Mold Design with Stereolithography

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3D printed mold making process

Transform Your Mold Design Process with 3D Printing Technology

How To Use Stereolithography 3D Printing To Create Better Molds

Mold making is an excellent application for 3D printing since it enables the creation of parts in a much larger range of materials than printing can offer on its own, while taking advantage of the precision, flexibility and speed of additive manufacturing. Stereolithography (SLA) is a particularly good fit for mold design applications because it has a very smooth surface finish, high surface detail, and is watertight. There are a few ways to leverage SLA for mold making.


3D Printing a Master Pattern

A master pattern is a replica of the final intended product which is used to produce molds. It is often made slightly larger to account for shrinkage of the casting material. It needs to be as refined and precise as possible since every subsequent process will inherit any flaws it has, and it needs to be durable so that it can be used repeatedly to make more molds.

SLA is a good candidate for master patterns since it is tough, rigid, and can be sanded and finished to a high level of quality - an important step for removing layer lines and other print artifacts.

Silicone is typically the material of choice for most casting applications since it is durable and flexible, making demolding easy. For a single part mold, the master pattern is simply placed in a mold box and then silicone is poured around it. For a multi part mold, a non-sulfur oil based clay can be used to define the boundaries of the first mold part and then the other parts can be built up against the previous parts, making sure to include registration keys where each part touches. 

Once demolded, the SLA part can be cleaned up and reused many times. It should be stored very carefully to ensure it remains undamaged. 

3D printed master pattern

The white part on the left is the SLA master pattern, which is used to create the silicone mold which then has resin poured into it.


3D Printing a Master Mold

Another approach is to print molds for each silicone mold part, which allows for more control over how the different mold parts interact with each other. The previous process involves a lot of hands-on work and in-the-moment decision making, but this method gives you the ability to design the finished part and the mold components in CAD up front. The geometry for each of these SLA parts would include a section of the master part as well as the parts of the mold that define the edges of that section, all combined into one body. Designing everything up front gives you more control and repeatability with the mold design and reduces the risk of mistakes being made during the mold making process. As with the master pattern, the master molds need to be highly refined and stored safely.  

3D printed master mold

Master Mold Created Using Stereolithography (SLA) 3D Printing


SLA Mold

SLA parts can also be used directly as mold parts, as long as they are designed in such a way that the casting can be released easily. This means including a fairly significant draft angle and using a release agent. Even with these precautions, parts may still adhere to the mold, so this is best reserved for materials and geometries that lend themselves to easy part release, like silicone. 

SLA mold

A multipart SLA mold can be used to cast parts out of silicone or other soft durometer materials.


3D Printing Fixtures

Sometimes castings need to be made around internal components and it can be tricky figuring out how to hold them in place. 3D printed mold parts lend themselves to internal fixtures since they are based on precise CAD models which means alignment and fixtures can be designed ahead of time and the printed parts can be relied upon to line up.

Fixtures and alignment jigs typically don't interact much with the finished surface of the casting so other 3D printed materials can be used. Selective Laser Sintering (SLS) and Fused Deposition Modeling (FDM) are good alternatives to SLA since their surface roughness actually improves the bond when surrounded by silicone, and they don't cause silicone to react the same way that SLA does (though that is easily fixed with a clearcoat). 

3D printed fixtures

Shown is a design for a fixture meant to hold a part suspended into the mold. There are indents in the silicone mold that align with the fixture for precise placement. 


Slip Casting

These same techniques can be used for slip casting, a method for making ceramics that uses plaster molds and slip, a liquid clay that dries in contact with plaster. In this case, SLA can be used as a master pattern or master mold, but not as a direct mold because the interaction between the slip and the plaster is integral to this process. 

 

Slip casting mold

A typical slip casting mold.


Investment Casting

It is also possible to use SLA parts for investment casting, which is a process used for making metal parts. Typically, a wax positive is made and then has several layers of a ceramic slurry applied to form the mold. The wax is then melted or burnt out, leaving a hollow mold which can have metal poured into it. The way that 3D printing improves this process is in the creation of the wax positive. It can be used directly as a mold for wax, or as a master pattern to make the mold used for wax. This makes it possible to easily translate a digital design concept into a metal part. Of course, metal can also be 3D printed using Direct Metal Laser Sintering (DMLS) technology.

 

Stainless Steel 316L Investment Cast using a 3D Printed Master

Stainless Steel 316L Investment Cast using a 3D Printed Master


Digital meets traditional

Mold making is one of the oldest manufacturing methods still used today and can easily be the focus of someone's entire life because of how much skill is involved. 3D printing can shorten the learning curve significantly for less seasoned mold makers, and it enables experienced mold makers to experiment with much more complex forms and allows for greater precision and repeatability while cutting down on production time.

If you're interested in using SLA for mold making, we've got you covered