It means that shape of injection molded product deviates from shape of mold cavity, which is one of common defects of plastic products. There are many reasons for warping and deformation, and it is often impossible to solve it by process parameters alone. Combining relevant information and actual work experience, following briefly analyzes factors that affect warpage of injection molded products.

 

In terms of molds, main factors affecting deformation of plastic parts are gating system, cooling system and ejection system.

Position, form and quantity of gate of injection mold will affect filling state of plastic in mold cavity, which will cause plastic part to deform.
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, warpage deformation will also be greatly reduced. For some flat plastic parts, if only one center gate is used, because shrinkage in diameter direction is greater than shrinkage in circumferential direction, molded plastic parts will be deformed; if multiple point gates or film gates are used instead, warping and deformation can be effectively prevented.
When using point gates for molding, also due to anisotropy of plastic shrinkage, position and number of gates have a great influence on degree of deformation of plastic part.
In addition, use of multiple gates can also shorten plastic flow ratio (L/t), thereby making melt density in cavity more uniform and shrinking more uniform. At the same time, entire plastic part can be filled under a smaller injection pressure. Smaller injection pressure can reduce molecular orientation tendency of plastic, reduce its internal stress, and thus reduce deformation of plastic part.

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 warps plastic part.
If temperature difference between mold cavity and core used in injection molding flat plastic parts (such as mobile phone battery shells) 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, and uneven shrinkage will warp plastic part. Therefore, cooling of injection mold should pay attention to balance of cavity and core temperature, temperature difference between the two should not be too large (in this case, two mold temperature machines can be considered).
In addition to considering that temperature of inside and outside 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 is balanced, shrinkage of all parts becomes more uniform, effectively preventing occurrence of deformation. Therefore, arrangement of cooling water holes on mold is very important. After distance between pipe wall and surface of 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 2 meters. Several cooling circuits should be installed in a large mold, inlet of one circuit is located near outlet of the other circuit. For long plastic parts, straight-through water channels should be used. (And most of our molds use S-type loops-it is not conducive to circulation, but also prolongs cycle.)

Design of ejector system also directly affects deformation of plastic parts. If ejection system is unbalanced, it will cause an imbalance in ejection force and deform plastic part. Therefore, when designing ejection system, it is important to balance ejection resistance. 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 demoulding 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 ejectors as possible should be set to reduce overall deformation of plastic parts (this is reason for changing ejector rod to ejector block).
When soft plastics (such as TPU) are used to produce 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 or even pierce or fold and cause plastic part to be scrapped. If you switch to a combination of multiple elements or a combination of gas (hydraulic) pressure and mechanical ejection, effect will be better (will be used later).

Plasticization stage is process of transforming glassy pellets into a viscous liquid melt (three-state change of raw material plasticization was discussed during training). In this process, temperature difference between polymer temperature in axial and radial directions (relative to screw) will cause stress to plastic; in addition, injection pressure, speed and other parameters of injection machine will greatly affect degree of molecular orientation during filling, which will cause warpage and deformation.

Under action of injection pressure, molten plastic is filled into mold cavity, cooled and solidified in the cavity. This process is a key part 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, as well as 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:
(1) Temperature difference between upper and lower surfaces of plastic part will cause thermal stress and thermal deformation;
(2) Temperature difference between different areas of plastic part will cause uneven shrinkage between different areas;
(3) 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 (described earlier). At the same time, stress "frozen" in plastic part during filling and cooling stages will be released in form of "deformation" due to loss of external constraints, resulting in warpage and deformation.

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, geometric shape of product will be very different from design requirements, serious deformation will cause product to be scrapped (ie, shrinkage problem).
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.
During 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 (ie each Anisotropy).
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 of crystalline plastic and anisotropy of shrinkage are superimposed on tendency to affect warpage and deformation of crystalline plastic parts than that of amorphous plastics.

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.

For injection molded products with inserts, since shrinkage rate of plastic is much greater than that of metal, it is easy to cause distortion (some even cracking); to reduce this situation, metal can be preheated (generally not less than 100 ℃), and then put into production.

There are many factors that affect warpage of injection molded products. Structure of mold, thermophysical properties of plastic material, conditions and parameters of molding process all have different degrees of influence on warpage of product. Therefore, above-mentioned factors must be considered comprehensively in treatment of warpage and deformation of injection molded products.