The low-pressure rotocasting process today is seeing a resurgence in popularity fostered, in part, by the development of improved process controls and easier-to-handle plastics. Moldmaking methods have improved as well. Specifically, the combination of high-quality silicone rubbers with rigid polyurethane systems is yielding glove molds that facilitate both casting and demolding of one-piece hollow parts with severe undercuts, fine surface detail and/or threads and handles.
Glove Mold Advantages
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When used for rotomolds, silicone rubber provides many of the same benefits for which the material has long been known. The material is easy to handle and provides outstanding reproduction of surface detail.
Because glove molds are built with few if any seams or parting lines, they prevent leakage during part molding and virtually eliminate mark-off and blemishes on final part surfaces. Furthermore, mold surfaces do not require releasing before use. As a result, final parts require no secondary cleaning before they can be painted.
Building A Glove Mold
Silicone rubber glove molds are comprised of a thin layer of low-viscosity, high- strength silicone rubber, a layer of thixotropic silicone and a thin coating of durable polyurethane as a jacket.
Step 1. Prepare the master model. Clean all surfaces to remove grease or grime that can inhibit curing or cause a blemish on the surface of the silicone mold. Wearing gloves as you work with the model will ensure “finger print free” surfaces and highest quality end-parts. Remember, the better the surface of the master, the better the surface of the mold and part will be.
Step 2. Fixture the master model. Before beginning the moldmaking process, the master must be fixtured securely so that it can be completely and evenly coated without being touched or moved. For example, to build the glove mold for a container, a dowel might be inserted into the mouth or opening at the top and then attached to a nearby table.
Photo 1 — Model on fixture.
Step 3. First Silicone Layer. Prepare a small batch of GI-1000 silicone adding both a Standard Activator and an Ultra Fast Catalyst to the mixture.
Recommended Silicone Mix Ratio for a Small Part
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GI-1000 Silicone Rubber Base 100 grams
Standard Activator 3 grams
Ultra Fast Catalyst 7 grams
The combination of two catalysts will produce a working time long enough for vacuum degassing of the silicone rubber, drizzling the system over the master model until it is completely encapsulated and setting up before the material begins to sag and drip off model surfaces.
NOTE: The blue-colored standard activator and clear Ultra Fast Catalyst provide a visual verification that system components are fully mixed.
Drizzle coating will help prevent air entrapment and promote complete transfer of details between the model surface and the mold. After the layer is complete, carefully monitor the silicone for at least 10 minutes. The material will sag slightly. If it begins running off and creating a dry area on the model, invert and turn the coated master model until the silicone flows back in place or use a paint brush to re-coat dry areas. Allow the silicone to set for about 45 minutes until it is almost tack-free before proceeding with the next layer.
Photo 2 – Coating master with silicone
4. Second Silicone Layer. Next, mix XT-493 silicone rubber with its activator, stirring thoroughly to ensure complete blending. The product is a paste-like, non-flowing silicone that has excellent hang-up on vertical surfaces. It can be easily applied to build up the next mold layer using a tongue depressor or paint brush (taking care to avoid damaging the silicone with sharp edges). The “glove” layer should be about ¼ to1/2-inch thick over the bulk of the mold. However, thicker areas can be built up in sections that may require greater strength such as planned split lines.
Photo 3 – Application of XT-493 Silicone
After the initial layer of XT-493 silicone rubber is applied, mix another small batch of material to create die-locks that will be used to prevent shifting of mold sections within the rigid polyurethane jacket during the rotocasting of parts.
When the silicone is tack-free (after about one hour), remove the support fixture shown in Photo 1 and coat any dry areas with XT-493 silicone. Allow the mold to cure overnight.
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5. Polyurethane Jacket Layer. Mix IE-3065 polyurethane resin and hardener and apply with a tongue depressor or a short, stiff bristle brush until all sections of the mold are covered except the die-locks created in Step. 4. NOTE: A standard paint brush with bristles cut to about ¼ inch in length is ideal if a brush is to be used. The silicone lock structures should protrude slightly above the jacket surface so that they can be pulled up, locking the mold into place within the jacket. During rotocasting, locks will prevent mold walls from collapsing.
The IE-3065-A/B polyurethane can be handled within one hour of application.
Photo 4 – PUR Application
6. Disassemble Mold Sections. Carefully cut a split line with a Dremel tool to divide the rigid jacket into two halves. Remove the jacket from the glove and then peel the glove off the master model.
NOTE: A two-part jacket can also be created by coating one half of the mold with IE-3065 polyurethane. After the material is completely cured, condition the surface by applying a coat of wax and/or silicone release agent. This will allow the second part of the jacket to easily split from the first half. Then, use additional polyurethane to build the second jacket half following the same process used for the first section.
Next, carefully cut a slit through the silicone glove large enough to remove the master. This area will be used for to inject the mold during rotocasting.
If the split is too long, it may cause serious distortion in the glove and contribute to mold leakage during casting. The solution: pin cut mold sides together and bond with silicone caulk, leaving only the desired fill port open.
NOTE: Another method for creating a slit is to mold a zipper into the silicone glove during Step 4 when XT-493 is being applied.
7. Prepare Mold for Casting. Place silicone glove mold back in one side of the polyurethane jacket. Add the second side of the jacket and pull up die-locks to secure the glove mold in place. The rotocasting material can now be injected into the mold to begin part production.
Photo 5 – Master and mold with finished part
For more information about silicone glove molding, contact RAMPF at 248.295.0223 or e-mail [email protected].
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