Structural Design Requirements for Rubber Performance Test Molds

Rubber performance test molds produce standard specimens for tensile test, tear test, compression‑set test and hardness measurement. Specimen dimensional accuracy and surface condition directly determine experimental‑data reliability. Different from mass‑production rubber molds, test‑mold design focuses on specimen dimensional stability, rubber compound flow, venting, uniform heating and convenient demolding. It shall match rubber compression‑vulcanization technology and guarantee test‑result repeatability for laboratory small‑batch sample preparation.

1. Cavity and Parting‑Surface Structural Requirements

Cavity dimensions must comply with national‑standard specimen specifications. Both rubber vulcanization shrinkage and steel thermal expansion shall be calculated into cavity size. Higher machining precision is required compared with commodity rubber molds, with key‑feature tolerance controlled within ±0.02 mm. Cavity surface roughness Ra ≤0.8 μm to avoid scratches and pits which increase test‑data dispersion.

Parting surface locates at maximum part contour plane. Complete specimens should stay within one mold half as much as possible. Parting line must never cross specimen loading‑bearing zones. Tensile and tear‑sample stressed sections shall be fully contained inside one single cavity half. For multi‑cavity test molds, wall thickness between cavities remains uniform and cavities keep proper distance from mold edges to ensure identical heating condition for every specimen. Single‑cavity molds fit small‑batch comparison tests. Excessive cavity quantity causes inconsistent vulcanization degree among distant cavities.

Surrounding overflow grooves are arranged on parting surfaces with depth 0.2‑0.4 mm for excess compound material. Connected exhaust channels discharge trapped air. Air entrapment inside cavities will generate bubbles and invalidate mechanical test results.

2. Feeding, Runner and Venting System Design

Most rubber test molds adopt compression charging workflow. Pre‑weighed rubber blanks are manually loaded into open cavities. Gate vestiges must not appear on specimen effective test zones. If injection‑compression test molds are applied, gates shall be placed at non‑stressed specimen ends. Gates should be thin and short for easy post‑vulcanization peeling without leaving notches or dents on specimen surfaces.

Venting is critical for rubber‑specimen molds. Vent slots of 0.05‑0.1 mm depth are arranged at melt‑flow terminals and melt‑converging positions. Excessive vent depth induces heavy flash; insufficient depth fails to release trapped gas and volatile components. Additional vent inserts eliminate dead‑zone air traps. Each cavity of multi‑cavity molds shall have independent vent passages to prevent cross‑contamination between different specimens.

3. Mold Body, Positioning and Heating‑Matching Structure

Mold overall thickness shall match hot‑plate dimension of laboratory compression vulcanizer. Upper and lower mold plates select low‑thermal‑deformation steel such as tempered 45# steel or P20 pre‑hardened steel to resist warpage under repeated high‑temperature vulcanization. High‑precision locating pins ensure mold‑closing alignment. Reasonable fitting clearance prevents specimen thickness variation and uneven flash caused by mold misalignment. Locating pins shall be easy to disassemble and maintain without high‑temperature seizure.

Stopper height‑limit blocks are installed at four mold corners. Lapped flat surfaces precisely control cavity closing thickness and counteract press‑pressure fluctuation. Thickness consistency is essential for hardness and compression‑set testing. Mold‑plate contact surfaces with vulcanizer hot‑plates maintain high flatness for uniform heat transfer. Mold weight takes manual laboratory handling into consideration; oversized heavy molds should be avoided.

4. Demolding, Insert and Specimen‑Protection Structure

Rubber specimens are soft and vulnerable. Sharp ejector pins are prohibited for direct ejection. Manual part removal after mold opening is the primary solution. For sticky thin specimens, flat ejector plates can be applied. Ejection force must act only on waste‑material margins outside valid test zones. All inner cavity corners adopt small radius fillets to eliminate sharp angles. Sharp corners cause mold chipping and micro‑cracks on rubber samples, distorting tear‑test and tensile‑test outcomes.

For tear specimens with pre‑cut notches, replaceable notch‑forming inserts are adopted. Inserts require secure positioning and convenient disassembly. Notch cutting edges receive high‑frequency quenching to maintain sharpness. Worn edges can be renewed by insert replacement without remaking the whole mold. Minimize tiny scattered inserts inside cavities to prevent rubber squeezing into assembly gaps and hard‑to‑remove flash.

5. Machining Tolerance and Auxiliary Supporting Design

Key cavity dimensions should be verified by coordinate‑measuring machines after machining. Cavity inner surfaces must be free of tool marks and scratches, polished following demolding direction. Lifting holes and hand grips facilitate mold transportation from vulcanizer. Mold outer surfaces receive blackening anti‑rust treatment; cavity surfaces avoid hard coating preventing coating debris mixing into rubber samples.

Sufficient clearance for cleaning is reserved in mold structural layout to remove vulcanized rubber residues without dead corners. Each test mold shall be permanently marked with specimen specification for laboratory classification and to prevent cross‑usage.

Summary

 Test molds for rubber performance differ greatly from mass‑production rubber molds. The core design objective is guaranteeing specimen geometric precision, surface integrity and vulcanization uniformity to reduce experimental deviation. Cavity and parting‑line design define specimen base geometry. Feeding and venting systems eliminate bubble and flash defects. Mold positioning and stopper blocks secure thickness consistency. Demolding and insert features protect specimens from mechanical damage. Strict machining and auxiliary details maintain long‑term service life under repeated vulcanization. Well‑implemented design specifications reduce repeated sample‑making work and deliver authentic and reliable tensile, tear and compression‑set test results.