Plastic Processing warpage causes and solutions

I. warpage refers to the shape of the injection molded article deviates from the shape of the mold cavity, it is one of the defects of plastic products common. Warpage occurs because many parameters alone are often powerless to solve. Combined with relevant information and practical experience, the following factors affecting the injection-molded articles warpage brief analysis.

Second, the structure of the impact of the injection mold warpage of. In molds, plastic deformation of the main factors affecting the gating system, cooling system and ejector system.

1. Gating system

Injection mold gate location, gate number and form will affect the plastic in the mold cavity filling state, causing plastic deformation. The longer the distance the flow, the greater the internal stress between the frozen layer and the central layer consists of flow-induced flow and feeding; the other hand, the shorter flow distance, the shorter the article flow from the gate to the end of the flow of time, freeze time filling layer thickness decreases, reduce internal stress, warpage will therefore greatly reduced. Some flat shaping parts, use only a center gate, due to shrinkage in the diameter direction is larger than the shrinkage in the circumferential direction, molded plastic parts will produce distortion; if the switch multiple-point gate or a film type gate, can effectively prevent warpage. When molded using point gate, also due to the plastic shrink anisotropy, gate location, number of all have a great influence on the degree of deformation of plastic parts. In addition, the use of multiple gates also enable the plastic flow ratio (L / t) shortened so that the density of the melt in the cavity become more uniform, more uniform shrinkage. Also, the entire plastic parts can be filled in less injection pressure. While smaller injection pressure can be reduced tendency of molecular orientation of the plastic, reducing its

2. Cooling system

In the injection process, the cooling rate is non-uniform plastic parts will be uneven shrinkage forming plastic parts, such differences lead to shrinkage leaving a bending moment produced warping plastic parts. If the mold cavity during injection molding flat shaping parts (such as mobile phone battery shell) is used, the core temperature difference is too large, as close to the cold mold cavity surface melt quickly cooled down, and close to the hot surface of the mold cavity the material layer will continue to shrink, uneven shrinkage will warp the plastic parts. Therefore, it should be noted injection mold cooling cavity, core temperature tends to balance the temperature difference between the two is not too large (in this case, consider using two mold temperature). In addition to considering the appearance of the temperature within the plastic tends to balance In addition, the plastic parts should be considered consistent with the temperature of each side, that you want to keep the cooling mold cavity, core temperature is uniform throughout, so that the entire plastic cooling rate balanced, so that the contraction become more uniform throughout to effectively prevent the generation of distortion. It is therefore crucial cooling water holes disposed on the mold. After the wall cavity surface to determine the distance, as far as possible so that the distance between the small cooling water holes, walls, in order to ensure uniform temperature. Meanwhile, since the temperature of the cooling medium increases with the length of the cooling water rises, so that the mold cavity, core temperature difference along the waterways. Therefore, the required length of each cooling circuit is less than 2 m waterways. In large mold cooling circuit should be set several import a loop located near the exit of another loop. For long shaped Part should be used straight-through waterways. (And we die mostly based S-type loop ---- is not conducive to cycling, but also extend the period. Top of the system design has a direct impact plastic deformation. If the top of the system layout imbalance will result in ejection force of unbalanced leaving the plastic deformation. Therefore, the ejection system design should strive to be balanced with the release resistance. In addition, the cross-sectional area of ​​the ejection lever can not be too small to prevent the plastic force per unit area is too large (especially in When the temperature is too high under the premise of the release) leaving the plastic deformation. jack should be arranged as close to a large release resistance area. Without prejudice to the quality of plastic (including requirements, dimensional accuracy and appearance) should When the jack is provided in order to reduce as much as possible the overall plastic deformation (in other rams for the top block is the truth). with plastic (such as TPU) to produce thin-walled plastic parts deep cavity, since the release resistance is large, The material has a softer, if at all using a single mechanical push-up, will produce plastic deformation, even top wear caused by folding or produce scrap plastic parts, such as switching to a multi-element joint or gas (liquid) pressure and mechanical Top of the combination would be better (will be used later).

Third, the impact of plastics stage WARPAGE

Plasticizing stage of the glass pellets into the viscous flow state of melt processes (talked about changes in raw material plasticized Tristate training). In this process, the temperature of the polymer in the axial, radial (screw relative terms) the temperature difference will produce plastic stress; In addition, the injection pressure of the injection machine, speed and other parameters will greatly affect the degree of molecular orientation during filling, thereby causing warpage.

Fourth, the plastic filling and cooling stages influence on warpage molten state under the action of the injection pressure, filled into the mold cavity and the cavity is cooled, solidified. This process is the key injection molding. In this process, temperature, pressure, speed of the three mutual coupling effects, the impact on the quality and productivity of plastic parts are great. Higher pressures and flow rates will produce a high shear rate, thereby causing difference in parallel to the flow direction and perpendicular to the flow direction of molecular orientation, while producing "freezing effect." "Freezing effect" will produce frozen stress, internal stress formed plastic parts. Effect of temperature on warpage is reflected in the following aspects:

(1) plastic parts, lower surface temperature difference causes thermal stress and deformation;

(2) Plastic temperature difference between the different regions will cause uneven shrinkage between different regions;

(3) different temperature conditions will affect the shrinkage of plastic parts.

Fifth, the effect of stripping stage WARPAGE

Plastic parts from the mold cavity and allowed to cool to room temperature in the process, mostly glassy polymer. Stripping force imbalance, launch mechanism movement is not smooth or stripping the top of the area is easy to make inappropriate product deformation (As already mentioned). Meanwhile, in the filling and cooling stages "frozen" stresses within the plastic parts due to loss of external constraints, it will be released in the "deformed" form, leading to warpage.

Sixth, the direct cause of the contraction injection products injection warpage warpage impact on that uneven shrinkage of plastic parts. If you do not consider the shrinkage of the filling process in the mold design stage, the geometry of the product design requirements will vary greatly, resulting in a serious distortion products will be scrapped (ie problem of shrinkage). In addition to filling stage will cause deformation, the mold temperature difference between the upper and lower wall will cause contraction of the upper and lower plastic surface differences, resulting in warpage. Warpage analysis, the contraction itself is not as important differences on shrinkage. In the injection molding process, the molten plastic in the injection mold filling stage because the polymer molecules are arranged along the flow direction of the plastic shrinkage in the flow direction is larger than the shrinkage in the vertical direction, leaving the injection produce warpage (ie each anisotropy). Generally uniform volumetric shrinkage caused only plastic parts, only uneven shrinkage will cause warpage. Differential shrinkage of crystalline plastics in the flow direction and the perpendicular direction than non-crystalline plastics large, and the shrinkage rate than non-crystalline plastics large, large shrinkage after its crystalline plastic shrinkage anisotropy effect superimposed result warpage tendency crystalline plastic parts is much greater than that of non-crystalline plastic.

Seven of the influence of the residual thermal stress warpage in injection molding process, the residual thermal stress is an important factor in causing warpage, and have a greater impact on the quality of injection products. Due to residual thermal stress on warpage is very complex, not repeat them here.

Eight, influence of warpage insert metal to put inserts plastic products, due to the shrinkage of plastic than the large metal, so easily lead to distortion (some even cracking); to reduce this case, First preheat metal (generally not less than 100 ℃), and then put into production. IX Conclusion Influence of Injection warpage many factors, structure, physical properties of plastic materials and heat molding process conditions and parameters of the mold are different degrees of influence on warpage articles. Therefore, warpage of injection molding process must consider these factors.

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