Plastic Performance Test Specimen Test Mold Design Key Points

Plastic performance test specimen molds are applied to produce standard test bars for tensile, impact, bending and heat‑distortion tests. The molding consistency of specimens directly determines the reliability of material test data. Defects including internal residual stress, weld lines, sink marks, bubbles and glass‑fiber floating on test specimens will cause deviation of test results. Mold design shall prioritize internal quality of test samples before molding efficiency. The key design points for plastic performance test specimen molds are elaborated as follows.

1. Cavity and Specimen Structural Design

The dimension of specimen cavities shall strictly comply with corresponding national testing standards. Thickness, gauge‑length and transition fillets of tensile specimens, Charpy impact specimens and bending specimens shall not be modified arbitrarily. Cavity machining precision shall reach the level of precision mold, and cavity surface roughness shall be controlled within Ra0.8μm to avoid interference of surface defects on impact and tensile test values.

Multi‑cavity layout is feasible for one set of test mold. Different types of standard specimens can be combined in layout. It is critical to guarantee identical gating condition for every cavity. Divergent data among batches of test samples will occur if molding conditions differ among cavities. Fillets at thickness transition zones of specimens shall be fully processed without sharp corners. Stress concentration will appear at sharp corners and lead to generally lower impact test figures.

Wall thickness of cavities shall be uniform. Local thick rubber area shall be avoided, for thick‑wall positions tend to generate sink marks and invisible internal vacuum bubbles. Those invisible bubbles will become fracture initiation points during mechanical tests. If specimens with multiple thickness specifications are required, interchangeable inserts are recommended instead of direct modification on cavity surface, for fast switching of specimens in different thickness.

2. Gating System Design

Gate selection exerts great influence on residual stress inside specimens. Tiny pin gates are not suitable for test specimens for high shear force will bring high internal stress. Gates shall be set at the clamping end of tensile specimens rather than gauge‑length sections to prevent gate traces on stressed test zones.

Side gates or fan gates are preferred. Gate thickness shall be no less than half of specimen wall thickness to reduce melt shear rate and cut down internal residual stress. Runner cross‑section shall be enlarged properly. Small‑size runners will raise shear heat and result in material degradation, which matters a lot for glass‑fiber reinforced modified materials, PC and PA66.

Weld lines shall be prevented from appearing on gauge‑length sections of specimens. Melt flow direction shall be well‑planned in layout to avoid melt convergence passing core test zones. If weld lines cannot be eliminated due to structural constraints, sufficient vents shall be opened at weld positions. Specimens with obvious weld lines shall be discarded for formal tests.

3. Venting System Design

Test specimen molds require better venting performance than common product molds. Gas trapped by melt front will cause micro‑burns and invisible micro‑bubbles, which will severely interfere with impact strength and elongation‑at‑break test results.

Venting slots shall be arranged at melt flow terminals and every corner of cavities. Venting depth shall be adjusted according to material features: 0.02‑0.03mm for ordinary non‑crystalline plastics, and 0.015‑0.02mm for crystalline materials including PA, PBT and PET. Each independent cavity shall be equipped with separate vents instead of shared vent channels. Good fitting of parting surface shall be guaranteed to avoid flash while realizing smooth gas exhaust. Venting depth shall not be too large for low‑viscosity high‑flow materials to prevent burrs.

4. Mold Temperature and Cooling System

Cooling channels shall be arranged close to cavities to realize even mold temperature. Temperature difference among cavities shall be controlled within ±5℃. Fluctuating mold temperature will cause inconsistent crystallinity of specimens, further bringing obvious deviation on impact, heat‑distortion and flexural modulus test data.

Distance between cooling channels and cavity surface shall be kept at 8‑12mm with proper channel spacing to eliminate local hot spots. For crystalline modified materials such as PA66+GF and PBT+GF whose crystallinity is highly sensitive to mold temperature, molds shall support stable high‑temperature mold operation at 80‑120℃. Partial rapid cooling shall be prohibited for test specimen molds. Uneven internal stress generated by sharp local cooling will lead to poor repeatability of test data even with perfect appearance.

5. Ejection, Guiding and Steel Grade Selection

For thin‑wall test specimens, ejection structure shall avoid specimen deformation. Ejector pins shall be arranged on clamping ends instead of gauge‑length test zones. Excessive ejection force will bend specimens and produce internal stress. More ejector pins shall be adopted for thin impact specimens to decentralize ejection force, and stripper‑plate ejection structure can be applied when necessary.

Reliable guiding and positioning shall be ensured with reasonable clearance between guide pillars and guide sleeves to realize accurate mold closing and prevent flash caused by cavity misalignment. Polishing‑friendly pre‑hardened steel such as S136H is preferred for cavity steel to acquire low‑roughness cavity surface. Wear‑resistant steel shall be selected for glass‑fiber‑reinforced materials to stop cavity erosion and surface quality deterioration in mass production.

6. Auxiliary Design Notes

Cold runner structure is recommended for test molds for convenient material and color switching without complicated cleaning. Specimen quality takes priority over sprue‑less molding. Semi‑hot runner can be adopted for high‑cost raw materials, while risks of material degradation caused by melt residence inside hot runners shall be noticed.

Mold base shall possess sufficient strength to ensure tight fitting of parting surface under clamping force, so as to avoid specimen thickness over‑tolerance caused by mold swelling. Reserved installation positions for temperature sensors help monitor actual cavity mold temperature in production.

Conclusion

The core objective for test specimen molds is to produce standard samples with low internal stress and high batch consistency rather than good‑looking appearance. Strict compliance with national standards for cavity dimension, gates away from stressed zones, sufficient venting, uniform mold temperature, reduced shear stress and ejection protection for gauge‑length sections will lower data dispersion and guarantee authenticity and validity of material test results.