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Mold Making and Casting
Mold making is suitable for making multiple duplicates of an object, or experimentation with materials that are unavailable through conventional fabrication methods in the Makerspaces.
Advantages of 3D Printed Molds
- Allows for rapid iteration of casted objects
- Higher degree of control over pour holes and vents
- Calculation of mold volumes for material efficiency
- Create molds with a high degree of accuracy
- Reusable mold boxes to create multiple molds.
Disadvantages of 3D Printed Molds
- Residual layer lines transferred to final casted objects
- Size constraints limited to the print volume of 3D printers
A positive mold is a replica of the desired final product, typically made of a rigid material to ensure accurate replication of the actual object. Whereas a negative mold, on the other hand, is the inverse of the final object, typically made of a pliable/soft material to facilitate ease of demolding.
Types of Mold
Depending on the complexity of geometry, you may need to consider designing for several molding components.
One part molds are suited for geometries that have a flat surface on one of the sides. Whereas two or more part molds are more suitable for geometries that have a higher level of detail.
- 1.Drafting AnglesAdding tapered walls to the mold design will aid the removal of your object from the model.
- 2.Orientation of the objectFor complex geometries with details such as overhangs and small details, proper orientation of the geometries will reduce stress points that may destroy the casted object when demolding.
Key Considerations of Mold Making
The most common 3D printing material at the NExTLab is PLA filament. With a low melting temperature and minimal warping, PLA is one of the easiest materials to 3D print with. However, its rigidity does not allow for many opportunities for design experimentation. So as an alternative to PLA, NExTLab stocks a range of experimental filaments that can be used with our open access printers such as the Prusa, Raise 3D and the Modix Big 60.
One of the experimental filaments the NExTLab stocks is TPU, also known as Thermoplastic polyurethane, which is a very elastic and flexible material. It is smooth to the touch, but at the same time extremely durable and strong. It offers high wear resistance and possesses excellent resistance to oils, greases, and solvents. This makes it very suitable for mold-making.
Nozzle Temperature Setting
TPU filaments typically print in the range of 220–250 degrees Celcius. But it is best to consult the filament manufacturer's specification on the side of the box or look up the settings on Google as some manufacturers may have different specifications for their filaments.
Print Bed Temperature Setting
An ideal bed temperature for printing TPU is in the range between 30-60 degrees Celcius.
Retraction is a feature meant to eliminate stringing/drooping by pulling back (retracting) filament when the printhead moves to a new location. Doing so counteracts the hot end pressure and helps prevent excess material from flowing when it’s not supposed to.
But since TPU is a flexible material, small and fast movements from retraction could cause the TPU filament to get caught in the stepper motor. The best practice is to minimise small fast movements by turning off the retraction settings altogether.
As part of the April holiday season, the project was to create a non-denominational chocolate treat. Since the final object was going to be eaten, we needed to create a mold for a food-safe mold. ~mold-ception~
The Chocolate Bunny was modelled using the SubD tools in Rhino to minimise hard edges and crevices that may get stuck in the mold.
Chocolate Bunny 3D Model
As the bunny will have to be casted in a food-safe mold, we need to create a two part food-grade silicone mold. The original bunny model was sliced in half at the section with the largest surface area and placed in a bounding box. Alignment points are modelled into the box for the silicone models to align with one another and become watertight.
As the mold will be 3D printed with TPU, the thicknesses of the print will determine which part of the print will be flexible and which will be rigid. Area such as the bunny negative and the alignment points, are printed to be thicker to ensure that there isn't any warpage. Whereas the shell walls will be thinner to allow for flexibility and ease of demolding.
Equipment and Materials
Depending on the requirements of your project, you may need different materials and equipment to the case study as shown above. But the main equipment that you will need are:
- 2.Mixing Container
- 3.Mixing Tool
- 4.Pouring Container
- 5.Digital Scale
Materials can be sourced from local or online vendors of your choice. But the NExTLab recommends Barnes as they have an extensive range of casting and molding products, and their staff are very knowledgeable to advise you with what you might need for your project.
Materials required for Silicone Casting
Depending on the type of silicone used, you will have a window of working time before the silicone cures. This will typically be specified on the side of the box or the SDS sheet.
When combining the silicone and activator together, it is very important to get the mixing ratio correct because the silicone may not cure properly. This could cause issues such as difficulty in demolding or the texture being tacky so it is unusable.
When combining the silicone mixture, there is a high chance of introducing air into the mixture. The trapped gasses will cause the silicone to dimple and create porous surfaces that will translate onto the casted object. To prevent this, make sure to fold the parts slowly until it becomes a homogenous mixture. In the case that there are a lot of trapped gas, firmly tap the mold against a hard surface to knock the gas bubbles to the surface.
Casted Chocolate Bunny in Silicone Mold