With the rapid development of the polymer materials industry, plastics are widely used in industrial manufacturing. Material mechanical performance testing is essential for quality control. National standard tensile and impact specimens are standard test samples used to measure tensile strength, elongation at break, impact toughness, and other physical and chemical indicators. Since test data is highly sensitive to specimen appearance, dimensions, and parallelism, the molds used to produce them must meet strict requirements with no flash, warpage, sink marks, or weld lines. Thus, national standard specimen molds are high-precision precision injection molds. This article analyzes the production process of these molds from design, material selection, precision machining, assembly, and trial molding to ensure the molded specimens meet national standard test requirements.
Ⅰ. Specimen Structural Characteristics and Mold Production RequirementsNational standard tensile and impact specimens are generally long, flat, thin-walled structures with simple geometry but strict dimensional tolerances. The surface must be mirror-smooth with straight, burr-free edges. The central test section has uniform thickness, while the end clamping sections are wider, leading to uneven heat buildup at the ends and faster cooling in the middle during injection molding. Since specimens are directly used in mechanical tests, even minor deformation, surface texture, or internal bubbles can skew test data. Therefore, mold production must meet high precision, low deformation, and high consistency requirements: cavity dimensional tolerance within ±0.02 mm, tight parting line fit to eliminate flash, and uniform weight, size, and hardness for every molded part.
Ⅱ. Mold Structural Design
The specimen mold uses a 4-cavity symmetrical layout, including both tensile and impact specimen cavities, arranged evenly for balanced flow. A flat, straight parting line is chosen for simplicity, tight fit, and ease of processing and maintenance. The gating system uses a balanced runner layout with a 5 mm diameter runner to ensure simultaneous filling and equal pressure in all cavities. Edge gates are located on the non-test end faces, allowing gentle melt entry without jetting and minimal gate vestige requiring no secondary grinding. The cooling system uses straight, conformal water lines with 8 mm diameter, 12 mm from the cavity, evenly spaced along the specimen length to ensure uniform cooling and prevent bending or twisting. The ejection system uses evenly spaced fine ejector pins to distribute force evenly, ensuring ejection without scratching or whitening.
Ⅲ. Mold Material Selection and Machining ProcessTo meet high-precision mirror requirements, the cavity and core are made of S136 stainless steel, heat-treated to 48-52 HRC for corrosion resistance, polishability, and long-term stability under mass production. The mold base is a standard reinforced type for rigidity and resistance to deflection. All processing is done with precision equipment: cavity profiles are cut with wire EDM, surface textures with mirror spark erosion, and key dimensions are machined to account for plastic shrinkage. Guide pins, bushes, and ejector pins are precision standard parts with tight fit clearances to eliminate wobble during mold opening and closing. After machining, the cavity is hand-polished to a mirror finish to ensure transparent, scratch-free specimens.
Ⅳ. Mold Assembly and Fitting AdjustmentAssembly is a critical step in specimen mold production. All iron filings must be removed before assembly, and parts inspected for compliance. The parting line is fitted with red lead and repeatedly lapped to ensure full contact with no gaps, eliminating flash at the source. Guide pins and bushes are lubricated with high-temperature grease for smooth mold operation. The ejection system undergoes multiple no-load tests to ensure accurate, synchronized ejection and reset without jamming or misalignment. Water lines are pressure-tested at 1.2 MPa for 30 minutes to detect leaks and prevent rust and unstable mold temperatures. After assembly, mold parallelism and perpendicularity are verified to ensure structural accuracy.
Ⅴ. Trial Molding Optimization and Mass Production
After assembly, the mold is tested with ABS material at 225°C barrel temperature and 50°C mold temperature. Initial trials observe melt flow, filling speed, and venting. Minor warpage or end sink marks are corrected by adjusting holding pressure and cooling time. Optimized specimens show no weld lines, bubbles, or surface defects, with all dimensions within national standard tolerances. Mass production maintains constant mold temperature and circulating cooling water for stable cycle times and consistent specimens with no need for trimming or grinding, suitable for direct testing. The mold shows minimal wear, retains polish, and has a service life exceeding 100,000 cycles.
| No. | Category | Parameter | Standard Value | Tolerance | Inspection Standard |
|---|
| 1 | Mold Structure | Cavity Layout & Base | 4-cavity symmetrical, standard thick base | None | Balanced flow, stable clamping, no deformation |
| 2 | Core & Cavity | Steel & Hardness | S136H, 48–52 HRC | ±2 HRC | Corrosion resistant, polishable, stable |
| 3 | Runner System | Runner & Gate Size | φ5mm runner, 1.5×0.8mm edge gate | ±0.05mm | Balanced filling, no weld marks |
| 4 | Cooling System | Cooling Line Layout | φ8mm, 10–15mm from cavity | ±2mm | Uniform cooling, no warping |
| 5 | Ejection System | Ejector Pin & Fit | φ3mm pin, H7/f7 fit | ±0.01mm | Smooth ejection, no scratch |
| 6 | Final Product | Dimensional Quality | ±0.02mm tolerance, flatness ≤0.2mm/100mm | No out-of-tolerance | Meets GB standard, mirror surface |
Conclusion
National standard tensile and impact specimen molds are high-precision test-grade injection molds requiring strict standards in structural design, material selection, precision machining, and assembly. By using a symmetrical multi-cavity layout, balanced gating, uniform cooling, and mirror polishing, specimen appearance and dimensional accuracy are ensured. Rigorous processing and debugging reduce warpage, deformation, and flash, producing specimens fully compliant with national standards. The mold is simple, durable, and stable for mass production of standard specimens in testing laboratories and compounding plants, serving as a reference for similar precision long thin-walled part molds.
