Fab Academy 2025 (Rwanda) – Molding & Casting

1. Introduction

This document outlines the plan for the molding and casting assignment, covering both group and individual components. The group assignment involves reviewing Safety Data Sheets (SDS), producing test casts, and comparing mold-making processes. The individual assignment focuses on designing and producing a three-part mold and documenting the casting process.

2. Group Assignment

2.1 Review Safety Data Sheets (SDS)

Objective: Review SDS for molding materials (silicone rubber, urethane rubber) and casting materials (epoxy resin, plaster).

Material	Example Product	Hazards	Precautions	First Aid	Disposal
Silicone Rubber	Smooth-On OOMOO 30	Skin/eye irritation; inhalation risk	Gloves, glasses, ventilation	Wash skin; Rinse eyes 15 min	Cure fully, dispose normally
Urethane Rubber	Smooth-On Mold Max 30	Respiratory sensitization	N95 mask, gloves, ventilation	Fresh air; Wash skin	Cure fully, dispose normally
Epoxy Resin	Smooth-On Crystal Clear	Flammable when mixed; irritation	Gloves, glasses, ventilation	Wash skin; Fresh air	Cure fully
Plaster	USG Hydrocal White	Dust irritation	Dust mask, gloves	Wash skin; Rinse eyes	Dispose as solid waste
Construction Cement (OPC)	General Purpose Cement	Dust irritation; alkaline when wet	Use gloves, dust mask, safety glasses	Skin: rinse with water; Eyes: rinse 15 min; Inhalation: fresh air	Cure completely; dispose as solid waste

2.2.1 Volume & Mass Calculation of the Cast

The mold cavity has the following dimensions:

Diameter: 45 mm
Depth: 30 mm
Radius: 22.5 mm (2.25 cm)
Height: 30 mm (3 cm)

Volume Calculation:

V = π r² h
V = 3.14 × (2.25 cm)² × 3 cm
V ≈ 47.8 cm³

Material Used: Construction cement (Portland-based), not Plaster of Paris.

Density of Portland Cement: 3.15 g/cm³

Theoretical Mass:

Mass = Density × Volume
Mass = 3.15 g/cm³ × 47.8 cm³
Mass ≈ 150 g

We used a mass of 170g in order to fill metal and silicon molds

Measured Mass: 170 g

This confirms that the cast was made using Portland cement, which has a much higher density than Plaster of Paris. The measured value matches the expected range when using cement instead of POP.

Casting Cement:
- Cement, Water, scale, silicon mold and metal mold ready
- Cement to water ratio (2:1) – cure 24 hours. I used Cement 170g and water mass @ 90g overfilled by 5g. I was supposed to be 85g
- I measured the mass of water
- Pouring into the metal and silicom molds
- Cement poured into both molds and left to cure
- Molds removed from molds
Casting Resin:
- I used Green Poxy 33 resin and SD4775 epoxy hardner. Epoxy resin (100g:27g ratio) – cure 24 hours. To ensure curing and mechanical perfomance of the epoxy system, the ratios of resin and hardner were verified using a digital scale. the process involved three steps: Taring the contatiner, weighing the resin and weighing the hardner
- I tared the scale, wighed the mass of resin and hardner
- Metal and silicon molds
- Pouring misture of resin and hardener into both molds and allowing to cure
- The cure product from both molds
Comparison:
- Epoxy: Clarity, strength, long cure. One thing to note was that there is some red color in the metal cast from resin and hardner. This was probably becasue of the exothemic heat trapped in the metal mold
- Plaster: Cheap, easy, brittle.

Process	Tool	Material	Pros	Cons	Surface Finish
3D Printing	FDM Printer	PLA	Fast, low cost	Layer lines, needs finishing	Smooth after sanding
CNC Milling	ShopBot CNC	Wax / MDF	Precise, smooth	Slower, material waste	Smooth with minimal work

Process

Tool

Material

Pros

Cons

Surface Finish

3D Printing

FDM Printer

PLA

Fast, low cost

Layer lines, needs finishing

Smooth after sanding

CNC Milling

ShopBot CNC

Wax / MDF

Precise, smooth

Slower, material waste

Smooth with minimal work

Fab Academy 2025

@ Fab Lab Rwanda, Kigali

Molding and Casting Assignment Plan

1. Introduction

2. Group Assignment

2.1 Review Safety Data Sheets (SDS)

2.2 Produce Test Casts

2.2.1 Volume & Mass Calculation of the Cast

2.3 Compare Mold-Making Processes