1. The barrel needs to be cleaned before injection molding
Before the newly purchased injection molding machine is used for the first time, or when the product needs to be changed, the raw material needs to be replaced, the color needs to be changed, or decomposition is found in the plastic during production, the barrel of the injection molding machine needs to be cleaned or disassembled. The barrel is generally cleaned by the heated barrel cleaning method. The cleaning material is generally made of plastic raw materials (or plastic recycled materials). For TPR materials, the transition cleaning material can be replaced by the processed new material.
2. TPR molding temperature
During the injection molding process, whether the temperature setting is accurate is the key to the appearance and performance of the product. The following are some suggestions for temperature setting when performing TPR processing injection molding.
The temperature of the feed area should be set quite low to avoid clogging of the feed port and allow entrained air to escape. When using masterbatch, in order to improve the mixing state, the temperature of the transition area should be set above the melting point of the masterbatch. The temperature of the area closest to the injection nozzle should be set close to the required melt temperature.
Therefore, after testing, the temperature setting range of each area of tpr products is usually: 160 degrees Celsius to 210 degrees Celsius for the barrel and 180 degrees Celsius to 230 degrees Celsius for the nozzle.
The mold temperature should be set higher than the condensation temperature of the injection molding area, which will avoid moisture contamination of the mold and the appearance of stripes on the surface of the product. Higher mold temperature usually leads to longer cycle times, but it can improve the appearance of welding lines and products, so the mold temperature range should be designed to be between 30 and 40. 3. Filling, holding pressure, and cooling of the mold
In the process of filling the mold cavity during product molding, if the filling performance of the product is not good, the pressure will drop too much, the filling time will be too long, the filling will not be full, etc., resulting in quality problems in the product. In order to improve the filling performance of the product during molding and improve the quality of the molded product, the following aspects can generally be considered:
Change to another series of products of Keyue; change the gate position; change the injection pressure; change the geometric shape of the part. Usually, the control of injection pressure is divided into the control of primary injection pressure, secondary injection pressure (holding pressure) or more than three injection pressures. Whether the timing of pressure switching is appropriate is very important to prevent excessive pressure in the mold, overflow or lack of material. The specific volume of the molded product depends on the melt pressure and temperature when the gate is closed during the holding stage.
If the pressure and temperature are consistent every time from the holding stage to the product cooling stage, the specific volume of the product will not change. Under constant molding temperature, the most important parameter that determines the size of the product is the holding pressure, and the most important variables that affect the product size tolerance are the holding pressure and temperature.
For example: After the mold filling is completed, the holding pressure is immediately reduced, and when the surface layer forms a certain thickness, the holding pressure rises again, so that a large thick-walled product can be formed with a low clamping force to eliminate pits and flash.The holding pressure and speed are usually 50%~65% of the highest pressure and speed when the plastic fills the mold cavity, that is, the holding pressure is about 0.6~0.8mpa lower than the injection pressure. Because the holding pressure is lower than the injection pressure, the oil pump load is low during the considerable holding time, the service life of the solid oil pump is extended, and the power consumption of the oil pump motor is also reduced.
A certain amount of metering is pre-adjusted so that a small amount of melt (buffer) remains at the end of the screw near the end of the injection stroke. According to the filling situation in the mold, further injection pressure (secondary or tertiary injection pressure) is applied to add a small amount of melt. In this way, the product can be prevented from sinking or the shrinkage rate of the product can be adjusted.
The cooling time mainly depends on the melt temperature, the wall thickness of the product and the cooling efficiency. In addition, the hardness of the material is also a factor. Compared with very soft varieties, harder varieties will solidify faster in the mold.
If cooling is performed from both sides, the cooling time required for every 0.100' wall thickness will usually be about 10 to 15 seconds. Products with encapsulation will require longer cooling times because they can be effectively cooled through a smaller surface area. The cooling time required for every 0.100' wall thickness will be about 15 to 25 seconds.
IV. Influence of injection molding process conditions
1. Incomplete plastic molding (1) Improper feed adjustment, lack of material or excess material. (2) Injection pressure is too low, injection time is short, and the plunger or screw is retracted too early. (3) Slow injection speed. (4) Too low material temperature.
2. Overflow (flash) (1) Injection pressure is too high or injection speed is too fast. (2) Too much feed causes flash. (3) Too high barrel, nozzle temperature or mold temperature will reduce plastic viscosity and increase fluidity, causing flash under smooth mold entry.
3. Silver streaks, bubbles and pores (1) Too high material temperature causes decomposition. (2) Low injection pressure and short holding time make the molten material not close to the cavity surface. (3) Too fast injection speed causes the molten plastic to decompose under large shearing action and produce decomposition gas; too slow injection speed cannot fill the cavity in time, resulting in insufficient surface density of the product and silver streaks. (4) Insufficient material quantity, too large feed buffer, too low material temperature or too low mold temperature will affect the flow and molding pressure of the molten material and produce bubbles. (5) The back pressure of the screw is too low and the speed is too high during pre-molding, causing the screw to retract too quickly, and the air is easily pushed to the front end of the barrel along with the material.
4. Burnt dark lines (1) The barrel and nozzle temperature are too high. (2) The injection pressure or pre-molding back pressure is too high. (3) The injection speed is too fast or the injection cycle is too long.
