Key Machining Points of Injection Molds for High-temperature Resistant Rubber Plugs
High-temperature resistant rubber plugs are mostly used in new energy, pipeline sealing, high-temperature equipment and other scenarios. Materials often include fluororubber, silicone rubber, PEEK and other high-temperature resistant plastic raw materials. These materials feature special melting characteristics and large fluctuation of molding shrinkage rate, which put higher requirements on mold sealing, exhaust, surface precision and heat resistance. The machining quality of molds directly determines the sealing performance and service life of rubber plugs. Combined with the product characteristics of small annular structure of rubber plugs that no scratches or flash are allowed on the sealing surface, the core points in mold machining stage are sorted out to avoid mass production risks brought by machining defects.
1. Mold steel selection and heat treatment processing requirements
When forming high-temperature resistant rubber plugs, the processing temperature of raw materials is generally higher than that of ordinary plastics. Some materials will release corrosive small molecule gas. The mold steel needs to have high-temperature strength, corrosion resistance and good polishing performance at the same time. S136, H13 and other heat-resistant anti-rust mold steel are preferred. Steel blanks need flaw detection to avoid internal sand holes and cracks and prevent mold cracking under high-temperature working conditions. Heat treatment is a key link, and the hardness needs to be controlled within a reasonable range. Insufficient hardness will cause deformation of mold cores under continuous high temperature and high pressure, while excessive hardness is easy to cause edge chipping. Stress relief treatment is required after heat treatment to eliminate residual stress inside the steel, preventing deformation and size deviation after finish machining and ensuring dimensional stability of repeated mold opening and closing for a long time.
2. Finish machining and polishing control of mold core molding surface
The sealing lip, inner and outer circular surfaces of rubber plugs belong to key molding surfaces. Machining marks will be directly reproduced on the product surface, resulting in sealing failure. Uniform finish machining allowance is reserved in the rough machining stage, and excessive unilateral allowance is not allowed to reduce cutting stress in finish machining. High-speed precision CNC machining is preferred for finish machining, with feed rate controlled to reduce tool marks. For circular arcs and fillet positions of annular rubber plugs, the tool path should transit smoothly to avoid sharp corner tool joint marks. The polishing process is carried out by step-by-step grinding. Rough grinding is used first to remove tool marks, then fine sandpaper and polishing paste are used step by step. The polishing grain follows the demolding direction, and transverse polishing lines are strictly prohibited. The final effect of the sealing molding surface reaches mirror or uniform matte finish without pitting, scratches, orange peel and other defects. Full inspection of molding dimensions is carried out after machining to ensure uniform wall thickness of rubber plugs.
3. Machining control of parting surface and sealing position
Rubber plug products have thin wall thickness and narrow sealing area. The flatness of parting surface machining directly determines whether flash is generated. The parting surface needs to be processed by surface grinding to ensure flatness and parallelism. No depressions, protrusions or scratches are allowed on the parting surface. A reasonable sealing width is controlled at the sealing position. The sealing area processed must be complete and continuous, and smooth fillet transition is made at corner positions to avoid stress concentration at sharp corners. After machining, fit grinding of the parting surface is carried out by fitters to ensure complete fitting of parting surfaces during mold closing and prevent high-temperature melt from squeezing into gaps to form flash. Once flash appears, burrs will form on the sealing edge of rubber plugs, which are easy to break during assembly and use and affect the sealing effect.
4. Key points for exhaust system machining
High-temperature resistant plastic materials will release gas during melting and molding. Rubber plugs are small closed annular structures with high risk of trapped gas inside the cavity. Trapped gas will cause product burning, short shot and bubbles, reducing the high-temperature resistance of rubber plugs. The depth of exhaust grooves must strictly match the characteristics of raw materials. Excessive depth will lead to flash, while insufficient depth causes poor exhaust. Exhaust grooves are opened at the end of material flow and the final filling position of the annular structure of rubber plugs. The direction of exhaust grooves remains smooth without dead corners. Exhaust grooves are polished and cleaned, and all iron scraps and burrs left by machining are removed to prevent iron scraps from being stuck in exhaust grooves and causing blockage during mold trial. For highly corrosive raw materials, machining allowance can be reserved at exhaust groove positions for later grinding and repair to facilitate maintenance in the production process.
5. Cooling channel machining and sealing treatment
High mold temperature is required when molding high-temperature resistant rubber plugs. The machining quality of cooling channels is related to the temperature uniformity of the cavity and reduces dimensional deviation caused by uneven product shrinkage. The hole wall must be smooth when drilling cooling channels, and burrs at hole openings are removed. The cooling channels are as close to the molding cavity as possible with uniform distance to ensure stable mold temperature. The machining precision of threads at cooling channel joints is strictly controlled, and thread slipping and burrs are not allowed to ensure no water leakage after assembly. Water pressure test must be carried out separately after cooling channel machining to check whether there are microcracks and leakage on the hole wall of cooling channels. Anti-rust treatment is carried out for cooling channels to prevent rust and blockage caused by alternating cold and heat for a long time, resulting in abnormal local mold temperature.
6. Machining control of ejection and demolding structure
Rubber plug products are small in size and made of soft materials. Improper machining of ejection positions is easy to cause ejection whitening, deformation and drawing marks. The straightness of ejector pin holes must be guaranteed during machining, and the reasonable matching clearance between ejector pins and holes is controlled. Excessive clearance causes flash, while too small clearance leads to jamming due to thermal expansion of ejector pins under high temperature. The end face of ejector pins is flush with the molding surface without depression or protrusion. The matching surfaces of demolding parts such as angle lifters and inserts are well ground, and the sliding clearance adapts to high-temperature working conditions without jamming after thermal expansion. Deburring is carried out for all moving parts after machining. Repeated mold opening and closing tests are conducted during fitter assembly to ensure smooth demolding movement without scratching the sealing surface of rubber plugs.
7. Post-machining inspection and pre-assembly key points
After all machining of the whole mold is completed, mold trial on the machine cannot be carried out directly. Dimension recheck, appearance inspection and pre-assembly inspection must be performed. CMM and calipers are used to detect cavity size, concentricity and wall thickness to check whether they meet the tolerance requirements of rubber plug drawings. Check whether the molding surface and parting surface have bumps and scratches, and clean all residual iron scraps and oil stains from machining. Repeated mold opening and closing of the mold is carried out in the pre-assembly stage to check all moving mechanisms and confirm no interference or jamming. For high-temperature molding working conditions, the heat resistance adaptability of all mold parts is checked in advance. If out-of-tolerance dimensions or surface defects are found, repair shall be carried out in time to reduce mold modification workload in the mold trial stage.
To sum up, the machining of injection molds for high-temperature resistant rubber plugs needs to meet multiple requirements including heat resistance, sealing, exhaust and precision molding. Every working procedure from steel heat treatment, molding surface polishing, parting surface fit grinding, cooling and exhaust channel machining to ejection mechanism machining will affect the final sealing quality of rubber plug products. Strictly implementing various working procedure standards, controlling machining stress and dimensional precision and reducing inherent defects brought by machining can improve the stability of molds under high-temperature molding working conditions, reduce product defect rate and extend mold service life.
