What is a warpage defect in injection molding?
Warpage defect in injection molding is one of the common defects in injection molding of thin-walled plastic parts.
When the warpage deformation exceeds the allowable error, it becomes a molding defect, which affects the product assembly. Accurate analysis of the warpage defect in injection molding of a large number of thin-walled products is a prerequisite for effective control of warpage defects.
Most of the warpage defect in injection molding analysis adopts qualitative analysis, and measures are taken from the aspects of product design, mold design, and injection molding process conditions to minimize the degree of warpage deformation.
1. The influence of mold gate on warpage defect in injection molding
The position, form, and number of gates of the injection mold will affect the filling state of the plastic in the mold cavity, resulting in the deformation of the plastic part.
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; conversely, the shorter the flow distance, the shorter the flow time from the gate to the end of the flow of the workpiece, and the frozen layer will be frozen during filling. The thickness is reduced, the internal stress is reduced, and the warpage deformation will be greatly reduced for this reason.
If only one center gate or one side gate is used, because the shrinkage rate in the diameter direction is greater than the shrink rate in the circumferential direction, the molded plastic part will be deformed; if you switch to multiple point gates, it will be effective to prevent warping and deformation.
2. The influence of mold cooling on warpage defect in injection molding
The uneven cooling rate of the plastic will also cause uneven shrinkage of the plastic. This difference in shrinkage results in the generation of bending moments and warpage of the plastic parts.
For example, in the injection molding of flat plastic parts, the temperature difference between the mold cavity and core used is too large, because the melt close to the cold mold cavity surface cools down quickly, while the material layer close to the hot mold cavity surface will continue shrinkage, uneven shrinkage will warp the plastic part.
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.
The arrangement of cooling water holes on the mold is also crucial. After the distance between the pipe wall and the surface of the cavity 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 temperature difference between the cavity and core of the mold is generated along the water channel. Therefore, the length of the water channel of each cooling circuit is required to be less than 2m.
3. The influence of mold ejection on warpage defect in injection molding
The design of the mold ejection also directly affects the deformation of the plastic part. If the ejection system is unbalanced, the ejection force will be unbalanced and the plastic part will be deformed. Therefore, when designing the ejection system, the stress should be balanced with the ejection 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. 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 the plastic parts, as many ejectors as possible should be set up to reduce the overall deformation of the plastic parts.
When soft plastics are used to produce large-scale deep-cavity thin-walled plastic parts, due to the large demolding resistance and softer materials, if a single mechanical ejection method is used completely, the plastic parts will be deformed or even topped.
Or the plastic parts will be scrapped due to folding, such as multi-element combination or the combination of gas (hydraulic) pressure and mechanical ejection, the effect will be better.
4. The influence of plasticizing stage on warpage defect in injection molding
In the plasticizing stage, the glassy pellets are transformed into a viscous fluid state to provide the melt required for mold filling. In this process, the temperature difference between the polymer temperature in the axial and radial directions will cause stress to the product; in addition, the injection pressure, speed, and other parameters of the injection molding machine will greatly affect the degree of molecular orientation during filling, which will cause warpage deformed.
Multi-level injection control can reasonably set the multi-stage injection pressure, injection speed, holding pressure, and sol method according to the structure of the runner, the form of the gate, and the structure of the injection molded part, which is beneficial to prevent warpage defect in injection molding.
5. The effect of product shrinkage on warpage defect in injection molding
In the injection molding process, during the injection and filling stage of the molten plastic, the shrinkage rate of the plastic in the flow direction is greater than the shrink rate in the vertical direction due to the arrangement of polymer molecules in the flow direction, which causes warpage and deformation of the injection molded part.
The multi-stage injection process selected on the basis of the analysis of the product geometry, due to the thin wall of the product and the longer flow length, the melt flow must pass through quickly, otherwise, it is easy to cool and solidify, and high-speed injection should be set for this.
However, the high-speed injection will bring a lot of kinetic energy to the melt. When the melt flows to the end, it will produce a large inertial impact, resulting in energy loss and edge overflow.
At this time, the melt must slow down the flow rate and reduce the filling pressure. It is necessary to maintain the so-called holding pressure so that the melt replenishes the shrinkage of the melt in the mold cavity before the gate solidifies, which puts forward multi-level injection speed and pressure requirements for the injection molding process.
6. The influence of residual thermal stress on warpage defect in injection molding
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 out of the mold, under the action of the uneven internal stress, warping deformation occurs.
The phase transformation and stress relaxation behavior of the plastic from liquid to solid during the cooling stage. For the uncured area, the plastic exhibits viscous behavior, which is described by a viscous fluid model; for the cured area, the plastic exhibits viscoelastic behavior, which is described by a standard linear solid model to describe.
Therefore, mold developers or product developers can use visco-elastic phase transition models and two-dimensional finite element methods to predict thermal residual stress and corresponding warpage deformation.
The velocity of the fluid surface should be constant. Quick glue injection should be used to prevent the melt from freezing during the glue injection process. The injection speed setting should take into account the rapid filling in the critical area (such as the runner) while slowing down the speed at the water inlet. The injection speed should be guaranteed to stop immediately after the cavity is filled to prevent over-filling, flashing, and residual stress.
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