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How to (2) Use urethane Fast Cast type resins

Captain Sci-Fi

Hi Guys,

Following on from the open type RTV mould making I thought I would run down the procedure for making successful castings in the garage kit makers favourite material Urethane resin. I will assume that you have downloaded the Health & Safety and technical specification sheets from the suppliers website and have taken the usual precautions.

Firstly you need determine how much resin to make up. I use cling film to cover my master parts and fill the cavity with raw dry rice is a good approximation for turning volume into resin weights. Empty the rice in to a measuring cup and note what volume it levels up to, this will be the total weight of resin to try first.

All the Urethane resin brands I have tried are mixed 50 / 50 by weight so we need half the volume shown in weight. If the scale read 100 that would be 50 of each part of A & B resin mixed together.

I buy the XD4566 resin from Jacobson Chemical and use the see thru graduated cups as shown, always mix by weight and use an accurate. Have everything ready before you add the second part as that will be the point of no return. Don't rush but work purposefully to complete pouring within the pot life time limit. I have found that temperature plays the major role in this length of open time you get with the material. There are signs that the curing process is taking place firstly in appearance, there is a marked change from a brown tinted clear liquid to milky clouds forming, like this:

Secondly you can sometimes feel warmth if you hover your palm over the open mould, this is the heat produced know as exotherm. As you can see it forms first where the material is thickest usually at the centre and continues outwards as the warmth builds up. This is why a warm day with warmer chemicals will speed up the process.

This whole process, the transformation from a liquid to a solid, takes place fairly quickly, I usually turn out the part you see here in a couple of hours.

Tricks and tips:

There are several ways to improve you part production even further without going to the expense and time of making a two part mould or even resorting to rotocasting to make a part semi hollow.

It is possible to make a plug that hangs in the mould to displace the greater bulk of material and leave just thin walled parts. This will work with every open cast mould if you need to consider the following conditions
1. The resin part needs to be light in it's final weight
2. You need to create room within a model for lights, supporting frames or interior details

3. You want to save resin material to keep the retail price as low as possible.

I initially filled the mould all the way to the top with resin to produce a solid part. I used the threaded inserts to fix a perspex bridge plate to the back of the part to aid removal later and suspend the plug leaving the cavity I needed to fill to make the finished part.

Once cast, I turn this in my lathe reducing the outside by 5 mm overall as the gap remaining would be the thickness of the finished part. I sprayed this with release agent (basically a vaseline gel in spray form) and then wrapped in cling film (the same product used to keep food fresh. This makes a waterproof layer to aid part removal later. Here it is replaced in the mould minus the bridge plate for clarity.

If I were to make 100's of this part I would make another plug from RTV but use it in exactly the same way just negating the use of cling film food wrap.

Glenn Allen

Before i cast i fill the mould with water then empty in to a measuring jug this gives tells me how much resin to use , if i use two part resin , 50/50 then it's easy to measure out the correct amount of each part of resin .