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Analysis of injection warpage deformation of thin-walled injection molded parts

Update:10-08-2020
Summary:

Warpage deformation means that the shape of injection m […]

Warpage deformation means that the shape of injection molded products is deformed and warped unevenly, which deviates from the shape accuracy requirements of the parts. It is one of the common and difficult product defects in injection mold design and injection production.

With the development of the plastic industry, especially the development of the electronic information industry, the appearance and performance requirements of plastic products are getting higher and higher. For plastic case parts such as notebooks, palmtop computers, and flat and thin mobile phones, the degree of warpage has been one of the important indicators for evaluating product quality, and has attracted more and more attention and attention from mold designers. It is hoped to predict the possible cause of warpage of plastic parts in the design stage, so as to optimize the design, reduce the warpage deformation of the product, and meet the accuracy requirements of product design.

1. The cause of warpage and deformation

Warpage deformation is caused by uneven stress and shrinkage during the injection process. Poor demolding, insufficient cooling, inappropriate shape and strength of the part, poor mold design and process parameters, etc. also make the plastic part warp.

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

The difference in wall thickness and uneven cooling of the plastic parts leads to the difference in shrinkage.

Condensing pressure difference and cooling rate difference in thickness direction of plastic parts.

When the plastic part is ejected, the temperature is too high or the ejection force is uneven.

Improper shape of plastic parts, curved or asymmetrical shapes.

Poor mold accuracy and unreliable positioning make the plastic parts easy to warp and deform.

Improper position of the feed port and poor injection process parameters make the shrinkage direction obvious and uneven shrinkage.

The difference in the orientation of the molecular chain between the flow direction and the direction perpendicular to the flow direction causes the shrinkage to be different.

The wall thickness of the convex and concave molds is cooled asymmetrically, the cooling time is insufficient, and the cooling is improper after demolding.

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

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

Gate design

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

The location of the gate should be selected 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 greater the internal stress caused by the flow and feeding between the frozen layer and the central flow layer, the greater the deformation of the plastic part;

Conversely, the shorter the flow distance, the shorter the flow time from the gate to the end of the flow of the workpiece, the thickness of the frozen layer during mold filling will be reduced, the internal stress will be reduced, and the warpage will also be 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 greater distortion; if you use multiple points instead Gates or film-type gates can effectively prevent warpage and deformation, so the flow ratio calculation and verification must be performed 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.

The distribution test of the flat box-shaped plastic parts in different gate numbers: 15% glass fiber reinforced PA66, plastic parts weighing 1450g, with many reinforcing ribs along the flow direction of the surrounding wall. 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 plastic flow ratio (L/t), 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, reducing the molecular orientation tendency of the plastic, reducing the internal stress, and reducing the deformation of the plastic part.

Cooling system design

During the injection process, the uneven cooling rate of the plastic part will also cause the uneven shrinkage of the plastic part. This difference in shrinkage leads to the generation of bending moment and warpage of the plastic part.

If the temperature difference between the mold cavity and core of the precision flat large plastic shell part is too large, the melt on the cold mold cavity surface will quickly cool down, while the material layer close to the hot mold cavity surface will continue to shrink. 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 cavity.

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

Secondly, the uniformity of the temperature on each side of the plastic part should be considered, that is, to keep the temperature of the core and cavity uniform, so that the cooling rate of the plastic part is balanced, the shrinkage is uniform, and the deformation is effectively prevented. The design of the cooling system is based on theoretical calculations and should be determined by rigorous process testing. Therefore, the setting of cooling water holes on the mold is very important.

After the distance between the pipe wall and the cavity surface is determined, the distance between the cooling water holes should be kept as small as possible. If necessary, use a dense and uneven arrangement, that is, where the material temperature is high, the cooling water holes are denser, and the material temperature is low The cooling water holes are sparsely arranged to maintain basically the same cooling rate. At the same time, since the temperature of the cooling medium increases with the increase in the length of the cooling water channel, the length of the water channel of the cooling circuit should not be too long.

Design of ejector mechanism

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

The arrangement of the ejector rod should be as close as possible to the part with high demolding resistance. For precision flat plastic shell parts, as many ejector rods as possible should be provided to reduce the deformation of the plastic parts, and a composite ejection mechanism that combines ejector ejection and ejector plate ejection is used.

When soft plastics are used to produce large-scale deep-cavity thin-walled plastic parts, because the demolding resistance is relatively large, and the material is softer, if the mechanical ejection method is completely adopted, the plastic parts will be deformed. If multiple components are used instead Combination or gas (hydraulic) pressure combined with mechanical ejection will be better.

3. Mold filling cooling and warpage 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 greater impact on the quality of plastic parts.

Higher pressure and flow rate will generate high shear stress, causing the difference in molecular orientation parallel to the flow direction and perpendicular to the flow direction, resulting in a 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 the plastic part will cause thermal stress and thermal deformation;

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

Different temperature conditions will affect the shrinkage of plastic parts.

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

4. Parts shrinkage and warpage deformation

The direct cause of the warpage of injection molded parts is the uneven shrinkage of plastic parts. For warpage analysis, shrinkage itself is not important, but the difference in shrinkage is important. In the injection molding process, due to the arrangement of polymer molecules along the flow direction of the melt during the injection molding stage, the shrinkage rate of the plastic in the flow direction is greater than the shrinkage rate in the vertical direction, causing warpage of the injection molded part.

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 crystalline plastics to warp and deform is much greater than that of amorphous plastics.

5. Residual thermal stress and warpage deformation

In the injection molding process, the residual thermal stress is an important factor that causes the warpage deformation. Because the residual thermal stress has a very complicated influence on the warpage deformation of the part, the mold design is usually analyzed and predicted with the aid of injection molding 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 thermal physical properties of the plastic material and the process parameters of the injection molding process all have different degrees of influence on the warpage of the part. Therefore, the experimental research on the warpage deformation mechanism of the workpiece must focus on many factors.

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