China Injection Molds Forming Specifications for Sanitary Fittings

Sanitary fittings such as shower shells, valve bases, toilet cover plates, water inlet joints and decorative covers are mostly produced from PP, ABS, PVC and modified nylon materials. These products have strict requirements including uneven wall thickness, high dimensional precision of sealing surfaces, resistance to stress cracking under long-term water immersion and mirror-grade flawless appearance without weld lines. Ordinary general injection molding standards cannot meet the special demands of sanitary products on waterproof performance, assembly matching and aging resistance. A dedicated forming specification covering pre-mold inspection, raw material control, molding parameters, defect treatment and post-processing is formulated to guarantee stable dimensional accuracy, reliable sealing performance and qualified appearance of mass-produced sanitary plastic parts.

1. Pre-Production Inspection Standards for Molds

All sealing ribs and assembly fitting surfaces of sanitary molds must be polished to Ra0.02 mirror finish without tool marks, scratches or pits. Cavities, cooling channels, ejector pins and inserts shall be cleaned thoroughly to remove rust and burrs, especially sealing grooves and thread positions, as residual oil stains will cause bubbles and flow marks on finished products. Complete water pressure test at 0.6MPa for mold cooling circuits without leakage. Independent cooling channels shall be arranged for thick-walled shower and valve components, with the distance between cooling pipes and cavity surfaces controlled at 8–12mm to balance cooling speed of thin and thick areas and prevent sink marks that damage sealing performance. Vent slots with depth 0.015–0.02mm and length 8–15mm shall be opened at weld lines, flanges, thread ends and rib terminals. Segmented vents are additionally required for transparent and glossy sanitary parts to avoid scorch and air marks. Ejector pins and sleeves shall slide smoothly without jamming, with unilateral ejection clearance no more than 0.01mm. High-temperature grease shall be applied to guide pins and guide sleeves to prevent mold misalignment that causes flash and uneven wall thickness.

2. Raw Material Drying and Mixing Control Standards

Different raw materials for sanitary fittings are equipped with fixed drying parameters: ABS glossy shells are dried at 80–85℃ for 3–4 hours with moisture content lower than 0.02%; modified PP joints are dried at 70℃ for 2 hours; nylon structural accessories are dried at 100–110℃ for 4 hours; soft PVC sealing parts shall be dried at 60℃ without high-temperature decomposition. UV-resistant color masterbatch is adopted for weatherproof and waterproof sanitary products with mixing time no less than 8 minutes. Glass fiber reinforced valve materials shall avoid long-time high-speed mixing to prevent exposed glass fibers from scratching sealing surfaces. The mixing ratio of recycled materials shall not exceed 20%, and all mixed materials must be fully dried to resist hydrolysis cracking. Dried raw materials shall be sealed and kept warm to avoid secondary moisture absorption from air. PVC materials shall use independent barrels to prevent corrosion and precipitates blocking cooling channels.

3. Multi-Stage Injection Molding Parameter Standards

Segmented barrel temperature is set according to material types: ABS glossy sanitary parts adopt rear zone 180℃, middle zone 200–210℃ and nozzle 215℃; PP thick-walled valve bases are set at 190–220℃; nylon structural parts range from 230–250℃; the maximum temperature of soft PVC shall not exceed 170℃ to avoid decomposition and silver streaks. Mold temperature is controlled by zones: 60–70℃ for glossy decorative parts to improve surface smoothness; 40–50℃ for thick-walled sealing bases to reduce internal sink marks through slow cooling; transparent sanitary parts require mold temperature fluctuation within ±3℃ to prevent uneven light transmittance. Multi-stage injection is adopted: low-speed filling for runners, medium-speed filling for main product bodies and low-speed passing through sealing ribs and thin ribs to avoid surface wash-out and thickened weld lines. Two-stage packing is applied: high-pressure primary packing to eliminate shrinkage depressions and low-pressure secondary packing to stabilize dimensions. Packing time increases by 2 seconds per 1mm wall thickness, and cooling time of thick-walled products is extended by 30% to reduce internal residual stress. Back pressure of 8–12bar is applied for ABS and PP, 12–18bar for nylon, with screw speed controlled at 40–60r/min to reduce shear heat and prevent stress cracking after long-term water immersion.

4. Dimensional and Shrinkage Compensation Standards for Key Structures

Single-side cavity compensation of 0.1–0.15mm is added to sealing ribs to offset depressions caused by thick-wall shrinkage. Threaded joints adopt anisotropic shrinkage calculation with separate cavity enlargement for radial and axial directions to ensure smooth screwing and zero water leakage. The wall thickness difference of products shall be controlled within 1:2 with arc transition on molds, and packing time shall be extended during molding. For valve bases thicker than 5mm, arch pre-compensation is reserved on cavities to prevent central concave surfaces leading to poor sealing. Two samples per mold are taken for size inspection of sealing thickness, thread outer diameter and buckle assembly dimensions, with dimensional deviation limited within ±0.03mm. Machine stop and parameter adjustment are required if dimensional tolerance exceeds the standard for 20 consecutive molds.

5. Prevention and Rectification Standards for Common Defects

Shrinkage depressions, penetrating weld lines and internal bubbles are the main causes of water leakage for sanitary fittings. Sink marks can be eliminated by increasing secondary packing pressure and mold temperature; thickened weld lines require enlarged vents and reduced injection speed; bubbles are solved by thorough raw material drying and increased screw back pressure. Flow marks, silver streaks and scratches on glossy surfaces are improved by full drying, cavity cleaning and lower barrel temperature. Flash overflow shall be solved by boosting mold clamping force, cleaning parting surfaces and trimming matching clearance of inserts. Internal stress cracking is avoided by post-mold annealing: ABS parts are soaked in 60℃ warm water for 30 minutes. Forcible ejection and picking shall be forbidden to prevent residual stress.

6. Demolding, Post-Processing and Warehousing Standards

Oil-based release agents are prohibited for sealing and transparent sanitary parts; only a tiny amount of water-based release agent can be used if necessary to avoid residual grease resulting in bonding failure and water leakage. Sealing ribs and threads shall only be polished with fine sandpaper without damage to flat sealing surfaces. All hard ABS and nylon sanitary fittings go through annealing treatment to lower the risk of cracking after water immersion. Finished products can only be packaged after full cooling to room temperature to avoid deformation from high-temperature sealing. Threads and sealing surfaces shall be equipped with protective sleeves during classified storage to prevent bump scratches that damage assembly tightness.

Conclusion

The injection molding of sanitary fittings differs from ordinary plastic parts, focusing on three core indicators: sealing performance, dimensional precision and crack resistance in water. This complete specification covers mold acceptance before production, raw material drying and mixing, segmented injection parameters, shrinkage compensation for structural features, defect rectification and finished post-processing. Strict implementation of zoned mold temperature control, multi-stage injection packing, raw material drying and annealing can effectively reduce hidden leakage risks such as sink marks, bubbles and stress cracks, stabilize long-term product quality and lower after-sales failure rate of water seepage.