Mold Life

While silicone RTV molding is a great solution in many applications, without a doubt the greatest obstacle to high cast part volumes is the limited life of the silicone tool.  Most silicone molds are good for about 25-35 parts, but that number can vary significantly based on the materials used, surface finish, and the geometry of the part being molded.  For example, a polished part will have a lower mold life than a textured Shore A part.  Assuming a higher number of parts per mold is desired, here are some design tips and rules of thumb for higher mold life.  Please keep in mind that this relates to silicone molding, not injection molding.

1:1 Aspect ratio: By keeping features as wide as they are deep, the features are more stable, and consequently the silicone that forms those features will be more stable as well.  This is particularly true with raised or recessed text, also channels that surround the edge of a part in a “tongue and groove” feature.  A groove that’s .050” wide, but .100” deep will not last as long as a groove that’s .050” wide and .050” deep, nor will it last as long as a groove .100” wide and .100” deep.  Keeping features such as this as close to 1:1 as possible will help the feature remain intact after several de-mold cycles, which will lead to improved mold life.

Thin floors: Just because a wall or floor can conceivably be built with Stereolithography, doesn’t mean it’s a great candidate for silicone molding.  Typically, floors thinner than .030” will not be very moldable, which is at the minimum thickness for what area will allow the material to flow through.  And floors thinner than .050” will be problems across larger areas.  Remember what’s forming that wall is soft silicone, that can stretch, bend, and sag with each molding cycle, causing thin areas to become even thinner, or shut off completely.  When possible, try to keep your nominal wall thickness close to .080” at a minimum, and keep the webbing of keypads at .030” for optimum silicone mold life.

Undercuts: It’s true, that one can get away with omitting draft angles, even molding undercuts with silicone tooling.  Even so, undercuts do put a tremendous strain on the silicone during the de-mold cycle, which can stretch, even tear the silicone.  If an undercut is absolutely necessary, try to provide ample room opposite the undercut feature, so the trapped silicone has somewhere to go when the part is being de-molded.

Fill in the holes..  One question your project manager may ask you, during your molding project at Protogenic, is “is it alright if we fill in this deep pocket area?” For good reason, you’ve designed your part with uniform wall thickness, and for added strength you’ve included some gussets to keep that corner boss in check.  Well, the result is a deep, narrow area in the corner, that will lead to reduced silicone mold life.  Any time we spot an area like this, we’ll be sure to ask if it can be minimized, so your mold life is as high as possible, and since we don’t have the sink issues that are common with injection molding, this is often a good solution.

These are only a few areas where silicone mold life can be an issue.  On every molding project that you do with Protogenic, you’ll get the opportunity to work one on one with your project manager, who in addition to documenting and highlighting all of your specific project needs, will help you look for ways to improve your mold life.  For answers to more specific questions, or to discuss a specific feature on your project, please contact a member of the Protogenic sales team, and we’ll be happy to help however possible.