Types of Silicone Molding

Author: Krissy Dempsey, Marketing Manager

There are a variety of types of medical component molding.  These types can be categorize by the type of material being converted, part size, volume, molding technology, just to name a few. In this post, we’ll focus specifically on silicone molding. 

There are two types of moldable silicone raw materials: High Consistency Rubber (HCR) and Liquid Silicone Rubber (LSR). 

High Consistency Rubber (HCR) 

HCR has the consistency of peanut butter.  The catalyst can be platinum or peroxide and is compounded with the base via a two-roll mill.  HCR can be molded via compression molding or transfer molding. With compression molding, material is placed between two plates. The heated plates are compressed, and the excess seeps out along the parting line.  This is an older technology but can still be a cost effective method for low volume parts.  Transfer molding differs from compression molding in that the press transfers the material through a runner, sprue and gate system into the cavity of the mold. Transfer molding can be a preferred processing method for silicone parts requiring unique material properties or low to mid-range annual volumes. 

Liquid Silicone Rubber (LSR) 
The most recent processing technology in silicone molding is liquid injection molding (LIM), which uses LSR material. LSR is the consistency of corn syrup, is supplied in drums, and uses a platinum catalyst. Within LIM molding, there are a variety of sub-categorizations and associated technologies including overmolding, two-shot molding and micromolding. 

Overmolding involves molding silicone over, or onto, a substrate such as a thermoplastic part, titanium part or perhaps a silicone tube.  In this process, a robot or an operator places the first component into the silicone mold.  The silicone is then molded onto the initial component thereby resulting in a combined, overmolded, single part. This capability only requires an LSR molding machine and appropriate tooling. Training of operators is essential in this operation as there is a risk of misplacement of the insert which could result in damage to the tooling. 

Two-shot silicone-thermoplastic molding is similar in that it is combining silicone and thermoplastic materials. However, with two-shot molding, it is done simultaneously. The initial thermoplastic component is molded in one half of the mold while the other half is overmolding the silicone onto the previously molded thermoplastic. When the mold opens, the silicone overmolded parts are removed, and the thermoplastic parts are rotated or transferred to the silicone overmold side of the tool. This technology requires specific equipment, a high-temperature thermoplastic material, a self-bonding grade LSR material, and a high level of experience and understanding of shrink and thermodynamics within each half of the mold as these materials are essentially opposites in regards to cure methods (thermoset heated and thermoplastic cooled). 

Finally, micromolding of LSR is rising in demand within the medical device component space as devices continue to get smaller and smaller.  Using high precision tooling, an appropriate press (shot size, screw and barrel size, etc.), robotics, and more, silicone parts with a mass of 0.01g are manufacturable. Some examples include integrated seals, soft tips for surgical tools, or micro-valves. 

For more information on molding types or specific capabilities of Saint-Gobain Medical Components, please contact us today. 

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