Why Isn’t My Urethane Plastic Part Curing?

If you’ve ever used urethane plastic for your part-making or prototyping projects and it won’t dry or cure, VFI has answers for you. From experience, we know this issue typically occurs in the application process when the chemical reaction between the liquid components fails to take place or does not occur correctly. Curing can be affected by improper mixing, cold temperatures, thin pours, and incompatible additives.

Every urethane plastic material will come with its own cure timeline. Depending on the formula, it can happen as quickly as 5 minutes or as slowly as 48 hours. Always refer to the manufacturer’s technical data sheet for product-specific instructions when curing and post-curing.

Reasons Urethane Plastic Will Not Cure

1. Incorrect Mix Ratio & Poor Mixing

Since thermoset polyurethane plastics are two-component materials, they must be measured using the correct ratio by weight or volume and mixed thoroughly. If you deviate from these ratios or do not mix it well enough, it may not have the materials to cure. These mistakes can happen because users rush to use the material before the working time ends.

Other factors that can influence off-ratio measurements and poor mixes include:

• Material separation. If your material has been stored for a while and you don’t premix it before combining the components, you might run into curing issues from separated ingredients.
• Bad weight scale. If you’re using a scale that isn’t 100% accurate, you may be measuring too much or not enough of each material, which can cause issues with curing.
• Bad dispensing equipment. Instead of pouring the material by hand, some part makers use dispensing equipment. If the dispensing equipment gets off ratio from blocked lines or an off-ratio pump, it can result in an improper cure.

If you undermix or don’t mix for long enough, it may result in a sticky or warped part. This usually occurs by not scraping the sides and bottom of the mixing container. If sticky residue is on your tooling material, it can be hard to remove, which prevents the tooling from being used for future castings. In some cases, improper mixing can even prevent the part from being able to accept paint.

If you don’t use the correct mix ratio, your part may leach or exude oil in certain spots once cured. This typically happens from using too much of the B (POLY) component. You can also tell if your “cured” part is heavy on the A (ISO) or B side because it will appear streaky or non-uniform in color. Some liquid components will be similar colors, so it’s not always easy to tell if the mix is uniform before cure.

Off-ratio mixes and poor mixing can also affect the final properties of the part. Typically, parts will not meet their designated Shore hardness level or provide the same durability specified on the technical data sheet.

2. Cold Temperatures

Cool and freezing temperatures slow down the curing process and can even inhibit the cure of some polyurethanes. Various elements in the part-making process can be influenced by these conditions.

• Tooling. If the tooling you’re using to cast your urethane part into is too cold, it could affect the material as it tries to cure. The heat the material generates might not be enough to elicit the curing reaction. Some manufacturers recommend using heated tooling, while others recommend at least a standard room temperature.
• Environment. If your workspace has low temperatures or your part is left in freezing conditions, it might not cure. While the part creates a heat reaction, it might not be able to warm up enough if ambient temperatures are too low.
• Material. If the material freezes in transit or storage, the ingredients in the B component may separate. If the separated material is combined with the A component, it might not cure properly, and once cured, it may exude an oily substance.

3. Pouring Too Thin

Most urethane plastics have a thickness minimum requirement of ¼ or ½ inch. A few products can go as thin as 1/8 of an inch. However, the thinner the pour, the longer it will take to cure. Extremely thin parts don’t create the heat necessary to fully cure, especially if tooling and ambient temperatures are not elevated. These parts will need to be post-cured in an oven to generate the heat reaction. Depending on the thickness, additional heat might not always work. Avoid working with small batches of material, as this can also inhibit curing.

4. Incompatible Additives

Certain wet foreign materials added to a urethane plastic may be incompatible with the mix and affect curing time or final properties. Some pigments and fillers can cause the resin to cure too soft, while others will make the part feel oily because the pigment is leaching out. Incorporating too much of these additives can also affect the mix ratio, resulting in a failed cure.

Solutions

Warm both components to at least 65°F before use and work in a room-temperature environment (77°F) with low humidity. Premixing the B component is typically recommended to re-blend ingredients where separation may have occurred.

Use the correct amounts of A and B side components as specified on the technical data sheet. If you’re measuring by weight, use an accurate gram scale. We recommend this method for the most accurate results. If you’re measuring by volume, ensure your mixing cups are accurately marked and you’re filling to the correct line. Spring scales and some graduate mixing cups should be avoided as they only offer approximates.

Thoroughly mix your materials together. Use clean, straight-sided mixing containers with flat bottoms and straight stir sticks or power mixing equipment. Scrape the sides and bottom of the mixing containers so material is distributed evenly. Be conscious of your working time, but if you have enough time, pour your liquid plastic into a clean mixing container and mix again to ensure uniformity.

When adding pigments or fillers to your plastic part, use compatible, chemically dry materials, and do not overuse them.

Before pouring or injecting your material, you might want to heat your tooling material in an oven to ensure the part cures properly. This process can also help dissipate bubbles generated in the mixing and pouring process. Only heat tooling if it is recommended by the manufacturer.

Ensure you pour the part thick enough as specified by the manufacturer’s instructions. Avoid pouring any material under 1/8 of an inch. Thinner materials will take longer to cure since they generate less heat and typically need to be post-cured.

Understand the curing stages of urethane. Some urethanes require a longer time to cure due to extended pot life or other factors. Most urethanes at room temperature won’t develop full properties for 3 to 7 days. Some resins recommend and even require post-curing in an oven for several hours at temperatures as high as 170°F to achieve full physical properties. If you do not follow the manufacturer’s instructions, your part might be weaker than anticipated and may never fully cure.

Contact VFI if you have any further questions when using urethane plastic for part-making applications.