Mold Design and Injection Molding Key Points for TPU Waterproof Sealing Rings
TPU material is widely used in various waterproof sealing rings due to excellent elasticity, hydrolysis resistance and sealing characteristics. Sealing rings are thin-wall elastic molded parts with strict requirements for mold structure, cavity precision and molding process. Improper design or parameter control easily leads to shrinkage, bubbles, flash, tearing during demolding and sealing failure after assembly. Detailed planning at mold design stage and stable control during molding directly determine the waterproof performance and service life of sealing rings.
1. Pre-evaluation of product structure
TPU waterproof sealing rings mostly have circular or special-shaped cross-sections. Demolding deformation shall be prioritized during design. Sharp corners of products should be avoided, and proper fillets are set at corners to reduce stress concentration. The wall thickness of sealing rings needs to stay uniform. Excessive wall thickness difference causes inconsistent shrinkage and triggers warpage deformation, affecting compression sealing effect. For large-size sealing rings, a small number of process notches can be set on non-sealing working surfaces of products to balance injection filling pressure. Notches are arranged away from waterproof contact areas without affecting sealing performance after assembly. Reserve dimensional compensation according to actual shrinkage rate of TPU raw materials. The shrinkage range varies greatly for TPU of different hardness. Lower hardness materials have relatively larger shrinkage rates, so measured shrinkage data of raw material grades shall be confirmed in advance.
2. Design of mold cavity and parting surface
The parting surface of sealing ring molds is preferably arranged at the neutral cross-section of products to ensure symmetry of upper and lower cavities, facilitating machining and product removal. The parting surface needs sufficient sealing width. Flatness of sealing zones is controlled within small tolerance range to reduce flash generation. Adopting insert structure for cavities benefits polishing, replacement and maintenance in later stages. Surface roughness of cavities for sealing working surfaces must be strictly controlled, because scratches on the surface will directly damage waterproof performance. Multi-cavity molds require basically consistent runner length of each cavity to realize balanced feeding and prevent large dimensional difference of products from different cavities. Grinding treatment is carried out after cavity machining to reduce tool marks on the surface and avoid tiny defects on product surface after molding.
3. Gating and venting system design
TPU melt has high viscosity and is sensitive to shearing. Gates should not be too small to prevent temperature rise and material decomposition caused by high-speed shearing. Gates are preferably arranged at non-sealing areas of sealing rings to avoid gate marks affecting waterproof contact surfaces. The cross-section of main runner and sub-runner prefer circular structure to reduce flow resistance. Keep runner surface smooth to reduce melt retention. Sealing rings are thin-wall products, so air inside cavities is hard to discharge. Vent grooves are set on parting surfaces and melt flow ends. The depth of vent grooves must be strictly controlled. Excessive depth easily generates flash while insufficient depth will cause burn marks and bubbles. Multi-point feeding can be adopted for long-circumference sealing rings. Meanwhile, weld line positions need to be well controlled. Weld lines cannot be located on main stress-bearing sealing positions of products to prevent water leakage caused by cracking at weld positions under compression during use.
4. Cooling system design
TPU molding is sensitive to mold temperature. Fluctuation of mold temperature causes unstable hardness and shrinkage of products. Cooling water channels are arranged close to cavity surfaces with uniform layout to reduce local temperature difference of molds. Seal water channel joints tightly to eliminate water leakage. Mold temperature needs to be maintained within a proper range. Low mold temperature increases melt flow resistance and leads to insufficient filling. High mold temperature extends molding cycle and causes sticking and deformation of products. Mold temperature controllers can be used for small sealing ring molds to realize constant temperature control, ensuring stable mold temperature in each shot and reducing dimensional fluctuation of mass products.
5. Demolding mechanism design
TPU material is soft and prone to tearing under tension. Single-point concentrated ejection cannot be adopted for ejection structures. Ejector sleeves or multiple ejector pins are preferred for uniform ejection to disperse ejection force and prevent local stretching damage of sealing rings. Polishing treatment is applied on ejector part surfaces to reduce friction resistance with products. Ejection speed is set gently to avoid instantaneous impact force pulling molded parts. Mold opening stroke is reasonably defined with segmented adjustment of mold opening speed. Reduce speed at the stage when products separate from cavities to reduce instantaneous deformation of elastic parts. If products have undercut structures, core pulling action needs to be slow and stable. Too fast core pulling will stretch sealing rings and create permanent deformation, losing original sealing dimensions.
6. Injection molding process control
Raw material drying is a key step for TPU molding. Moisture absorbed by materials will produce bubbles and silver streaks after molding. Set drying temperature and duration according to requirements of material grades. Use dried materials timely after drying to avoid re-absorption of moisture in air. Injection temperature needs reasonable control. Barrel temperature cannot be too high to prevent thermal degradation of TPU, which reduces material elasticity and aging resistance. Injection speed adopts segmented control. Low speed is used for filling in the front section, and parameters are adjusted properly at melt flow ends to avoid short shot or excessive shearing at thin-wall positions. Packing pressure should not be too large. Excessive pressure increases internal stress of products and continuous deformation after demolding. Cooling time matches product wall thickness. Sealing rings taken out with insufficient cooling are easy to deform, while overly long cooling time will reduce production efficiency. Products need to be placed for a period after molding before dimensional inspection to reserve stress release cycle and guarantee reliability of dimensional measurement results.
7. Trial molding and mass production quality control
The first trial molding prioritizes verification of filling status, weld line positions and demolding effect. Record appearance and dimensional changes of products under different parameters to gradually lock stable process window. Sealing rings focus on detection of cross-section dimensions, surface defects and compression rebound performance. Appearance inspection mainly checks scratches, bubbles and tiny flash on sealing surfaces, and any minor defect may affect waterproof capacity. Inspect mold status regularly during mass production, check wear of parting surfaces and cavity surfaces, and confirm whether vent grooves are blocked by accumulated materials. Manage raw material batches in mass production. Hardness and shrinkage of TPU differ slightly among batches, and process parameters need fine adjustment when necessary to maintain stable sealing performance of products.
