Application Review: RTV Rubber Tooling

Process:
RTV Molds (Room Temperature Vulcanizing Rubber)

Applicable Industries:
Aerospace, Toy Manufacturers, Medical, Sports Equipment Automotive, Computer & Consumer Products

Types of Masters Used:
Rapid Prototypes
Machined or Fabricated Patterns
Wax Sculptings
Types and Quantities of Parts Made:
Polyurethane 20 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+

Procedure:
Room temperature vulcanized molds are fast, easy and are a relatively inexpensive way to create prototype and production tooling. New silicone products offering greater tear strength, reduced shrink and larger useful temperature ranges have helped this mold making technique gain wide acceptance in such industries as aerospace, sporting goods, toys and decorative furniture.

There are a number of steps in the silicone mold making process. One must first create a pattern; an SLA or LOM pattern (or Master) is ideal. Because castable silicones reproduce surface detail and textures exactly, the pattern must be free of chips, burrs and scratches as well as grease and oil. Typically the master must be hand finished to the desired quality before making the mold. Choosing a logical parting line is the next step in the process. Pattern geometry determines the number of pieces needed for the mold; undercuts or internal geometry necessitate a one, two or multiple piece mold. A mold box or container needs to be constructed to hold the pattern in place and contain liquid silicone. Plastic, wood or aluminum bar stock are suitable materials. The parting line can be set by imbedding the pattern in modeling clay, surrounding the pattern with wood, or breaking the pattern apart and mounting the separate pieces on plates. Details that are not included in the first mold half need to be masked with clay.

Curing time 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.

Properties of silicone such as tear strength and hardness vary considerably from product to product making some better suited for geometry's which require extensive stretching. The most important consideration is the intended casting material. Many silicones will not work well for casting epoxy or low melt metals. Knowing the part geometry and end application will help us to choose the right silicone to ensure a successful mold.

Some considerations for the silicone mold making process. Many materials such as sulfur in modeling clay, vinyl or neoprene will inhibit the cure of silicone. Either patch test the surface or choose a silicone that is known to work on that surface. Some silicones will shrink up to .6% after a period of days. For accuracy, choose a lower shrinkage silicone, cast parts as soon as possible, and avoid using heat to cure the mold.

These types of molds can be used for very simple to very complex parts. By establishing a parting line with wood or clay, slides and loose pieces can be made very much the same as done in prototype injection molds. To date HPDC has made very intricate molds with as many as 13 pieces to pick up undercuts and mechanical details.