Why Is My Urethane Plastic Part Shrinking?

If you’re new to working with urethane materials, you may not know much about their dimensional stability. While most urethanes are dimensionally stable, they will shrink to a degree due to the exothermic reaction they generate as they cure. Linear shrinkage–found on the individual technical data sheet (TDS)–should be considered before working with the material.

If your urethane plastic part is shrinking more than what is listed on the TDS, something may be affecting it, such as size and part thickness, temperature, or the tooling material. Shrinkage from these factors can affect the final part’s performance and appearance.

Reasons Urethane Plastic Shrinks

It’s safe to expect at least .15% linear shrinkage since most urethane plastics are designed to produce minimal shrink. However, they will shrink at different rates because not every formulation is the same. If you don’t pay attention to the linear shrinkage, you might plan for your part to shrink to a certain degree, but it may shrink more or less than you expect.

An increase in the specified shrinkage can be a result of several factors. To obtain high-quality, dimensionally stable parts, you must consider the following when casting urethane plastic:

#1 Casting Size & Wall Thickness

Shrinkage can be negligible, but it will depend on the material’s exotherm and the amount used. Some thermoset materials may generate a higher exothermic reaction than others, which can increase shrinkage. The more material you use, the more it will heat up due to the exothermic reaction, which also causes an increase in shrinkage. So, if you are making large or thick-walled parts, you may see greater shrinkage than in small or thin-walled parts.

If the part’s wall thickness is not consistent, you should expect uneven curing times, which can result in increased, non-uniform shrinkage. Because the walls shrink unevenly, it can lead to warping or deformation in areas that cure more slowly. It can also lead to cracks in the part if the walls are thin enough.

#2 Temperature

A part made in a cold workshop compared to one made in a warm workshop will not turn out the same. Polyurethane dimensions can change significantly with ambient temperature changes. Urethane expands when heated and contracts when cooled.

When working in excessive temperatures, you might think the part will cure to the right size, but once it cools to room temperature, its dimensions will change. Making the part in higher temperatures will cause the material to shrink faster and to a greater extent once cooled and cured.

If your tooling material or the material itself is not at the recommended temperature, the plastic may also behave differently. It can contract more quickly and increase shrinkage.

#3 Tooling Material

The type of tooling used can also impact the shrinkage of parts. As previously stated, urethane experiences thermal expansion. If the tooling used is made of a flexible material like silicone or urethane, it will respond to that expansion. Depending on other factors, this can cause increased deformation and shrinkage during the curing process.

The shape or design of the tooling may also affect part size. If you don’t plan for some amount of shrinkage, the size of your part will be slightly smaller based on the design of your tooling. Undercuts or complex-shaped cavities can lead to a non-uniform shrink, which means some areas may shrink more than others.

#4 Adding Materials

Using different materials within a casting can also cause an uneven cure and shrinkage. The most common material to place into a plastic part or casting is metal, which will act as a heat sink and change how the material will shrink or crack. Other materials that can affect the shrink are wood, foam, and other plastic materials.

How to Plan for Shrinkage

The most important aspect of making a part that meets your size requirements and tolerances is adjusting the dimensions of the tooling material. If you know the urethane plastic you are using will shrink to a degree at a certain volume, you’ll want to increase the dimensions of the cavity by the estimated shrink amount.

It is essential to account for this if you have high tolerances or critical dimensions. Anticipating the shrinkage will ensure that once the part has fully cured, it will meet the specified dimensions, even though the tooling is oversized.

Also, when designing tooling, you want to make sure the wall thickness is consistent. This will ensure that any shrinkage that does occur is uniform and will reduce deformities in the part, such as warpage or sink marks.

Providing a consistent temperature throughout the tooling and materials with ovens or heated tables will prevent an uneven cure rate. This temperature should be a minimum of 90°F.

When making large parts, you want to avoid using materials with a high exothermic reaction to prevent shrinkage from excessive heat. It is possible to pour some plastics in layers to combat mass shrinkage. This may not be possible if you use a clear urethane resin. UV-stable clear plastics are typically poured in mass to prevent the creation of seams in the see-through part.

Work in a temperature-controlled environment during the part-making and curing process. Make sure the tooling and material temperatures are also consistent. This will keep the liquid urethane at a consistent temperature, so it doesn’t expand or shrink more than it is already designed to do.

Accounting for these shrinkage problems can help reduce waste, rework, and delays when making urethane parts.

Shrinkage Rates of VFI Plastics

VFI products are tested using 12” x 0.5” x 0.5” samples and follow ASTM D2566 testing standards. The environment, casting material, and tooling are maintained at room temperature (72°F-77°F). Standard testing methods may not be fully reliable for non-controlled settings using larger part dimensions.

Products	Shrinkage Linear
VFI-110 75 D Injectable Plastic	<0.010 in/in
VFI-1678 80 D Injectable Plastic	<0.010 in/in
VFI-4170 70 D Fast Casting Plastic	0.0625 in/in
VFI-4171 70 D Casting Plastic	0.03125 in/in
VFI-4172 75 D Slow Casting Plastic	<0.005 in/in
VFI-4280 75 D Polyurethane Casting Plastic	<0.010 in/in
VFI-4281 78 D Polyurethane Casting Plastic	<0.010 in/in
VFI-4282 78 D Polyurethane Casting Plastic	<0.010 in/in
VFI-4285 85 D Polyurethane Casting Plastic	<0.010 in/in
VFI-4286 85 D Polyurethane Casting Plastic	<0.010 in/in
VFI-4478 85 D Prototyping Polyurethane Plastic	<0.010 in/in
VFI-4580 82 D UV Stable Clear Polyurethane	<0.005 in/in
VFI-4581 82 D UV Stable Clear Polyurethane	<0.005 in/in
VFI-4582 82 D UV Stable Clear Polyurethane	<0.005 in/in
VFI-4585 84 D UV Stable Clear Polyurethane	<0.005 in/in
VFI-4586 84 D UV Stable Clear Polyurethane	<0.005 in/in

Contact VFI if you need help finding the right urethane plastic for your part-making, prototyping, or rotocasting project.