analysis warpage of thin wall injection molded parts

analysis warpage of thin wall injection molded parts

Warpage is one of the common defects in injection mold design and production, which is difficult to solve.

With the development of plastic industry, especially the development of electronic information industry, the appearance and performance of plastic products are required to be higher and higher. For example, the degree of warpage of plastic shell parts such as notebook, PDA, flat and thin mobile phone has been regarded as one of the important indexes to evaluate the product quality, which has been paid more and more attention by mold designers. It is hoped that the possible warpage causes of plastic parts can be predicted in the design stage, so as to optimize the design, reduce the warpage deformation of the products and meet the precision requirements of product design.
1. Causes of warpage
Warpage is caused by uneven stress and shrinkage during the injection process. Poor demoulding, insufficient cooling, unsuitable shape and strength of parts, poor mold design and process parameters also make plastic parts warped.
The mold temperature is uneven, and the temperature inside the plastic part is uneven.
The difference of wall thickness and uneven cooling lead to the difference of shrinkage.
The thickness condensation pressure difference and cooling rate difference of plastic parts.
When the plastic part is ejected, the temperature is too high or the ejection force is uneven.
The shape of the plastic part is improper, with bending or asymmetric shape.
The mold precision is poor and the positioning is not reliable, which makes the plastic parts easy to warp and deform.
The improper position of the feeding port and the poor injection process parameters lead to obvious shrinkage directivity and uneven shrinkage.
The difference of molecular chain orientation between the flow direction and that perpendicular to the flow direction leads to different shrinkage.
The wall thickness of punch and die is unsymmetrical, the cooling time is insufficient, and the cooling is improper after demoulding.
2. Effect of mold structure on Warpage of injection molded parts
In mold design, there are three main factors affecting the warpage of plastic parts, which are gating system, cooling system and ejection system.
Gate design
The gate of injection mold is the key part of the whole gating system. Its location, form and number of gates directly affect the filling state of the melt in the mold cavity, resulting in the abnormal changes of the solidification, shrinkage and internal stress of the plastic. The common gate types are side gate, point gate, latent gate, straight gate, fan gate and film gate.
The gate location should be chosen to minimize the plastic flow distance. The longer the flow distance is, the greater the flow difference between the internal flow layer and the external frozen layer, and the greater the internal stress caused by the flow and feeding between the frozen layer and the central flow layer, the greater the plastic deformation;
On the contrary, the shorter the flow distance, the shorter the flow time from the gate to the flow end of the part, the thickness of the frozen layer will be thinner, the internal stress will be reduced, and the warpage will also be reduced.
For example, if the precise thin-walled plastic parts use a central gate or a side gate, because the radial shrinkage rate is greater than the circumferential shrinkage rate, the molded plastic parts will produce large distortion deformation; if multiple point gates or film gates are used, the warping deformation can be effectively prevented, so the calculation and verification of flow ratio must be carried out in the design.
When the point gate is used, the location and quantity of gate have great influence on the deformation degree of the plastic part due to the heterotropism of plastic shrinkage.
The distribution test of flat box shaped plastic parts in different gate numbers is as follows: 15% glass fiber reinforced PA66, 1450g plastic part, with many reinforcing ribs along the flow direction around the wall. The same process parameters are used. Gate method: (a) straight gate, (b) 5 ~ 4 point gate, (c) 9 ~ 8 point gate. The test results show that setting gate according to B has the best effect and meets the design requirements. The gate designed according to C is worse than that of straight gate, and the warpage deformation exceeds the design requirement by 3.6 ~ 5.2mm.
The results show that the flow ratio (L / T) of plastics can be shortened by multiple gates, so that the density and shrinkage of melt in the mold are more uniform. At the same time, the plastic part can fill the mold cavity under a small injection pressure, reduce the molecular orientation tendency of the plastic, reduce the internal stress and deformation of the plastic part.
Design of cooling system
During the injection process, the uneven cooling rate of the plastic part will also lead to the uneven shrinkage of the plastic part, which leads to the generation of bending moment and warpage of the plastic part.
If the temperature difference between the cavity and the core of the precision flat large plastic shell parts is too large, the melt on the cold die cavity surface will cool down quickly, while the material layer close to the hot mold cavity surface will continue to shrink, and the uneven shrinkage will make the plastic part warped. Therefore, the cooling system design of injection mold should strictly control the temperature balance of core and cavity.
For precision flat plastic shell parts, the temperature difference should not exceed 5 ° to 8 ° for materials with large shrinkage and easy deformation.
Secondly, the same temperature on each side of the plastic part should be considered, that is, the temperature of the core and cavity should be kept uniform, so that the cooling rate and shrinkage of the plastic parts can be balanced and the deformation can be effectively prevented. On the basis of theoretical calculation, the design of cooling system should be determined by strict process test. Therefore, the setting of cooling water hole on the mold is very important.

After the distance between the pipe wall and the surface of the mold cavity is determined, the distance between the cooling water holes should be as small as possible. If necessary, the uneven density of the cooling water holes should be adopted, that is, the cooling water holes at the high material temperature should be drained more closely, and the cooling water holes at the low material temperature should be sparsely drained to maintain the same cooling rate. At the same time, because the temperature of cooling medium increases with the increase of cooling channel length, the length of cooling water channel should not be too long.

Design of ejection mechanism
The design of ejection mechanism also directly affects the deformation of plastic parts. If the layout of ejection mechanism is unbalanced, the unbalanced jacking force will be caused and the plastic part will be deformed. Therefore, the ejection mechanism should be balanced with the demoulding resistance. The sectional area of ejector pin should not be too small to prevent deformation caused by excessive stress per unit area of plastic parts.
The ejector should be arranged as close as possible to the part with high demoulding resistance. For the precision flat plastic shell parts, more ejector bars should be set as much as possible to reduce the deformation of the plastic parts, and a compound demoulding mechanism combining ejector rod demoulding and ejector plate demoulding should be adopted.
When soft plastics are used to produce large deep cavity and thin-walled plastic parts, due to the large demoulding resistance and soft materials, if the mechanical ejection method is completely adopted, the plastic parts will be deformed. If the multi-element combination or the combination of gas (liquid) pressure and mechanical ejection is used, the effect will be better.
3. Mold filling cooling and warping deformation
Under the action of injection pressure, the molten plastic is filled into the mold cavity and cooled and solidified in the mold cavity. In this process, the coupling of temperature, pressure and velocity has a great influence on the quality of plastic parts.
Higher pressure and flow rate will produce high shear stress, which will cause the difference of molecular orientation parallel to and perpendicular to the flow direction, resulting in larger internal stress of the plastic part. The influence of temperature on warpage is mainly reflected in the following aspects:
The temperature difference between the inner and outer surfaces of plastic parts will cause thermal stress and thermal deformation;
The temperature difference between different regions of the plastic part causes uneven shrinkage;
Different temperature conditions will affect the shrinkage of plastic parts.
Therefore, strictly controlling the appropriate injection process parameters is an important means to reduce warpage.
4. Shrinkage and warpage of parts
The direct cause of warpage of injection molded parts is the uneven shrinkage of plastic parts. For warpage analysis, shrinkage itself is not important, but the difference of shrinkage is important. In the process of injection molding, due to the arrangement of polymer molecules along the flow direction, the shrinkage of plastic in the flow direction is larger than that in the vertical direction, resulting in warpage of the injection molded parts.
Generally, uniform shrinkage only causes the change of the volume of plastic parts, and only the uneven shrinkage can cause warpage deformation. The difference of shrinkage rate between the flow direction and vertical direction of crystalline plastics is larger than that of amorphous plastics, and the shrinkage rate of crystalline plastics is larger than that of amorphous plastics. Therefore, the warpage tendency of crystalline plastics is much larger than that of non crystalline plastics.
5. Residual thermal stress and warpage
In the process of injection molding, residual thermal stress is an important factor causing warpage. Because the influence of residual thermal stress on warpage is very complex, mold design is usually analyzed and predicted by injection CAE software.
6. Summary
There are many factors that affect the warpage of precision flat and thin plastic parts. The structure of the mold, the thermophysical properties of plastic materials and the process parameters of injection molding process have different degrees of influence on the warpage of the parts. Therefore, the experimental research on the warpage mechanism of parts must be focused on, and many factors should be considered comprehensively.