4 Things To Know Before Selecting Your Polyurethane Casting Resin

You have something that you want to create with resin, but what resin do you use?  This is a question many ask and in this article, I will aim to answer that question with 4 things you need to know before you select that resin.

• Feel
• Mixing requirements
• Viscosity (how thick is the liquid)
• Work time (gel time, cure time)

How do you want your part to feel?

One of the best features of casting parts with polyurethane is the variety of cured properties that can be achieved.  From foam to soft gels, rubber to rigid plastic, there are a lot of options to consider.  Maybe the most important property to look at to determine the feel is the shore hardness value.  The vast majority of polyurethanes will come equipped with either a “Shore A” or “Shore D” value. 

This chart will help to interpret the hardness value of your product.

Here are some common products and their hardness value.

• Rubber Bands – 20 shore A
• Pencil Eraser – 50 shore A
• Tire Treads – 80 shore A
• Golf Ball – 50 shore D
• Plastic Caster Wheel – 60 shore D
• Cell phone – 80 shore D
• Bone – 90 shore D

What are the mixing requirements?

The mix ratio of the product tells you how much of each side (Part A and Part B) need to go into the final mixture.  Mix ratios will come with a “by weight” and a “by volume” option. 

Consider the Simple Resin product below.

With this product, you have a mix ratio of 1:1 by weight and 1:1 by volume.  So if you added 100 grams/100 ml of Part A then it would require 100 grams/100 ml of Part B in order to be the proper mix ratio.

Mixing the polyurethane at a different ratio than what the product recommends can have a variety of impacts on the cured part.  As a general rule, if you mix it with extra Part A, the product will be brittle and potentially break more easily (semi-rigid and rigid plastics).  If mixed with extra Part B the product can be softer than expected and often can be tacky to the touch.  When you put way too much hardener in, the product can remain in a gooey liquid state and you end up cleaning out your mold.

If you don’t have access to an accurate scale, then selecting a product with a straightforward mix ratio by volume is a huge advantage.

How thick is the product? (Viscosity)

Polyurethane casting resins come in a wide range of viscosities, but for the most part, you’ll find them in the range of 0-5000 cps.  Viscosity can have an effect on mixing, removing air bubbles, and how fine of details the product will flow into in your mold.

Below is a list of common household items and their viscosities for your reference.

• Water – 1 cps
• Olive Oil – 100 cps
• Maple Syrup – 150 cps
• SAE 30 Motor Oil – 500 cps
• Latex House Paint – 1500 cps
• Shampoo – 3000 cps
• Karo Syrup – 5000 cps

What is the work time (gel time, cure time) of the product?

Work time and gel time are two terms used to describe a relatively similar event.  

Upon mixing the two parts of the material you’ve selected you have a certain amount of time to actually work with the material (work time).  After you’ve reached your work time the material stops being workable or pour able.  Once you’ve reached the gel time the material has actually gone through the first stage of solidifying which generally takes place shortly after the work time has expired.   

There are many factors that can change the work time of a product.  Below are a few of the most common factors.

• Temperature
  • At Simple Resin, we test our gel times at 77F.  The warmer you get, the shorter your work time and if your product gets cold you’ll have a longer work time.  Increasing or decreasing the temperature of the three items below will impact your work time/ demold time.
    • Work area
    • Product
    • Mold surface (impacts your demold time)
• Part thickness
  • The standardized way to measure work time is in a cup that is 2.5 inches in diameter and 2 inches tall.  If your part thickness varies from this (which it almost certainly will) then your work time/ demold time will vary too.  Take for example a product with a 7 minute work time.  If you mix that product and pour it onto a table it might take a few hours before it starts to harden up and maybe overnight before it starts to approach the hardness that its intended to have.  If you take that same product and pour a 12″x12″ cube it might be to full hardness in less than an hour.
• Volume of your mix
  • Just like your part thickness, your mix volume can alter the work time you have as well.  If you’re mixing up 1 oz of material at a time you will get a longer work time then if you mix up 2 gallons of material at a time.  The extra mass causes the reaction to gain temperature faster causing the faster in work time.

If you have any further questions about polyurethane and getting into casting parts, feel free to send an email to info@simpleresin.com.  Check back regularly for more posts and videos that explore the possibilities of polyurethane casting!