Quality Urethane / Vacuum / Silicon / RTV Casting requires two things:
- RTV Silicone Tooling; and
- Vacuum Casting Machines
Void free urethane parts are produced through vacuum casting which can be molded in texture and color. In the Room Temperature Vulcanized (RTV) process, the liquid silicone rubber is poured around the master pattern and the resulting mold is cured after being pulled from the master pattern. The urethane parts are then subsequently casted.
It is very different to rapid tooling or injection molding as no high pressure or temperature is involved. Batch prototypes for marketing research are thus produced in an economical way.
Room Temperature Vulcanizing (RTV Vacuum Casting)
The most common silicone compounds used for mold making are RTV or “Room Temperature Vulcanizing” silicones which are mixed in two parts (a base and a catalyst) to induce curing. The silicone mixture is poured or spread over a prepared specimen, with gauze or other reinforcing cloth added between pours for increase strength and tear resistance if desired.
After curing and removal, the mold may be replaced and covered with rigid jacket or mother mold (often composed of plaster, resin, or urethane foam) to preserve the original mold shape during storage and casting.
Normal curing time for most silicones is between 18 and 24 hours, but cure times may be greatly reduced by using fast-acting catalysts. When making molds in a laboratory, vacuum Dearing may be performed to remove trapped air bubbles; however, when working in the field or without Dearing equipment, alternative techniques (discussed below) may be used to minimize air bubbles.
Let us study the process in a diagrammatic fashion:
Silicone Mold-making Procedures
One can follow the steps to for Silicone Mold-Making:
- Clean and prepare specimen (apply consolidant, sealant, or release agent if necessary).
- Build retaining wall around specimen.
- Stir silicone base to loosen any settled material.
- Decant desired amount of silicone into mixing container.
- Mix in appropriate amount of catalyst.
- Apply to specimen.
- If reinforced mold is desired, gently press in gauze after pouring first layer.
- When cured, gently remove mold from specimen.
- If needed, create rigid backing or jacket over mold with plastic, urethane foam, or plaster.
- Remove backing material and mold. Store mold inside rigid backing.
Considerations in Vacuum Casting
- No special sands or binders are required in this process.
- Metal Casting Industry’s major problem of recovery and reconditioning isn’t observed in Vacuum Casting due to absence of binders in the mold.
- Moisture related effects of metal casting is absent as the mold doesn’t contain water.
- It is however a relatively slow process.
Advantages of RTV Vacuum Casting
RTV Vacuum casting has higher advantages over traditional methods. Out of the long list, few of the major ones are listed below:
- Higher quantity of prototypes at relatively low cost can be produced with the process
- It provides premium and precision plastic prototypes
- It assists in engineering design prior hard tool
- It allows easy modification and edition
- Allows low cost tooling
- Reduces time to market
- It helps to evaluate potential markets
- Assistance for in house testing
- Provides aesthetic application where appearance holds criticality
- Used in marketing shows and sales presentation
Applications of Vacuum Casting
Ideal applications of Vacuum Casting are as below:
- Prosthetic and medical devices
- Part integration and function testing
- Automotive and aerospace parts
- Production of plastic prototype parts for a range of industries, including consumer goods
- Decorative objects, such as wall plaques or short run custom ornaments
- Product marketing