Analysis of common problems in injection molding of TPR thermoplastic elastomer
Time：2021-03-13Hits：2063
Generally, TPR should inspect the appearance and process performance of materials before molding according to their properties and supply. The provided particles often contain different degrees of moisture, flux and other volatile low molecular substances, especially TPR with hygroscopic tendency, and its moisture content always exceeds the process requirements.
Therefore, drying treatment and moisture measurement must be carried out before processing. When the temperature is high, the water content of TPR is lower than 5%, even lower than 2%-3%, so vacuum drying oven is often used for drying at 75℃-90℃ for 2 hours. The dried materials must be properly sealed to prevent the materials from absorbing moisture again from the air and losing the drying effect. Therefore, the hopper of the drying room can continuously supply the hot materials to the injection molding machine to simplify the operation, keep clean, improve the quality and increase the injection rate. The loading of dry hopper is generally 2.5 times of the hourly material of injection molding machine.
TPR staining
TPE based on SBC is superior to most other TPR materials in color. Therefore, they only need less color masterbatch to achieve a certain color effect, and the color produced by them is purer than others. Generally speaking, the viscosity of color masterbatch should be lower than that of color masterbatch because the melt index is higher than that of color masterbatch, which is beneficial to the dispersion process of color masterbatch and makes the color distribution more uniform.
In SBS TPE, polystyrene vehicle is recommended.
It is suggested to use polypropylene (PP) as color carrier and hard SEBS as base material.
On the basis of soft SEBS, low density polyethylene or ethylene-vinyl acetate copolymer can be used. PP vehicle is not recommended for soft materials, because the hardness of its composites will be affected by it.
In some injection molding applications, polyethylene (PE) carrier may have a bad influence on the adhesive strength of the substrate.
It is necessary to clean the cartridge before injection.
Before the newly purchased injection molding machine is used, or when the products, raw materials and colors need to be changed or the barrel of the injection molding machine is found to be damaged, the injection molding machine barrel must be cleaned or disassembled first.
Usually, the cleaning machine barrel is cleaned by heating machine barrel. Cleaning materials usually use plastic raw materials (or plastic recycled materials). For materials, the processed new materials can be used to replace the transitional cleaning materials.
Hot pressing temperature.
In the molding process, whether the temperature is set accurately plays a vital role in the appearance and performance of products. The following are some suggestions on setting the injection temperature in the process.
The temperature of the feeding area should be set very low to avoid blockage of the feeding port and overflow of entrained air. When using color masterbatch, in order to improve the mixing state, the transition zone temperature should be set above the melting point of color masterbatch. The temperature setting in the area near the injection nozzle should be close to the required melting temperature. Therefore, after testing, the temperature setting range of general products in each area is: barrel temperature 160-210℃, nozzle temperature 180-230℃.
The mold temperature should be set at the high condensation temperature in the injection molding area, so as to avoid the contamination of the mold caused by moisture and the appearance of streaks on the product surface. The higher the die temperature, the longer the cycle time, but it can improve the appearance of the welding line and products. Therefore, the die temperature range should be designed to be 30~40 degrees.
Images.
Filling, maintaining pressure and cooling the mold.
When the product is molded and filled into the mold cavity, if the filling performance of the product is not good, the pressure drop is too large, the filling time is too long, and the filling is not satisfactory, which leads to the quality problems of the product.
In order to improve the filling performance of products in the molding process and improve the quality of molded products, the following aspects can be considered:
First, change other series of products of Keyue;
Second, change the gate position;
Third, change the injection pressure;
Fourth, change the geometric parameters of parts.
Generally, injection pressure control is divided into primary injection pressure control, secondary injection pressure control and more than three injection pressure control. Whether the timing of pressure conversion is appropriate or not is very important to prevent excessive die pressure, overflow or shortage of materials, etc. The specific capacity of mold products depends on the melt pressure and temperature of the closed gate in the pressure maintaining stage.
If the pressure and temperature are consistent every time during the cooling stage of the product, the specific volume of the product will remain unchanged. When the molding temperature is determined to be constant, the important parameter of product size is packing pressure, and the variables that affect product size tolerance are packing pressure and temperature.
For example, when the mold filling is finished, the holding pressure drops immediately, and when the surface layer has a certain thickness, the holding pressure rises again, so that thick-walled large pieces can be molded with low mold clamping force, and pit collapse and flash can be eliminated.
Generally, the pressure and speed of packing are 50%-65% of the high pressure and speed during injection molding, that is, 0.6-0.8MPa lower than the injection pressure. Because the packing pressure is lower than the injection molding pressure, the oil pump load is lower in a long packing time, the service life of the solid oil pump is prolonged, and the energy consumption of the oil pump motor is reduced.
By pre-adjusting a certain range, there is still a small amount of melt (buffer amount) at the end of the screw near the end of the injection stroke, and further injection pressure (secondary and tertiary injection pressure) can be applied to a small amount of melt according to the filling situation in the mold. In this way, it is possible to prevent the product from sagging or adjust the shrinkage of the product.
Melt temperature, product wall thickness and cooling efficiency are the main factors affecting cooling time. In addition, material hardness is also an important factor. Harder materials solidify faster in the mold than very soft materials. When both sides are cooled, the cooling time required for every 0.100' wall thickness is usually 10-15 seconds. Encapsulated products need longer cooling time because they can be cooled effectively with smaller surface area. A cooling time of 15 to 25 seconds is required for every 0.100' wall thickness.
Influence of injection molding conditions
1. Incomplete plastic molding.
Improper feed adjustment, insufficient or excessive feed.
The injection pressure is too low, the injection time is too short, and the plunger or screw returns prematurely.
(1) slow injection speed.
(2) The material temperature is too low.
2. Spill (flash)
Injection pressure is too high, or injection speed is too fast;
(1) flash caused by excessive feeding.
(2) If the temperature of the barrel and nozzle is too high or the temperature of the mold is too high, the viscosity of the plastic will decrease, the fluidity will increase, and the flash will occur when the mold enters the assembly line.
Third, lines, bubbles and pores.
If the material temperature is too high, it will lead to material decomposition;
The injection pressure is low and the holding time is short, so that the melt will not adhere closely to the cavity surface.
(3) The injection speed is too fast, so that the molten plastic is decomposed under the action of large shear and decomposed gas is produced; The injection speed is too slow, which leads to the failure to fill the cavity in time, resulting in the formation of silver lines on the surface of the product and insufficient density.
Insufficient material consumption, too large feeding cushion, too low material temperature or too low die temperature will affect the flow and molding pressure of melt and cause bubbles.
When the screw is pre-molded, the back pressure is too low and the rotating speed is too high, which makes the screw rebound too fast and easily pushes air into the front end of the barrel.
德创化工 (edit：dechuang)
