13. Molding and Casting¶
group assignment:
- Review the safety data sheets for each of your molding and casting materials.
- Make and compare test casts with each of them.
- Compare mold making processes.
individual assignment:
- Design a mold around the process you’ll be using
- Produce it with a smooth surface finish that does not show the production process toolpath
- Use it to cast parts
- Extra credit: use more than two mold parts
A long time ago, I tried to purchase silicone molds and use gypsum materials and metal tin to make products. This course has given me a deeper understanding of making silicone molds. I learned how to make master molds, silicone molds, and replicate products using silicone molds.
This time I use five materials (Casting Wax、Fast Cast Polyurethane Casting Resin ,silicone, gypsum,Pinch silicone by hand) to make molds and products for this table.
Group Assignment.¶
1.Reviewing the safety data sheets for molding and casting materials.¶
1.1 Using CNC machine to carve Brick Wax and make master mold¶
Brick wax is very suitable for making master molds with CNC machines:
1. Ultra high precision, eliminating the need for subsequent polishing
2. Compared to manual carving molds, it is more efficient and saves a lot of molding time
3. Waste wax can be 100% recycled with extremely low material loss
In addition to CNC engraving, it can also quickly produce product prototypes like gypsum materials. Indirectly heat at 65-70 ℃ for 30 minutes to fully liquefy.
Brick Wax Properties Table¶
| Category | Description of Brick Wax |
|---|---|
| Physical State | Solid block at 25℃, becomes liquid when melted (melting point 52-58℃) |
| Release Agent | Not required (self-releasing) |
| Safety Notes | ① Heat-resistant gloves when melting ② Work in ventilated area ③ Keep away from open flames |
| Melting Ratio | 100% pure wax (no mixing required) |
| Heating Time | 30 minutes indirect heating at 65-70℃ for complete liquefaction |
| Setting Time | At 25℃: - Surface skin: 8-10 minutes - Full solidification: 40-50 minutes |
| Notes | reusable 3-5 times |
1.2 Food-Grade AB Silicone Usage Specifications¶
| Category | Food-Grade AB Silicone |
|---|---|
| Solid/Liquid | Both A/B components are liquid , curing to form elastic solid after mixing |
| Mold Release Required | Not mandatory, the master mold cannot be made of silicone. |
| Safety Precautions | ① Wear nitrile gloves ② Operate in ventilated area |
| Mixing Ratio | 1:1 (volume ratio), precision applications require weight ratio (A:B=1:1.05) |
| Mixing Time | 2-3 minutes (scrape container walls ≥3 times), Usually within half an hour |
| Curing Time | At 25℃: - Initial cure: 2 hours (demoldable) - Full cure: 24 hours (food-grade strength) |
| Notes | The higher the degree value, the higher the hardness (e.g. A50 is harder than A30)， suitable for long-term use at -40℃~200℃ |
1.3 Quick-Dry Hand-Kneading Silicone Properties Table¶
| Category | Description of Quick-Dry Hand-Kneading Silicone |
|---|---|
| Solid/Liquid | Both components are paste-like , sets in 3-5 minutes after hand kneading |
| Mold Release Required | Not needed (self-releasing) |
| Safety Precautions | ① Wear PE gloves when handling ② Avoid eye contact |
| Mixing Ratio | 1:1 (volume ratio), knead by hand until uniform color |
| Mixing Time | Hand kneading 2-3 minutes (until color streaks completely disappear) |
| Curing Time | At 25℃: - Initial cure: 5-8 minutes (shapeable) - Full cure: 30 minutes (reaches final hardness) |
| Notes | can be sanded/painted after curing, suitable for DIY crafts |
1.4 Quick-Set Gypsum Powder Properties Table¶
| Category | Description of Quick-Set Gypsum Powder |
|---|---|
| Physical State | Powder, becomes slurry when mixed with water, final solid state |
| Mold Release Required | oil-based for metal/plastic molds, water-based for silicone molds |
| Safety Precautions | ① Wear mask to prevent dust inhalation ② Avoid eye contact with dry powder ③ Exothermic reaction during curing (50-60℃) |
| Water-Powder Ratio | 100:28-35 (100g gypsum powder requires 28-35ml of water, excess water reduces strength) |
| Mixing Time | 2-3 minutes (until lump-free), 1 minute deaeration |
| Curing Time | At 25℃: - Initial set: 4-6 minutes - Final set: 10-15 minutes - Full drying: 24 hours |
| Notes | Suitable for quick mold flipping |
1.5 Fast Cast Polyurethane Casting Resin Properties Table¶
| Category | Description of Fast Cast Polyurethane Casting Resin |
|---|---|
| Physical State | Part A viscosity 300-500cps, Part B viscosity 400-600cps |
| Mold Release Required | non-silicone release agents like PVA or Teflon spray recommended |
| Safety Precautions | ① Wear N95 mask + goggles ② Nitrile glove protection ③ Forced ventilation in work area |
| Mixing Ratio | 1A:1B (volume ratio), for high-precision applications use weight ratio 1A:0.95B |
| Mixing Time | 3 minutes (mechanical mixing at 200rpm), 1 minute deaeration |
| Curing Time | At 25℃: - Demold time: 30 minutes - Full cure: 4 hours - Post-cure: 24 hours (maximum strength) |
| Notes | shrinkage <0.3%, suitable for precision part replication ， Exothermic peak up to 80℃ |
2.then make and compare test casts with each of them;¶
(Picture X)
3.Mold Material Comparison Table¶
| Category | CNC Carved Brick Wax | Fast Cast Polyurethane | Silicone | Gypsum | Pinch Silicone by Hand |
|---|---|---|---|---|---|
| Process | CNC carving (<25℃)→Lost-wax casting | 1:1 AB mix→30min demold | AB mix→vacuum degas→4-24h cure | Water:powder (100:30)→15min set | Hand kneading→3min定型 |
| Key Features | Ultra precision (±0.05mm) | High strength/wear resistance | Best detail reproduction | Lowest cost | Instant shaping (no mold needed) |
| Best For | Precision metal casting patterns | Engineering part replication | Complex texture copying | Quick simple shapes | Rapid prototyping |
| Limitations | Requires CNC equipment | Exothermic (needs layered pour) | High material cost | Brittle | Lower accuracy (±0.5mm) |
Symbols（I am not sure）:
▸ Accuracy ranking: CNC wax > Polyurethane ≈ Silicone > Pinch silicone > Gypsum
▸ Cost ranking: Gypsum < Pinch silicone < CNC wax < Polyurethane < Silicone
individual assignment¶
1.Design a mold around the process you’ll be using¶
I have decided to use CNC machines to carve brick wax and 3D print the master mold, then use silicone to make the mold, and finally use gypsum to make the product prototype.
（This photo needs to be replaced with a physical photo）
This is my first time making a master mold, and I plan to make the simplest single-sided mold, so it also has some limitations when using CNC carved wax molds.
1. Firstly, the size should be within 80MM * 80mm * 35MM;
2. The bottom of the mold cannot be larger than the top;
3. The edges of the mold should be tilted at an angle to prevent damage to the underlying structure during milling.
4. The distance between the final model and the wall at the edge should be at least greater than the diameter of the milling head(3mm);
5. It is better to use CNC to create curved shapes rather than flat structures. Flat structures can be made using laser cutting machines.
For me, it may be because it’s my first time making a master mold, and I find it difficult to understand the techniques of making molds for silicone molds. After practice, I finally understand this mystery: the method of reverse engineering, starting from the plaster prototype, designing a silicone model, and then designing a model for the silicone model.
① I used Fusion 360 to create the design, and I referred to this tutorial for this 3D model(Click on the image to play the video).
② Add 0.5mm thickness to the bottom of the model through stretching or other methods.
③ Design the structure of silicone molds through surface modeling. Be sure to ensure that the bottom width is greater than the milling head diameter (3MM) and the top is about 2MM higher than the model.
④ Design the master mold for silicone molds. Design a square container with its top aligned, and use Boolean operations to easily obtain the master model.
2.Produce it with a smooth surface finish that does not show the production process toolpath¶
2.1 Equipment and tools¶
I use Roland CNC machines from Fablab Shanghai laboratory to make wax molds.
(Picture X)
At the same time, I use a flat head milling head with a diameter of 4MM for rough machining and a spherical milling head with a diameter of 3MM for precision machining.
2.2 FILES¶
① Set Model, Default size, origin at top center¶
② Set Modeling Form¶
Set the margin to 0, adjust the depth red line to bring it closer to the bottom, and keep the rest of the settings as default.
③ Set new process(roughing)¶
Set new process-Roughing-Top-tool(3mm squre)-Partial(adjustment range)-contour lines-Change the material acrylic to modeling wax and import parameters-Process Name(Roughing 1)
④ Set new process(finshing)¶
Set-new process-Finshing-tool(R0.5 ball)-Partial-Spiral-Modeling WaxR0.5 Ball(default)-Finshing（Later）
⑤ Preview¶
2.3 machine operation¶
① Fixed brick wax¶
This is a fixture designed and made by my teacher, which can firmly fix the wax brick in the work area.
② Replace the milling head¶
Use a wrench to install a flat head milling with a diameter of 4MM, taking care to protect the tool from falling and causing damage to the milling head.
③ focus¶
This machine has a focusing sensor that can achieve automatic focusing. Two types of milling machines need to be replaced with CNC machines, so focus twice. Therefore, it is recommended to place the focus sensor in the non cutting area to ensure accurate focal length.
(Picture X)
④ Carving processing¶
Import the file and start carving. Special attention: Do not open the machine cover during operation.
- If you make this foolish mistake and have to carve it again, it will not only waste a lot of time.
- It may also lead to significant errors and require starting over with new materials, resulting in material waste.
- Click VIEW to pause the machine normally. You can clean up the waste, view the carving effect, and take photos.
(Picture X)
3.Use it to cast parts¶
I use this beautiful wax brick mother mold to cast silicone molds, and the material is AB food grade silicone.
- Clean the master mold to ensure there is no dust or oil stains.
- Mix AB silicone gel, weigh it in a ratio of A: B=1:1, and stir thoroughly
until the color is uniform (about 2-3 minutes). - Slowly pour in silicone gel and let it sit for 1 minute to naturally defoam
(or use a vacuum machine to remove bubbles). - Room temperature curing (usually 4-8 hours), carefully peel off the master
mold after complete curing, and complete the mold.
After several days of curing, I obtained a very satisfactory silicone mold.
(Picture X)
With the help of this silicone mold, I use gypsum for rapid casting.
- Apply release agent (e.g., soapy water or specialized agent to prevent sticking)
- Mix plaster slurry: Adjust water-to-plaster ratio (similar to yogurt consistency), stir quickly until smooth
- Pour into mold: Slowly fill the silicone mold, gently vibrate or tap to remove bubbles
- Cure: Let sit for 30 mins-1 hour (depends on plaster type)
- Demold: Carefully peel off the silicone mold after hardening, trim edges
(Picture X)
I also tried using a 3D printed master mold, which is faster, but obviously its accuracy is not as good as CNC machine engraving.
(Picture X)
I also tried to design a complex 2-piece master mold, but there was a low-level mistake. My mentor (Saverio) suggested modifying the details of the master mold by simplifying the two symmetrical models into one, which can save wax brick materials and production time. I think this is a genius idea that has been very helpful to me!
(Picture X)
Summary:¶
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I used to buy silicone molds, and through this class, I learned how to make master molds for silicone molds, which was very interesting! I can’t wait to try more mold manufacturing.
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Making molds is fun, but there are also limitations and challenges. Only by learning more about 3D modeling can one have the opportunity to create any product shape they like. Therefore, I need to continue to work hard to learn 3D modeling (surface modeling).