Plastic shrinkage is a problem related to plastic shrinkage.
1. Plastic shrinkage occurs in four ways: thermal shrinkage, phase change shrinkage, orientation shrinkage, compression shrinkage, and elastic recovery. The shrinkage process consists of three parts: shrinkage before gate solidification, cooling shrinkage, and shrinkage after demolding.
2. Main causes of shrinkage: 1. Insufficient injection volume 2. Excessive melt temperature 3. Insufficient injection and holding pressures 4. Insufficient injection and holding times 5. Excessive injection speed 6. Improper mold temperature
3. Main causes of shrinkage: 1. Insufficient injection volume 2. Injection pressure too low 3. Improper injection speed 4. Low mold temperature
4. Injection molded parts with insufficient plastic or incomplete mold saturation
Cause analysis: The plastic melt does not completely fill the mold cavity. The plastic material has poor fluidity.
Solution: The product is not properly matched to the injection molding machine, or the machine's plasticizing capacity or injection volume is insufficient. The material and mold temperatures are too low, making it difficult for the plastic to flow under the current pressure. The injection speed is too slow, and the holding pressure or holding pressure is too low. Insufficient plastic melting and poor fluidity result in significant injection pressure loss. Increase the number of gates, arrange the gates appropriately, and avoid uneven filling of multiple cavities. Insufficient or improperly reserved cold wells in the runners allow cold slugs to enter the cavity, hindering normal plastic flow and increasing cold slug pockets. The nozzle, runner, and gate are too small, and the flow path is too long, resulting in excessive plastic filling resistance. Poor mold venting prevents air from being removed.
5. Burst
Cause Analysis: This occurs when the plastic melt flows into the parting surface or the insert mating surface. Sufficient clamping force may result in a film of excess plastic at the junction of the main runner and the branch runner.
Solution: Insufficient clamping force allows the high-pressure plastic injected into the cavity to create a gap between the parting surface or the insert mating surface, allowing the plastic melt to overflow into this gap. The mold (fixed side) is not fully contacting the machine nozzle, creating a gap between the male and female molds. (Not tightened) Mold temperature affects the crankshaft clamping system. Improve the mold platen's strength and parallelism. The mold guide sleeve is worn, the mold mounting plate is damaged, or the tie rod (coring rod) is insufficiently strong and bent, causing the parting surface to shift. Foreign matter is adhering to the parting surface. The venting groove is too deep. The cavity projection is too large, the plastic temperature is too high, or the pressure is too high.
6. Surface Shrinkage and Voids
Cause Analysis: Surface depressions occur. These are caused by volumetric shrinkage of the plastic and are commonly found in thick areas, such as ribs or at the junction of the pillar and the surface. These vacuum bubbles formed during cooling in thick areas of the part due to volumetric shrinkage are called voids. When air, moisture, and volatile gases enter the plastic melt during the injection molding process, the resulting cavities are called bubbles.
Countermeasures: Increase the gate and runner dimensions to ensure effective pressure application to the thickest areas of the molded part. Adjust the gate position if necessary. Increase the holding pressure and hold time. Increase the filling speed to achieve full compression before the plastic cools and solidifies. The transition from injection to holding pressure is too fast. Smoother the thickness transition and improve cooling efficiency in this area. Thoroughly dry the pellets beforehand to remove moisture. Setting the barrel temperature appropriately to prevent the generation of decomposition gases from the plastic can effectively prevent the generation of gases. Use a smaller screw or machine to prevent excessive screw shear. Increase the back pressure to allow gases to escape from the barrel. Appropriately reduce the filling speed to allow sufficient time for gases to escape.
7. Silver Streaks (Sprays, Water Splashes)
Cause Analysis: Silvery-white streaks appear on or near the surface of the part, along the direction of plastic flow. Silver streaks are generally caused by the vaporization of moisture or volatiles in the plastic, or moisture adhering to the mold surface. Air entrained in the injection molding machine screw can also produce silver streaks. Material impurities.
Countermeasures: The plastic contains moisture, volatiles, or is insufficiently dried. The plastic melt overheats or remains in the barrel for too long, decomposing and generating large amounts of gas. This incomplete exhaust during solidification results in silver streaks. Mold temperature is too low, causing the plastic melt to solidify rapidly, leading to incomplete exhaust. Oil, moisture, or mold release agent adhering to the mold surface evaporates into a gaseous state, which liquefies as the plastic melt cools and solidifies. Air is entrained in the screw. If the lower part of the hopper is not sufficiently cooled, the temperature on the hopper side is low, creating a temperature difference with the barrel. Plastic pellets often scratch the screw, leading to air entrainment. Poor exhaust during the initial injection phase. The plastic melt solidifies rapidly during the initial injection phase, resulting in incomplete exhaust and silver streaks. Injection pressure is too high or the injection speed is too fast. When the thickness of the mold changes dramatically, the compressed plastic melt in flow experiences rapid decompression and expansion. The volatilized decomposition gases liquefy upon contact with the mold cavity.
8. Burn Marks and Gas Marks
Cause Analysis: Burn marks generally refer to discoloration on the part surface caused by plastic degradation and a blackened appearance at the end of the mold filling process. Burn marks occur when air trapped in the mold cavity fails to escape quickly during plastic melt filling (trapped air), resulting in compression and a significant increase in temperature, which burns the material. This indicates poor venting.
Solution: Enhance venting in the trapped area to allow for timely air discharge. Reduce injection pressure, but be aware that a decrease in pressure will also slow the injection speed, which can easily lead to flow marks and weld marks.
1. Use multi-stage controlled filling and employ a multi-stage deceleration method at the end of the molding process to facilitate gas evacuation.
2. Use a vacuum pump to extract air from the cavity, ensuring vacuum filling. Clean the venting slots to prevent blockage.
3. A gate that is too thin or too long can cause plastic degradation. Vent grooves, vent inserts, etc.
9. Surface Flow Marks and Water Ripples
Cause Analysis: Flow marks are traces of plastic melt flow, appearing as streaks and waves centered at the gate. Numerous fine lines perpendicular to the flow occur on the surface, resulting in ripples similar to fingerprints on the product surface.
Countermeasures: Flow marks are caused by the initial flow of plastic melt into the mold cavity cooling too quickly, creating a boundary between it and the subsequent flow. Cold material remaining at the tip of the injection molding machine nozzle directly entering the mold cavity can cause flow marks. Flow marks occur when the plastic melt temperature is low and the viscosity increases. Low mold temperature removes a large amount of heat from the plastic melt, causing the melt temperature to drop and the viscosity to increase, resulting in flow marks. Slow injection speeds cause the plastic melt temperature to drop and the viscosity to increase, resulting in flow marks. During the mold filling process, the temperature of the molten plastic in the cavity drops, causing it to fill at a high viscosity. The molten plastic contacting the mold surface is pressed in a semi-solidified state, creating numerous vertical lines on the surface, resulting in ripples similar to fingerprints on the part. When the molten plastic temperature drops further, it solidifies before being completely filled, causing underfill. Ripples often occur near the edges of the part and at the end of the filling process.
10. Water Inclusion Marks (Weld Marks) and Jetting Marks (Snake Marks)
Cause Analysis: When the mold uses multiple gates for gating, the flow fronts of the plastic merge. The flow fronts can also be split in two by holes and obstructions. Uneven wall thickness can also cause weld marks. The plastic melt, passing through the gate at high speed, enters the cavity directly, solidifies upon contact with the cavity surface, and is then pushed out by the following plastic melt, leaving a serpentine mark. With side gates, spray marks are more likely to occur when there is no or insufficient material retention area after the plastic passes through the gate.
Solution: Reduce the number of gates. Add a material overflow well near the weld, move the weld line to the overflow well, and then remove it. Adjust the gate position. Change the location and number of gates to relocate the weld line. Enhance venting in the weld line area to quickly evacuate air and volatiles. Increase material and mold temperatures to enhance the plastic's fluidity and raise the material temperature during fusion. Increase injection pressure and appropriately increase the size of the gating system. Increase injection 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 so that the plastic melt hits pins or walls after passing through the gate. Change the gate type to use overlapping gates or lug gates, and provide sufficient material retention area in the gate area. This can slow the initial injection speed of the plastic melt. Increase the gate thickness/cross-sectional area to allow the flow front to form immediately. Increase the mold temperature to prevent rapid solidification of the material.
