The Ultimate Buyer's Guide for Purchasing Urethane Casting Services

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Several components are necessary for the processing of urethanes. Both the prepolymer and curative must be prepared separately. They then get mixed and introduced into the casting mold. Finally, it has to set for a predetermined amount of time and may need a post-curing step as well. 

1. Hand Processing

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Small-batch urethane production is normally performed using hand-processing techniques. The process is described in detail below.

• Prepolymer Preparation - Depending on the type of urethane, the prepolymer may arrive as a solid or a liquid, if it is a solid it needs to be melted down in an oven. Next, the component masses can be calculated; the mixing ratio is normally specified by the material supplier. This ratio can be varied slightly in order to favor certain mechanical properties such as abrasion resistance or compressive strength, though in general this is not required. The correct amount of prepolymer must then be weighed out in preparation for the mix. Most prepolymers should be degassed by placing them under vacuum to remove any air bubbles from the material and thus ensure a high-quality end product free of bubbles.
• Curative Preparation - Next, the curative must be prepared. Once again, this component may be provided in liquid or solid form, each of which has its own set of advantages and disadvantages. The curative can then be weighed out according to the ratio calculation in preparation for mixing. If the urethane needs to be colored, pigment is most often added to the curative mix. Experts generally recommend mixing the pigment into the curative to make it easy to see if all the curative has been properly mixed in with the prepolymer. Very little pigment is required to color a part; as little as 2% of the total mix mass will successfully color the part.
• Mold Preparation - Next, the mold can be prepared. The mold can be made of fiberglass, steel, or even silicone. It needs to be lubricated with a mold release agent to prevent the part from sticking to the mold&#;s surfaces. Molds for hot cure urethanes should be heated before casting as this helps prevent the formation of surface defects.
• Casting - Once everything is ready, the material can be mixed. In the case of hot cure urethanes, the prepolymer and curative must first be heated to the recommended temperature then thoroughly mixed. Mixing must be done in a way that limits the introduction of air into the mix. After mixing, a second degas process is performed and the material can be cast into the mold. Urethane set times can vary from a couple of minutes to an hour. Note that materials that have short set times are best processed by machine rather than by hand.

2. Machine Processing

Urethane can be mixed using a machine called a dispenser. This machine mixes the prepolymer, curative, and pigment automatically and injects it into a mold. The whole process is similar to injection molding and does not require any hand processing. 

3. Post Curing

Most urethanes need to be post-cured in an oven to achieve their final properties. In some cases, a part may be demolded after an hour but must be left in an oven for up to 16 hours for the remaining cross-linking to occur.

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