PolyJet 3D Printing for LSR Molding and Soft-Touch Jigs and Fixtures

   By on July 27, 2020

The Stratasys PolyJet 3D printing technology lets you produce high-quality, high-accuracy parts and prototypes at a reduced production cost. Many times, however, your prototypes might need a little hand to help you achieve the desired result. In this blog article, we review some of the processes that can help you create the ideal casting molds for your parts or prototypes, such as Liquid Silicone Rubber Molding (LSR), using the PolyJet technology. We review their differences and which materials are suitable for each of these processes to aid you in getting your ideal end-result.

3D Printed Rubberized Parts and Overmolds

With Stratasys PolyJet 3D printers, we can easily produce rubberized components and simulate overmolding with a wide range of material properties. This is partially due to realistic rubberized material performance with materials like Agilus30 – which can be blended with Vero materials to produce parts with hardnesses (or more literally, softness) ranging from a Shore 30A to a Shore 95A. The second part of this equation is the improved ability to assign color and material properties to parts and assemblies using the GrabCAD Print 3D printer software. This extremely user-friendly and powerful software tool leads the industry in both features and ease of use. But even with all these abilities, sometimes we must still mold and cast parts. Perhaps, we need higher heat or chemical resistance than can be 3D printed, or our prototypes will go through testing which requires the exact material used in the final production part – regardless of the reason, sometimes 3D printed parts just don’t cut it. Don’t fret, because your Stratasys PolyJet printer still has a place even with these non-printed prototypes.

3D Printed Tooling

For times when 3D printed prototypes won’t produce a satisfactory result, we can usually leverage 3D printing to produce molds and patterns to aid in the casting process. Often, we see this with overmolding on existing items like electrical connectors, medical devices, and electronics cases. In these applications, we do not have the liberty of producing the desired result directly from a printed job, but we can find a large cost and time savings by producing 3D printed tooling – and few processes are as well suited for this as Stratasys PolyJet. The PolyJet process is known for its industry-leading color replication, accuracy, and surface finish – for tooling applications the latter two are key. With its high accuracy and smooth surface finish, it is possible to directly print tooling using a PolyJet printer.

mold master 4

Liquid Silicone Rubber Molding (LSR)

For the above applications, we can directly print tooling in which to inject a wide range of materials. In many instances, we see the need to use rubberized urethanes and silicone rubbers. Silicones are particularly of interest in these applications because it has properties such as high heat, non-stick and high-tear resistance that make them ideal for high-performance applications where simulated materials from 3D printers will not work. Silicones also have important applications as they can hold medical and food contact certifications that might be instrumental to the successful testing of a prototype part. By 3D printing prototype tooling, it is possible to produce prototypes using these specific rubbers and silicones without the need for producing expensive aluminum tooling. This saves both time and money during the prototyping stage and allows for fast iteration changes.

The Liquid Silicone Rubber molding process (LSR) does not differ much from other types of molding, and in many ways it is very similar to RTV silicone. The major differences here are that instead of printing a pattern out of a rigid PolyJet material to cast a rubberized mold out of, with LSR we are printing a rigid mold and casting our part directly out of that. The ease of this process is directly related to PolyJet’s accuracy and smooth surface finish which both allows for printing mold halves that have perfect alignment and a glossy surface finish. These tools are typically suitable for prototype parts without any additional sanding, polishing or post-processing. The accuracy and repeatability of the PolyJet process makes it ideal for complex molds with the need of inserts and cores for overmolding applications.

LSR Mold>> Learn more about the role of PolyJet Technology in silicone molding

Different Types of PolyJet Materials for Silicone Molding

Depending on what type of silicone you are injecting into the mold, there are several materials and machines available for this task. If a low temperature (non post-cure) RTV silicone is needed, it is possible to utilize nearly any PolyJet printer in the Stratasys product line. These silicones do not require the molds to be heated nor post-cured in a heated environment. The lack of heat with these silicones means that a high-temperature material is not needed, opening the door for the use of a Vero PolyJet material. Vero is Stratasys’s basic rigid plastic that is available in a wide range of colors and transparencies. The great thing about being able to use Vero is that it is available on all Stratasys PolyJet machines from their smallest Desktop Objet to the full-color large-format J850.

When a post-cure procedure or a rapid cure silicone (that has an elevated kick-off temperature as cures) is used, a higher temperature material is required. For these instances, we use a PolyJet material called Digital ABS+ (DABS in hip slang), a material that is designed to simulate the performance of ABS plastic. This means that it has greater toughness and temperature resistance than the standard Vero materials. This elevated temperature resistance allows it to withstand exothermic kick-off and post-cure temperatures of most common silicones. DABS is composed of two different materials that are blended during the PolyJet printing process; however, not all PolyJet machines are capable of blending materials. If DABS is required for you LSR molding application, a Connex3 or J-series printer will be needed.

Prepping these printed mold halves for casting is extremely easy and requires few extra steps, and because sanding or polishing the mold is not necessary, it is often as simple as buffing the mold with wax and spraying it with a release agent. Through testing various molds, we have found that additional cure platinum RTV silicones (which are typically susceptible to cure inhibition when in contact with various surfaces) have fully cured when in contact with PolyJet materials.

Injecting Mold

Injecting the molds with silicone is typically an easy (although sometimes messy process) and you should always follow the silicone manufacturer’s recommendations on mixing ratios and handling instructions. With most manual silicone molding applications that have a pot life of greater than ten minutes, I would strongly recommend de-gassing the silicone under vacuum to prevent any excess air bubbles forming in the silicone after mixing. This is a simple process of putting the mixed silicone in a vacuum chamber and allowing it to come to full vacuum until most of the bubbles in the mixture have expanded and collapsed (typically less than 2 minutes at full vacuum). While de-gassing is not required, it does help aid in reducing the chances of having air pockets in the final molded part. In a more DIY setting you will most likely be injecting your silicon with a large disposable syringe or if you are lucky enough to be in a larger shop that frequently does RTV molding, it will be done with either an automated mixing gun or injecting gun (sort of a chalk gun on steroids).

Admire Your Hard Work

With the part fully cured (based on your silicone manufacture time estimates, you can now demold the part and check your work. It is normal to have some flashing along the parting line of the mold which can easily be trimmed with a small blade, along with removing any fill gates or air vent protrusions. If you need more parts, simply apply some more mold release to the mold components and cast another silicone part.

Demolded Part
Depending on your mold geometry and heat requirements for the curing process – it is possible to get tens or even hundreds of parts from your 3D printed mold. Watch our video below for more details on this process.

PolyJet 3D printing can help you simplify your molding process using different techniques and 3D printing materials. If you have any questions on how 3D printing can aid in your molding needs, please reach out to TriMech’s 3D printer applications engineering team.

3D printing molds with PolyJet technology can help you save time and money. Download our Technical Application Guide: PolyJet Molds for Silicone Parts, to learn all the steps and details of this process.

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