Key Points for Controlling TPR Injection Molding Process Parameters
As a commonly used thermoplastic elastomer, TPR material combines the elasticity of rubber with the processability of plastics, and is extremely sensitive to parameters such as temperature, pressure, speed and cooling. Unreasonable parameter settings can easily lead to defects such as mold sticking, shrinkage, short shots, warpage and poor resilience. Controlling injection molding process parameters is the key to ensuring product performance. The following systematically elaborates on TPR injection molding processing parameters and control points from the core process links.
1. Drying and Preheating Process Control
TPR materials have a certain hygroscopicity, and residual moisture will cause silver streaks, bubbles and surface roughness on the product during molding, so pre-drying treatment is required before injection molding. The recommended drying temperature is 60–80℃, the drying time is 1–2 hours, and the moisture content is controlled below 0.1%. For hygroscopic TPR grades with high rubber content, the drying temperature can be appropriately increased to 80–90℃, and the drying time extended to 2–3 hours, while avoiding high-temperature drying for a long time to prevent material aging and performance degradation. Preheating can not only remove moisture, but also improve the fluidity of the melt, reduce injection pressure and shorten the molding cycle.
2. Barrel Temperature and Nozzle Temperature Control
TPR is a temperature-sensitive material, and the barrel temperature directly determines the melt fluidity and plasticizing quality. The temperature should be set stepwise from the feed zone to the nozzle: the feed zone temperature is 140–160℃ to ensure smooth feeding without bridging; the middle zone temperature is 160–180℃ to complete the melting and homogenization of the material; the front zone and nozzle temperature are 170–190℃ to ensure the melt has good fluidity and no melt drooling. For high-hardness TPR, the temperature can be appropriately increased by 5–10℃; for soft TPR with high rubber content, the temperature should be strictly controlled below 190℃ to avoid thermal decomposition and odor. The nozzle temperature should be 5–10℃ lower than the front zone temperature to prevent drooling, and a temperature-controlled nozzle can be used for precise control.
3. Injection Pressure, Speed and Holding Pressure Control
Injection pressure is the key to ensuring that the melt fills the cavity smoothly. For TPR with good fluidity, the injection pressure is generally set at 40–80 MPa; for thick-walled or complex structural products, the pressure can be increased to 80–120 MPa, but excessive pressure should be avoided to prevent flash, internal stress and product deformation. The injection speed should be matched with the product structure: for thin-walled, small-sized products, high-speed injection is used to ensure complete filling before the melt cools; for thick-walled, large-sized products, medium-low speed injection is used to avoid air trapping and weld lines. The holding pressure is set at 30–50% of the injection pressure, and the holding time is determined according to the product wall thickness, generally 5–15 seconds, to compensate for the shrinkage caused by melt cooling and ensure the dimensional stability of the product.
4. Mold Temperature and Cooling System Control
Mold temperature directly affects the crystallization rate, surface quality and dimensional accuracy of TPR products. The conventional mold temperature is controlled at 20–40℃. For products requiring high surface gloss and low shrinkage, the mold temperature can be appropriately increased to 40–60℃ to improve the melt replication performance and reduce surface defects; for soft TPR products, the mold temperature should be controlled below 30℃ to accelerate cooling, shorten the molding cycle and prevent mold sticking. The cooling system should be designed with uniform water channels to ensure consistent cooling of each part of the mold, avoid local overheating or undercooling, and prevent product warpage and deformation. For precision products, a temperature control machine can be used to accurately control the mold temperature difference within ±2℃.
5. Back Pressure, Screw Speed and Cycle Time Control
Back pressure is used to improve the plasticizing uniformity of the melt, and the general setting is 0.5–1.5 MPa. Appropriate back pressure can eliminate bubbles in the melt, improve the dispersion of pigments and additives, and ensure the consistency of product performance; excessive back pressure will increase the melt temperature, cause material decomposition and increase energy consumption. The screw speed is set at 50–100 rpm, and the plasticizing time is controlled within 10–20 seconds to avoid high-speed rotation causing shear heat and material aging. The molding cycle should be reasonably matched according to the product structure and process parameters, generally 20–40 seconds, to ensure the product is fully cooled and shaped while improving production efficiency.
6. Common Defects and Parameter Adjustment Solutions
Mold sticking: Reduce the barrel and nozzle temperature, lower the mold temperature, increase the holding pressure and time, and check whether the mold demolding slope and polishing meet the requirements.
Shrinkage and sink marks: Increase the injection pressure and holding pressure, extend the holding time, appropriately increase the mold temperature, and optimize the gate and cooling system design.
Short shots and insufficient filling: Increase the injection pressure and speed, raise the barrel and nozzle temperature, improve the exhaust of the mold, and check whether the material fluidity meets the requirements.
Warpage and deformation: Optimize the cooling system to ensure uniform cooling, reduce the injection pressure and internal stress, adjust the mold temperature, and improve the product structure design.
Poor resilience: Avoid high-temperature molding and long-term heating, control the drying temperature and time, and select TPR materials with suitable hardness and formula.
