Solutions to the shrinkage problem of thick-walled plastic parts
Shrinkage in thick-walled plastic parts is caused by uneven volumetric shrinkage during melt cooling, leading to surface depressions or internal shrinkage cavities. The key to solving this problem is to optimize the holding parameters and reduce shrinkage differences through continuous pressure compensation. Thick-walled plastic parts (usually those with a wall thickness exceeding 4mm) have a long cooling time, and the melt shrinks significantly within the mold cavity. If the holding pressure is insufficient or the holding time is too short, the melt cannot be replenished in time, which can easily cause shrinkage in the thick wall areas. The holding pressure should be set according to the thickness of the plastic part, generally 60%-80% of the injection pressure. For example, when the injection pressure is 120MPa, the holding pressure should be 80-90MPa. For plastic parts 5-10mm thick, the holding time needs to reach 50%-70% of the cooling time. For example, if the cooling time is 30 seconds, the holding time should be set to 18-20 seconds. Using multi-stage holding pressure allows for more precise shrinkage control. The first stage (0-5 seconds) uses higher pressure (90 MPa) to fill the shrinkage space, the second stage (5-15 seconds) uses medium pressure (70 MPa) to maintain and compensate, and the third stage (15-20 seconds) uses low pressure (50 MPa) to reduce internal stress. A thick-walled PP plastic part (8mm thick) shrank due to insufficient holding pressure. By increasing the holding pressure from 60 MPa to 80 MPa and extending the holding time from 10 seconds to 20 seconds, the shrinkage depth was reduced from 0.5 mm to less than 0.1 mm.
Adjusting mold and melt temperatures is a crucial technique for addressing shrinkage in thick-walled plastic parts. A reasonable temperature gradient can slow melt cooling, allowing time for pressure-holding compensation. Excessively low mold temperatures can cause the part’s surface to harden rapidly, preventing internal melt shrinkage from being compensated, resulting in surface shrinkage. Excessively high temperatures prolong cooling time and reduce production efficiency. Mold temperatures for thick-walled parts are typically 10-20°C higher than those for thin-walled parts. For example, the mold temperature for thick-walled PC parts is 80-100°C, 20°C higher than for thin-walled PC parts. This ensures more uniform cooling across the surface and interior. The melt temperature should be controlled 10-30°C above the material’s flow temperature to ensure good melt fluidity during the pressure-holding phase. For example, the melt temperature for PA66 should be set at 260-280°C to avoid premature solidification due to low temperatures. Furthermore, a mold temperature controller should be used to precisely control the mold temperature, keeping fluctuations within ±2°C, to ensure temperature stability. The incidence of shrinkage defects in a certain ABS thick-walled plastic part (wall thickness 10mm) was reduced from 25% to 5% by increasing the mold temperature from 50℃ to 70℃ and the melt temperature from 220℃ to 240℃.
Optimizing gate design and location can improve melt filling and holding pressure in thick-walled parts, reducing shrinkage. Gate size must match part thickness. Thick-walled parts should use a larger gate. For example, for an 8mm-thick part, a gate diameter of 3-5mm (approximately 0.5 times the wall thickness) ensures sufficient melt enters the cavity to compensate for shrinkage during the holding phase. The gate should be located at the thickest part of the part or closest to the thickest area. This allows the melt to preferentially fill the thicker areas and compensate for shrinkage during the holding phase. For example, for a circular, thick-walled part (100mm diameter, 10mm center thickness), placing the gate at the center can improve holding efficiency by 40% compared to using an edge gate. For complex, thick-walled parts, multi-point gating can be used. For example, a car armrest (6-12mm wall thickness) can have three gates, each corresponding to a different thick-walled area. By balancing the holding pressure across each gate, shrinkage is evenly distributed across the part. In addition, the freezing time of the gate needs to be later than the maximum shrinkage time of the plastic part. For example, the freezing time of the gate of thick-walled plastic parts should be controlled at more than 30 seconds to ensure sufficient pressure compensation. The freezing can be delayed by increasing the gate length (such as from 5mm to 10mm).
Gas-assisted injection molding (GAIM) is an effective method for addressing shrinkage in thick-walled plastic parts. High-pressure gas is used to create hollow structures in thick-walled areas of the part, reducing melt consumption and achieving internal pressure retention, thereby suppressing shrinkage. The key to GAI lies in controlling the timing and pressure of gas injection. Gas is typically injected when the melt is 70%-90% full, with a pressure 5-10 MPa higher than the holding pressure. For example, at an injection pressure of 100 MPa, the gas pressure should be 80-90 MPa. This allows the gas to push the melt away in the thick-walled areas, forming a gas cavity and simultaneously applying pressure to the surrounding melt to compensate for shrinkage. For parts with large variations in wall thickness (e.g., 15 mm thick in one area and 3 mm thick elsewhere), gas preferentially enters the thick-walled areas, preventing shrinkage due to excessive shrinkage. For example, using GAI on a household appliance handle reduced shrinkage in the thick-walled area from 3% to 0.5%. Gas-assisted molding also reduces internal stress in plastic parts, minimizing the risk of warpage. For example, after gas-assisted molding of a thick-walled PC lampshade, shrinkage was eliminated and warpage was reduced from 2mm to 0.5mm. When using this technology, careful attention must be paid to gas channel design to ensure uniform gas diffusion and prevent gas from penetrating the surface of the plastic part and forming bubbles.
Improving raw material drying and formula optimization can reduce shrinkage in thick-walled plastic parts caused by material problems. Moisture or volatiles in the raw materials can form bubbles during the molding process, which can easily cause shrinkage depressions around the bubbles, manifesting as shrinkage. Hygroscopic plastics (such as PC and PA) require thorough drying. For example, PC should be dried at 120°C for 4-6 hours with a moisture content below 0.02%, and PA6 should be dried at 80-100°C for 8 hours with a moisture content no more than 0.03% to avoid shrinkage caused by hydrolysis or bubbles. Adding an appropriate amount of nucleating agent or filler to the raw material formula can improve the shrinkage uniformity of crystalline plastics. For example, adding 0.2% talc to PP as a nucleating agent accelerates and makes crystallization more uniform, reducing shrinkage caused by excessive localized shrinkage. For thick-walled plastic parts requiring a high gloss, specialized low-shrinkage materials can be used. For example, medical housings use low-shrinkage ABS, which reduces molding shrinkage from 1.5% to 0.8%, significantly reducing shrinkage risk. In addition, the melt flow rate (MFR) of the raw material needs to match the thickness of the plastic part. For thick-walled plastic parts, it is advisable to use raw materials with higher MFR (such as PP’s MFR is 15-20g/10min) to improve melt fluidity and ensure smooth pressure compensation.
