Warping is one of the common defects of plastic products because the shape of the custom plastic injection profiles deviates from the shape of the mold cavity. There are many reasons for the warping deformation, which cannot be solved by the process parameters alone. The following is a brief analysis of the factors affecting the warping deformation of injection products.
1. The effect of mold structure on warpage of products.
In terms of molds, the main factors affecting the deformation of plastic injection profiles are the pouring system, cooling system and ejection system.
Gating system. The position, form, and quantity of the gate of the injection mold will affect the filling state of the plastic in the mold cavity, resulting in deformation of the plastic product. The longer the melt flow distance, the greater the internal stress caused by the flow and shrinkage between the frozen layer and the central flow layer; the shorter the flow distance, the shorter the flow time from the winding to the end of the product flow, and the thickness of the frozen layer during filling Thinning reduces internal stress and warping deformation. For some flat plastic parts, if only one core gate is used, it is because of the diameter.
Plastic Injection Profiles
(1) Twisted deformation will occur in the plastic parts after molding; if multiple point gates or film gates are used instead, warpage and deformation can be effectively prevented. When using point gates for molding, due to the anisotropy of plastic shrinkage, the position and number of gates have a great impact on the degree of deformation of plastic products. In addition. The use of multiple flexures can also shorten the flow ratio (L / t) of the plastic, thereby making the melt density in the mold cavity more uniform and the shrinkage more uniform. For ring products, the same degree of the final product is also affected due to the different gate shapes. When the entire plastic product can be filled under a small injection pressure, a smaller injection pressure can reduce the molecular orientation tendency of the plastic and reduce its internal stress. Therefore, the deformation of the plastic part can be reduced.
(2) Cooling system. During the injection process, the uneven cooling speed of the plastic product will also affect the uneven shrinkage of the plastic part. This shrinkage difference will cause the bending moment to cause the product to warp. If the temperature difference between the mold cavity and the core used in the injection molding of a flat-shaped product (such as a cell phone battery case) is too large, the melt close to the surface of the cold mold quickly cools down, and the material close to the surface of the hot mold cavity The shell will continue to shrink, and uneven shrinkage will warp the product. Therefore, the cooling of the injection mold should pay attention to the temperature of the cavity and the core tend to balance, and the temperature difference between the two should not be too large (then consider using two mold temperature machines).
In addition to considering the temperature of the product inside and outside tend to balance. The temperature consistency of each side should also be considered, that is, the temperature of the cavity and the core should be kept as uniform as possible when the mold is cooled, so that the cooling speed of the plastic parts is balanced, so that the shrinkage everywhere is more uniform and effective. To prevent deformation.
Therefore, the arrangement of the cooling water holes on the mold is very important, including the cooling water hole length d, the distance between the water holes b, the distance c from the tube wall to the cavity surface, and the product wall thickness w. After the distance from the tube wall to the cavity surface is determined, the distance between the cooling water holes should be made as small as possible. In order to ensure that the temperature of the mold wall is uniform, the problem that should be paid attention to when determining the diameter of the cooling water hole is that no matter how large the mold is, the diameter of the water hole cannot be greater than 14mm, otherwise the cooling liquid will be difficult to form turbulent flow. The diameter of general water holes can be determined according to the average wall thickness of the product, when the average wall thickness is 2mm. The diameter of the water hole is 8-10mm; when the average wall thickness is 2-4mm, the diameter of the water hole is 10-12mm; when the average wall thickness is 4-6mm, the diameter of the water hole is 10-14mm. At the same time, due to the cooling medium The temperature rises with the increase of the length of the cooling water channel, which causes a temperature difference between the cavity of the mold and the core along the water channel. Therefore, the length of the water channel of each cooling circuit is required to be less than 2m. In large molds, several cooling circuits should be set up. The inlet of one circuit is located near the outlet of the other circuit.
For long plastic parts, straight-through channels should be used. However, most of our current molds use S-shaped circuits, which are not conducive to circulation and extend the cycle.
(2) Eject the system. The design of the ejection system also directly affects the deformation of the plastic product. If the ejection system is unbalanced, it will cause an unbalanced ejection force and deform plastic products. Therefore, when designing the ejection system, the ejection force should be balanced with the ejection resistance. In addition, the cross-sectional area of the ejector must not be too small, so as to prevent the plastic product from deforming due to excessive force per unit area of the plastic product (especially when the demolding temperature is high). The ejector rod should be arranged as close as possible to the part with high demolding resistance. Under the premise of not affecting the quality of plastic products (including use requirements, dimensional accuracy, appearance, etc.), as many poles as possible should be provided to reduce the overall deformation of the plastic products (this is the reason for changing the top rod to the top block).
When soft plastics (such as TPU) are used to produce plastic parts with deep cavity walls, due to the large demolding resistance and the material is relatively soft, if only a single-mechanical ejection method is used, plastic products will be deformed. Even the plastic products are scrapped because they are pushed through or folded. In this case, it is better to use a combination of multiple elements or a combination of gas (liquid) pressure and mechanical ejection.