Dimensional Specification and Machining Requirements for Chinese‑Standard Plastic Test Specimen Molds
Plastic test specimen molds are critical tooling for laboratories and material manufacturers to obtain repeatable mechanical‑property data. All cavity dimensions and machining quality must comply with Chinese national standards including GB/T 1040, GB/T 1843, GB/T 17037 series, which govern injection‑molded tensile, impact and flexural specimens. Minor dimensional deviation, poor surface finish or assembly error may cause obvious dispersion of test figures and make experimental results invalid. Unlike mass‑production molds, test specimen molds prioritize high dimensional accuracy, low internal stress and zero invisible defects over high‑speed cycle time. This article introduces core dimensional rules, steel selection, machining tolerance and assembly specifications for Chinese‑standard test‑bar molds.
1. Core Cavity Dimensional Specifications for National‑Standard Specimens
Cavity design must strictly follow corresponding GB/T standards without arbitrary modification on key geometry. For GB/T 1040 Type‑1A dumbbell tensile specimens, the parallel gauge section, transition fillet radius and clamping‑zone dimension shall conform to standard drawings. The simply‑supported‑beam Charpy impact specimen complies with GB/T 1043.1, while Izod cantilever‑beam impact specimen follows GB/T 1843. The V‑notch on impact specimens is one of the most critical features: notch depth, 45‑degree angle and root fillet radius of 0.25 mm shall be machined as an integrated cavity feature instead of manual post‑grinding, because hand‑finished notches introduce irregular stress concentration and distort impact strength data.
For multi‑cavity test molds, each cavity shall keep identical dimensional parameters. Position deviation between different cavities shall be controlled below 0.015 mm. Cavity machining tolerance for key functional dimensions shall be kept within ±0.02 mm. Excessive tolerance will change actual cross‑section area of finished bars and bring more than 5 % error for tensile‑strength measurement. Wall‑thickness uniformity across gauge sections must be guaranteed; local thick spots are forbidden to avoid invisible vacuum bubbles and sink marks inside specimens. Interchangeable‑insert structure is recommended when multiple specimen thicknesses are required, so users can switch sample thickness without re‑machining the whole mold base.
2. Mold Steel Selection and Surface Machining Requirements
Cavity steel grade directly influences long‑term dimensional stability and surface quality. Pre‑hardened corrosion‑resistant steel such as S136H is widely recommended for standard test molds. For glass‑fiber‑reinforced engineering plastics, wear‑resistant quenched steel shall be adopted to prevent gradual cavity erosion caused by glass‑fiber flow during continuous production. Ordinary P20 steel is not suitable for long‑run specimen mold service, as surface scratches will transfer onto test‑bar surfaces and interfere with impact test results.
Cavity surface roughness shall reach Ra 0.8 μm or better. Polishing direction shall be perpendicular to melt‑flow direction to minimize flow‑induced surface defects. Polishing shall not generate local concave or convex deformation on gauge‑length zones. All sharp inner corners shall be processed into smooth transition fillets, eliminating hidden stress‑concentration points on molded specimens. Burrs and tool marks left by CNC or EDM must be completely removed during polishing. Any residual machining trace on cavity surface will be replicated on plastic samples and affect final test repeatability.
3. Gating, Venting and Cooling Related Machining Rules
Gate location shall avoid gauge‑length test zones. Gates are normally arranged at specimen clamping ends. Fan gate or side gate is preferred; tiny pin gates shall be avoided to reduce high‑shear‑induced residual stress inside test bars. Runner cross‑section shall be properly enlarged to lower melt shear rate. All runners and gates require smooth surface finish without tool marks or sharp corners.
Venting slots are critical for eliminating micro‑bubbles and micro‑burns which cannot be observed by naked eyes. Each independent cavity shall be equipped with separate vent channels instead of shared exhaust paths. Vent depth is set according to polymer types: 0.015‑0.020 mm for crystalline polyamides and PBT, and 0.020‑0.025 mm for non‑crystalline plastics. Vent width should be maximized within flash‑free limits to improve gas discharge efficiency.
Cooling channels shall be evenly distributed close to each cavity surface, keeping cavity‑to‑cooling‑hole distance between 8‑12 mm. Temperature difference among multiple cavities shall be limited within ±5 ℃. Uneven mold temperature will cause inconsistent crystallinity for semi‑crystalline plastics and raise data fluctuation for modulus, heat‑distortion and impact tests. Reserved sensor mounting holes help operators monitor real mold temperature on cavity area during sample preparation.
4. Assembly, Inspection and Acceptance Standards
High‑precision guiding components are required for test‑specimen molds. Guide‑pillar and guide‑sleeve clearance shall be strictly controlled to prevent cavity misalignment and uneven flash around specimens. Ejector pins are only permitted on specimen clamping sections; ejection structures shall never cover gauge‑length test zones to avoid deformation and extra internal stress during demolding.
After CNC machining, EDM and polishing, full‑dimension inspection shall be carried out for every cavity feature, including notch geometry, fillet radius, wall thickness and parallelism. Dimensional inspection report should be kept as mold‑delivery document. Before formal material testing, trial injection shall be performed, and molded test bars shall be checked for sink marks, bubbles, weld lines and flash. If any invisible inner defect is suspected, sampling verification can be conducted by sectioning representative specimens.
Conclusion
Chinese‑standard test‑specimen molds demand strict control over cavity dimension, surface quality and assembly precision. Complying with GB/T national‑standard dimension requirements, applying proper mold steel, limiting machining tolerance and optimizing gating‑venting‑cooling structures can effectively reduce specimen internal defects and data deviation. Even small machining errors will accumulate into large experimental uncertainty. Therefore, every dimensional and processing detail shall be fully considered in mold manufacturing, to guarantee reliable, comparable and traceable plastic‑material test results.
