Navigator #9, July 2001
RTV Molding and Urethane Castings
Urethane Castings have put the function in "fit, form, and function" of the RP world. People have been using RTV (room temperature vulcanizing) molding for years to fill the void between prototype and production. Let's take a look to see the vast improvements in this application.
How Does RTV Work?
The RTV mold is created from a master pattern. Typically the pattern is a SLA. The SLA is used to create a silicone rubber mold. The silicone is poured around the pattern to produce a core and cavity of a mold. The SLA is then removed which leaves the negative image for polyurethane to be cast into. These molds can produce up to 20 parts per mold and these molds can be made as family or multi-cavity molds.
When to Use RTV
When is RTV Molding useful for rapid prototyping? RTV is very helpful when you need a spec-like material for multiple sets in a short time frame for less cost than a
An over mold with two different colors & two different hardnesses. The red part is 80 shore D and the black over mold is 60 shore A.
Materials
The materials that can be cast in a rubber mold are 2-part polyurethane, silicone rubber, and wax. The most typical is urethane due to the large selection of properties. Materials can match similar hardnesses from 10 shore A to 80 shore D. The properties of heat temperatures have improved to hold up to 220 degrees F. The silicone materials can be shot in Shore A durometers for prototyping. While waxes can be shot for investment casting patterns.
Colors and Textures
Parts can be cast in color instead of painted to prevent colors from chipping or flaking off. These colors can be matched by a Pantone number or a color chip. White is more difficult to match, but depending on the material, it can be done. Texture is added to the SLA patterns so when the mold is created, the texture is transferred directly to the mold. Different textures are available and multiple textures can be placed on the same part for cosmetic purposes.
Over Molding
RTV works wonderfully for insert or over molding. Two patterns are produced, one with the over mold, and one without the over mold. Then, two rubber molds are produced and the urethane is cast. The first part is cast into the rubber mold without the over mold. That part is then pulled from the mold and inserted into the rubber mold with the over mold. Then the over mold is cast around the existing cast part. Once that part is pulled, it has both materials cast to complete the two part over mold.
Water Clear Parts
There are now urethanes that can be cast water clear. These applications work great for light pipes and lenses. The transparency depends highly on the finish of the master pattern. Because mold release is not used, only 5-7 parts can be pulled from each mold. After these quantities, mold release is needed which will create a slight haze to clarity.
A water clear urethane lens placed over tree leaves.
SLA's can also be color tinted in a transparent look for last minute meetings. For other surface finishes, SLA is not limited. SLA's can be chrome plated, vacuum plated, or plated with nickel, zinc, or copper. Customized texture can be added to imitate leather, wood grain, or an EDM finish.
Advantages
- Can mold undercuts due to the
flexible nature of the silicone molds.
- Can cast different colors and/or
urethane properties each shot.
- Can mold threaded inserts directly into the cast parts.
- First parts in 7-10 days without
paying for production tooling costs.
Disadvantages
- Limited materials. (It is only spec-like material)
- Only 20 parts per mold.
- Modifications of original pattern require a new master pattern to create a new RTV mold.
Please contact your sales representative to see if RTV molding can help you get your product to market sooner.
Vista Travels to Get and Give Education
Emily Berg (programmer) and Dan Mishek (Sales Manager) attended the 2001 3DNASUG in Destin, Florida. The annual conference was held March 18-22 and the weather never got above 45°F so plenty of work got done! Vista feels attendance at this conference is important. This is the center stage where new equipment, software, finishing techniques, and resins are introduced. Both Emily and Dan brought back interesting information. They were excited about the introduction of a new resin! Call Emily or Dan to find out more about it.
In April, Jim Mishek attended the Rapid Prototyping & Manufacturing Show in Cincinnati, Ohio. Jim not only was an attendee of the show, but a guest speaker as well, presenting on the topic of metal castings in rapid prototyping. He has also presented at Amcon Minneapolis, Amcon Phoenix, SPE, and FMA.
Quick Quip
“The greatest mistake a person can make is to be afraid of making one.”
- ELBERT HUBBARD
“If you make a mistake, we would be happy to prototype it again, and again, and again...”
- DAN MISHEK
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