Warpage refers to the deviation of the shape of the injection molded product from the shape of the mold cavity. It is one of the common defects of plastic products. There are many reasons for the warpage and deformation, which cannot be solved by process parameters alone. The following is a brief analysis of the factors that affect the warpage and deformation of injection molded products.
1. The influence of mold structure on product warpage and deformation.
In terms of molds, the main factors affecting the deformation of plastic parts are pouring system, cooling system and ejection system.
(1) Pouring 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 feeding 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 mold filling Thinning, the internal stress is reduced, and the warpage deformation will also be greatly reduced. For some flat plastic parts, if only one core gate is used, it is due to the diameter direction. The shrinkage rate of BU is larger than the shrinkage rate in the circumferential direction, and the molded plastic parts will be deformed; if multiple point gates or film-type gates are used, warping deformation can be effectively prevented. When point gates are used for molding, also due to the anisotropy of plastic shrinkage, the location and number of gates have a great influence on the degree of deformation of plastic products. In addition. The use of multiple flexures can also shorten the plastic flow ratio (L/t), thereby making the melt density in the cavity more uniform and shrinking more uniform. For annular products, due to the different gate shapes, the same degree of the final product is also affected. When the entire plastic product can be filled under a smaller injection pressure, the smaller injection pressure can reduce the plastic's molecular orientation tendency and reduce its internal stress. Therefore, the deformation of plastic parts can be reduced.
(2) Cooling system.
During the injection process, the uneven cooling rate of plastic products will also affect the uneven shrinkage of the plastic parts. This difference in shrinkage leads to the generation of bending moments and warpage of the products. If the temperature difference between the mold cavity and the core used in the injection molding of flat products (such as mobile phone battery shells) is too large, the melt close to the cold mold cavity will quickly cool down, while the material close to the hot mold cavity The layer shell will continue to shrink, and the uneven shrinkage will cause the product to warp. Therefore, the cooling of the injection mold should pay attention to the balance between the temperature of the cavity and the core, and the temperature difference between the two should not be too large (in this case, two mold temperature machines can be considered).
In addition to considering the internal and external temperature of the product tends to balance. The temperature consistency on 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 rate of the plastic parts can be balanced, so that the shrinkage of the various parts is more uniform and effective Ground to prevent deformation. Therefore, the arrangement of cooling water holes on the mold is very important, including cooling water hole diameter d, water hole spacing b, pipe wall to cavity surface distance c and product wall thickness w. After the distance between the pipe wall and the cavity surface is determined, the distance between the cooling water holes should be as small as possible. In order to ensure the uniformity of the temperature of the molded rubber wall; the problem that should be paid attention to when determining the diameter of the cooling water hole is that no matter how big the mold is, the diameter of the water hole cannot be greater than 14mm, otherwise the coolant will hardly form turbulent flow. Generally, the diameter of the water hole 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, as shown in Figure 4-3 Shown. At the same time, since the temperature of the cooling medium rises with the increase of the length of the cooling water channel, the temperature difference between the cavity and the core of the mold is generated along the water channel. Therefore, the water channel length of each cooling circuit is required to be less than 2m. Several cooling circuits should be installed in a large mold, and the inlet of one circuit is located near the outlet of the other circuit. For long plastic parts, straight-through water channels should be used. Most of our current molds use S-shaped loops, which is not conducive to circulation and prolongs the cycle.
(3) Ejection system.
The design of the ejector system also directly affects the deformation of plastic products. If the ejection system is unbalanced, it will cause an imbalance in the ejection force and deform the plastic product. 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 rod cannot be too small to prevent the plastic product from being deformed due to excessive force per unit area (especially when the demolding temperature is high). The arrangement of the ejector rod should be as close as possible to the part with high demolding resistance. On the premise of not affecting the quality of plastic products (including use requirements, dimensional accuracy, appearance, etc.), as many items as possible should be set up 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 deep-cavity thin-walled plastic parts, due to the large demolding resistance and the softer materials, if only the single-mechanical ejection method is used, the plastic products will be deformed. Even top wear or folds cause plastic products to be scrapped. In this case, it will be better to switch to a combination of multiple elements or a combination of gas (hydraulic) pressure and mechanical ejection.
