1. Warping deformation means that the shape of injection molded products deviates from the shape of mold cavity, which is one of the common defects of plastic products. There are many reasons for warping deformation, and process parameters alone are often not enough to solve them. Based on the relevant data and practical work experience, the factors affecting the warpage of injection molded products are briefly analyzed.
Secondly, the influence of mold structure on warpage of injection molded products.
In the mold, the main factors affecting the deformation of plastic parts are gating system, cooling system and ejection system.
1. gating system
The position, form and number of injection mold gates will affect the filling state of plastic in the mold cavity, which will lead to the deformation of plastic parts. The longer the flow distance, the greater the internal stress caused by the flow and feeding between the frozen layer and the central flow layer; On the contrary, the shorter the flow distance, the shorter the flow time from the gate to the flow end of the part, the thinner the frozen layer, the lower the internal stress and the smaller the warping deformation during mold filling. Some flat plastic parts, if only one central gate is used, will be deformed because the shrinkage in the diameter direction is greater than that in the circumferential direction. If multi-point grid or thin film grid is used instead, warping deformation can be effectively prevented. When using point gate molding, due to the anisotropy of plastic shrinkage, the position and number of gates have great influence on the deformation degree of plastic parts. In addition, the use of multiple gates can also shorten the flow ratio (L/t) of plastics, thus making the melt density and shrinkage in the mold cavity more uniform. At the same time, the whole plastic part can be filled with less injection pressure. The smaller injection pressure can reduce the molecular orientation tendency and internal stress of plastics, thus reducing the deformation of plastic parts.
2. Cooling system
In the process of injection molding, the uneven cooling speed of plastic parts will also lead to uneven shrinkage of plastic parts, which will lead to bending moment and warping of plastic parts. The temperature difference between the mold cavity and the core used in the injection molding of flat plastic parts (such as mobile phone battery case) is too large, and the melt near the cold mold cavity surface will cool quickly, while the material layer near the hot mold cavity surface will continue to shrink, and uneven shrinkage will warp the plastic parts. Therefore, the cooling of injection mold should pay attention to the temperature balance between cavity and core, and the temperature difference between them should not be too large (at this time, two mold temperature machines can be considered). In addition to the temperature balance of the internal and external surfaces of plastic parts, the temperature consistency of all surfaces of plastic parts should also be considered, that is, when the mold is cooled, the temperature of cavity and core should be kept as consistent as possible, so that the cooling speed of plastic parts can be balanced everywhere, so that the shrinkage of plastic parts can be more uniform and deformation can be effectively prevented. Therefore, the arrangement of cooling water holes on the mold is very important. 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 to ensure the uniform temperature of the cavity wall. At the same time, because the temperature of the cooling medium increases with the increase of the length of the cooling water channel, the temperature difference between the cavity and the core of the mold appears 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 the large mold, and the inlet of one circuit is located near the outlet of the other. For long plastic parts, direct waterway should be adopted. (And most of our molds use S-rings-which is not conducive to circulation and prolongs the cycle. The design of ejection system also directly affects the deformation of plastic parts. If the layout of the ejection system is unbalanced, the ejection force will be unbalanced and the plastic parts will be deformed. Therefore, when designing the ejection system, we should strive to strike a balance with the demoulding resistance. In addition, the cross-sectional area of the ejector pin should not be too small to prevent the plastic parts from being deformed due to excessive stress per unit area (especially when the demoulding temperature is too high). The ejector pin should be arranged as close as possible to the parts with high demoulding resistance. On the premise of not affecting the quality of plastic parts (including use requirements, dimensional accuracy and appearance, etc. ), set as many thimbles as possible to reduce the overall deformation of plastic parts (this is why thimbles are used instead of thimbles). When using high-quality plastics (such as TPU) to produce thin-walled plastic parts with deep cavities, the plastic parts will be deformed, even pushed out or folded if a single mechanical ejection method is completely adopted, which will cause the plastic parts to be scrapped. For example, it is better to use a multi-component combination or a combination of gas (liquid) pressure and mechanical ejection (which will be used later).
Thirdly, the influence of plasticizing stage on warping deformation of products.
The plasticizing stage is the process of transforming glassy particles into viscous melts (the three-state change of raw material plasticization is mentioned in the training). In this process, the temperature difference between the polymer in the axial direction and the radial direction (relative to the screw) will make the plastic stress; In addition, the injection pressure, speed and other parameters of the injection machine will also greatly affect the orientation of molecules in the filling process, and then cause warping deformation.
Fourthly, the influence of filling and cooling stages on the warping deformation of products.
Under the action of injection pressure, molten plastic is filled into the mold cavity and cooled and solidified in the mold cavity. This process is the key link of injection molding. In this process, temperature, pressure and speed are coupled, which has great influence on the quality and production efficiency of plastic parts. Higher pressure and flow rate will produce higher shear rate, which will cause the difference of molecular orientation parallel to the flow direction and perpendicular to the flow direction, and at the same time produce "freezing effect", which will produce freezing stress and form internal stress of plastic parts. The influence of temperature on warping deformation is reflected in the following aspects:
(1) The temperature difference between the upper and lower surfaces of plastic parts will cause thermal stress and thermal deformation;
(2) The temperature difference in different areas of plastic parts will cause uneven shrinkage between different areas;
(3) Different temperature states will affect the shrinkage of plastic parts.
5. Effect of demoulding stage on warping deformation of products
Plastic parts are mostly glassy polymers when they leave the cavity and cool to room temperature. Unbalanced demoulding force, unstable movement of ejection mechanism or inappropriate ejection area are all easy to deform the product (as mentioned above). At the same time, the stress "frozen" in plastic parts during mold filling and cooling will be released in the form of "deformation" due to the loss of external constraints, which will lead to warping deformation.
6. Effect of shrinkage of injection molded products on warpage.
The direct cause of warpage of injection molded products lies in uneven shrinkage of plastic parts. If the influence of shrinkage is not considered in the mold design stage, the geometric shape of the product will be greatly different from the design requirements, and serious deformation will lead to the scrapping of the product (that is, shrinkage problem). In addition to the deformation caused by the filling stage, the temperature difference between the upper and lower walls of the mold will also cause the difference in the shrinkage of the upper and lower surfaces of the plastic parts, resulting in warping deformation. For warping analysis, shrinkage itself is not important, but the difference of shrinkage is very important. In the process of injection molding, due to the arrangement of polymer molecules in the flow direction, the shrinkage of molten plastic in the flow direction is greater than that in the vertical direction, which leads to the warpage (i.e. anisotropy) of injection molded parts. Generally, uniform shrinkage will only cause the volume change of plastic parts, and only uneven shrinkage will cause warping deformation. The difference between the shrinkage rates of crystalline plastics in the flow direction and the vertical direction is greater than that of amorphous plastics, and its shrinkage rate is also greater than that of amorphous plastics. After the large shrinkage of crystalline plastics and the anisotropic superposition of shrinkage, the warping deformation trend of crystalline plastics is greater.
Seven. Influence of Residual Thermal Stress on Warping Deformation of Products
In the process of injection molding, residual thermal stress is an important factor causing warping deformation, which has a great influence on the quality of injection molded products. Because the influence of residual thermal stress on product warping deformation is very complicated, I won't go into details here.
Eight. Influence of Metal Inserts on Warping Deformation of Products
Because the shrinkage rate of plastic is much greater than that of metal, the injection products with inserts are prone to distortion (some even crack); In order to reduce this situation, metal parts can be preheated before production (generally not lower than 100℃).
Nine. conclusion
There are many factors that affect the warpage of injection molded products. The structure of the mold, the thermophysical properties of plastic materials, the conditions and parameters of molding process all have different effects on the warping deformation of the products. Therefore, the above factors must be considered comprehensively when dealing with the warping deformation of injection molded products.