Warpage deformation is one of the common defects in injection molding of thin shell plastic parts. Most of the warpage deformation analysis adopts qualitative analysis, and measures are taken from the aspects of product design, mold design and injection molding process conditions to avoid large warpage deformation as far as possible.For example, some common plastic products, plastic shoe racks, plastic clips, plastic brackets, etc
In terms of mold, the position, form and number of gates of injection mold will affect the filling state of plastic in the mold cavity, resulting in the deformation of plastic parts. Since warpage deformation is related to uneven shrinkage, the relationship between shrinkage and product warpage is analyzed by studying the shrinkage behavior of different plastics under different process conditions. It includes the influence of residual thermal stress on the warpage deformation of products, and the influence of plasticization stage, mold filling and cooling stage and demoulding stage on the warpage deformation of products.
Effect of shrinkage of injection molded products on warping deformation solution:
The direct cause of warpage deformation of injection molded products lies in the uneven shrinkage of plastic parts. For warpage analysis, shrinkage itself is not important. What matters is the difference in shrinkage. In the process of injection molding, due to the arrangement of polymer molecules along the flow direction, the shrinkage of molten plastics in the flow direction is greater than that in the vertical direction, resulting in warpage and deformation of injection parts. Generally, uniform shrinkage only causes changes in the volume of plastic parts, and only uneven shrinkage can cause warpage deformation. The difference between the shrinkage rate of crystalline plastics in the flow direction and vertical direction is larger than that of amorphous plastics, and its shrinkage rate is also larger than that of amorphous plastics. After the superposition of the large shrinkage rate of crystalline plastics and the anisotropy of shrinkage, the tendency of warping deformation of crystalline plastics is much larger than that of amorphous plastics.
Multistage injection molding process selected based on the analysis of product geometry: due to the deep cavity and thin wall of the product, the mold cavity is a long and narrow channel. When the melt flows through this part, it must pass quickly, otherwise it is easy to cool and solidify, which will lead to the risk of filling the mold cavity. High speed injection should be set here. However, high-speed injection will bring a lot of kinetic energy to the melt. When the melt flows to the bottom, it will produce a great inertial impact, resulting in energy loss and edge overflow. At this time, it is necessary to slow down the flow rate of the melt and reduce the mold filling pressure, and maintain the commonly known pressure holding pressure (secondary pressure, follow-up pressure) to make the melt supplement the shrinkage of the melt into the mold cavity before the gate solidifies, which puts forward the requirements of multi-stage injection speed and pressure for the injection process.
Solution to warpage and deformation of products caused by residual thermal stress:
The velocity of the fluid surface should be constant. Rapid glue injection shall be adopted to prevent the melt from freezing during glue injection. The setting of glue injection speed should take into account the rapid filling in the critical area (such as flow channel) and slowing down at the water inlet. The glue injection speed should ensure that it stops immediately after the mold cavity is filled to prevent overfilling, flash and residual stress.
Post time: May-17-2022