Warpage-deformation: 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 different shapes are very different. Big. When amount of warpage deformation exceeds allowable error, it becomes a forming defect, which affects product assembly. It is a prerequisite for effective control of warpage defects to accurately predict warpage deformation of a large number of thin-walled parts (wall thickness less than 2mm). Qualitative analysis is mostly used for warpage analysis, and measures are taken from product design, mold design and injection molding process conditions to avoid large warpage deformations as much as possible.

 

Cause Analysis

Position, form and quantity of gate of injection mold will affect filling state of plastic in mold cavity, which will lead to deformation of plastic part.
The longer flow distance, the greater internal stress caused by flow and feeding between frozen layer and central flow layer; conversely, the shorter flow distance, the shorter flow time from gate to end of flow of part, thickness of frozen layer during mold filling is reduced, internal stress is reduced, and warpage deformation will also be greatly reduced. If only one center gate or one side gate is used, since shrinkage rate in diameter direction is greater than shrink rate in circumferential direction, molded plastic part will be deformed; if multiple point gates are used instead, warping deformation can be effectively prevented.
When using point casting for molding, also due to anisotropy of plastic shrinkage, location and number of gates have a great impact on degree of deformation of plastic part. Because 30% glass fiber reinforced PA6 is used, large injection molded part weighing 4.95 kg is obtained, 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 plastic flow ratio (L/t), 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 under a small injection pressure. The smaller injection pressure can reduce molecular orientation tendency of plastic, reduce its internal stress, thus reduce deformation of plastic part.
Mold temperature: Mold temperature has a great influence on internal performance and apparent quality of product. Mold temperature is determined by presence or absence of crystallinity of plastic, 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 product quality.
Using experimental methods to study warpage deformation of plastic products is mainly reflected in study of effects of material properties, product geometry and size, and injection molding process conditions on warpage deformation of products. A large number of experiments were designed as early as possible to obtain influence of gate geometry, holding pressure parameters (holding pressure and holding time) and elasticity of mold on final size of product. As a polymer base, PET is used to study warpage characteristics of different materials and flat plates with different wall thicknesses. Relationship between reinforcement ratio of 33% glass reinforced fiber PA66 injection molded disk, anisotropy of linear thermal expansion coefficient, thickness of product and warpage was experimentally studied. Concept of warpage index was proposed for the first time, warpage index was used to study warpage characteristics of PA66 plastic products, warpage index, relationship between warpage and fiber orientation, relationship between yield and warpage index were studied.
Experimental methods to study warpage deformation are often limited to a specific geometric shape, specific materials and process conditions, cannot fully consider influence of many factors on warpage deformation, and it cannot predict magnitude of warpage deformation that may occur in product design stage. In actual use, limitations of empirical formula are also obvious. It is not only affected by experimental conditions, but also related to many factors such as processing method of experimental data, application conditions of empirical formula, an empirical formula is only suitable for production processes that are quite close to experimental conditions.

Since warpage deformation is related to uneven shrinkage, we start with studying shrinkage behavior of different plastics under different process conditions to analyze relationship between shrinkage and product warpage. Based on simulation of injection flow, holding pressure, and cooling, a model for predicting shrinkage of injection molded products is proposed through experiments and linear regression methods. Based on shrinkage prediction, deformation of product is calculated through a structural analysis simulation program.
It is difficult to obtain products with high dimensional accuracy using materials with high shrinkage rates. In order to achieve high accuracy, amorphous resins and resins with consistent shrinkage in all directions should be used as much as possible. For many materials, shrinkage of product can be measured under conditions of changing flow speed, holding pressure, holding time, mold temperature, mold 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 method for volume shrinkage, crystalline content, mold restriction, plastic orientation, etc., uses 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 leads to generation of bending moments and warpage of plastic part.
If temperature difference between mold cavity and core used in injection molding flat plastic parts is too large, melt close to cold mold cavity surface will cool down quickly, while material layer close to hot mold cavity surface will continue to shrink, uneven shrinkage will cause plastic part to warp. Therefore, cooling of injection mold should pay attention to temperature of cavity and core tending to balance, 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, that is, temperature of cavity and core should be kept as uniform as possible when mold is cooled, so that cooling rate of plastic part can be balanced, so that shrinkage of all parts becomes more uniform, effectively prevent occurrence of deformation. Therefore, arrangement of cooling water holes on mold is very important. After distance between pipe wall and surface of the cavity 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, temperature difference between cavity and core of mold is generated along water channel. Therefore, length of water channel of each cooling circuit is required to be less than 2m. Several cooling circuits should be set in a large mold, and inlet of one circuit is located near outlet of the other circuit. 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 ejector system also directly affects deformation of plastic parts. If ejection system is unbalanced, ejection force will be unbalanced and plastic part will be deformed. Therefore, it is important to balance ejection resistance when designing ejection system. In addition, cross-sectional area of ejector rod should not be too small to prevent plastic parts from being deformed due to excessive force per unit area (especially when demolding temperature is too high). Arrangement of ejector rod should be as close as possible to part with high demolding resistance. Under premise of not affecting quality of plastic parts (including use requirements, dimensional accuracy and appearance, etc.), as many ejector rods as possible should be provided to reduce the overall deformation of plastic parts.
When soft plastics are used to produce large-scale deep-cavity thin-walled plastic parts, due to large demolding resistance and softer materials, if a single mechanical ejection method is used completely, plastic parts will be deformed, even plastic parts will be pierced or folded. If parts are scrapped, effect will be better if it is changed to a multi-element combination or a combination of gas (hydraulic) pressure and mechanical ejection.

In injection molding process, residual thermal stress is an important factor that causes warpage and has a greater impact on quality of injection molded products. Because residual thermal stress has a very complicated influence on warpage and deformation of product, mold designers can analyze and predict with 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 products from a mechanical point of view. In some foreign documents, warpage is considered to be caused by residual stress caused by uneven shrinkage.
In cooling stage of injection molding, when temperature is higher than glass transition temperature, plastic is a viscoelastic fluid with stress relaxation: when temperature is lower than glass transition temperature, plastic becomes 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 stage. For uncured area, plastic exhibits viscous behavior, which is described by a viscous fluid model. For cured area, plastic exhibits viscoelastic behavior, which is described by a standard linear solid model. Visco-elastic phase transition model and two-dimensional finite element method are used to predict thermal residual stress and corresponding warpage deformation.

In plasticizing stage, glassy pellets are transformed into a viscous fluid state to provide melt required for mold filling. In this process, temperature difference between axial and radial (relative to screw) temperature of polymer will cause stress to plastic; in addition, injection pressure, speed and other parameters of injection machine will greatly affect orientation of molecules during filling, which will cause warpage and deformation.
Low-speed injection is used at initial stage of injection, high-speed injection is used when filling cavity, and low-speed injection is used when filling is close to the end. Through control and adjustment of injection speed, it is possible to prevent and improve appearance of products such as burrs, jet marks, silver bars or scorch marks.
Multi-level injection control program can reasonably set multi-stage injection pressure, injection speed, holding pressure and melting method according to structure of runner, form of gate and structure of injection molded part, which is beneficial to improve plasticization effect, improve product quality, reduce defect rate and extend life of mold/machine.
Controlling oil pressure, screw position, and screw speed of injection molding machine through multi-level programs can improve appearance of molded parts, improve shrinkage, warpage, and burrs, reduce uneven size 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, temperature, pressure, and speed are coupled with each other, which greatly affects quality and production efficiency of plastic parts. Higher pressures and flow rates will produce high shear rates, which will cause differences in molecular orientation parallel to flow direction and perpendicular to flow direction, and at the same time produce a "freezing effect". "Freezing effect" will produce freezing stress, forming internal stress of plastic part. Influence of temperature on warpage is reflected in following aspects.
A. Temperature difference between upper and lower surfaces of plastic part 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 conditions will affect shrinkage of plastic parts.

Plastic parts are mostly glassy polymers during process of leaving cavity and cooling to room temperature. Unbalanced demolding 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 warpage deformation.
True three-dimensional method to calculate residual stress and final shape (shrinkage and warpage). They considered impact of packing stage, divided product into three layers, analyzed residual stress and deformation with a three-dimensional grid. A numerical simulation model of residual stress and deformation caused after packing stage is proposed. When calculating residual stress, a thermoviscoelastic model (including volume relaxation) is used. Finite element method adopted is based on shell theory assembled by plane elements, which is suitable for thin-walled injection molded products with complex shapes.

Direct cause of warpage of injection molded products is uneven shrinkage of plastic parts. If impact of shrinkage during filling process is not considered in mold design stage, geometry of product will be very different from design requirements, 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, what is important is difference in shrinkage. In injection molding process, during injection molding stage of molten plastic, shrinkage rate of plastic in flow direction is greater than shrink rate in vertical direction due to arrangement of polymer molecules in flow direction, which causes warpage and deformation of injection molded part. 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, its shrinkage rate is also larger than that of amorphous plastics. Large shrinkage rate of crystalline plastics and anisotropy of shrinkage are superimposed on tendency of crystalline plastics to warp and deform compared to amorphous plastics.
Multi-stage injection molding process selected on the basis of analysis of product geometry: due to deep cavity and thin wall of product, mold cavity forms a long and narrow runner. Melt must pass quickly when flowing through this part, otherwise it will be easy to cool and solidify, which will cause danger of not filling mold cavity. 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 great inertial impact, resulting in energy loss and edge overflow. At this time, melt must be slowed down and filling pressure is reduced. Maintaining so-called holding pressure (secondary pressure, subsequent pressure) allows melt to replenish shrinkage of melt in mold cavity before gate solidifies, which puts forward multi-level injection speed and pressure requirements for injection molding process.

Velocity of fluid surface should be constant. Quick glue injection should be used to prevent melt from freezing during glue injection process. Injection speed setting should take into account rapid filling in critical area (such as runner) while slowing down speed at water inlet. Injection speed should be guaranteed to stop immediately after cavity is filled to prevent over-filling, flashing and residual stress.