Warpage deformation is one of common defects in injection molding of thin-shell plastic parts, because it involves accurate prediction of amount of warpage deformation, warpage deformation laws of injection molded parts of different materials and shapes are very different. When warpage deformation exceeds allowable error, it becomes a forming defect, which in turn affects product assembly. Accurately predicting warpage deformation of a large number of thin-walled parts (thickness less than 2mm) is premise of effectively controlling warpage defects. Most of warpage deformation analysis adopts qualitative analysis, and measures are taken from aspects of product design, mold design and injection molding process conditions to avoid large warpage deformation as much as possible.

 

Location, form and number of gates in injection mold will affect filling state of plastic in mold cavity, resulting in deformation of plastic part.
The longer flow distance, the greater internal stress caused by flow and feeding between frozen layer and central flow layer; on the contrary, the shorter flow distance, the shorter flow time from gate to flow end of part, thickness of frozen layer is reduced when mold is filled, internal stress is reduced, and warpage deformation will be greatly reduced. If only one center gate or one side gate is used, since shrinkage rate in diameter direction is greater than shrinkage rate in circumferential direction, molded plastic part will be distorted; if multiple point gates are used instead, warpage deformation can be effectively prevented.
When point casting is used for molding, also due to anisotropy of plastic shrinkage, location and number of gates have a great influence on degree of deformation of plastic part. Since 30% glass fiber reinforced PA6 is used, a large injection molded part with a weight of 4.95kg is obtained, so there are many reinforcing ribs along flow direction of surrounding wall, so that each gate can be fully balanced.
In addition, use of multiple gates can also shorten flow ratio (L/t) of plastic, so that density of material in cavity becomes more uniform and shrinkage is more uniform. At the same time, the entire plastic part can be filled with less injection pressure. The smaller injection pressure can reduce molecular orientation tendency of plastic, reduce its internal stress, and thus reduce deformation of plastic part.
Mold temperature: Mold temperature has a great influence on intrinsic properties and apparent quality of product. Temperature of mold depends on presence or absence of plastic crystallinity, size and structure of product, performance requirements, and other process conditions (melt temperature, injection speed and injection pressure, molding cycle, etc.)
Pressure control: Pressure in injection molding process includes plasticizing pressure and injection pressure, directly affects plasticization of plastics and quality of products
Experimental method to study warpage deformation of plastic products is mainly reflected in study of influence of material properties, product geometry and size, and injection molding process conditions on warpage deformation of products. Early through design of a large number of experiments, influence of gate geometry, holding pressure parameters (holding pressure and holding time) and elasticity of mold on final size of product was obtained. Using PET as polymer base, warpage characteristics of different materials and slabs with different wall thicknesses were investigated. Relationship between reinforcement ratio, anisotropy of linear thermal expansion coefficient, thickness of product and warpage of 33% glass-reinforced fiber PA66 injection-molded magnetic disk was experimentally studied, and concept of warpage index was proposed for the first time. Warpage characteristics of PA66 plastic products were studied by warpage index, relationship between warpage index, warpage and fiber orientation state, relationship between yield and warpage index were also studied.
Experimental method to study warpage deformation is often limited to a specific geometric shape, specific material and process conditions, cannot comprehensively consider influence of many factors on warpage deformation, and cannot predict magnitude of warpage deformation that may occur in product design stage. In actual use, limitations of empirical formulas are also obvious. They are not only affected by experimental conditions, but also related to many factors such as processing method of experimental data and application conditions of empirical formula.

Since warpage deformation is related to uneven shrinkage, relationship between shrinkage and product warpage is analyzed by studying shrinkage behavior of different plastics under different process conditions. Based on simulation of injection flow, pressure holding and cooling, through experiments and linear regression methods, a model for predicting shrinkage of injection molded products is proposed.

 

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, shrinkage of product is measured under conditions of changing flow speed, holding pressure, holding time, mold temperature, filling time, product thickness and other parameters. According to test results, shrinkage of 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 constraints, plastic orientation, etc., using flow and cooling analysis results to predict shrinkage strain.

During injection process, uneven cooling rate of plastic part will also cause uneven shrinkage of plastic part, this difference in shrinkage will lead to generation of bending moment and plastic part will warp.
If temperature difference between mold cavity and core used in injection molding of flat plastic parts is too large, melt close to cold cavity surface will cool down quickly, while material layer close to hot cavity surface will continue to shrink, uneven shrinkage will warp plastic part. Therefore, when cooling injection mold, it should be noted that temperature of cavity and core tends to be balanced, temperature difference between the two should not be too large.
In addition to considering that temperature of inner and outer surfaces of plastic part tends to be balanced, temperature on each side of plastic part should also be considered to be consistent, that is, when mold is cooled, it is necessary to keep temperature of cavity and core as uniform as possible, so that cooling rate of plastic parts is balanced, shrinkage everywhere is more uniform, and occurrence of deformation is effectively prevented. Therefore, arrangement of cooling water holes on mold is very important. After distance between tube wall and cavity surface is determined, distance between cooling water holes should be as small as possible to ensure that temperature of cavity wall is uniform.
At the same time, since temperature of cooling medium increases with increase of length of cooling water channel, a temperature difference is generated between cavity and core of mold along water channel. Therefore, water channel length of each cooling circuit is required to be less than 2m. Several cooling circuits should be set up in large molds, with inlet of one circuit near outlet of the other. For long plastic parts, a cooling circuit should be used to reduce length of cooling circuit, that is, to reduce temperature difference of mold, so as to ensure uniform cooling of plastic parts.
Design of ejection system also directly affects deformation of plastic parts. If ejection system is arranged unbalanced, ejection force will be unbalanced and plastic parts will be deformed. Therefore, when designing ejector system, a balance should be sought with demolding resistance. In addition, cross-sectional area of ejector rod should not be too small to prevent plastic part from being deformed due to excessive force per unit area of plastic part (especially when demolding temperature is too high). Arrangement of ejector pins should be as close as possible to parts with large demoulding resistance. Under premise of not affecting quality of plastic parts (including use requirements, dimensional accuracy and appearance, etc.), as many ejectors as possible should be installed to reduce the overall deformation of plastic parts.
When using soft plastic to produce large-scale deep-cavity thin-walled plastic parts, due to large demoulding resistance and soft material, if a single mechanical ejection method is used completely, plastic parts will be deformed, or even ejection or folding will cause plastic parts to be scrapped, effect will be better if combination of multiple components or combination of pneumatic (hydraulic) and mechanical ejection is used.

In process of injection molding, residual thermal stress is an important factor that causes warpage deformation, and has a greater impact on quality of injection molded products. Since influence of residual thermal stress on warpage deformation of product is very complex, mold designer can analyze and predict with the help of injection molding CAE software.
During molding process of plastic melt, due to uneven orientation and shrinkage, internal stress is uneven, so after product is out of mold, under action of uneven internal stress, warping deformation occurs. Therefore, many scholars analyze and calculate internal stress and warpage of product from a mechanical point of view. In some foreign literatures, warpage is considered to be caused by residual stress caused by uneven shrinkage.
During cooling phase of injection molding, when temperature is above glass transition temperature, plastic is a viscoelastic fluid with stress relaxation: when temperature is below glass transition temperature, plastic becomes a solid. Liquid-solid phase transition and stress relaxation of plastic during cooling process have a great influence on accurate prediction of residual stress and residual deformation of product.
Phase transition and stress relaxation behavior of plastics from liquid to solid during cooling. For uncured region, plastic exhibits viscous behavior, described by viscous fluid model, and for cured region, plastic exhibits viscoelastic behavior, described by standard linear solid model, using visco-elastic phase transition model and the two-dimensional finite element method to predict thermal residual stress and corresponding warping deformation.

In plasticizing stage, vitreous pellets are transformed into a viscous fluid state, providing melt required for mold filling. In this process, temperature difference of polymer in axial and radial directions (relative to screw) will cause stress to plastic; in addition, parameters such as injection pressure and speed of injection machine will greatly affect degree of molecular orientation during filling , resulting in warpage deformation.
Use low speed in initial stage of injection, use high speed in cavity filling, and use low speed injection when filling is nearing the end. Through control and adjustment of injection speed, various undesirable phenomena such as burrs, jet marks, silver streaks or scorch marks can be prevented and improved in appearance of product.
Multi-stage injection control program can reasonably set multi-stage injection pressure, injection speed, pressure holding pressure and melting method according to structure of runner, form of gate and structure of injection molded parts, which is beneficial to improve plasticizing effect, improve product quality, reduce defect rate and prolong life of mold/machine.
Controlling hydraulic pressure, screw position and screw speed of injection molding machine through multi-stage programs can improve appearance of molded parts, improve corresponding measures for shrinkage, warpage and burrs, and reduce uneven size of each injection molded part of each mold.
By controlling oil pressure, screw position and screw speed of injection molding machine through multi-stage programs, it can seek to improve appearance of molded parts, improve corresponding measures for shrinkage, warpage and burrs, and reduce size non-uniformity of each injection molded part of each mold.

Under action of injection pressure, molten plastic is filled into mold cavity, cooled and solidified in cavity, which is key link of injection molding. In this process, interaction of temperature, pressure and speed has a great influence on quality and production efficiency of plastic parts. Higher pressures and flow rates create high shear rates that induce differences in molecular orientations parallel to and perpendicular to flow direction, with a "freezing effect". "Freezing effect" will generate freezing stress, forming internal stress of plastic part. Influence of temperature on warpage deformation is reflected in following aspects.
A. Temperature difference between upper and lower surfaces of plastic parts will cause thermal stress and thermal deformation;
B. Temperature difference between different areas of plastic part will cause uneven shrinkage between different areas;
C. Different temperature states will affect shrinkage rate of plastic parts.

Plastic parts are mostly glassy polymers as they come out of cavity and cool to room temperature. Unbalanced demoulding force, uneven movement of ejection mechanism or improper ejection area can easily deform product. At the same time, stress frozen in plastic part during filling and cooling stages will be released in the form of deformation due to loss of external constraints, resulting in warping deformation.
True 3D method to calculate residual stress and final shape (shrinkage and warpage). They considered effect of packing stage, divided product into three layers, analyzed residual stress and deformation by 3D mesh, and proposed a numerical simulation model of the residual stress and deformation caused after the holding pressure stage. To calculate residual stress, a thermoviscoelastic model (including volume relaxation) was used. Finite element method used is based on shell theory composed of plane elements, which is suitable for thin-walled injection molded products with complex shapes.

Direct cause of warpage and deformation of injection molded products is uneven shrinkage of plastic parts. If influence of shrinkage during filling process is not considered in mold design stage, geometry of product will be very different from design requirements, and serious deformation will cause product to be scrapped. In addition to deformation caused by filling stage, temperature difference between upper and lower walls of mold will also cause difference in shrinkage of upper and lower surfaces of plastic part, resulting in warping deformation.
For warpage analysis, shrinkage itself is not important, it is difference in shrinkage that matters. During injection molding process, molten plastic will warp and deform due to arrangement of polymer molecules along flow direction during injection filling stage, so that shrinkage rate of plastic in flow direction is larger than shrinkage rate in vertical direction. Generally, uniform shrinkage only causes changes in volume of plastic parts, and only uneven shrinkage can cause warpage deformation. Difference between shrinkage rate of crystalline plastics in flow direction and vertical direction is larger than that of amorphous plastics, and its shrinkage rate is also larger than that of amorphous plastics. Crystalline plastics have a much greater tendency to warp than amorphous plastics.
Multi-stage injection molding process selected on the basis of analysis of geometric shape of product: because cavity of product is deep and wall is thin, mold cavity forms a long and narrow flow channel, and melt must flow quickly through this part. Otherwise, it will be easy to cool and solidify, which will lead to danger of not filling mold cavity, so high-speed injection should be set here. However, high-speed injection will bring a lot of kinetic energy to melt. When melt flows to the end, it will produce a large inertial impact, resulting in energy loss and overflow phenomenon. At this time, flow rate of melt must be slowed down and filling pressure must be reduced. Maintaining so-called holding pressure (secondary pressure, follow-up pressure) enables melt to supplement shrinkage of melt into mold cavity before gate solidifies, which requires multi-stage injection speed and pressure for injection molding process.

Velocity of fluid surface should be constant. Rapid injection should be used to prevent melt freezing during injection. Shot speed setting should take into account slowing down at water inlet while fast filling in critical areas such as runners. Injection speed should be stopped immediately after cavity is filled to prevent overfilling, flash and residual stress.