Injection shrinkage voids, also known as vacuum bubbles, are a common defect in the injection molding process. These voids appear as holes inside or on the surface of a plastic part, severely impacting its mechanical properties and appearance. These voids are typically circular or oval in shape, hollow within, and may be surrounded by signs of stress concentration. Not only do these voids reduce the strength of the part, making it susceptible to breakage during use, but they also compromise its sealing properties, a critical flaw for products requiring waterproofing and airtightness. Therefore, thoroughly analyzing the causes of voids and implementing effective solutions are crucial to ensuring the quality of injection molded products.
Shrinkage holes are primarily caused by the uneven shrinkage of the plastic melt during the cooling and solidification process. Once the plastic melt fills the mold cavity, due to the lower mold temperature, the melt begins to cool and solidify from the outer layer, forming a hard shell. However, the melt inside remains molten and continues to shrink as the temperature drops. If insufficient melt is replenished at this point, a vacuum forms due to the volume reduction, leading to shrinkage holes. This is particularly common in areas with uneven wall thickness, as the melt in thicker areas cools more slowly and shrinks more, making insufficient melt replenishment more likely. For example, shrinkage holes are common where the ribs connect to the main body of a part, due to the sudden increase in wall thickness.
In addition to uneven cooling and shrinkage, improper setting of injection molding process parameters is also an important cause of shrinkage holes. Insufficient injection pressure or too low holding pressure will prevent the melt from obtaining sufficient pressure to push new melt for replenishment during cooling and shrinkage, thus forming shrinkage holes. Too short a holding time will also lead to insufficient replenishment, because when the holding time is over, the gate has solidified and the melt can no longer be replenished into the cavity. Too fast an injection speed will cause the melt to cool prematurely in the cavity, forming a hard shell that hinders the flow and replenishment of subsequent melt; while too slow an injection speed may cause the melt to begin cooling during the filling process, increasing the probability of shrinkage holes. In addition, too high a barrel temperature will reduce the viscosity of the plastic melt and increase its fluidity, but it will also prolong the cooling time, increase the amount of shrinkage, and increase the possibility of shrinkage holes.
Improper mold design can also exacerbate the formation of shrinkage cavities. The mold’s gate position and size have a significant impact on the melt filling and holding process. If the gate is too far from the thick-walled area, the melt will have cooled by the time it reaches the thick wall, and its fluidity will decrease, making it difficult to effectively hold and replenish the pressure, which can easily lead to shrinkage cavities. If the gate size is too small, the melt flow resistance will increase, and the melt will not be able to smoothly enter the cavity to replenish the shrinkage during holding pressure, which will also cause shrinkage cavities. Improper design of the mold’s cooling system is also an important factor. Uneven distribution of the cooling water channels will cause inconsistent cooling speeds in various parts of the plastic part, too slow cooling in thick walls, and excessive shrinkage, which will result in shrinkage cavities. In addition, high surface roughness of the mold cavity will increase the melt flow resistance, affect the filling and replenishment of the melt, and indirectly lead to the formation of shrinkage cavities.
Solutions to injection molding shrinkage voids require multiple approaches, including process parameter adjustment, mold optimization, and raw material control. Regarding process parameters, the injection and holding pressures can be appropriately increased to ensure sufficient pressure for melt replenishment during cooling and shrinkage. The holding time can be extended to allow the gate to solidify after the melt has fully replenished. The injection speed can be appropriately adjusted. For thick-walled parts, a staged injection method, starting slow and then increasing the speed, can be employed to ensure that the melt is fully filled into all areas. Regarding mold optimization, the gate should be positioned appropriately, maximizing proximity to thick-walled areas to facilitate melt replenishment. The gate size should be appropriately increased to reduce flow resistance. The cooling system should be optimized to ensure evenly distributed cooling water lines, particularly in thick-walled areas, to accelerate cooling and minimize shrinkage. Regarding raw material control, plastic raw materials with good fluidity and stable shrinkage should be selected, and the raw materials should be thoroughly dried to avoid excessive moisture content that affects the melt’s fluidity and shrinkage properties. These comprehensive measures can effectively reduce or even eliminate injection shrinkage voids and improve part quality.
