China Injection Molds: Strength Control Rules for Drill Housing Molds

Drill housings are commonly manufactured with glass fiber reinforced PA, ABS+PC and other engineering plastics. The injection mold bears long-term high injection pressure, glass fiber abrasion and alternating opening-closing impact loads. Insufficient mold strength will lead to cavity deformation, insert cracking, template bending and continuous flash defects. Combined with the production characteristics of China injection molds, the whole-process strength control standards are formulated covering steel selection, structural design, heat treatment, trial mold verification and mass production maintenance.

Steel Material and Heat Treatment Strength Control

Mold cores and cavities adopt different steels according to production volume. NAK80 and S136H pre-hardened steel are used for medium and small batch production with hardness controlled at HRC32-38. SKD61 and H13 quenched steel are selected for over 100,000 shots and raw materials containing more than 20% glass fiber, with hardness HRC48-52 after two tempering processes to eliminate quenching stress and reduce cracking risks. S50C thick pre-hardened plates are used for mold bases instead of thin inferior plates. All steel incoming materials shall be attached with material certificates, and hardness testing shall be carried out one by one after core machining; assembly is forbidden if hardness deviation exceeds ±2HRC. Molds processing glass fiber materials must be treated with nitriding or TD coating to enhance surface compression and wear resistance. Weld repaired areas need secondary tempering followed by surface strengthening treatment.

Structural Strength Control of Cavities and Cores

Sufficient solid steel thickness shall be reserved for mold cores. The single-side thickness of small drill housing molds shall not be less than 35mm, and large-size models shall reach over 45mm to relieve expansion deformation under high injection pressure. Transition fillets R2-R5 shall be added at sharp corners of buckles, ribs and battery slots to eliminate stress concentration. Thin cores deeper than 15mm adopt double-end fixing and auxiliary support columns to avoid bending offset caused by melt impact. Thin steel areas are split into independent inserts, which are fixed by interference fit plus screws to prevent displacement under high pressure. The minimum residual steel thickness of local core positions shall be above 8mm, and pressure bearing blocks are added at weak parts to share clamping force.

Strength Control of Mold Base, Guide and Ejection System

The basic thickness of A and B plates shall be no less than 60mm, and large molds shall be thickened to 80mm. Support columns with diameter above 25mm are evenly arranged around the cavity with a spacing of 80-120mm to avoid permanent arch deformation of templates under central pressure. Guide pins and guide sleeves are made of SUJ2 bearing steel quenched to HRC58-62. Auxiliary guide pins are added for molds over 400mm in length and width to counteract lateral torsion during mold opening and closing. Ejector pins smaller than 2mm are equipped with sleeves; flat ejectors and sliding blocks replace tiny pins at deep ribs and undercuts. Ejector plates are fitted with small guide pins, and the thickness of fixed and bottom ejector plates is more than 25mm to prevent plate deformation and pin breakage under concentrated ejection force.

Molding Process Matching Control for Mold Strength

A 20% safety margin shall be reserved for calculated clamping force; long-term full-load clamping is prohibited to avoid mold base fatigue cracking. Segmented injection is adopted: low-speed filling in the early stage, and holding pressure limited within 80MPa after 90% cavity filling to cut down lasting compression on mold cores. Gates shall avoid facing thin cores directly to reduce continuous melt impact load. Cooling channels are 12-18mm away from cavities with hole distance twice the aperture to prevent weakening core strength. Uniform cooling reduces thermal stress generated by cold and hot alternation. A 10-15mm low-speed buffer section is reserved during mold closing to eliminate impact loss of guide parts and pressure bearing blocks.

Strength Verification Control During Assembly and Trial Mold

12.9 grade high-strength bolts are used for locking. The fitting gap between inserts and pressure bearing blocks is controlled within 0.01mm, and locking shall be conducted evenly in batches to avoid assembly internal stress from unilateral force. Trial mold shall boost parameters in gradient: observe mold expansion traces under 50% standard process first, then lift to mass production parameters step by step. Stop inspection after 50 consecutive shots to check hidden defects including core offset, loose inserts and bent ejector pins. Re-measure parting line clearance after 2,000 test shots; core offset over 0.02mm indicates insufficient rigidity, which shall be solved by mold structural reinforcement rather than increasing clamping force.

Periodic Strength Maintenance Control in Mass Production

Clean parting lines, pressure bearing blocks and guide parts every day to remove glass fiber dust and carbonized residues, which may leave indentations leading to cavity collapse after long-term clamping. Check the matching clearance of guide pins and ejector pins weekly; replace accessories in time if the gap exceeds 0.03mm. Conduct comprehensive inspection every 10,000 shots to check micro cracks and edge chipping at core corners and insert roots. Recheck surface strengthening layer every 50,000 shots for glass fiber molds and re-nitride once steel substrate is exposed. Loosen partial locking screws to release internal stress before mold storage with ejection mechanism fully reset. Adjust process parameters preferentially instead of polishing mold core largely to avoid thickness reduction. All welding areas shall be tempered to eliminate stress before polishing and surface strengthening.

Special Reinforcement for Weak Positions of Drill Housing Molds

High-failure positions include long rib cores on handles, battery buckle inserts, undercut slides of housing, parting lines at housing corners and core areas facing gates. Slide base plates are thickened with wedge locking blocks and wear-resistant strips installed. Small undercut inserts are fixed with additional pressure strips, and delayed ejection is adopted to avoid forced breakage. Annular pressure bearing rings are added at corner positions to disperse concentrated clamping force and prevent collapse and expanding flash after long-term mass production.