Brief introduction to preventing injection screwslipping
2018-09-12 10:00:49
When the injection screw is difficult to press at the feeding port, or cannot form enough adhesive force along the length of the barrel to convey the material, the screw slips. During the screw pre-molding stage, the screw will slip when it rotates in the barrel and conveys the material along the direction of the screw and backs up to accumulate the material for the next injection. If the screw begins to slip during the preforming stage, the axial motion of the screw stops as the screw continues to rotate. Usually screw slippage will lead to material degradation before injection molding, but also short shot, prolonged processing cycle and other product quality problems.
The causes of screw slippage include high back pressure, overheating or overcooling in the back of barrel, wear of barrel or screw, shallow screw groove in screw feeding section, improper hopper design, hopper missing or blocking, resin humidity, excessive lubricant content in resin, fine particle size of material, poor cutting shape of resin used or recycled material.
Influence of technological parameters
The low temperature behind the barrel is usually the main reason for the slippage of injection screw. The barrel of the injection molding machine is divided into three sections. At the back of the feeding section, a thin layer of melted polymer is formed during the heating and compression process. The melt film is pasted on the cylinder. If there is no such thin layer, it will be difficult for the pellets to move forward.
The material in the feed section must be heated to critical temperature to produce a key melt film. However, too often the material in the cylinder is too short to allow the polymer to reach the temperature. This may be due to the size of the equipment is too small, the corresponding configuration of smaller barrels and screws caused. Short residence time is easy to cause polymer melting or insufficient mixing, which will lead to screw slip or stop.
Now we introduce two simple ways to deal with this problem. Starting from the end of the cylinder, add a small amount of material to clean it and check the melting temperature. If Dongguan Huahong brand has a short residence time, the melting temperature will be lower than the set value of the barrel temperature. The second method is to observe the molding products, if found such as marble markings, black spots or light stripes, it shows that the material in the barrel has not been very good mixing.
One of the solutions to the screw slip attempt is to gradually raise the temperature of the feeding section until the screw is rotated and retreated in harmony. Sometimes we have to increase the barrel temperature to the recommended set point to reach this range.
Setting too high back pressure will also cause screw to stop or slip. Increasing the setting value of backpressure also increases the energy to enter the material. If the back pressure is set too high, the screw may not produce enough forward melt pressure to overcome the back pressure, and the screw will rotate in a certain position without retreating, which will do more work on the melt and significantly increase the melt temperature, thereby adversely affecting product quality and cycle life. The back pressure exerted on the melt can be adjusted by the control valve on the injection cylinder to Dongguan Huahong brand.
Impact of equipment
If the screw slippage is caused by processing equipment rather than process parameters, then screw and barrel wear is likely to be the key problem. As with the feeding section, the resin adheres to the wall of the cylinder when it is melted in the screw compression section. When the screw is rotated, the material is cut off and left the cylinder wall and conveyed to the front. If there is wear area on the screw and barrel, the screw can not effectively transport the material forward. If the equipment is suspected of wear and tear, the screw and barrel should be checked and the fit clearance between the two should be checked. If the clearance between the screw and the barrel exceeds the standard value, the replacement or repair work should be started.
The design parameters of screw, especially the compression ratio (the depth ratio of feeding section to the depth of homogenizing section), play an important role in plasticizing uniformity. Too shallow feeding section (with a smaller compression ratio) will reduce production and cause screw slippage due to insufficient feeding. Suppliers of various resins generally have the best compression ratio for injection materials.
Failure of the check ring (Dan Xiangfa) will also cause screw slippage.
When the screw rotates and plasticizes the material, the stop ring should be in the front (open) position, contacting the fixed ring seat. If the stop ring is in a backward (closed) state, or in a state between forward and backward, the melt polymer is passing through the stop ring.
