RTV Molding / Urethane Casting - Rapid Prototyping - Rapid Tooling

Room Temperature Vulcanization (RTV) rubber molds are the fastest, most accurate, and least expensive way to create up to a dozen or so duplicates of a prototype part. Rubber molds can faithfully duplicate details and textures present on the original part, and can be very forgiving of part geometry when it comes time to remove it from the mold. After the RTV Molds are completed, they are then typically used to produce limited quantity polyurethane castings with a wide variety of material properties.

Urethane casting material is very popular for product prototypes. It can be formulated to imitate elastomers, and can have structural properties similar to high-impact stryrene. It can mimic production parts in material properties, thermal properties, color, and surface texture. It can also be machined, sanded, glued, and painted readily.

PROCESS DESCRIPTION

A Rapid Prototype (usually SLA) is used as a master pattern to form the rubber mold. The SLA part is made 0.003 inch/inch oversize to compensate for rubber shrinkage. The pattern is sanded and primed. The master pattern is surrounded by a parting surface that establishes the parting line of the mold. The liquid RTV rubber is poured over the pattern and parting surface combination. Once the rubber has solidified, the parting block is removed and rubber is poured over the pattern and mold half combination. The pattern is removed to yield the mold halves of a rubber tool.

Curing time for the mold is dependent on the product and curing agent. Times range from 30 minutes to over 40 hours. Adding heat will speed up the curing process significantly. Aging the mold at room temperature for up to 72 hours, if possible, will increase the productive life of the mold.

Liquid urethanes, thermoset materials, are then poured or injected into the rubber mold. The urethane cures to solidify. The RTV tool is then separated to remove the cast urethane prototype. Casting methods include:

Gravity casting: The liquid urethane is poured into the RTV mold. This method relies on gravity to feed the material into the cavity of the tool.
Vacuum Cast Molding (VCM): Vacuum Cast Molding is a new technology for producing functional thin-walled complex prototypes parts in plastic materials which stimulate ABS, Nylon, Polycarbonate and other production materials. Features as small as a finger print can be reproduced with the vacuum casting process. Unlike conventional RTV molding, the Vacuum Casting process produces quantities of 15-25 parts per week and fills complex thin-walled parts completely void free.
Thin Wall RIM: Thin-wall reaction injection-molding (TW-RIM) can produce copies of rapidly prototyped parts at a rate of about three or four/hour. The short cycle time comes from use of fast-setting polyurethanes and other materials that gel in less than a minute and which can be demolded after about 10 min.
One factor that makes such fast cure times possible is the development of inexpensive disposable mixing heads. They automatically mix two-part resins in a vacuum and shoot the material into the mold. Previously, molders had to mix the materials by hand, put the mixture in a vacuum chamber to de-air it, then send the material to an injector head. These time-consuming steps made it impractical to shoot material that gelled in less than a minute.
The cost of injection equipment for fast-setting resins has dropped, boosting the technique's popularity all the more. When it first debuted, the least expensive injection system ran about $20,000. Newer equipment for handling fast-setting resins carries a price tag of around $4,000.
An additional benefit of quick curing is that it can substantially lengthen the life of a mold. This is because the curing process plays a significant role in disintegrating the mold. Thus a mold that might normally last through 25 to 50 shots could be expected to provide 50 to 100 shots when used with quick-curing resins.

Specifications

Time:: From pattern to first part: 3 - 7 days
Production Rate:: Typically a mold can be shot 1-3 times per day. A single mold can sometimes produce up to 10 pieces at once depending on the part complexity and size.
Types and Quantities of Parts Made:

Polyurethane 15 to 60

Polyurea 10 to 60

Epoxy 10 to 30

Investment Wax Patterns 50 to 300+

Low Melt Metal Alloy 20 to 75

Polyurethane Foam 0 to 200+

Silicone Rubber 20 to 80+

Accuracy: As accurate as the SLA prototype (+/-0.005"); however, tolerances of +/-0.002" have been achieved.