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Some reasons for the deformation of plastic products

Views: 64 Author: WJM Publish Time: Origin: WJM

1. Causes of warping deformation

(1) Warpage deformation is caused by uneven stress and shrinkage of the product during the injection process. Poor demoulding, insufficient cooling, unsuitable shape and strength of parts, and poor mold design and process parameters also cause plastic parts to warp.

(2) The mold temperature is uneven, and the internal temperature of the plastic part is uneven.

(3) Differences in wall thickness of plastic parts and uneven cooling lead to differences in shrinkage.

(4) Thickness-to-thickness condensation pressure difference and cooling speed difference of plastic parts.

(5) When the plastic parts are ejected, the temperature is too high or the ejection force is uneven.

(6) Improper shape of plastic parts, with curved or asymmetrical shapes.

(7) The mold precision is poor and the positioning is unreliable, which makes the plastic parts easy to warp and deform.

(8) The position of the feeding port is improper, and the injection process parameters are not good, so that the shrinkage direction is obvious and the shrinkage is uneven.

(9) The difference in molecular chain orientation between the flow direction and the direction perpendicular to the flow direction results in different shrinkage rates.

(10) The wall thickness of the convex and concave die is cooled asymmetrically, the cooling time is insufficient, and the cooling after demolding is improper.

2. Influence of mold structure on warpage and deformation of injection molded parts

In terms of mold design, there are three main factors that affect the warpage and deformation of plastic parts, which are gating system, cooling system and ejection system.

(1) Gate design

The injection mold gate is a key part of the entire gating system. Its location, form and the number of gates directly affect the filling state of the melt in the mold cavity, resulting in plastic solidification, shrinkage and variation in internal stress. Commonly used gate types include side gates, point gates, submerged gates, straight gates, fan gates, and film gates.

The gate location should be chosen to minimize the flow distance of the plastic. The longer the flow distance, the greater the flow difference between the inner flow layer and the outer frozen layer, so that the internal stress caused by the flow and feeding between the frozen layer and the central flow layer is greater, and the deformation of the plastic part also increases; 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 is reduced when the mold is filled, the internal stress is reduced, and the warpage deformation is also reduced.

Such as precision thin-walled large plastic parts, use a center gate or a side gate, because the radial shrinkage rate is greater than the circumferential shrinkage rate, the molded plastic parts will have a large distortion; if multiple points are used instead Gate or film gate can effectively prevent warpage deformation, so the flow ratio calculation and check must be carried out during design.

When using point gate molding, also due to the anisotropy of plastic shrinkage, the location and number of gates have a great influence on the degree of deformation of the plastic part.

Distribution test of flat box-shaped plastic parts at different gate numbers: 15% glass fiber reinforced PA66 plastic parts with a weight of 1450g are provided with many reinforcing ribs along the flow direction of the surrounding walls.

Basically the same process parameters are used. Gate method: (a) Straight gate, (b) 5~4 point gate, (c) 9~8 point gate. According to the test results, setting the gate according to b has the best effect and meets the design requirements. The gate designed according to c is worse than the straight gate, and the warpage deformation exceeds the design requirement by 3.6~5.2mm.

Multiple gates can shorten the flow ratio (L/t) of the plastic, thereby making the melt density and shrinkage in the mold 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 reduce the deformation of the plastic part.

(2) Design of cooling system

During the injection process, the uneven cooling rate of the plastic part will also cause the uneven shrinkage of the plastic part, and this difference in shrinkage will lead to the generation of bending moment and the plastic part will warp.

For example, the temperature difference between the mold cavity and the core of the precision flat and large plastic shell part is too large, the melt on the surface of the cold mold cavity will cool down quickly, and the material layer close to the surface of the hot mold cavity will continue to shrink, and the shrinkage will not continue. Uniformity will warp the plastic part. Therefore, the cooling system design of the injection mold should strictly control the temperature balance of the core and the cavity.

For precision flat plastic shell parts, materials with large molding shrinkage and easy deformation, production tests show that the temperature difference should not exceed 5°~8°.

Secondly, the consistency of the temperature on each side of the plastic part should also be considered, that is, to keep the temperature of the core and the cavity uniform, so that the cooling speed of the plastic part is balanced, the shrinkage is uniform, and the deformation is effectively prevented. The design of the cooling system should be determined by strict process die test on the basis of theoretical calculation. Therefore, the setting of cooling water holes on the mold is very important.

After the distance between the pipe wall and the surface of the cavity is determined, the distance between the cooling water holes should be as small as possible, and if necessary, the arrangement of uneven density should be adopted, that is, the cooling water holes should be denser at the high material temperature, and the cooling water holes should be denser at the lower material temperature. The cooling water holes are sparsely arranged to keep the cooling rate basically the same. At the same time, since the temperature of the cooling medium increases with the length of the cooling water channel, the water channel length of the cooling circuit should not be too long.

(3) Design of ejector mechanism

The design of the ejector mechanism also directly affects the deformation of the plastic parts. If the ejection mechanism is arranged unbalanced, the ejection force will be unbalanced and the plastic parts will be deformed. Therefore, when designing the ejector mechanism, a balance with the demolding resistance should be sought. The cross-sectional area of ​​the ejector rod should not be too small to prevent the plastic parts from being deformed due to excessive force per unit area.

The arrangement of the ejector pins should be as close as possible to the parts with large demoulding resistance. For precision flat plastic shell parts, as many ejector pins as possible should be set up to reduce the deformation of the plastic parts, and a composite demoulding mechanism combining ejector pin release and pusher plate release should be used.

When using soft plastic to produce large-scale deep-cavity thin-walled plastic parts, due to the large demoulding resistance and the soft material, if the mechanical ejection method is completely adopted, the plastic parts will be deformed. Combined or pneumatic (hydraulic) combined with mechanical ejection will work better.

3. Mold 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 cavity. In this process, temperature, pressure and speed are coupled with each other, which has a great impact on the quality of plastic parts.

Higher pressure and flow rate will generate high shear stress, causing differences in molecular orientation parallel to and perpendicular to the flow direction, resulting in greater internal stress in the plastic part. The influence of temperature on warpage deformation is mainly reflected in the following aspects:

(1) The temperature difference between the inner and outer surfaces of the plastic parts will cause thermal stress and thermal deformation;

(2) The temperature difference between different areas of the plastic part causes uneven shrinkage;

(3) Different temperature states will affect the shrinkage rate of plastic parts.

Therefore, strict control of suitable injection process parameters is an important means to reduce warpage deformation.

4. Parts shrinkage and warpage deformation

The direct cause of warpage and deformation of injection molded parts is the uneven shrinkage of plastic parts. For warpage analysis, shrinkage itself is not important, it is the difference in shrinkage that is important. During the injection molding process, due to the arrangement of polymer molecules along the flow direction, the shrinkage rate of the plastic in the flow direction is larger than the shrinkage rate in the vertical direction during the injection and filling stage, resulting in warpage and deformation of the injection molded parts.

Generally, uniform shrinkage only causes changes in the volume of plastic parts, and only uneven shrinkage can cause warpage deformation. The difference between the shrinkage rate of crystalline plastics in the flow direction and the vertical direction is larger than that of amorphous plastics, and its shrinkage rate is also larger than that of amorphous plastics. Therefore, the tendency of warpage deformation of crystalline plastic parts is much greater than that of amorphous plastics.

5. Residual thermal stress and warpage deformation

In the process of injection molding, residual thermal stress is an important factor that causes warpage deformation. Because the influence of residual thermal stress on the warpage deformation of parts is very complex, mold design is usually analyzed and predicted with the help of injection molding CAE software.

6. Summary

There are many factors that affect the warpage deformation of precision flat and thin plastic parts. The structure of the mold, the thermophysical properties of the plastic material and the process parameters of the injection molding process all have different degrees of influence on the warpage deformation of the parts. Therefore, the experimental research on the warpage deformation mechanism of the workpiece must focus on comprehensive consideration of many factors.


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