Shrinkage problems in plastic parts (surface sink marks and internal cavities) are defects caused by insufficient molten plastic replenishment during the cooling of thicker areas. We often encounter this problem. Do you know how injection molded parts manufacturers solve product shrinkage issues? HUIWEN Plastics will explain.
Among commonly used raw materials, PC material is notoriously difficult to solve due to its rapid cooling rate, and PP material also faces challenges with sink marks and cavities. Therefore, when encountering severe shrinkage problems in thick, large parts, unconventional injection molding techniques are necessary to resolve the issue.
Firstly, while ensuring the injection molded part does not deform upon demolding, the cooling time is minimized, allowing the part to be demolded earlier at a high temperature. At this point, the outer layer of the injection molded part is still very hot, and the surface is not overly hardened. Therefore, the temperature difference between the inside and outside is relatively small, which facilitates overall shrinkage and reduces concentrated shrinkage within the injection molded part.
Since the overall shrinkage of an injection molded part is constant, the greater the overall shrinkage, the smaller the concentrated shrinkage, thus reducing internal shrinkage cavities and surface depressions.
Sinking problems occur because the mold surface heats up, reducing cooling capacity. The surface of the freshly solidified injection molded part is still relatively soft. Unresolved internal shrinkage cavities create a vacuum, causing the injection molded part surface to compress inward under atmospheric pressure. Combined with the shrinkage force, this results in sinking problems. Furthermore, the slower the surface hardening rate, the more prone to sinking, such as with PP materials; conversely, the faster the surface hardening rate, the more prone to shrinkage cavities.
Therefore, after premature demolding, the injection molded part should be appropriately cooled to maintain a certain surface hardness, making it less prone to sinking. However, if the sinking problem is severe, moderate cooling will not eliminate it. In such cases, freezing water quenching must be used to rapidly harden the injection molded part surface, which may prevent sinking, but internal shrinkage cavities will still exist. For materials with a relatively soft surface, such as PP, the effects of vacuum and shrinkage forces may still cause sink marks in injection-molded parts, although the degree of sink marks is greatly reduced.
While taking the above measures, extending the injection time instead of the cooling time will further improve the improvement of surface sink marks and even internal cavities.
When addressing cavities, excessively low mold temperatures will worsen the problem. Therefore, use machine water cooling for the mold, not chilled water. If necessary, further increase the mold temperature; for example, when injection molding PC, raising the mold temperature to 100 degrees Celsius will improve the effect of reducing cavities. However, if the goal is to address sink marks, the mold temperature should not be increased but rather lowered.
Furthermore, sometimes the above methods may not completely solve the problem, but they will provide significant improvement. If it is absolutely necessary to completely eliminate surface sink marks, adding an appropriate amount of anti-shrinkage agent is a good, albeit unavoidable, solution. Of course, this method cannot be used for transparent parts.
If shrinkage marks still exist on the surface of thick-walled parts, or if there are uneven wall thicknesses in plastic parts, then gas-assisted injection molding can solve this problem.
Gas-assisted injection molding is a novel plastic molding technology that introduces high-pressure gas into the thick-walled sections of the part, creating a hollow cross-section inside the molded part to complete the filling process. This achieves gas pressure holding and eliminates shrinkage marks.
Traditional injection molding processes cannot combine thick and thin walls, and the parts often have high residual stress, are prone to warping and deformation, and frequently exhibit shrinkage marks. The newly developed gas-assisted injection molding technology, by hollowing out the interior of thick-walled parts, successfully produces thick-walled and uneven-walled products with excellent surface properties, low internal stress, and high strength despite being lightweight.
Televisions, home appliances, automobiles, furniture, daily necessities, office supplies, toys, and other products have opened up entirely new application areas for plastic molding. Gas-assisted injection molding technology is particularly suitable for pipe-shaped products, thick-walled and uneven-walled parts (parts composed of cross-sections of varying thicknesses), and large flat structural parts. Gas-assisted systems include a nitrogen generator and pressurization system, a pressure control unit, and an air intake element. Gas-assisted processes can be integrated with traditional injection molding processes (injection molding machines).
Product weight can be reduced by up to 40% (material savings), molding cycle time shortened by up to 30%, shrinkage marks eliminated, and yield increased; injection pressure reduced by up to 60%, allowing the production of large parts using smaller tonnage injection molding machines, reducing operating costs; mold life extended, manufacturing costs reduced, and the use of more robust structures such as thicker ribs, thicker reinforcing bars, and connecting plates allowed, increasing the freedom of mold design.
3.500+ Material Choices Custom Injection Molding
