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13. Molding and Casting

Assignment

Group assignment: - Review the safety data sheets for each of your molding and casting materials, then make and compare test casts with each of them compare mold making processes.

safety data sheets for Molding and Casting Material

i. Liquid Polyurethane Rubber and Liquid Polyurethane Rubber We referee this [product description](https://www.amazon.in/FLEXON-Liquid-Polyurethane-Rubber-PUR-40/dp/B0BFJ543SS) to study the details of the product.

Liquid Polyurethane Rubber is the two component resins. which cures at room temperature. Its cold casting product which will be useful for both molding and casting applications.

TECHNICAL GUIDELINES:

Stir/Shake the part A & B well before use. Mixing Ratio will be depended on the hardness require. Our Lab has PUR 40 which has mixing ratio of 100:40 by weight.

Weigh the desire amount of Part A into a clean mixing container and also Part B. Mix the Part A & B together by stirring with a stick until a uniform color is obtained. Pour the deaired material slowly in a steady stream from one end of the mould so that the material flows evenly over the pattern. A mold release agent may be applied on the pattern first to improve release. Allow the rubber to cure for 24 hours at 25 Deg C before removing the cured rubber mold from the pattern.

ii. Liquid Silicone Rubber (LSR-225) and Catalyst LSR-2series

We referee this product description to study the details of the product.

Product Overview

  • Brand: SILOCZEST
  • Manufacturer: CHEMZEST ENTERPRISES
  • Origin: India
  • Type: Two-part liquid silicone rubber (base + curing agent)
  • Color: White
  • Hardness (Shore A): 25°A (medium-soft)
  • Density: 1.09 g/cm³
  • Viscosity (@25°C): 20,000 ± 2,000 cps (low viscosity, easy pouring)

Curing & Processing

  • Mixing Ratio (Curing Agent): 3–5% by weight
  • 3%: Medium-fast setting (~3–5 hours cure time)
  • 5%: Instant setting (~1 hour cure time)
  • Pot Life (@25°C): 5–15 minutes
  • Cures at Room Temperature: 1–12 hours (depending on catalyst %)
  • Low shrinkage, easy demolding, no vacuum degassing required.

Mechanical Properties

  • Tensile Strength: ≥43 kgf/cm²
  • Tear Strength: ≥26 kgf/cm
  • Elongation at Break: ≥500%

Applications

  • Mold Making: Fiber casting, statues/idols, shoe molds, candles, soap, architectural elements.
  • Casting Materials: Plaster, resin, wax, gypsum (POP), cement.
  • Industrial/Art Uses: Prototypes, sculptures, furniture replicas.

Key Features

✔ Fast curing (adjustable with catalyst %)
✔ Excellent detail reproduction
✔ High flexibility & tear resistance
✔ Cost-effective for beginners/professionals
✔ Can be thickened with THIX additive for brush-on applications.

Packaging

  • Available in 1 kg packs.
  • Storage: Keep at room temperature.

iii. Hydrostone and Water.

Product Description:

USG Hydro-Stone® Brand Gypsum Cement is a high-performance gypsum-based material designed for casting architectural elements, art pieces, novelties, and statuary. It delivers exceptional hardness, high compressive strength (up to 10,000 psi when dry), and superior water resistance while capturing fine details. Ideal for professional and artistic applications, it ensures durability and precision.

User Instructions:

  1. Mixing:
  2. Use 32 lbs. water per 100 lbs. product (or equivalent ratio).
  3. Sift Hydro-Stone slowly into water—avoid dumping to ensure full dispersion.
  4. Mix mechanically for optimal strength (longer mixing = stronger casts but shorter set time).

  5. Set Time: 19–25 minutes (adjust water/slurry temperature as needed—warmer = faster set).

  6. Pouring:

  7. Pour gently into the mold’s deepest area to minimize air bubbles.

  8. Agitate/vibrate molds to eliminate surface air pockets.

  9. Drying:

  10. Dry casts at ≤120°F (49°C) to prevent calcination.
  11. Ensure strong air circulation (15–30 fps) and vent humidity for efficiency.

  12. Cool molds to room temperature before handling.

  13. Storage:

  14. Keep in a 65–75°F (18–24°C), 45–55% RH environment.
  15. Use within 6 months of manufacture; rotate stock.

Safety Precautions:

⚠ WARNING:
- Wear gloves, eye protection, and a respirator—dust causes skin/eye irritation and respiratory issues.
- Work in well-ventilated areas; avoid inhaling dust.
- Never encase body parts—setting generates heat and hardens rapidly.
- If contact occurs:
- Skin: Wash immediately with water.
- Eyes: Rinse for 15+ minutes, remove contacts, seek medical help.
- Store out of reach of children.

For full details, refer to the SDS (Safety Data Sheet) at usg.com.

Note: Properties may vary under different conditions. For specific applications, consult a USG Sales Representative.

Molding Making Processes

Comparison Between Milling a Mold and 3D Printing for Molding and Casting

When creating molds for injection molding and silicone molding, two primary methods are used: CNC milling (subtractive manufacturing) and 3D printing (additive manufacturing). Each has distinct advantages, disadvantages, and ideal use cases.

1. CNC Milling (Subtractive Manufacturing)

Process:

  • A block of material (metal, aluminum, or high-grade plastic like wax) is cut away using CNC machines to form the mold cavity.
  • High-precision machining ensures tight tolerances.

Advantages:

✔ High Precision and Surface Finish – Ideal for tight-tolerance industrial molds.
✔ Durability – Metal molds (steel, aluminum) withstand high pressures and temperatures.
✔ Long Lifespan – Best for mass production.
✔ Material Options – Can machine hard metals, composites, and engineering plastics.

Disadvantages:

❌ High Cost – Expensive setup, especially for complex geometries.
❌ Long Lead Time – Programming, fixturing, and machining take time.
❌ Design Limitations – Undercuts and intricate internal features may require EDM or additional processes.
❌ Material Waste – Subtractive process generates scrap.

Best Use Cases:

✅ High-volume production (injection molds, die-casting).
✅ When extreme durability is needed (metal molds for repeated use).
✅ Critical surface finish requirements (optical lenses, automotive parts).

2. 3D Printing (Additive Manufacturing)

Process:

  • Mold is printed layer-by-layer using materials like resin, plastic, or metal (via SLS, SLA, or DMLS).
  • Often used for prototyping molds or short-run production.

Advantages:

✔ Fast and Low-Cost Prototyping – Quick iteration of mold designs.
✔ Complex Geometries – Easily handles undercuts, conformal cooling channels, and intricate details.
✔ Minimal Waste – Additive process uses only necessary material.
✔ No Tooling Required – Direct digital manufacturing reduces setup time.

Disadvantages:

❌ Lower Durability – Plastic/resin molds degrade faster than metal.
❌ Limited Material Choices – Most 3D-printed molds can’t handle high temps/pressures like metal.
❌ Post-Processing Needed – Often requires sealing/polishing for smooth surfaces.
❌ Lower Precision – Some 3D printing methods have layer lines or slight warping.

Best Use Cases:

✅ Rapid prototyping (testing mold designs before CNC machining).
✅ Low-volume production (silicone casting, urethane casting).
✅ Conformal cooling molds (3D-printed metal molds with optimized cooling channels).
✅ Custom one-off molds (jewelry, dental, art).

Comparison Summary

Factor CNC Milled Molds 3D Printed Molds
Cost High (tooling, setup) Low (no tooling)
Lead Time Longer (hours-days) Faster (hours)
Durability Excellent (metal) Limited (plastic/resin)
Complexity Limited (needs multi-axis machining) High (handles intricate designs)
Surface Finish Very smooth (machined) May need post-processing
Best For Mass production, high-temp/pressure Prototyping, short runs, complex shapes

Here are some Molds which are already there in our lab form previous work. We decide to use this for our group assignment.

  • Negative Silicon Mold

  • Wave Positive Mold.

  • Negative Silicon Mold.

i. Liquid Polyurethane Rubber and Liquid Polyurethane Rubber

  • Liquid Polyurethane Rubber part A and part B.

  • Poring Part A Liquid Polyurethane Rubber in a plastic cup and weighing.

  • We poured 50.2g of Polyurethane part A.

  • Since we use 50.2g of Polyurethane Part A, we have to calculate how much Polyurethane Part B we have to use. The right proportion mentioned on the data sheet is 100:40. 

    100:40
Therefore for 50.2g of part A, 40% of 50.2g = 20.08g

  • We then mix the two part properly.

  • Poring the Part A and Part B mixture into the Mold.

  • Here are final results

ii. Liquid Silicone Rubber and Catalyst LSR-2series

  • Liquid Silicone Rubber and Catalyst LSR-2 series.

  • Pouring the Silicone in the plastic cup and measuring its weight.

  • We decided to use 178.4g of Silicon.

  • Now measuring the right proportion of Catalyst for Liquid Silicone Rubber. The ratio specified on their data sheet is 3-5%. 

    178.4g * 5% = 8.73g

  • Then we mixed them well and we did that very fast and carful because it gets easily cure once they are mixed.

  • We then put the mixture into the mold.

  • Final Results

iii. Hydro-stone and Water. - Hydro-stone

  • We mixed Hydro-stone with water. The right proportion is

  • We then poured the mixture into the mold.

  • We left it to dry.

  • Here is the Result

Comparing test cast

i. Liquid Polyurethane Rubber and Liquid Polyurethane Rubber - The result is quite hard but it is not too hard. - Very hard to take out from 3D printed Mold. - Quite easy to take out from Silicone Mold.

ii. Liquid Silicone Rubber and Catalyst LSR-2series - Very Soft result. - It is very easy to take out from both wax mold and 3D printed Mold.

iii. Hydrostone and Water. - Result is quite hard. - We did not try it on 3D printed mold as it will be very hard to take out from hard mold. - Very easy to take out from Silicone Mold.