How to Select Plastic Shore Hardness? Mold Design & Product Application

Shore hardness reflects the rigidity, toughness, and feel of plastic materials. It directly influences mold structure design, injection parameters, demolding performance, and final product functionality. Improper hardness selection often leads to cracking, deformation, sticking, or failure to meet application requirements. Matching hardness with mold design and usage scenarios is essential for high-quality mass production.

1. Classification of Shore Hardness

Shore A measures soft and elastic materials such as TPE, TPR, silicone, and soft PVC. The range is 0–100A; lower values mean softer texture. Materials below 60A are highly flexible and suitable for sealing and anti-slip parts. Those between 60A–95A balance softness and strength for handles, buttons, and cable jackets.

Shore D applies to rigid engineering plastics including PP, ABS, PC, PA, and POM. Most structural parts fall within 40D–90D. Higher values indicate greater rigidity, wear resistance, and load-bearing capacity. Hardness above 95A is generally expressed in Shore D.

Material hardness affects melt flow, shrinkage, impact resistance, and demolding. Mold designers must confirm hardness in advance to optimize runners, gates, ejection, cooling, and venting systems.

2. Mold Design Based on Hardness Levels

For soft materials below 60A, molds require large gates, wide runners, and high-polished surfaces to prevent sticking and tearing. Ejection should use large-area ejector pins or stripper plates to avoid deformation. Venting channels must be enlarged to prevent burning and short shots.

Materials from 60A to 95A adapt to standard mold designs. Side gates or pin-point gates are applicable. Ejection systems can use conventional layouts. Uniform cooling ensures consistent shrinkage and dimensional stability. These materials cause low mold wear and are compatible with standard mold steels.

Hard materials above 40D demand high mold strength and precision. Gates should avoid direct impact on cavity walls to reduce stress cracking. Runner systems must ensure smooth flow with low pressure loss. Ejection must be balanced to prevent white marks or fractures. Cavities and cores require hardened steel and proper venting to eliminate burning and weld lines.

3. Hardness Selection by Application

Sealing, shock-absorbing, and soft-touch products such as gaskets, foot pads, earplugs, and tool grips use 20A–60A materials for flexibility and comfort.

Buttons, grips, cable skins, and decorative parts typically use 60A–95A materials for appropriate toughness, stability, and appearance.

Structural components, housings, gears, brackets, and load-bearing parts require materials over 50D to maintain rigidity, dimensional accuracy, and durability.

4. Common Mistakes and Optimization

Many designers only focus on hand feel rather than mold feasibility. Over-soft materials with small gates easily tear during demolding. Over-hard materials in deep-cavity structures lead to filling problems and cracking. Ignoring shrinkage differences and uneven cooling also causes quality defects.

The correct approach is to determine hardness based on functional requirements, then design mold structure accordingly. Prototyping and sample testing help verify matching between material, mold, and process. Establishing material hardness standards improves development efficiency and reduces mold modifications.