3D Printing, Applications & Projects

Faster, Cheaper RTV Silicone Molding Using FDM and PolyJet 3D Printing

By Juan Carlos Gandiaga on July 13, 2015

Are you creating any parts that are cast using silicone molding? If so, consider how 3D printing can dramatically speed up the process of creating the pattern. Here’s a quick guide to how it works—and the potential benefits.

RTV-Molding-Examples

Casting parts from silicone molds is a three-stage process.

  1. Create a positive physical part of the final design—the pattern. For the typical RTV molding process, the material for the pattern is not subjected to high heat or pressures. However, the surface quality of the final part (assuming no final post-processing) is dictated by the surface finish of this pattern.
  2. Cover the pattern in silicone to form a silicone mold. The mold will be designed to divide into pieces to allow for the pattern to be removed and the final material to be injected.
  3. Casting of the part using the silicone mold. Note that the original pattern is not used for this step. This step can be repeated using the same silicone mold for multiple final parts.

By far, the first step of creating a pattern takes the most time and expense. Traditionally, patterns were machined using a hard or soft material. Due to the often-complex part design desired with silicone molding, this machining process usually takes longer than creating the mold or casting parts.

Waiting a week, for example, for the machine shop to complete the part creates several problems. First, it can be an expensive step. Secondly, due to the time and cost investment the designer will naturally tend to be conservative in the design to minimize the risk of tooling revisions. Finally, while the machined piece can be used for functional testing, the design is already committed and any revisions won’t be in the first molded pieces.

Advantages of 3D Printing the Pattern

Pattern-and-moldInstead of CNC machining the pattern, 3D printing it using FDM or PolyJet technology from Stratasys saves cost—often 50-70% over CNC machining. What’s even better is the turnaround savings of an overnight print (time savings up to 90%). Designers can also use the 3D printed pattern to do quick functional testing, knowing they have time to iterate, allowing for more design freedom and ultimately better designs. The process can also be kept in-house during the prototyping stage.

Creating the pattern on an FDM printer like the Fortus 450mc results in a stable, durable and accurate part made from engineering thermoplastic. To address the surface finish requirements, FDM parts are easily post-processed for the desired finish, including glossy or textures. Simliar performance is possible using PolyJet printers, with the advantage of higher native surface resolution.

In the mold-making process, FDM patterns won’t deflect, even after being stored for months or years. Complex geometries are a best-fit for FDM, especially when there is positional for several design revisions. The part is also able to withstand higher temperatures when needed to accelerate curing of molds.

Final-harmonica-part

The steps after creating a 3D printed pattern are identical to traditional silicone mold making and casting. The use of Stratasys technology doesn’t change the process—it just makes it faster and less expensive by fixing a bottleneck. Designers benefit too with more freedom and ultimately, better designs.

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