HPA

Hi
I want to produce several lightweight accs for our models such as pilots engine cylinders  and many other items. There is an ample availabillity of plugs and prototypes to use. My question is. The process pf using 2 component silicone for the mould is simple and easy.What sort of PU resin should I use in the silicone mould in order to produce a porous free as smooth as possible product? Is there a special release agent I have to use?

Hi Nickhawk

I have done a fair bit of PU molding in the past. I used a two part silicon rubber for the mold and a high flow two part PU resin. The silicon rubber flows very slowly so you must be patient when pouring. Before starting, decide the best approach for avoiding entrapment of air bubbles. Pouring around the sides of the pattern is best. Do not pour over the top of the pattern.

Ideally you need to apply a vacuum the the mold box to extract air bubbles but I found placing the mold in a bucket with a rope attached to the bucket handle and then using the rope to swing the bucket in a vertical circle works quite well. Starting the swing is easy and stopping does require care.

I also used the bucket swing method when casting the PU in the mold. The PU resin I used started to gell two minutes after mixing so you do not have much time to mix, pour and swing.

Use nitrile gloves to pretect your skin otherwise you may become sensitive to the newly cured PU. IF GLOVES ARE NOT AN OPTION, DO NOT HANDLE THE FRESH MOLDING. Leave it in the mold for a few hours. This is no guarantee because the PU deep in the mold will take quite some time to cure.

Hope that helps

Paul

I forgot to answer the question regarding release agent. When casting the silicon rubber mold, I coated the pattern with petroleum jelly thinned with white petrol. When I cast a mold in two or more parts, I coated the pattern and the previously cast silicon rubber part line with the thinned petroleum jelly.

I did try using a silicon oil release agent on the mold when casting the PU but found it to be unnecessary. Even though the release agent was supposed to be paintable, I decide to not accept the risk.

Another issue I discovered is that the silicon rubber may stick to the pattern if it is painted and you do not use the forementioned petroleum jelly.

Paul

Thank you Paul i actually do have experience with making the silicone molds and I did yesterday create the two halfs of a small Williams bros pilot bust. But I got lost in the search of the correct PU resin as there are so many around to choose .I want a product which will be flexible and smooth ,non porous and paintable. So you tell me a release agent is not necessary. I think I will give it a try and report the results here.
There are so many ready made plugs in the static modelling shops one could use to produce the moulds especially pilot busts of all scales and sizes.

Cheers
Nick