The low rigidity and low viscosity also prevent the use of knockout pins which can damage parts and cause flash and pin binding. The inherent flexibility and high elongation does provide a means for removal of parts with significant undercuts.
Thermoplastic pellets are melted and conveyed through a hot screw and barrel and injected into a cool cavity where the melt hardens into the final product shape. The products are typically much more rigid than silicone and can be pushed out of the mold by knockout pins within the mold at the end of each cycle. This rigidity also causes binding of some vertical walls and most undercut features.
Silicones and thermoplastics continue to expand and include many materials with diverse properties that offer more possibilities to designers. Good communication between designers and manufactures can ease start up and improve product quality and function.
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