1. There are four kinds of plastic shrinkage: thermal shrinkage, phase change shrinkage, orientation shrinkage, compression shrinkage and elastic recovery. The shrinkage process consists of three parts: shrinkage before solidification of gate, cooling shrinkage and shrinkage after demoulding.
2. The main reasons for shrinkage: 1. The injection volume is not enough; 2. The melt temperature is too high; 3. The injection pressure and pressure holding pressure are too low; 4. The injection time and pressure holding time are too low; 5. The injection speed is too high; 6. The mold temperature is improper
3. Main reasons for shrinkage cavity: 1. Insufficient injection volume 2. Too low injection pressure 3. Improper injection speed 4. Too low mold temperature
4. Injection molded parts are short of glue and not full of mold
Cause analysis: The plastic melt does not completely fill the cavity. The fluidity of plastic materials is poor.
Countermeasures: Improper matching between products and injection molding machine, insufficient plasticizing capacity or injection volume of injection molding machine. The material temperature and mold temperature are too low, the plastic is difficult to flow under the current pressure, the injection speed is too slow, and the pressure holding or pressure holding is too low. Insufficient melting and poor fluidity of the plastic lead to large injection pressure loss. The number of gates shall be increased, the gate position shall be arranged reasonably, and the multi cavities shall be arranged and filled unevenly. The cold charging well in the runner shall be reserved insufficiently or improperly, and the cold charging head shall enter the mold cavity to block the normal flow of plastic, so as to increase the cold charging hole. The nozzle, runner and gate are too small, the process is too long, and the plastic filling resistance is too large. When the mold is poorly vented, the air cannot be removed.
5. Peak (rough edge)
Cause analysis: The plastic melt will flow into the parting surface or insert mating surface. The clamping force is sufficient, but the film like excess rubber is produced at the junction of the main runner and the splitter
Countermeasures: The clamping force is insufficient, and the high-pressure plastic injected into the cavity causes a gap between the parting surface or the fitting surface of the insert, and the plastic melt overflows into this gap. The die (fixed side) does not fully contact the nozzle of the machine, and the male and female die have a gap. The influence of mold temperature on the crankshaft type clamping system. Improve the strength and parallelism of the formwork. The mold guide post sleeve is worn/the mold mounting plate is damaged/the pull rod (Corinthian column) is not strong enough to bend, resulting in deviation of the parting surface. Foreign matters attach to the parting surface. The exhaust slot is too deep. The projection surface of the cavity is too large/the plastic temperature is too high/the pressure is too maintained
6. Surface shrinkage and cavity (vacuum bubble)
Cause analysis: the surface of the product is sunken. It is caused by the volume shrinkage of plastic, and is commonly found in local areas with thick meat, such as stiffeners or the junction area between column position and surface. The vacuum bubble caused by the volume shrinkage of the local meat thickness of the product during the cooling process is called void. When the plastic melt contains air, water and volatile gas, the air, water and volatile gas enter the product during the injection molding process and the residual cavity is called bubble
Countermeasures: increase the size of gate and runner, so that the pressure can effectively act on the thick part of the molded product. If necessary, the position of the rubber mouth can also be adjusted. Increase the holding pressure and extend the holding time. The filling speed is increased, and the full compression can be achieved before the plastic is cooled and solidified. Glue injection and pressure maintaining are too fast. Make the meat thickness change smooth, and improve the cooling efficiency of this part. The colloidal particles are fully dried in advance to remove moisture. The barrel temperature should not be set too high, which can effectively prevent the generation of plastic decomposition gas. Replace with small screw or machine to prevent over shearing of screw. Raise the back pressure so that the gas can be discharged from the barrel. Reduce the filling speed properly, and the gas will have enough time to discharge.
7. Silver grain (material flower, water flower)
Cause analysis: silvery white stripes along the plastic flow direction on or near the product surface. The silver wire is generally caused by the gasification of moisture in the plastic or volatile matter or moisture attached to the mold surface. Sometimes the injection molding machine screw gets involved in the air to produce silver bars. The material has impurities.
Countermeasures: The plastic contains water, volatile matter and is not dry enough. When the plastic melt is overheated or remains in the barrel for too long, it decomposes, generating a large amount of gas, which is incompletely discharged, and silver wire is generated during solidification. The mold temperature is too low, and the plastic melt solidifies rapidly, resulting in incomplete exhaust. There is oil or water or release agent on the surface of the mold, which evaporates into gas and liquefies with the cooling and solidification of the plastic melt. If the screw is drawn into air and the lower part of the hopper is sufficiently cooled, the temperature at the side of the hopper is low and there is a temperature difference with the barrel. The rubber particles often scratch the screw and are easy to bring in air. Poor exhaust at the beginning of injection. The plastic melt ejected at the initial stage solidified rapidly, so the gas was not completely discharged and silver lines occurred. The injection pressure is too high and the injection speed is too fast. When the meat thickness changes violently, the flowing compressed plastic melt rapidly reduces pressure and expands, and the volatile decomposition gas is liquefied after contacting the mold cavity.
8. Scorching and air ripple
Cause analysis: The so-called burn mark generally includes the discoloration of the product surface due to plastic degradation and the phenomenon that the filling end of the product is burnt black. Scoring means that the air retained in the mold cavity cannot be discharged rapidly (trapped air) when filling the plastic melt, and is compressed to significantly increase the temperature, which will scorch the material. Poor exhaust.
Countermeasures: Strengthen the exhaust in the trapped area to exhaust the air in time. Reduce the injection pressure, but it should be noted that the injection speed will slow down after the pressure drops, which is easy to cause flow marks, weld lines and weld lines to deteriorate.
1. Multi segment control filling is adopted, and multi segment deceleration mode is adopted at the end of the molding process to facilitate gas discharge.
2. Use a vacuum pump to extract the air in the mold cavity, so that the mold cavity can be filled in a vacuum state. Clean the exhaust slot to prevent blockage.
3. The gate is too thin or too long, resulting in plastic degradation. P Exhaust grooves, exhaust inserts, etc.
9. Surface streaks (flow marks) and water ripples
Cause analysis: the trace of plastic melt flow, the stripe wave pattern with the gate as the center. Numerous fine lines with vertical flow direction occur on the surface, resulting in ripples similar to fingerprints on the product surface.
Countermeasures: The flow mark is caused by the boundary between the plastic melt flowing into the mold cavity at first and the plastic melt flowing into the mold cavity. If the cold material left in the front of the injection molding machine nozzle directly enters the mold cavity, it will cause flow marks. If the temperature of plastic melt is low, the viscosity will increase and flow marks will occur. Low mold temperature will take away a lot of heat from the plastic melt, which will reduce the temperature of the plastic melt, increase the viscosity and cause flow marks. If the injection speed is too slow, the temperature of the plastic melt will decrease and increase during the filling process, and the viscosity will increase and flow marks will occur. During the filling process of the mold, the temperature of the plastic melt in the mold cavity drops, and the plastic melt is filled in a high viscosity state. The plastic melt in contact with the mold surface is pressed in a semi solidified state. The surface has numerous fine lines in a vertical direction, resulting in ripples similar to fingerprints on the surface of the product. When the temperature of the plastic melt drops again, the incomplete filling will solidify, resulting in insufficient filling. Ripple often occurs near the edge of the product and at the end of the filling.
10. Water inclusion (weld line), spray pattern (snake pattern)
Cause analysis: When the mold adopts the multi gate feeding scheme, the rubber flow fronts converge with each other; In the hole position and obstacle area, the rubber flow front will also be divided into two parts; Uneven wall thickness can also lead to weld lines. The plastic melt that passes through the gate at a high speed directly enters the cavity, contacts the cavity surface and solidifies, and then is pushed by the subsequent plastic melt, leaving traces of snaking. For side gate, there is no stagnant material area after the plastic passes through the gate or when the stagnant material area is insufficient, it is easy to produce spray marks.
Countermeasure: reduce the number of gates. Add a material overflow well near the fusion part, move the fusion line to the overflow well, and then cut it off. Adjust the gate position. Change the gate position and number, and move the position where the fusion line occurs to another place. Strengthen the exhaust in the fusion line area, quickly evacuate the air and volatile matter in this part, increase the material temperature and mold temperature, enhance the fluidity of plastic, and improve the material temperature during fusion. Increase the injection pressure and appropriately increase the size of the gating system. Increase the shooting speed. Shorten the distance between the gate and the weld area. Shorten the distance between the gate and the weld area. Reduce the use of mold release agent. Adjust the gate position to make the plastic melt collide with the pin or wall after passing through the gate. Change the gate form, adopt overlapping gates or lug gates, and set up sufficient stagnation area in the gate area. It can slow down the initial injection speed of plastic melt. Increase the gate thickness/cross-sectional area so that the flow front is formed immediately. Raise the mold temperature to prevent rapid curing of materials.