Accuracy:
Cast urethane is a highly accurate way to create prototypes quickly and relatively inexpensively. However, comparing it to parts made from SPI class 102 injection mold tooling is unreasonable.
Since cast urethanes create exothermic (heat) reactions while curing there can be some variation in finished part size. This is particularly apparent when time pressures are high and many parts are required per day from each mold. As the mold warms up, from several back to back castings, the mold may swell due to heat expansion. This results in parts that are slightly smaller in size from parts cast when the mold was cool. Predicting this change is difficult and therefore unreliable. The variations are so small, however, that this is generally not a concern on parts smaller than 12". On larger parts, more time is required between casts to attain a greater level of consistency.
Mold stability or stiffness also affects part accuracy. Based upon our previous experience, we carefully calculate the required wall thicknesses and build RTV molds suitable for the intended production requirements.
Parting lines are another area that can lead to poor part quality. At Metropolis we usually build the more complicated splitter plate design molds rather than hand cut parting lines. This affords us more control of the part and consequently creates better fitting parts and assemblies.
