1. Warping and deformation
It is one of the common defects in the injection molding of thin-walled plastic parts, and when the warpage deformation exceeds the allowable error, it becomes a molding defect, which in turn affects the product assembly. Accurate analysis of warpage deformation of a wide and increasing number of thin-walled products is a prerequisite for effective control of warpage defects. Warpage deformation analysis mostly adopts qualitative analysis, and measures are taken from the aspects of product design, mold design and injection molding process conditions to minimize the warpage deformation.
2. The influence of mold gate on warpage
The location, form, and number of gates of the mold gate will affect the filling state of the plastic in the mold cavity, resulting in deformation of the plastic part. The longer the flow distance, the greater the internal stress caused by the flow and contraction 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 thickness of the frozen layer is reduced during filling, the internal stress is reduced, and the warpage deformation is greatly reduced. If only one center gate or one side gate is used, the molded plastic parts will be twisted and deformed because the shrinkage in the diameter direction is greater than the shrinkage in the circumferential direction; If multiple point gates are used instead, warpage deformation can be effectively prevented.
3. The influence of mold ejection on warpage
The design of the mold ejection also directly affects the deformation of the plastic part. If the ejection system is unbalanced, it will cause an imbalance in the ejection force and deform the plastic parts. Therefore, the stress should be balanced with the release resistance when designing the ejection system. 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 release temperature is too high). The arrangement of the ejector bar should be as close as possible to the part with high demolding resistance. Under the premise of not affecting the quality of plastic parts (including use requirements, dimensional accuracy and appearance, etc.), as many ejectors as possible should be set to reduce the overall deformation of plastic parts. When using soft plastics to produce large deep-cavity thin-walled plastic parts, due to the large resistance of demolding and the softer material, if a single mechanical ejection method is completely adopted, the plastic parts will be deformed, and even the top penetration or folding will cause the plastic parts to be scrapped, such as switching to the combination of multi-element combination or gas (liquid) pressure and mechanical ejection The effect will be better.
4. The influence of plasticizing stage on warpage
In the plasticizing stage, the glass grains are converted into a viscous flow state, providing the melt required for mold filling. In this process, the temperature difference between the axial and radial directions of the temperature of the polymer will cause stress to the product; In addition, the injection pressure, rate and other parameters of the injection molding machine will greatly affect the orientation of the molecules during filling, which will cause warpage deformation. Multi-stage injection control can reasonably set the multi-stage injection pressure, injection speed, holding pressure pressure and sol mode according to the structure of the flow channel, the form of the gate and the structure of the injection molded parts, which is conducive to preventing warpage deformation.
5. The solution to product shrinkage affecting warpage
The shrinkage of the product itself is not important for warping, what is important is the difference in shrinkage. In the injection molding process, the molten plastic in the injection filling stage due to the arrangement of polymer molecules in the flow direction makes the shrinkage rate of the plastic in the flow direction greater than the shrinkage rate in the vertical direction, and the injection molded parts are warped and deformed. Generally, uniform shrinkage will only cause changes in the volume of plastics, and only uneven shrinkage will cause warpage deformation. The difference between the shrinkage rate of crystalline plastics in the flow direction and the vertical direction is larger than that of amorphous plastics. The multi-stage injection process selected on the basis of the geometry analysis of the product, due to the long flow ratio of the thin wall of the product, the melt flow must pass through quickly, otherwise it is easy to cool and solidify, and high-speed injection should be set. However, high-speed injection will bring great kinetic energy to the melt, and the melt flow to the bottom will produce a large inertial impact, resulting in energy loss and overflow phenomenon, at this time, the melt must slow down the flow rate, reduce the mold filling pressure and maintain the commonly known holding pressure so that the melt in the gate before solidifying to supplement the shrinkage of the melt in the mold cavity, which puts forward multi-stage injection speed and pressure requirements for the injection molding process.
6. The solution to warping the product due to residual thermal stress
During the molding process of the plastic melt, due to the uneven orientation and shrinkage, the internal stress is uneven, so after the product is molded, warping and deformation occurs under the action of uneven internal stress. Phase transformation and stress relaxation behavior of plastics from liquid to solid in the cooling stage, for the uncured area, the plastic shows viscous behavior, which is described by the viscous fluid model, and the viscoelastic behavior of the plastic in the cured area is described by the standard linear solid model. Therefore, mold developers or product developers can use viscoelastic phase conversion models and 2D finite element methods to predict thermal residual stresses and corresponding warpage deformations. The velocity of the fluid surface should be constant. Rapid injection should be used to prevent the melt from freezing during the injection process. The injection speed setting should take into account that critical areas (e.g. runners) are filled quickly while slowing down at the inlet level. The injection speed should be stopped immediately after the cavity is filled to prevent overfilling, flashing and residual stress.
