PolyJet 3D Printing’s Role in the World of Silicone Molding

By Ricky Shannon on

For many decades, mold making and casting were the go-to methods to produce realistic prototypes of smaller plastic goods. Often referred to as Room Temperature Vulcanization (RTV) or silicone molding, this is the process of producing a master part and making a flexible mold off that master. Then, the mold is filled with a two-part urethane resin, which cures into the shape of the mold. This produces a part that exactly mimics the original master’s shape and surface finish. This process is extremely versatile and is in use in many industries. Still, it requires a skilled model maker to produce a master and understand the proper molding techniques to achieve the desired result. Today, 3D printing is changing the way RTV molding is approached and no 3D printing technology is better suited for the task than Stratasys’ PolyJet technology. In this article, we will illustrate the process of RTV molding and explore how PolyJet printing can augment the various steps.

The Mold Master

Contrary to its name, mold master is not someone who is in control of the molds in a model shop, but instead, a mold master is an original model used to build the mold. The master could be made from any material and it is typically finished (sanded, polished, textured, etc.) to mimic the desired look of the final part. However, the material used to make the master does not matter as the final part will be cast in a urethane resin. You can think of the master as a mold to cast a mold. The actual mold pulled from the master is produced from a flexible rubber-like material, sometimes a urethane rubber is used, but more common than not, a silicone rubber. Silicone has several benefits for mold making, including high tear resistance (easy to stretch to de-mold parts), high temperature resistance (can withstand the high temperatures of the chemical reaction from resin curing) and a naturally non-stick surface (which also aids in part removal).

Producing the mold master is very time-consuming and requires a great deal of talent. Often a model maker is traditionally using wood and fillers to carve and sand an replica of an engineered part or the master will be CNC machined from a non-porous material like rigid plastic or aluminum. Both processes are time-consuming and require highly skilled labor – this is where 3D printing comes into play. Using the same CAD data that an engineer or designer has used to model the concept part, a mold master can be designed with no loss of design accuracy or intent. The designer will need to produce the two halves of the master using their original part design this also allows the engineer or designer to determine where a seam should be placed on the part. Also, they can produce highly accurate fill and vent gates in the mold along with creating alignment features so the final mold halves will perfectly align when clamped together.

mold master

With the CAD generated mold master, a PolyJet printed master can be produced using Vero rigid materials. Because of the tapered shapes used for plastic part design, the master can be printed using glossy settings directly off the PolyJet printer. This means there is little to no post-processing need as the part will have a perfectly smooth surface directly from the printer. If additional surface treatments are needed, often spraying matte or textured paints onto the 3D printed mold master will allow for a wide range of surfaces quickly and easily.

The Mold

This is where the rubber meets the road, if you need a quality flexible mold, you need RTV silicone. I suggest the use of a platinum cure silicone over the lower performing tin-cured silicones. Platinum silicones outperform tin-cured silicones and produce much less odor while mixing and curing. Some platinum cured silicones are also food-grade and can be used to produce parts requiring contact with skin and food. While there are flexible PolyJet materials with a surface finish ideal for directly printing molds, I have yet to get repeatable success from a directly printed mold. If I develop a process for using Agilus30 as the mold, I will update this blog and clue everyone into the necessary steps.

While we aren’t currently producing directly 3D printed molds, we are greatly reducing the time to make a mold by printing the mold master. Remember how I mentioned the engineer or designer can setup the parting line, the gates and alignment features in CAD and print the Mold Master with a PolyJet 3D printer? All those steps would traditionally need to be produced in clay. Yes, you heard that right, clay. The Mold Master is typically embedded halfway into a block of clay and the alignment features and gates were manually produced in the clay. Then, a silicone mold was cast off just one side of the clay embedded master at a time, after the first half, was poured the clay was unpacked from around the master and the second half was poured. If this sounds like a confusing explanation, well, it is. Mold making is a complicated and confusing process that takes time and skill to perfect. However, with the two 3D printed mold masters halves, it is possible to produce both molds at the same time without the need to create the parting line, alignment features and gates manually. This greatly reduces your mold production time by 6+ hours per mold as you can pour both halves of the mold at the same time (because you printed both sides of the master). This means you can produce two full molds in less time than it would take to create one traditionally produced mold using a clay divided master.
Silicone mold 3D prints
Casting the Parts

With an RTV silicone mold, a two-part urethane resin is mixed and poured into the mold. To be done properly, it is important to degas the two parts of the resin using a vacuum chamber and then very carefully mix the two to reduce the chance of bubbles being introduced into the mixture. Depending on the resin, it may have a working time of 2-5 minutes before it begins to gel and will no longer be able to be poured. It is during this short period that the material must be mixed, poured/injected and then ideally the mold should be placed into a pressure pot where it will be held under 50-60psi (consult the resin specs and the pressure pot manufactures recommendations). This pressure helps to crush any air bubbles in the mixture so they will not be present after the part cures and is removed from the mold.

3D printed silicone molds

With this stage there is a lot to go wrong, and unfortunately there are not many applications for 3D printing to help out, except of one.

Skip the RTV process and directly print our parts!

If you do not require a specific high-performance urethane to produce your parts, skip all the work and directly print your parts using a Stratasys PolyJet 3D printer. Even in their simplest Desktop Objet form, PolyJet parts are extremely accurate rigid models that can replicate the finest of details in rigid plastics. Expand out further into the PolyJet portfolio with the Connex3 and J8 (J750, J8350 and J826) series PolyJet printers and you can produce colored parts with textures, and even rubberized features like overmolds. By leveraging GrabCAD Print’s abilities when paired with a J8 Series PolyJet printer, you can even call out specific PANTONE colors directly in the software and avoid needing to put parts in the paint booth. By leveraging CAD programs like SOLIDWORKS, you can add physical surface textures and visual textures directly onto models without the need for hydrographic or screen printing.

silicone molding

Stratasys PolyJet printing can simplify the RTV molding process through many of its steps by reducing the amount of manual labor required, but why stop there? If the parts don’t require a highly specific resin, then directly printing the parts cuts out the need to design the mold master, produce molds, cast the part and then conduct any necessary finishing to produce the proper color or finish needed on the parts. 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.

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