Analysis of common problems in TPE injection molding will give you a better understanding of TPE materials
Time：2021-04-16Hits：1931
TPE is a common injection molding material. At present, TPE injection molding products have been widely used in transparent toys, sports equipment, home appliance shells, handles, grips and other industries. In order to ensure the quality of finished products, we must pay attention to the following tasks before injection molding TPE.
Clean the barrel thoroughly
Before the injection molding machine is just bought and used for the first time, or when it is in production, it is necessary to change products, raw materials and colors, or if there is decomposition in plastics, it is necessary to clean the barrel of the injection molding machine first. When cleaning, the machine barrel is basically heated.
Selection principle of cleaning material: TPR of SBS substrate, it is recommended to clean the barrel with PS material; TPE, the SEBS substrate, is recommended to be cleaned with PP material for higher hardness, and PE or EVA material for lower hardness is recommended to be used to clean the charging barrel.
Ensure good drying
For some overmolding processes, the materials must be dried before processing, and the water content must be measured. At high temperature, the moisture content of TPR should be below 5%, even 2%-3%, so vacuum drying oven is often used for drying at 75℃-90℃ for 2-3 hours. The dried materials must be properly sealed and stored to prevent the materials from absorbing moisture from the air and losing the drying effect. The use of the hopper in the drying chamber can continuously provide dry hot materials for the injection molding machine, which is beneficial to simplify the operation, keep clean, improve the quality and increase the injection rate. The loading capacity of drying hopper is generally 2.5 times of the hourly material consumption of injection molding machine.
Select color matching mother
TPE based on SEBS is superior to most other TPR materials in color. It only needs a small amount of color masterbatch to achieve a certain color effect, and the color produced is purer than TPR. Generally speaking, the viscosity of color masterbatch should be lower than that of TPR, because the melt index of TPR is higher than that of color masterbatch, which is beneficial to dispersion and makes the color distribution more uniform. For TPR based on SBS, polystyrene color carrier is recommended. For TPE based on SEBS, polypropylene (PP) is recommended. For TPR based on SBS, low density polyethylene or ethylene vinyl acetate copolymer can be used.
Typical temperature setting
When processing injection molding, the accuracy of temperature setting will have great influence on the appearance and performance of products. The temperature at the feeding place should not be too high, which can prevent blockage at the feeding place and escape of entrained air. And the temperature near the injection nozzle should be relatively close to the temperature of the melt. Therefore, the temperature range of TPE injection molding equipment will be different, the barrel is between 160℃ and 210℃, and the nozzle is between 180℃ and 230℃.
In addition, the mold temperature is higher than the condensation temperature in the injection molding area, which can prevent some moisture from polluting the mold and causing various stripes on the surface of TPE products. The high temperature of the mold often prolongs the cycle, but it can improve the appearance of the welding line and products, so the temperature of the mold should be set in the range of 30℃ to 40℃.
Pressure/speed setting
The packing pressure should be as small as possible. Generally, the packing pressure should be smaller than the injection pressure. The ejection pressure (P1) is set at 60% of the maximum value, and the holding pressure (PH) is set at 30% of the maximum value. The back pressure is determined by the viscosity and thermal stability of the material. Too high back pressure will increase the rotational shear force and easily overheat the material. Generally, it is better to use 5-15kg/cm2. The ejection speed (V1) is set at 40% of the maximum value, and if it is a multi-stage injection, the speed is generally from slow to fast. Screw speed (VS) is set at about 60RPM.
德创化工 (edit：dechuang)
