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Model Railroading > Moldmaking failure

Date: 09/20/22 14:22
Moldmaking failure
Author: Jeff_Johnston

I make a variety of silicone rubber molds to cast parts I need, in multiple, with urethane resin. I've had great success for many years with this process.

Until now ... I tried using a different type of moldmaking rubber, in this case the Tap Plastics Urethane RTV casting system, so the basic mold is urethane rubber instead of Tap's silicone RTV material. The urethane rubber material is extremely adhesive compared to the silicone rubber system. Used properly, it works great and makes a durable, flexible mold with fine detail reproduction capability. In this example, I failed to use adequate mold release on the patterns, which are small wood-framed boxes that represent the "house" part of the Willamette electric donkeys used by the Sugar Pine Lumber Company. The patterns are basswood cubes sheathed in scale lumber, pretty easy patterns, or they should be. I didn't use any mold release on the wood sheathing because I didn't want to lose any grain detail.

It took some extreme prying to get the pattern from the mold rubber, and the mold and pattern were both damaged in the process. Even the parts of the mold that were coated with Johnson't Paste Wax, my usual go-to for general mold release, were stuck pretty tight to the rubber. I managed to pry one of the patterns from the mold and decided to stop while I was ahead and make three new patterns (OK, I just like making hobby things with little sticks) and re-cast them in the silicone mold material.

The point of all this is, if you dive into this type of moldmaking and casting for your hobby projects, make sure to read the instructions regarding mold release additives for the mold patterns and the molding process in general. The urethane moldmaking system works, but you need to pay attention to the rules.

To address some FAQs ...

- No - I will not be having someone make these via 3D printing because I don't need to do that. It's not the right process for this project.
- Yes, I'm aware that Tap Plastics has a variety of moldmaking and casting manuals and videos. I use those, and have done so for the last 15 or more years when I've been successfully making silicone molds and urethane castings.

Hopefully, live and learn and not have to make casting patterns more than once.

Jeff Johnston
SPLCo/M&WRy circa Sept. 1927

Date: 09/20/22 15:51
Re: Moldmaking failure
Author: tehachapi-dave

We use Smooth On products because they are 40 minutes drive from my home and can go and talk to them directly in the retail store and get great advice and yes we now follow the directions exactly as prepared.  

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