Injection defects | thin-wall Injection Molding Parts warping deformation defect solution

Warping deformation is one of the common defects in the injection molding of thin shell plastic parts, because it involves the accurate prediction of the amount of warping deformation, and the warping deformation law of different materials and different shapes of injection parts is very different. When the amount of warping deformation exceeds the allowable error, it becomes a forming defect, and then affects the product assembly.

Accurate prediction of warpage deformation of various kinds of increasing thin-walled parts (wall thickness less than 2mm) is the premise of effective control of warpage defects. The warping deformation analysis mostly adopts qualitative analysis, and measures are taken from product design, Mold Design and injection molding process conditions to avoid large warping deformation as far as possible.

Cause analysis

Mold aspect

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 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, the thinner the frozen layer thickness during mold filling, the lower the internal stress, and the warpage deformation will be greatly reduced. If only one center gate or one side gate is used, because the shrinkage rate in the diameter direction is greater than the shrinkage rate in the circumference direction, the molded plastic part will be distorted and deformed; Warping deformation can be effectively prevented by using multi-point gate.

When the point casting molding, also due to the anisotrophicity of plastic shrinkage, the location and number of the gate have a great impact on the degree of deformation of the plastic parts. Because the use of 30% glass fiber reinforced PA6, and the obtained weight is 4.95kg of large injection parts, so there are many reinforcing ribs along the flow direction of the surrounding wall. Full balance is achieved for each gate.

In addition, the use of multiple gates can also shorten the flow ratio of plastics (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 whole plastic part can be filled with less injection pressure. The smaller injection pressure can reduce the molecular orientation of the plastic, reduce its internal stress, and thus reduce the deformation of the plastic parts.

Mold temperature: Mold temperature has a great influence on the internal performance and apparent quality of the product. The mold temperature depends on whether the plastic is crystalline, the size and structure of the product, the 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 two kinds of plasticizing pressure and injection pressure, and directly affects the plasticization and product quality of plastics

The study of warping deformation of plastic products by experimental method is mainly reflected in the study of material properties, product geometry and size, injection molding process conditions on the influence of warping deformation of products. A large number of experiments have been designed to obtain the influence of gate geometry, holding pressure parameters (holding pressure and holding time) and mold elasticity on the final size of the product.

The warping properties of plates with different materials and wall thickness were studied by using PET as a polymer base. The relationship between reinforcement ratio, anisotropy of linear thermal expansion coefficient, product thickness and warpage of 33% glass reinforced fiber PA66 injection molded disk was studied experimentally. The concept of warpage index was proposed for the first time and the warpage characteristics of PA66 plastic products were studied by using the warpage index. The relationship between warp index, warp and fiber orientation, and the relationship between yield and warp index are also studied.

Experimental research on warping deformation is often limited to a specific geometric shape, a specific material and process conditions, and can not fully consider the influence of many factors on warping deformation, and can not predict the size of possible warping deformation in the product design stage. In actual use, the limitations of the empirical formula are also obvious, not only affected by experimental conditions, but also related to many factors such as the processing method of experimental data and the application conditions of the empirical formula, and an empirical formula is only applicable to the production process that is quite close to the experimental situation.

Shrinkage/warping

Since warping deformation is related to non-uniform shrinkage, the relationship between shrinkage and warping is analyzed by studying the shrinkage behavior of different plastics under different process conditions. Based on the simulation of injection molding flow, pressure holding and cooling, a model for predicting the shrinkage of injection molding products is put forward through experiment and linear regression method.

It is difficult to obtain products with high dimensional accuracy with high shrinkage materials, and strive for high precision, amorphous resins and resins with consistent shrinkage in all directions should be applied as far as possible. Many materials can measure the shrinkage of products under the conditions of changing flow speed, 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, non-uniform shrinkage caused by molecular orientation and non-uniform shrinkage caused by unbalanced cooling. Shrinkage prediction methods for volume shrinkage, crystal content, mold limitation, 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 form the uneven shrinkage of the plastic part, and this contraction difference leads to the generation of bending torque and the bending of the plastic part.

If the temperature difference between the mold cavity and the core used in the injection molding of flat plastic parts is too large, the melt close to the cold mold cavity will soon cool down, and the material layer close to the hot mold cavity will continue to shrink, and the uneven shrinkage will warp the plastic parts. Therefore, the cooling of the injection mold should pay attention to the temperature of the cavity and the core, and the temperature difference between the two should not be too large.

In addition to considering the temperature of the internal and external surfaces of the plastic parts tends to be balanced, the temperature on each side of the plastic parts should also be considered to be consistent, that is, when the mold is cooled, the temperature of the cavity and the core should be kept uniform as far as possible, so that the cooling speed of the plastic parts is balanced, so that the contraction of the parts is more uniform, and the deformation is effectively prevented. Therefore, the layout of cooling water holes on the mold is very important. After the distance between the tube 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 rises with the increase of the length of the cooling channel, the mold cavity and core have a temperature difference along the water channel. Therefore, the length of each cooling loop is required to be less than 2m. Several cooling circuits should be set up in the large mold, and the inlet of one loop is located near the outlet of the other loop. For the long plastic parts, the 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 ejector system also directly affects the deformation of plastic parts. If the ejecting system layout is not balanced, it will cause the imbalance of the ejecting force and cause the deformation of the plastic parts. Therefore, the ejector system should be designed to balance with the stripping resistance.

In addition, the cross-sectional area of the ejector rod can not be too small to prevent the unit area of the plastic parts from being stressed too much (especially when the demoulding temperature is too high) and the plastic parts from deformation. The arrangement of the ejector rod should be as close as possible to the part with large demoulding resistance. Under the premise of not affecting the quality of plastic parts (including the use of requirements, dimensional accuracy and appearance, etc.), as much as possible should be set up to reduce the overall deformation of plastic parts.

When soft plastics are used to produce large deep-cavity thin-walled plastic parts, due to the larger demoulding resistance and the softer material, if a single mechanical ejection method is completely used, the plastic parts will be deformed, and even the top through or folding will cause the plastic parts to be scrapped, such as the use of multiple parts combined or gas (liquid) pressure and mechanical ejection will be better.

Effect 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, and has a great influence on the quality of injection molding products. Because the influence of residual thermal stress on product warping deformation is very complicated, mold designers can use injection molding CAE software to analyze and predict.

During the molding process, due to the uneven orientation and shrinkage, the internal stress of the plastic melt is uneven, so the product warps and deforms after the mold is produced under the action of the uneven internal stress. Therefore, many scholars analyze and calculate the internal stress and warping of products from the perspective of mechanics. In some foreign literatures, warping is regarded as the residual stress caused by non-uniform shrinkage.

During the cooling phase of injection molding, when the temperature is higher than the glass transition temperature, the plastic is a viscoelastic fluid with stress relaxation: when the temperature is lower than the glass transition temperature, the plastic becomes solid. The liquid-solid phase transition and stress relaxation in the cooling process of plastics have great influence on the accurate prediction of residual stress and residual deformation of products.

Phase transition and stress relaxation behavior of plastic from liquid to solid during cooling phase. For the uncured region, the viscous behavior of the plastic is described by the viscous fluid model. For the cured region, the viscoelastic behavior is described by the standard linear solid model. The viscoelastic phase transition model and the two-dimensional finite element method are used to predict the thermal residual stress and the corresponding warping deformation.

Influence of plasticizing stage on warping deformation of product

In the plasticizing stage, the glassy particles are converted to a viscous fluid state, providing the melt required for mold filling. In this process, the temperature difference between the axial and radial (relative to the screw) of the polymer 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, and then cause warping deformation.

Low speed is used in the initial stage of injection, high speed is used in cavity filling, and low speed injection is used when filling is near the end. Through the control and adjustment of the injection speed, the appearance of the product can be prevented and improved, such as rough edges, jet marks, silver bars or coke marks and other undesirable phenomena.

The multi-stage injection control program can reasonably set the multi-stage injection pressure, injection speed, pressure holding pressure and melting mode according to the structure of the flow channel, the form of the gate and the structure of the injection part, which is conducive to improving the plasticizing effect, improving product quality, reducing the defect rate and extending the life of the mold/machine.

By controlling the oil pressure, screw position and screw speed of the injection molding machine by multistage program, the appearance of the molded parts can be improved, the corresponding measures of shrinking, warping and burring can be improved, and the size of the molded parts injected by each mold can be reduced.

By controlling the oil pressure, screw position and screw speed of the injection molding machine by multistage program, the appearance of the molded parts can be improved, the corresponding measures of shrinking, warping and burring can be improved, and the size of the molded parts injected by each mold can be reduced.

Effect of filling and cooling stage on warping deformation of product

The molten plastic is filled into the mold cavity under the action of injection pressure, and the process of cooling and solidification in the mold cavity is the key link of injection molding. In this process, temperature, pressure and speed interact with each other, which has a great impact on the quality and production efficiency of plastic parts.

Higher pressures and flow rates produce high shear rates, which cause differences in molecular orientation parallel to and perpendicular to the flow direction, while creating a "freezing effect". "Freezing effect" will produce freezing stress, forming the internal stress of plastic parts. 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 plastic parts will cause uneven shrinkage between different areas;

C. Different temperature states will affect the shrinkage rate of plastic parts.

Influence of demoulding stage on warping deformation of Plastic Products

Plastic parts are mostly glassy polymers when they are removed from the cavity and cooled to room temperature. It is easy to deform the product because of unbalance of demoulding force, uneven movement of ejecting mechanism or improper ejecting area. At the same time, the stress frozen in the plastic part during the mold filling and cooling stage will be released in the form of deformation due to the loss of external constraints, resulting in warping deformation.

True three-dimensional method to calculate residual stress and final shape (shrinkage and warping). They took into account the effect of the holding stage, divided the product into three layers, and analyzed the residual stress and deformation by a three-dimensional grid. A numerical simulation model of residual stress and deformation caused after the holding stage is presented.

The thermal viscoelastic model (including volume relaxation) was used to calculate the residual stress. The finite element method is based on the shell theory composed of planar elements, which is suitable for thin-walled injection molded products with complex shapes.

Injection products shrinkage on the influence of warping deformation solution

The direct cause of warping deformation of injection molded products is the uneven shrinkage of plastic parts. If the influence of shrinkage during the filling process is not considered in the mold design stage, the geometry of the product will be very different from the design requirements, and serious 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 the shrinkage of the upper and lower surfaces of the plastic parts, resulting in warping deformation.

For warping analysis, the contraction itself is not important, but the difference in contraction is important. In the process of injection molding, due to the arrangement of polymer molecules along the flow direction, the shrinkage rate of the plastic in the flow direction is larger than that in the vertical direction, which causes the warping deformation of the injection part. Generally, uniform shrinkage only causes changes in the volume of plastic parts, and only uneven shrinkage will 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 the shrinkage rate of crystalline plastics is also larger than that of amorphous plastics, and the superposition of the shrinkage rate of crystalline plastics and the anisotropy of shrinkage leads to the warping tendency of crystalline plastic parts is much larger than that of amorphous plastics.

The multi-stage injection molding process selected on the basis of the geometric shape analysis of the product: because the cavity of the product is deeper and the wall is thinner, the mold cavity forms a long and narrow flow channel, and the melt must pass quickly when flowing through this part, otherwise it is easy to cool and solidified, which will lead to the danger of filling the mold cavity, and high-speed injection should be set here.

However, high-speed injection will bring a lot of kinetic energy to the melt, and a great inertial impact will be generated when the melt flows to the end, resulting in energy loss and overflow phenomenon. 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 called pressure holding pressure (secondary pressure, subsequent pressure) to supplement the melt shrinkage into the mold cavity before the melt solidify at the gate. This puts forward the requirement of multi-stage injection speed and pressure in the injection molding process.

Solution of warping deformation caused by residual thermal stress

The velocity of the fluid surface should be constant. Rapid injection should be used to prevent melt freezing during injection. The injection speed should be set to allow for rapid filling in critical areas (such as flow channels) while slowing down at the inlet. The injection speed should ensure that the die cavity is stopped immediately after filling to prevent overfilling, flash and residual stress.