Solution to the problem of severe shrinkage at the column position on the outer surface of the injection molded part
Shrinkage at the pillar position on the outer surface of injection molded parts is a common appearance defect in production. It manifests as depressions and shrinkage pits on the outer surface corresponding to the pillar position. This not only affects the aesthetics of the plastic part, but may also reduce the structural strength of the pillar position. This defect is especially important for plastic parts with high appearance requirements, such as automotive interiors and home appliance housings. This defect can lead to product scrapping and increase production costs. The main reason for pillar shrinkage is that the melt cooling rate at the pillar position differs from that in the surrounding area. The melt inside the pillar position does not receive sufficient melt replenishment during cooling and shrinkage, causing the outer surface to sag inward. Solving the problem of pillar shrinkage requires comprehensive measures from multiple aspects, including mold design, process parameter optimization, and material selection.
Optimizing mold design is the fundamental solution to shrinkage at the pins. Improving the mold’s cooling and gating systems can effectively reduce differential shrinkage at the pins. Regarding cooling system design, sufficient cooling water channels should be placed near the pins to ensure rapid and even cooling of the melt at these locations, minimizing differential shrinkage caused by uneven cooling. Cooling water channels should be located as close to the pins as possible, generally 1.5 to 2 times the channel diameter from the pin surface. The channel diameter can range from 8 to 12 mm depending on the pin size. The inlet and outlet water temperatures should not exceed 5°C to improve cooling efficiency. Regarding gating system design, ensure that the melt fully fills the pins and provides sufficient melt replenishment during the holding phase. Placing the gate close to the pins can shorten the melt flow path and reduce pressure loss. For thicker pins, auxiliary gates can be installed at the base of the pins to enhance melt filling and holding. Furthermore, reinforcing ribs or rounded corners at the base of the pins can disperse stress, reduce shrinkage concentration, and alleviate shrinkage.
Adjusting injection molding process parameters is an effective way to address pillar shrinkage. By optimizing parameters such as injection pressure, holding pressure, holding time, melt temperature, and mold temperature, the filling and holding effects of the melt can be improved, and shrinkage depression can be reduced. Increasing the holding pressure and extending the holding time are key to resolving pillar shrinkage. The holding pressure should be set according to the size and material properties of the pillar, generally 10%-20% higher than the normal holding pressure to ensure sufficient melt replenishment as the pillar melt cools and shrinks. The holding time should be extended until the pillar surface is basically solidified, usually 20%-30% longer than the normal holding time to avoid shrinkage caused by insufficient holding pressure. Appropriately increasing the melt temperature and mold temperature can reduce the viscosity of the melt, improve its fluidity and shrinkage-filling ability, making it easier for the melt to fill the space created by pillar shrinkage during the holding phase , thereby alleviating shrinkage. For example, for shrinkage at the column position of polypropylene parts, increasing the melt temperature from 200°C to 210-220°C and the mold temperature from 50°C to 60-70°C, while also increasing the holding pressure, often achieves good results. However, it is important to note that the melt and mold temperatures should not be too high, as this will extend the cooling time, reduce production efficiency, and even cause other defects such as flash and deformation in the plastic parts.
Improving the pillar structure design can fundamentally reduce the likelihood of shrinkage. While meeting application requirements, optimizing the size, shape, and distribution of the pillars can reduce differential cooling shrinkage. For taller, thicker-diameter pillars, the diameter can be appropriately reduced or the wall thickness can be increased to uniformize to avoid excessive shrinkage caused by excessive melt inside the pillar. For example, reducing the diameter of a 10mm diameter, 30mm height pillar to 8mm while maintaining uniform wall thickness can reduce melt usage and shrinkage, alleviating shrinkage. Introducing hollow structures or reinforcing ribs within the pillars not only reduces part weight but also reduces the melt volume inside the pillars, lowering shrinkage stress and minimizing shrinkage. For example, a 4mm diameter through-hole in the center of the pillar can reduce the melt volume within the pillar by approximately 40%, significantly reducing shrinkage. Furthermore, the pillars should be distributed appropriately to avoid concentrating multiple pillars in the same area to prevent severe shrinkage caused by the accumulation of shrinkage stress. If this is unavoidable, connecting ribs should be installed between the pillars to distribute shrinkage stress.
Choosing the right plastic material and additives can improve melt flow and shrinkage properties, reducing the occurrence of pillar concavity. For plastic parts prone to shrinkage, materials with low shrinkage and good flowability should be preferred. For example, crystalline plastics such as polyethylene and polypropylene have relatively high shrinkage, while amorphous plastics such as polycarbonate and ABS have lower shrinkage and are more suitable for parts with high aesthetic requirements and a high number of pillars. If high-shrinkage materials are necessary, appropriate fillers such as glass fiber and calcium carbonate can be added to reduce shrinkage. For example, adding 30% glass fiber to polypropylene can reduce shrinkage from 1.5%-2.5% to 0.5%-1.0%, effectively reducing pillar concavity. Furthermore, adding plasticizers or lubricants to the material can improve melt flow and shrinkage feeding, making it easier for the melt to fill the spaces created by pillar shrinkage during the holding phase, thereby alleviating shrinkage. However, it is important to note that the addition of additives may affect the mechanical properties and appearance of the plastic part, so the proportion of additives should be appropriately selected based on actual requirements.
Auxiliary process measures can alleviate the problem of pillar shrinkage to a certain extent, providing a temporary or supplemental solution for production. In situations where mold and structural optimization cannot be timely implemented due to small production batches and high mold modification costs, localized heating or cooling methods can be used to adjust the cooling rate of the pillar area. For example, installing a heater rod on the outside of the mold corresponding to the pillar can appropriately increase the mold temperature in the pillar area, slowing the cooling rate of the melt inside the pillar, allowing more time for melt replenishment during the holding phase and reducing shrinkage. For plastic parts that have already experienced slight shrinkage, surface treatment methods such as polishing, puttying, and painting can be used to improve the appearance quality. However, this method is only suitable for plastic parts with low aesthetic requirements and increases production costs and labor. During the production process, the detection and monitoring of pillar shrinkage should be strengthened to promptly identify problems and adjust process parameters to prevent the expansion of defects and the occurrence of batch problems. For example, an online visual inspection system can monitor the shrinkage of the pillar on the outer surface of the plastic part in real time. If shrinkage exceeds the allowable range, an automatic alarm will be issued and a prompt will be given to adjust the holding pressure or holding time to ensure stable product quality.
