Why Is My Urethane Mold Deforming?

So, you’ve done everything right in the process of making your mold, but after you’ve demolded it, it’s begun to deform. We’ve seen this happen to urethane rubber users periodically, and there can be several reasons this defect occurs. The biggest reason is time. You’re either demolding your mold or casting into it too quickly.

How you make the mold, use it, and store it can also affect its ability to hold shape over time. Once a mold becomes deformed, its usability is greatly reduced. This leads to a shortened lifespan, which increases production costs and downtime from having to remake the mold. The best way to maintain your mold is by reading the technical data sheet (TDS) and adhering to the instructions and guidelines listed by the manufacturer.

Reasons Your Mold is Deforming

1. Demolding Too Early

Many factors affect how fast your mold cures, including temperature, mold wall thickness, mold size, etc. If you don’t adhere to the recommended guidelines when making your mold, the pot life and demold time can change. If you lose track of them, you may run into issues.

If you demold too soon, the rubber might not have hardened enough to maintain its shape, causing it to warp or deform when you remove it from the mold box. Early removal can also cause damage because it is soft and prone to tearing. Even if the mold seems cured on the surface, it probably hasn’t cured internally.

At this point, since the mold is distorted, your future casting probably won’t be an accurate representation of the original object. If you want to demold it faster, you’ll have to use a faster material.

2. Casting Too Soon After Demolding

Many newer polyurethane users think that once the mold can be demolded, it’s ready to use. However, if you use it right after you pull it out of the mold box or form, you can deform or distort it.

Urethane initially cures after 16 hours and can be demolded, but it still needs to develop the necessary strength for various casting scenarios. Most manufacturers will recommend an additional waiting period after demolding before you use the mold. For products with demold time of 16 hours it is recommended to wait 72 hours.

Note: The longer you wait to cast into your newly made mold, the stronger it will be. Urethane develops full physical properties after 7 days. If you don’t want to wait, heating the mold can typically increase its physical properties faster.

3. Not Storing the Mold Properly

If you aren’t casting into your mold for a length of time, you’ll probably want to store it for later use. Do not store it vertically or on its side. Urethane can distort if it’s not stored on a flat surface. If a corner is curled, the mold is on top of another object, or something is lying on top of it, it may never go back to its original shape.

Also, don’t store it outside or in direct sunlight. Elevated temperatures and UV rays can negatively affect the mold. This may prevent you from getting accurate castings the next time you use it because the mold may have shrunk, expanded, or degraded.

4. Using the Mold in Extreme Temperatures

The type of deformation extreme temperatures cause will not ruin your mold, but it will change its shape. These extreme temperatures cause the mold to change because the rubber will expand and contract with the weather.

In most cases, this change might only be temporary. If the mold has expanded, it’s probably due to high temperatures, but if it has shrunk, it’s probably due to low temperatures. Dimensional changes often occur because you are not using the mold in the same conditions that you made it.

If you are constantly working in extreme temperatures, your mold may never return to its original size. This results in your castings being an inaccurate representation of the original model(s).

Note: Some solvent and oil-based release agents can also cause your urethane mold to expand. Unlike temperature, this will deform the mold permanently and cannot be reversed.

5. Mold Design Does Not Accommodate the Casting Material

Before you even begin to make your mold, you need to consider the design, what materials you are using to make it, and what materials you will be casting into it.

The thickness of your mold walls plays a big part in preventing your mold from distorting. We know you want to save money on material costs, but if you don’t make your walls thick enough, they may bulge in the casting process. Molds with complex shapes are prone to warping as well.

If you’re making a mold with thin walls, you must be sure it will hold up to the material you’re casting into it. Materials like concrete are heavy, and if your mold isn’t strong enough, it will distort during the casting process.

The hardness of the mold can also determine how it might deform if not designed well. If you use a softer rubber (25-45 A), you cannot pour the walls thinner than ¾ – 1 inch thick. Otherwise, it might not accommodate the weight of the casting material, especially if you are casting something big.

Also, thinner areas of the mold can take longer to cure. You might think you’ve given your mold enough time before casting, but it takes a little longer for thinner parts to gain the strength necessary to do so.

Solutions

Take your time, and don’t rush when demolding your newly made mold or casting into it. Check the manufacturer’s TDS to be sure you are following recommended cure times and temperatures.

When making the mold and casting, do so at room temperature (77°F) for the best results. All VFI products are tested with properties obtained at room temperature. Anything outside of that temperature will change the pot life, demold time, and dimensional stability of the rubber.

VFI recommends leaving your demolded urethane mold on a level surface at room temperature (77°F) for an extra 3 days. This will maximize the performance of the mold and allow it to gain enough properties for casting. You’ll be able to repeatedly cast into it without worrying about distortion.

A good rule of thumb is to use a harder rubber when making large castings. This way you can make the mold walls thinner because the rubber will have more tear resistance and can support heavier loads. On the other hand, if you use a softer material, make the mold with thicker walls to help cure it quicker and to give it additional structural support.

Urethane will cure quicker in thicker areas. Be sure you’re following the thickness requirements recommended by the manufacturer to withstand the pressure of casting materials. VFI’s recommended wall thicknesses are listed on every TDS. If you’re pouring under what’s recommended, you’ll need a rubber with a higher Shore hardness.

Since urethane rubber is meant to be reusable, you want to do everything you can to keep using it. The best way to extend the life of your mold is with proper care and storage. That means when you’re not using the mold, you want to store it flat, in a dry, temperature-controlled location, away from direct sunlight.

Do not stack urethane molds on top of or underneath other molds. They might stick to each other, which will require force to tear them apart and weaken the rubber. If you store them in contact with molds made of different materials, this can cause swelling, shrinking, and distortion from the transferring of oils or plasticizers.

Contact VFI if you are still having problems with your urethane mold distorting or if you have other technical issues.