The reason and solutions of defective injection products -- warp

1. Unbalanced molecular orientation. The warping deformation of thermoplastics largely depends on the difference between the radial and tangential shrinkage of the plastic part, which is caused by the molecular orientation.

þIn order to minimize the warping deformation caused by the difference of molecular orientation, conditions should be created to reduce the flow orientation and ease the relaxation of orientation stress. The most effective method is to reduce the melt temperature and mold temperature. When adopting this method, it is best to combine it with the heat treatment of plastic parts, otherwise, the effect of reducing the molecular orientation difference is often temporary. Because when the material temperature and mold temperature are low, the molten material cools quickly, and a large amount of internal stress will remain in the plastic parts, so that the plastic parts will still be warped in the future use or when the ambient temperature rises.

þIf the plastic parts are heat treated immediately after demoulding, and kept at a higher temperature for a certain time and then slowly cooled to room temperature, the orientation stress in the plastic parts can be largely eliminated. The heat treatment method is as follows: After demoulding, the plastic parts shall be immediately placed in 37.5~43 ℃ warm water for slow cooling.

2. Improper cooling. If the cooling system of the mold is not designed properly or the mold temperature is not controlled properly, the plastic parts will be warped and deformed due to insufficient cooling. In particular, when the thickness of the plastic parts varies greatly, the plastic parts are particularly prone to warpage due to the inconsistent cooling shrinkage of each part of the plastic parts.

þTherefore, when designing the shape structure of plastic parts, the section thickness of each part should be as consistent as possible.

þIn addition, the plastic parts must have enough cooling and setting time in the mold. For example. The thermal conductivity of UPVC is small. If the central part of the plastic part is not completely cooled, it will be demoulded, and the heat in the central part of the plastic part will be transferred to the outside, which will soften and deform the plastic part.

For the mold temperature control, the temperature difference between the male mold and the female mold, the mold core and the mold wall, and the mold wall and the insert shall be determined according to the structural characteristics of the molded parts, so as to offset the orientation shrinkage difference by controlling the difference in the cooling shrinkage speed of each part of the mold, so as to avoid the plastic parts from warping and deforming according to the orientation law.

þFor plastic parts with completely symmetrical shape and structure, the mold temperature should be kept consistent to make the cooling of all parts of the plastic parts balanced.

þIt is worth noting that when controlling the temperature difference between the mold core and the mold wall, if the temperature at the mold core is high, the plastic part will bend in the direction of the mold core traction after demoulding. For example, when producing frame shaped plastic parts, if the temperature of the mold core is higher than the cavity side, the frame edge of the plastic part will bend inward after demoulding. Especially when the material temperature is low, the bending phenomenon is more serious due to the large shrinkage of the melt flow direction. It should also be noted that the mold core is easy to overheat and must be cooled properly. When the temperature at the mold core cannot be reduced, appropriately increasing the temperature at the cavity side is also an auxiliary means.

þFor the design of the mold cooling system, attention must be paid to setting the cooling pipe at the parts where the temperature is easy to rise and the heat is concentrated. For those parts which are easy to be cooled, slow cooling should be carried out as far as possible to make the cooling of all parts of the plastic parts balanced. Generally, the cavity and core of the mold shall be cooled separately, and the distance between the cooling hole and the cavity shall be moderate, not too far or too close, and generally controlled within the range of 15~25mm; The diameter of the water hole shall be greater than 8mm, the depth of the cooling hole shall not be too shallow, the inner diameter of the water pipe and pipe joint shall be equal to the diameter of the cooling hole, the water flow in the cooling hole shall be turbulent, the flow rate shall be controlled within the range of 0.6~1.0m/s, and the total length of the cooling water hole shall be less than 1.2~1.5m, otherwise the pressure loss is too large; The temperature difference between the inlet and outlet of cooling water shall not be too large, especially for the first mock examination with multiple cavities, the temperature difference shall be controlled below 2 degrees.

3. The mold pouring system is unreasonable. The structural parameters of mold gating system are important factors that affect the shape and position of plastic parts, especially the design of mold gate involves the flow characteristics of molten material in the mold, the formation of internal stress in plastic parts and thermal shrinkage deformation.

þIf the gate position and gate type are reasonably determined, the deformation of plastic parts can be greatly reduced. When determining the gate position, do not make the molten material directly impact the core, but make the stress on both sides of the core uniform; For rectangular flat plastic parts with large area, when resin raw materials with large molecular orientation and shrinkage are used, film gate or multi-point side gate shall be used, and straight gate or point gate distributed in a straight line shall not be used as far as possible; For circular plastic parts, multi-point needle gate or direct center gate should be used, and side gate should not be used as far as possible; For annular plastic parts, disc gate or spoke cross gate shall be used, and side gate or needle gate shall not be used as far as possible; For shell shaped plastic parts, the straight gate shall be used, and the side gate shall not be used as far as possible.

þIn addition, when designing the gating system of the mold, the molten material should be kept parallel during the filling process according to the flow characteristics of the molten material. In this way, although the shrinkage of the molded plastic parts in the vertical direction is different, it will not cause great warpage deformation.

4. The design of mold demoulding and exhaust system is unreasonable. If a plastic part is subjected to a large unbalanced external force in the demoulding process, its physical structure will have a large warping deformation. For example, the mold cavity has insufficient demoulding slope, the plastic parts are difficult to eject, the ejection area of the ejector rod is too small or the distribution of the ejector rod is uneven, the ejection speed of each part of the plastic parts is inconsistent and the ejection is too fast or too slow during demoulding, the core pulling device and insert of the mold are improperly set, the core is bent or the mold strength is insufficient, the accuracy is too poor, and the positioning is reliable, which will lead to the warping and deformation of the plastic parts.

þIn this regard, in the aspect of mold design, we should reasonably determine the demoulding slope, the position and number of ejector rods, and improve the strength and positioning accuracy of the mold; For small and medium-sized molds, anti warping molds can be designed and made according to the warping law. The cavity can be made into a curved surface opposite to the warping direction in advance to offset the orientation deformation. However, this method is difficult to master and requires repeated trial production and mold repair. It is generally used for large batches of plastic parts.

þIn terms of mold operation, the ejection speed should be reduced or the ejection stroke should be increased.

þIn addition, poor mold exhaust also has a certain impact on the warping deformation of plastic parts, which should be paid attention to. For plastic parts that are easy to warp and deform, plastic parts can be forced to be shaped by putting them into wooden clamps that are suitable for their external structure. However, it should be noted that the plastic parts in the clamps should not be stressed and should be allowed to shrink freely. Cooling can be appropriately supplemented to promote the plastic parts to be shaped as soon as possible; For box plastic parts such as turnover boxes, support plates or frames can be used for shaping to prevent their shrinkage or expansion.

5. Improper process operation. During the process operation, if the injection pressure is too low, the injection speed is too slow, the holding time and injection under the condition of not over filling the mold, the cycle is too short, the melt plasticization is uneven, the drying temperature of raw materials is too high during the drying treatment, and the plastic annealing process is not properly controlled, the plastic parts will be warped and deformed.

þIn this regard, the corresponding process parameters shall be adjusted according to specific conditions.