Weighing and Mixing Liquid Tooling Plastics

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In order to guarantee the performance of any tooling plastic, you must weigh and mix the materials properly before using them. Poorly prepared liquid tooling materials is our number two source of tech calls and is usually the cause of materials not performing to their specifications.

Calculating Material Requirements

One question new users often ask is “how do I know how much material is needed to make a mold or fill a mold cavity” because if we mix too much, we end up wasting material, and if we mix too little, we may ruin our mold.

To provide a useful estimate of the amount of material needed, we need two pieces of information.

First, we need to know the volumetric yield of the material we’ll be using. This number is available on our website and is expressed In cubic inches per pound.

Second, we need to know the volume of the mold or casting. This is measured in cubic inches and can be obtained by using some simple math.

For rectangular box shapes, we simply multiply the length by the width by the height. For example, if your mold box is 6” x 5” x 1.5” inches, the total volume would be 45 cubic inches

For cylindlical shapes, we simply multiply the radius of the circle, by the radius of the circle, by the height of the cylinder by 3.14. For example, if your radius is 2 inches and your height is 1.5 inches, then your volume would be 18.8 cubic inches

Once we have both the volumetric yield and the volume, we simply divide the volume by the volumetric yield. This answer gives us the total amount of material in pounds that we’ll need to fill our mold box, or mold. Therefore, our mixture of both parts A and B together should equal or exceed this amount.

There are two additional considerations. First, if we are measuring the volume of a mold box, then we can assume there will be a model inside which will take up some of the volume, therefore decreasing the amount of material needed.

Secondly, because models and molds rarely have perfect angles or circles, we always over-estimate the volume to ensure we have enough material.

Agitating Materials

Before being weighed and mixed together, most materials need to be mixed individually in their can. Unless they ship far or sit for a long time, most materials won’t settle very much in the can. Therefore, they can be mixed manually with a paint paddle.

Some materials, such as our
Repro 83 fast-cast urethane, contain fillers which reduce shrink and add to the wear and machining characteristics. These fillers often settle during shipment and must be agitated mechanically with a plunge mixer shown here attached to a drill, or better yet, a Red Devil Paint mixer, shown here.

After 6 minutes in a Red Devil Mixer, the fillers are in suspension and the material is ready to be weighed and mixed together. Note that our three newest Repro formulations are all non-settling, meaning they can be mixed thoroughly without a mechanical aid.

Weighing and Mixing 1:1 Ratio Materials

One of the advantages of a 1 to 1 mix ratio material is that it can be measured without a scale. All of our Repro and a few of our
Freeman Polyurethane Elastomers can be weighed and mixed using this three-cup procedure.

Here, we poured enough material of each side into
two lined cups and eyeballed it so that each cup had about equal amounts.

Then, we poured the material from one side into the other and mixed it with a
paint paddle, making sure we scrape all of the material along the sides of the cup.

Finally, we poured the material into a third container because it is physically impossible to completely scrape the sidewalls of the cup and it is very important that all material is mixed before using it.

When we’re done with our material, we add a quick spray of
Magic Blanket into each can before resealing the lids. This shot of nitrogen helps to preserve the material for later use.

Weighing and Mixing Plastics with an uneven ratio

The process for mixing tooling plastics with uneven mix ratios, such as most epoxies and polyurethane elastomers, is different than a 1 to 1 mix ratio urethane like Repro.

Here we’ll demonstrate the proper mixing procedure with
Freeman 705 Epoxy Surface Coat, but this procedure applies to all plastics that have an uneven, or non 1 to 1 ratio.

The mix ratio we’re following here is 100 parts resin to 14 parts hardener by weight.

Once we put the lined cup on the scale, we zeroed the scale and then pour 100 grams of the epoxy resin in the cup.

We use a paddle to add or remove material towards the end since is it easier to control.

Most uneven ratio materials are much more sensitive to their ratio than a 1:1 system like Repro. Since we will want to be within 1% of the required ratio, we are using an electronic scale.

Once the resin is poured, we re-tared the scale to zero and added the harder directly on top of the resin.

It is important that you don’t use a separate cup for the hardener or you may lose too much material when you combine them.

After mixing the two materials together thoroughly, we pour the material into a second cup because it is physically impossible to completely scrape the sidewalls of the first cup and it is very important that all material is mixed before using it.