A flexible process to replicate parts for prototyping purposes or final products purposes
Polyurethane (PU) is a polymer composed of organic units joined by carbamate (urethane) links. Most polyurethanes are thermosetting polymers that do not melt when heated. Polyurethane polymers are traditionally and most commonly formed by reacting a polyisocyanate with a polyol. Through adjusting liquid polymer composition, we can finally produce solid parts of thermoset material with different performance, similar with ABS, Polycarbonate, PMMA, Rubber and so on.
The status of Polyurethane raw material is liquid, so casting method can be used to produce PU parts. VC (Vacuum casting) method can produce parts with high quality much more better than atmospheric pressure casting method.
RTV Silicone (room temperature vulcanization silicone) is a type of silicone rubber made from a two-component system available in a hardness range of very soft to medium - usually from 15 Shore A to 40 Shore. RTV silicones can be used to make silicone rubber mold. Using this type of soft mold can produce many complex Polyurethane products. SRM means Silicone Rubber Molding.
SRM+VC are the least expensive and fastest way to create about a dozen of products, suitable for the projects with quantity of prototypes/products less than 100 pcs, high quality demands, and designs with complex structures, such as undercut, which is hard to be demoulded from a metal mold. These parts are able to be removed through distortion of the soft mold.
SRM+VC (Silicone Rubber Molding plus Vacuum Casting) are used to replicate parts for prototyping purposes or final products purposes. Below information of Polyurethane Casting and any parts list in this chapter, are all from SRM+VC methods.
Illustration of the polyurethane casting process
An pattern is used to create a master male pattern as shown. The prototyping technique most often used is the SLA/CNC process.
The master pattern is fitted with a sprue and gate and then surrounded by a parting surface which establishes the parting line for the mold. Alternatively, the cured mold can be cut carefully to form the parting line.
The assembly is fixed in a vat, and liquid RTV silicone rubber is then poured over the pattern and parting surface combination as shown right.
Once cured, the RTV tools is removed from the vat and separated from the pattern and parting line surface to yield the two halves of the molding tool. Thermoset resin such as polyurethane is poured into the mold.
The finished part is removed from the mold. Undercuts are overcome by distorting the mold, which springs back to its original shape as long as distortion is not too severe.
The part must then be post-processed by trimming any flash, and the gate and sprue must also be removed.