Deformation of injection molded products
Deformation is one of the common defects in injection molding of thin-shell plastic parts, because it involves accurate prediction of warpage deformation, and the warpage deformation laws of injection molded parts of different materials and shapes vary greatly. When the amount of warpage exceeds the allowable error, it becomes a forming defect, which in turn affects product assembly.
Accurate prediction of warpage deformation of a large number of increasingly thin-walled parts (wall thickness less than 2mm) is a prerequisite for effective control of warpage defects. Warpage deformation analysis mostly adopts qualitative analysis, and measures are taken from product design, mold design and injection molding process conditions to avoid large warpage deformation as much as possible.
Cause Analysis
Mold
The position, form and number of gates of the injection 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 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, and the mold will freeze when filling The thickness of the layer is thinned, the internal stress is reduced, and the warping deformation is also greatly reduced. If only one center gate or one side gate is used, the molded plastic part will be distorted because the shrinkage rate in the diameter direction is greater than that in the circumferential direction; if multiple point gates are used instead, it can be effectively Prevent warping and deformation.
When spot casting is used for molding, also due to the anisotropy of plastic shrinkage, the position and number of gates have a great influence on the degree of deformation of plastic parts. Because 30% glass fiber reinforced PA6 is used, the obtained is A large injection molded part with a weight of 4.95kg, so there are many reinforcing ribs along the flow direction of the surrounding walls, so that each gate can be fully balanced.
In addition, the use of multiple gates can also shorten the plastic flow ratio (L/t), so that the material 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 small injection pressure. The lower injection pressure can reduce the molecular orientation tendency of plastics and reduce its internal stress, thus reducing the deformation of plastic parts.
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Mold temperature: The mold temperature has a great influence on the internal performance and apparent quality of the product. The temperature of the mold depends on the presence or absence of plastic crystallinity, the size and structure of the product, performance requirements, and other process conditions (melt temperature, injection speed and injection pressure, molding cycle, etc.)
Pressure control: The pressure in the injection molding process includes plasticizing pressure and injection pressure, and directly affects the plasticizing of plastics and product quality
The use of experimental methods to study the warpage of plastic products is mainly reflected in the study of the effects of material properties, product geometry and size, and injection molding process conditions on product warpage. A large number of experiments were designed to obtain the influence of gate geometry, packing parameters (holding pressure and holding time) and mold elasticity on the final size of the product.
PET was used as the polymer base, and the warpage characteristics of different materials and different wall thickness panels were studied. The relationship between the reinforcement ratio of 33% glass-reinforced fiber PA66 injection molded disk, the anisotropy of linear thermal expansion coefficient, the thickness of the product and the warpage was experimentally studied, and the concept of warpage index was proposed for the first time. Warpage characteristics, and the relationship between warpage index, warpage and fiber orientation state, and the relationship between yield and warpage index were studied.
The experimental method to study warpage deformation is often limited to a specific geometric shape, specific material and process conditions, and cannot fully consider the influence of many factors on warpage deformation, and cannot predict possible warpage during the product design stage. The size of the deformation. In actual use, the limitations of the empirical formula are also obvious, not only affected by the experimental conditions, but also related to many factors such as the processing method of the experimental data and the application conditions of the empirical formula, and an empirical formula is only suitable for the experimental conditions. close to the production process.
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shrink/warp
Since warping 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. On the basis of injection molding flow, holding pressure and cooling simulation, through experiments and linear regression methods, a model for predicting the shrinkage of injection molded products is proposed. On the basis of shrinkage prediction, the deformation of products is calculated through structural analysis simulation programs.
It is difficult to obtain products with high dimensional accuracy with materials with high shrinkage rate. To strive for high precision, amorphous resins and resins with consistent shrinkage in all directions should be used as much as possible. For many materials, the shrinkage of the product is measured under the conditions of changing the flow rate, holding pressure, holding time, mold temperature, filling time, product thickness and other parameters.
According to the test results, the shrinkage of the product is divided into three parts: volume shrinkage, uneven shrinkage caused by molecular orientation, and uneven shrinkage caused by unbalanced cooling. Shrinkage prediction methods for volumetric shrinkage, crystalline content, mold confinement, plastic orientation, etc., use flow and cooling analysis results to predict shrinkage strain.
Cooling System Design
During the injection process, the uneven cooling rate of the plastic part will also cause the uneven shrinkage of the plastic part. This difference in shrinkage will lead to the generation of bending moment and warp of the plastic part.
If the temperature difference between the mold cavity and core used in injection molding flat plastic parts is too large, the melt close to the surface of the cold mold cavity will cool down quickly, while the material layer close to the surface of the hot mold cavity will continue to shrink , the uneven shrinkage will warp the plastic part. Therefore, the cooling of the injection mold should pay attention to the temperature balance of the cavity and the core, and the temperature difference between the two should not be too large.
In addition to considering that the temperature on the inner and outer surfaces of the plastic part tends to be balanced, the temperature on each side of the plastic part should also be considered to be consistent, that is, when the mold is cooled, try to keep the temperature of the cavity and core uniform throughout, so that the cooling speed of the plastic part Balanced, so that the shrinkage is more uniform everywhere, effectively preventing deformation. Therefore, the arrangement of cooling water holes on the mold is very important. After the distance from the pipe wall to 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 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 2m. Several cooling circuits should be set up in large molds, and the inlet of one circuit is located near the outlet of the other circuit. For long plastic parts, a cooling circuit should be used to reduce the length of the cooling circuit, that is, to reduce the temperature difference of the mold, so as to ensure uniform cooling of the plastic parts.
The design of the ejection system also directly affects the deformation of the plastic part. If the layout of the ejection system is unbalanced, it will cause an imbalance in the ejection force and deform the plastic part. Therefore, when designing the ejection system, it should strive to balance with the demoulding 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 demoulding temperature is too high). The ejector pin should be arranged as close as possible to the part with the largest demoulding resistance. Under the premise of not affecting the quality of plastic parts (including use requirements, dimensional accuracy and appearance, etc.), as many ejector pins as possible should be installed to reduce the overall deformation of plastic parts.
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When soft plastic is used to produce large deep cavity and thin-walled plastic parts, due to the high demoulding resistance and soft material, if a single mechanical ejection method is completely adopted, the plastic parts will be deformed or even pushed through. Or the plastic part will be scrapped due to folding. It will be better to use a multi-component combination or a combination of gas (hydraulic) pressure and mechanical ejection.
Influence of Residual Thermal Stress on Warping and Deformation of Products
In the injection molding process, residual thermal stress is an important factor that causes warping and deformation, and has a greater impact on the quality of injection molded products. Since the influence of residual thermal stress on product warpage is very complex, mold designers can analyze and predict it with the help of injection molding CAE software.
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 released from the mold, it will warp and deform under the action of uneven internal stress. Therefore, many scholars analyze and calculate the internal stress and warpage of products from the perspective of mechanics. In some foreign literatures, warpage is considered to be caused by residual stress generated by uneven shrinkage.
In the cooling stage of injection molding, when the temperature is higher than the glass transition temperature, the plastic is a viscoelastic fluid, accompanied by stress relaxation: when the temperature is lower than the glass transition temperature, the plastic becomes solid. This liquid-solid phase transition and stress relaxation of plastics during cooling has a great influence on the accurate prediction of residual stress and residual deformation of products.
The phase transition and stress relaxation behavior of plastics from liquid to solid during the cooling phase. For the uncured area, the plastic exhibits a viscous behavior, which is described by a viscous fluid model; for the cured area, the plastic exhibits a viscoelastic behavior, which is described by a standard linear solid model, using a visco-elastic phase transition model and a two-dimensional finite element method to predict thermal residual stresses and corresponding warpage deformations.
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Influence of plasticizing stage on product warping deformation
In the plasticizing stage, the glass particles are transformed into a viscous fluid state to provide the melt required for filling the mold. In this process, the temperature difference of the polymer in the axial direction and radial direction (relative to the screw) will cause stress in the plastic; in addition, the injection pressure, speed and other parameters of the injection machine will greatly affect the degree of molecular orientation during filling. , causing warping deformation.
Use low speed at the beginning of injection, high speed when filling the mold cavity, and low speed injection when filling is near the end. Through the control and adjustment of the injection speed, various undesirable phenomena such as burrs, spray marks, silver bars or burnt marks can be prevented and improved.
The multi-stage injection control program can reasonably set the multi-stage injection pressure, injection speed, holding pressure and melting method according to the structure of the runner, the form of the gate and the structure of the injection molded part, which is conducive to improving the plasticizing effect and improving Product quality, reduce defect rate and prolong mold/machine life.
By controlling the oil pressure, screw position, and screw speed of the injection molding machine through a multi-level program, it can seek to improve the appearance of the molded parts, improve the corresponding measures for shrinkage, warping and burr, and reduce the size unevenness of each injection molded part of each mold. .
By controlling the oil pressure, screw position and screw speed of the injection molding machine through a multi-level program, it can seek to improve the appearance of the molded parts, improve the corresponding measures for shrinkage, warping and burr, and reduce the unevenness of the size of each injection molded part of each mold. .
Influence of mold filling and cooling stages on product warpage
Under the action of injection pressure, the molten plastic is filled into the mold cavity, cooled and solidified in the cavity, which is the key link of injection molding. In this process, temperature, pressure, and speed are coupled with each other, which has a great impact on the quality and production efficiency of plastic parts.
Higher pressures and flow velocities generate high shear rates, which cause differences in the orientation of molecules parallel to and perpendicular to the direction of flow, creating a "freezing effect". The "freezing effect" will generate freezing stress and form the internal stress of the plastic part. The influence of temperature on warping deformation is reflected in the following aspects.
A. The temperature difference between the upper and lower surfaces of plastic parts will cause thermal stress and thermal deformation;
B. The temperature difference between different areas of the plastic part will cause uneven shrinkage between different areas;
C. Different temperature states will affect the shrinkage of plastic parts.
Influence of the demoulding stage on the warping deformation of the product
Plastic parts are mostly glassy polymers during the process of leaving the cavity and cooling to room temperature. Unbalanced demoulding force, unstable movement of the ejection mechanism or improper ejection area of the demoulding can easily deform the product. 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, resulting in warping deformation.
True 3D approach to calculate residual stresses and final shape (shrinkage and warpage). They considered the influence of the packing stage, divided the product into three layers, and analyzed the residual stress and deformation by a three-dimensional mesh. , a numerical simulation model for the induced residual stress and deformation after the packing phase is proposed.
When calculating the residual stress, a thermoviscoelastic model (including volume relaxation) is used. The finite element method it adopts is based on the shell theory composed of planar elements, which is suitable for thin-walled injection molded products with complex shapes.
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The solution to the effect of shrinkage of injection molded products on warping deformation
The direct cause of warpage of injection molded products is the uneven shrinkage of plastic parts. If the impact of shrinkage during the filling process is not considered in the mold design stage, the geometric shape of the product will differ greatly from the design requirements, and severe deformation will cause the product to be scrapped. 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 shrinkage between the upper and lower surfaces of the plastic part, resulting in warping deformation.
For warpage analysis, shrinkage itself is not important, but the difference in shrinkage is important. In the injection molding process, the shrinkage rate of the plastic in the flow direction is greater than that in the vertical direction due to the arrangement of the polymer molecules along the flow direction during the injection molding stage of the molten plastic, resulting in warping deformation of the injection molded part. Generally, uniform shrinkage only causes changes in the volume of plastic parts, and only uneven shrinkage can cause warping 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, and its shrinkage rate is also larger than that of amorphous plastics. The superposition of the large shrinkage rate of crystalline plastics and the anisotropy of shrinkage leads to Crystalline plastics have a much greater tendency to warp than amorphous plastics.
The multi-stage injection molding process selected based on the analysis of the geometric shape of the product: because the cavity of the product is deep and the wall is thin, the mold cavity forms a long and narrow flow channel, and the melt must flow through this part very quickly Otherwise, it is easy to cool and solidify, which will lead to the danger of filling the mold cavity, so 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 large inertial impact, resulting in energy loss and overflow. At this time, the melt must be slowed down and the filling pressure must be reduced. Maintain the so-called 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 requirements for multi-stage injection speed and pressure on the injection molding process.
Solution to product warping and deformation due to residual thermal stress
The velocity of the fluid surface should be constant. Fast injection should be used to prevent the melt from freezing during the injection process. The shot speed setting should allow for fast filling in critical areas (such as runners) while slowing down at the water inlet. The injection speed should ensure that the mold cavity is filled and stops immediately to prevent overfilling, flash and residual stress.
