Analysis and solutions to warping deformation of plastic products 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 warpage, and it is often difficult to solve it by relying on process parameters alone. Combining relevant information and actual work experience, the following is a brief analysis of the factors affecting the warpage of injection molded products.
- The influence of mold structure on the warpage of injection molded products.
In terms of molds, the factors that affect the deformation of plastic parts mainly include the pouring system, cooling system and ejection system. - Pouring system
The position, form and number of the injection mold gate will affect the filling state of the plastic in the mold cavity, thereby causing deformation of the plastic part.
The longer the flow distance, the greater the internal stress caused by the flow and shrinkage between the frozen layer and the central flow layer; conversely, the shorter the flow distance, the shorter the flow time from the gate to the end of the flow of the part, the thinner the thickness of the frozen layer during mold filling, the lower the internal stress, and the warpage will be greatly reduced. For some flat plastic parts, if only one central gate is used, the plastic parts will be distorted after molding because the shrinkage rate in the diameter direction is greater than the shrinkage rate in the circumferential direction; if multiple point gates or film gates are used instead, warping deformation can be effectively prevented.
When point gates are used for molding, the position and number of gates also have a great influence on the deformation degree of the plastic parts due to the anisotropy of plastic shrinkage.
In addition, the use of multiple gates can also shorten the flow ratio (L/t) of the plastic, so that the melt density in the mold cavity is more uniform and the shrinkage is more uniform. At the same time, the entire plastic part can be filled under a smaller injection pressure. The smaller injection pressure can reduce the molecular orientation tendency of the plastic and reduce its internal stress, thereby reducing the deformation of the plastic part. - Cooling system
During the injection process, the uneven cooling speed of the plastic part will also form uneven shrinkage of the plastic part. This shrinkage difference leads to the generation of bending moment and causes the plastic part to warp.
If the temperature difference between the mold cavity and the core used in injection molding of flat plastic parts (such as mobile phone battery shells) is too large, the melt close to the cold mold cavity surface will cool down quickly, while the material layer close to the hot mold cavity surface will continue to shrink. The uneven shrinkage will cause the plastic part to warp. Therefore, the cooling of the injection mold should pay attention to the temperature of the cavity and the core tending to balance, and the temperature difference between the two should not be too large (at this time, two mold temperature controllers can be considered).
In addition to considering the temperature of the inner and outer surfaces of the plastic part tending to balance, the temperature of each side of the plastic part should also be considered. That is, when the mold is cooled, the temperature of the cavity and the core should be kept uniform as much as possible, so that the cooling speed of each part of the plastic part is balanced, so that the shrinkage of each part is more uniform, and deformation is effectively prevented. Therefore, the arrangement of cooling water holes on the mold is crucial. After the distance from the tube wall to the cavity surface is determined, the distance between the cooling water holes should be as small as possible to ensure that the temperature of the cavity wall is uniform. At the same time, since the temperature of the cooling medium increases with the increase of the length of the cooling water channel, the cavity and core of the mold will have a temperature difference along the water channel. Therefore, the water channel length of each cooling circuit is required to be less than 2 meters. Several cooling circuits should be set in large molds, and the inlet of one circuit is located near the outlet of another circuit. For long plastic parts, straight-through water channels should be used. (Most of our molds use S-shaped circuits, which are not conducive to circulation and extend the cycle.) - Ejector system
The design of the ejector system also directly affects the deformation of plastic parts. If the ejector system is unbalanced, it will cause an imbalance in the ejection force and cause the plastic parts to deform. Therefore, when designing the ejector system, we should strive to balance it with the demolding resistance. In addition, the cross-sectional area of the ejector rod should not be too small to prevent the plastic part from being deformed due to excessive force per unit area (especially when the demolding temperature is too high). The ejector rod should be arranged as close as possible to the part with large demolding resistance. Without affecting the quality of the plastic part (including use requirements, dimensional accuracy and appearance, etc.), as many ejector rods as possible should be set to reduce the overall deformation of the plastic part (this is the reason for replacing the ejector rod with an ejector block).
When using soft plastics (such as TPU) to produce deep-cavity, thin-walled plastic parts, due to the large demoulding resistance and the soft material, if a single mechanical ejection method is used, the plastic parts will be deformed, even penetrated or folded, causing the plastic parts to be scrapped. If a combination of multiple components or gas (liquid) pressure and mechanical ejection is used, the effect will be better (will be used later). - The influence of the plasticization stage on the warpage of the producthttps://solidcomould.com/product-category/injection-mould/office-furniture-mould/
The plasticization stage is the process of converting glassy particles into viscous melt (the three-state change of raw material plasticization was discussed during the training). In this process, the temperature difference of the polymer in the axial and radial directions (relative to the screw) will cause stress in the plastic; in addition, the injection pressure, rate and other parameters of the injection machine will greatly affect the orientation of the molecules during filling, thereby causing warpage. - The influence of the filling and cooling stages on the warpage of the product
The molten plastic is filled into the mold cavity under the action of the injection pressure and cooled and solidified in the cavity. This process is the key link of injection molding. In this process, the coupling of temperature, pressure and speed has a great impact on the quality and production efficiency of plastic parts. Higher pressure and flow rate will produce high shear rate, which will cause differences in molecular orientation parallel to the flow direction and perpendicular to the flow direction, and produce a “freezing effect”. The “freezing effect” will produce frozen stress and form internal stress of the plastic part. The influence of temperature on warpage is reflected in the following aspects:
(1) The temperature difference between the upper and lower surfaces of the plastic part will cause thermal stress and thermal deformation;
(2) The temperature difference between different areas of the plastic part will cause uneven shrinkage between different areas;
(3) Different temperature conditions will affect the shrinkage rate of the plastic part.
V. The influence of the demolding stage on the warpage of the product
Plastic parts are mostly glassy polymers in the process of being separated from the cavity and cooled to room temperature. Unbalanced demolding force, unstable movement of the ejection mechanism or improper demolding ejection area can easily cause product deformation (as mentioned above). At the same time, the stress “frozen” in the plastic part during the filling and cooling stages will be released in the form of “deformation” due to the loss of external constraints, thus causing warpage.http://Analysis and solutions to warping deformation of plastic products - The influence of shrinkage of injection molded products on warpage
The direct cause of warpage of injection molded products is the uneven shrinkage of plastic parts. If the influence of shrinkage during the filling process is not considered in the mold design stage, the geometric shape of the product will be very different from the design requirements, and severe deformation will cause the product to be scrapped (i.e. the problem of shrinkage rate). In addition to deformation during the filling stage, the temperature difference between the upper and lower walls of the mold will also cause the difference in shrinkage between the upper and lower surfaces of the plastic part, resulting in warpage. For warpage analysis, shrinkage itself is not important, but the difference in shrinkage is important. During the injection molding process, the shrinkage rate of the plastic in the flow direction is greater than the shrinkage rate in the vertical direction due to the arrangement of polymer molecules along the flow direction during the injection and filling stage, which causes the injection molded part to warp (i.e. anisotropy). Generally, uniform shrinkage only causes changes in the volume of plastic parts, and only uneven shrinkage can cause warpage. The difference between the shrinkage rates of crystalline plastics in the flow direction and the vertical direction is greater than that of non-crystalline plastics, and their shrinkage rate is also greater than that of non-crystalline plastics. The large shrinkage of crystalline plastics and the anisotropy of their shrinkage superimpose on each other, which leads to a much greater tendency of crystalline plastic parts to warp and deform than non-crystalline plastics.
VII. The influence of residual thermal stress on product warpage
During the injection molding process, residual thermal stress is an important factor causing warpage and has a great influence on the quality of injection molded products. Since the influence of residual thermal stress on product warpage is very complicated, it will not be elaborated here.
VIII. The influence of metal inserts on product warpage
For injection molded products with inserts, since the shrinkage rate of plastics is much greater than that of metals, it is easy to cause distortion (some even cracking); to reduce this situation, the metal parts can be preheated (generally not less than 100°C) before being put into production.
