Core Differences Between Hard‑Plastic and Soft‑Elastomer Mold Design

Hard plastics such as ABS, PC, PA and POM feature high rigidity and limited deformation during ejection. Soft elastomers including TPE, TPU and silicone are highly flexible, prone to sticking to mold surfaces and easy to stretch under force. Directly applying hard‑plastic mold structures for soft‑material production frequently causes sticking, part tearing, permanent deformation and excessive flash. Distinct design rules should be followed for cavity dimensioning, gating, venting, parting‑line sealing and ejection systems to achieve stable mass production.

1. Cavity Dimensioning and Shrinkage Compensation

Hard plastics deliver relatively stable molding shrinkage values. Typical shrinkage rates are 0.4‑0.7% for ABS, 0.5‑0.7% for PC and 1.5‑2.0% for POM. Cavity dimensions can be compensated according to standard material data, with small dimensional fluctuation and manageable tolerance control. Usable cavity tolerance can reach ±0.02~0.03 mm for hard‑plastic molds.

Soft‑elastomer shrinkage varies significantly with material hardness, injection pressure, packing pressure and wall thickness. TPE and TPU shrinkage ranges from 0.8‑2.5%, while silicone can reach 1.0‑3.0%. Lower hardness brings larger shrinkage fluctuation. Standard shrinkage figures cannot be copied directly; mold modification allowance shall be reserved at the design phase. Wall‑thickness uniformity must be strictly controlled to avoid severe dimensional drift caused by uneven shrinkage. Considering elastic rebound of soft parts, overall drawing tolerances should be moderately loosened.

2. Gating, Runner and Venting System Layout

Hard‑plastic molds support diverse gate types including side gate, submarine gate and pinpoint gate. Submarine gates realize automatic gate break‑off with proper shear effect. For soft elastomers, submarine gates are strongly discouraged. Their high toughness leads to stringing and uneven tearing at gate locations. Side gates or fan gates with enlarged cross‑sections are preferred to reduce shear stress and surface tearing.

For venting, hard‑plastic vent depth is normally 0.02‑0.03 mm. Soft materials tend to flash into tiny gaps, so vent depth shall be reduced to 0.008‑0.015 mm. Wider but shallower vents effectively evacuate trapped gas while suppressing flash generation. Additional vents are required at melt‑flow ends and insert clearances to prevent burn marks.

3. Parting‑line Sealing, Inserts and Sliding Mechanisms

Standard sealing width and 0.015‑0.025 mm fitting clearance for inserts and slides work well for hard‑plastic molds. Soft elastomers easily flow into micro gaps. Wider sealing bands are required on parting surfaces. Clearances for inserts and slides must be tightened to 0.005‑0.012 mm to avoid stubborn thin flash.

Hard plastics can tolerate limited forced demolding for undercut features. Soft materials may suffer elongation, scratch or tearing under forced ejection. Undercuts of considerable size should adopt slides or angle‑lifter structures instead of forced stripping. Minimize small scattered inserts to reduce flash‑generating gaps.

4. Ejection System Design Principles

Rigid hard‑plastic components can use small‑diameter ejector pins concentrated on ribs or thick sections. Soft elastomeric parts are vulnerable to piercing and indentation under localized force. Large‑area ejection components such as blade ejectors, ejector blocks or stripper plates shall be adopted to distribute ejection pressure. Increase ejector quantity to avoid local over‑stress.

Soft materials have high adhesion to steel surfaces and tend to stick to the cavity half. Mold structures must ensure finished parts stay on the core half during mold opening. Core‑pulling features or secondary cavity‑side stripping mechanisms can be applied. Draft angles should be increased by 1‑2 degrees compared with hard‑plastic standards to reduce frictional scratching during demolding.

5. Mold Steel Grade, Surface Treatment and Temperature Control

For hard‑plastic molds, P20, 718H or S136 are widely selected with standard polishing or nitriding treatment. Soft‑elastomer molds prioritize S136 corrosion‑resistant mirror steel. High‑grade polishing eliminates tool marks that increase sticking risk. Conventional nitriding is not recommended because surface micro‑pores trap elastomer residues. PTFE coating can be applied for severe sticking cases with careful anti‑scratch protection.

Cooling strategy also differs. Hard‑plastic molds place cooling channels close to cavities for fast cycle time. Soft materials require slower uniform cooling; cooling channels shall keep larger distances from cavity walls and moderate mold temperature settings to relieve internal residual stress and post‑mold deformation.

Summary 

Hard‑plastic mold design focuses on filling efficiency, dimensional accuracy and complex geometry realization depending on material rigidity. Soft‑elastomer mold design centers on anti‑sticking, flash suppression and scratch‑free demolding, featuring unstable shrinkage, tight fitting clearances and large‑area ejection requirements. Copying hard‑plastic mold structures directly for soft materials will result in numerous molding defects. Evaluating undercut release modes according to shore hardness and reserving mold‑revision allowance greatly improves trial‑out efficiency and production stability.