12. Molding and Casting: Richard Shan & Connor Cruz¶
Material Properties¶
Mold Star 15 Slow is a platinum silicone rubber with a pot life of 50 minutes and a cure time of 4 hours at room temperature. It has a Shore A hardness of 15, making it very soft and flexible. The silicone cures to a green color and is characterized by very low shrinkage and long-term stability, with a useful temperature range up to 450°F (232°C). This material is suitable for casting a variety of materials, including wax, gypsum, and resins. It can also be used directly on most machined molds, since it can be removed best from a hard mold.
Here is a cast which used Mold Star 15 Slow:
TASK 8 is a polyurethane resin with a very short pot life of 2.5 minutes and a quick cure time of 10-15 minutes. It features a much harder Shore D hardness of 80, indicating a rigid final product. This resin is designed for high-performance use, particularly where heat resistance is needed, up to 263°F (129°C) after heat curing. TASK 8 cures to an off-white color and is known for its high strength and durability, making it suitable for applications such as machine housings and general prototyping. Note that since TASK 8 produces a hard cast, a soft mold is necessary in order to remove the final product. As a result, one might need to go through a 3-step process to create a cast with TASK 8 (create a hard mold, cast a soft mold, cast the final product from the soft mold).
Here is a cast which used TASK 8:
Safety¶
Whenever working with Moldstar 15 or Task 8, everyone was required to wear single-use gloves. We would dispose of these gloves after we finished working with the mold. We also use individual plastic containers for mixing and pouring which are each thoroughly rinsed after usage. The molds are all stored in a secured cool and dark cabinet area. All work with the molds occurs in the same room as our laser cutter and soldering, meaning that there is an efficient and good ventiliation system.
Moldstar 15 is an exothermic process, but the heat released is very moderate and does not pose any immediate safety risks. Task 8 is also exothermic, and releases a significantly greater amount of heat, but its danger level mostly depends on the amount and volume for usage.
Cast Applications Comparison¶
Mold Star 15 Slow is ideal for general mold making, especially for creating molds that will be used repeatedly due to its tear-resistant nature and ability to replicate fine details. The low hardness allows for easy de-molding of cast parts without damaging either the mold or the part. Mold Star is better for flexible casts with lots of finer details or small parts.
TASK 8, on the other hand, is used for creating parts that must withstand higher temperatures and stress, such as prototypes that undergo thermal testing. Its rapid setting time and high rigidity make it less suitable for delicate or intricate molds but excellent for durable, functional parts. Task 8 is good for creating parts that will actively be in motion and cannot flex at all.
Datasheet Comparison¶
| Property | Mold Star 15 Slow | TASK 8 |
|---|---|---|
| Type of Material | Platinum silicone rubber | Heat resistant polyurethane resin |
| Mix Ratio | 1A:1B by volume | 1A:1B by volume (120A:100B by weight) |
| Pot Life | 50 minutes | 2.5 minutes |
| Cure Time | 4 hours | 10-15 minutes |
| Shore Hardness | 15A | 80D |
| Viscosity (cps) | 12,500 | 100 |
| Color | Green | Off-white |
| Useful Temperature Range | -65°F to 450°F (-53°C to 232°C) | Up to 263°F (129°C) after heat curing |
| Specific Gravity (g/cc) | 1.18 | 1.09 |
| Tensile Strength (psi) | 400 | 5,840 |
| Elongation at Break (%) | 440% | 4% |
| Flexural Strength (psi) | Not specified | 8,280 |
| Compressive Strength (psi) | Not specified | 8,760 |
| Heat Deflection Temperature | Not specified | 194°F/90°C (before curing), 263°F/129°C (after curing) |
| Applications | Casting wax, gypsum, resins | Machine housings, prototyping, thermal casting |
Mold Production Method Comparison¶
Machining molds is great for creating tight tolerances and can have a really small stepover, meaning that the molds will be more accurate and have a smoother surface. The surface finish can be controlled through a variety of variables such as stepover and different tools when machining a mold. Additionally, machining molds means that you can use a lot more different types of materials. However, in our experience, machining molds tended to use a lot of extra material that we had to melt down and recast into wax blocks which was a hassle. Additionally, setting up the process for machining took a lot longer and was more error-prone than 3D printing, and especially since we were only making a couple of molds, machining was the less practical option.
3D printing results in a surface that is less smooth since 3D printing inherently has small bumps and ridges due to the plastic extrusion going back and forth in lines. The surface finish can be changed by modifying variables such as printing speed and thickness, but post-processing like sanding down the mold is usually needed to produce a super smooth finish. However, 3D printing is drastically easier to set up and print, especially when you only need to make a single or a few molds. 3D printing also is easier to test and debug at any point in the process.
A resin 3D printer can be used to have a more smooth initial mold, but it might require more supports due to the possibility of the print cupping. As a result, supports might have a risk of being hard to remove from the mold and cause inaccuracies in the mold’s shape.