Stereolithography (SLA) is a great method for making accurate rapid prototypes to check for form, fit, and function, but the process has another use. SLA parts can also be used to make prototype rubber parts via a casting mold.
To recap: SLA is an additive manufacturing or 3D-printing technology that uses a vat of liquid, ultraviolet, curable resin and an ultraviolet laser to build parts one layer at a time. For each layer, the laser traces the cross-section of the part onto the surface of the liquid resin. Once the liquid is touched by the laser beam, the resin cures and solidifies to the layer below it. As each layer is finished, the platform the part is being created upon lowers a small amount so the next layer can be traced out above it. When the part is done, it is removed and cleaned of excess resin or supports, then cured in an oven.
A casting mold can be fabricated directly using this SLA process. The first step is to evaluate the part and decide on the best way to mold it. Parting lines and filling need to be considered. The mold will be designed in CAD and printed using an SLA machine. The mold will usually consist of two parts (core and cavity), but could have more removable sections if there are undercuts features in the cast part. The SLA process produces a clean, accurate mold with smooth inner surfaces – perfect for molding with little to no extra steps.
Once the mold is built, cleaned, and assembled, it is time to make the liquid silicone rubber parts (LSR). The liquid silicone rubber is usually made from low viscosity, two-component materials. After the two components are mixed together in the proper ratios it is injected into the SLA mold. The LSR part is removed from the mold after it cures. This process is then repeated to create the required number of parts.
The LSR parts created with this process have similar characteristics of mass-production molded parts. They have all the details of the final parts as well as great accuracy and aesthetics, combined with the desirable mechanical and thermal properties of production-grade material. The process has proven invaluable to test out a rubber part and save large sums of money compared with machining an aluminum prototype tool. Another advantage is that if there are changes that need to be made, just update the design, and print a new SLA mold. The new part should be ready for testing in a couple of days.
This process does have its limits though. A SLA mold has a limited life, and it is labor intensive to hand case each part. Once the mold is worn out another mold needs to be made, so it can get expensive if more than a low-volume run is needed. Also the part size is limited to the size of mold that can be printed in a 3D printer. Multiple mold sections can be glued together to make a large mold but this gets expensive and inaccurate. Also, only elastomers that are room temperature cure can be used for casting (mixing part A & B). The practical part size range is normally from a 0.25” to a 12” cube being about the largest part size. However, much larger parts have been successfully cast.
Often when rubber parts need to be prototyped and tested making SLA molds are the best option. Let Pongratz Engineering steer you in the right direction on which rapid prototyping method is best for your part or project.