Introduction
Prototypes produced using vacuum injection molding (overmolding) must not only resemble the final product in appearance but, more importantly, simulate the mechanical properties of the final product for effective functional and structural verification. Understanding the differences in hardness, strength, and toughness among different overmolding materials is essential to ensuring that prototypes meet testing requirements. This article will explain the key mechanical properties of commonly used overmolding materials.
Detailed Mechanical Properties of Various Overmolding Materials
All overmolding PU resins have very low volume shrinkage, typically between 0.1% and 0.6%, ensuring high dimensional accuracy in replicated parts.
Soft Elastomers
- TPU/TPE-like Soft Rubber: This material is used to simulate the elastic properties of rubber. It offers a wide range of hardness, from very soft Shore A30 to harder A95. It has low tensile strength (approximately 5–25 MPa) but very high elongation at break, ranging from 150–500%, demonstrating excellent softness and flex resistance.
- True Silicone Rubber: Silicone rubber is softer than soft PU, with a hardness starting at Shore A10. Its tensile strength is the lowest of all materials (approximately 3–10 MPa), but its elongation at break is the highest, reaching 150–700%, making it ideal for applications requiring extreme softness and flexibility.
Hard Plastic
- ABS-like Hard Plastic: This is the most standard hard prototyping material. Its hardness is approximately Shore D70–D80, with good tensile strength (approximately 35–60 MPa) but low elongation at break (approximately 5–15%), resulting in a hard yet slightly tough property similar to real ABS plastic.
- PC-like Transparent Parts: This material offers superior strength and toughness compared to ABS-like materials while maintaining high transparency. Its hardness is approximately Shore D75–D80, its tensile strength is higher (approximately 40–65 MPa), and its elongation at break is also increased to 10–20%.
- PMMA-like (Acrylic) Transparent Parts: This material pursues extreme hardness and transparency. Its hardness is the highest among all overmolding materials (Shore D80–D85), and its tensile strength is also quite high (50–70 MPa). However, this comes at the cost of poor toughness, with an elongation at break of only 3–6%, making it a typical hard and brittle material.
- PP/PE-like Tough Plastics: This material is characterized by its toughness. Its hardness is lower (approximately Shore D55–D65) and its tensile strength is also relatively low (20–35 MPa), but its elongation at break can reach 20–100%, making it ideal for structural parts such as clips that require resistance to bending.
Key Concepts
- Shore Hardness: A measure of a material’s ability to resist indentation. Shore A (for soft rubber and elastomers) and Shore D (for hard plastics) are scaled differently and cannot be directly converted. Higher values indicate harder materials.
- Tensile Strength: This refers to the maximum stress (force) a material can withstand before breaking. Higher values indicate a stronger material and less prone to breaking.
- Elongation at Break: This refers to the percentage increase in length at break. This value directly reflects its toughness; higher values indicate a more flexible material, greater resistance to bending, and less prone to brittle fracture.
Conclusion
Vacuum injection molding materials offer a wide range of performance options, from extremely soft to extremely hard, and from strong to brittle. At the initial stages of a project, you can select the most appropriate molding material based on the product’s functional requirements—whether an impact-resistant housing (e.g., ABS/PC), a bend-resistant snap (e.g., PP/PE), or a soft button (e.g., TPU)—for the most cost-effective design verification.