Alexei Volkov
Stress marks on plastic are an inherent result of the plastic injection moulding process. They occur when molten polymer is cooled and shaped. These residual stresses can sometimes be invisible but may cause field failures. The residual stress causes damage to the plastic and can result in cracks or distortion. These stress marks can be caused by cutting too close to the plastic or by using a knife. While there is no way to fix stressed plastic, you can return it to its original shape and paint over it.
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What You'll Learn
- Plastic stress marks are actual damage to the plastic itself
- Residual stress is an inherent result of the plastic injection moulding process
- Stress marks occur when the molten polymer is cooled and shaped
- Stress marks can be purposely formed to obtain desirable characteristics
- Ways to improve plastic stress marks include modifying product and mould design
Plastic stress marks are actual damage to the plastic itself
The residual stress can cause cracking under normal load conditions, which can lead to field failures. These cracks can be observed as micro-cracks on the product's surface. Distortion can also occur when plastic parts with excessive residual stress are subjected to post-processing thermal cycles such as sterilisation or heat sealing.
Additionally, products exposed to varying ambient temperatures during transportation or storage are also susceptible to distortion. To avoid these issues, it is crucial to reduce the holding pressure and time during the manufacturing process. This can be achieved by modifying the product and mould design, such as adjusting wall thickness or adding rubber to the surface.
While it is challenging to fix stressed plastic completely, there are ways to mitigate its appearance. One option is to paint over the stress marks or use a matte coat to blend them in and make them less noticeable. Another potential solution is to use plastic cement, which can sometimes melt and re-bond the stressed plastic, reducing its visibility.
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Residual stress is an inherent result of the plastic injection moulding process
There are several factors that can contribute to residual stress in injection-moulded products. One key factor is the flow rate, which refers to the rate at which the heated, liquefied material is pushed into the mould cavity. Different materials have different flow rates, and non-uniform or unexpected flow rates can create unpredictable residual stress. This, in turn, can affect the performance of the finished product.
Another factor that can affect residual stress is cooling time. The cooling process can be influenced by the flow rate, but other factors such as temperature gradients and cooling rates can also play a role. When the molten material enters the mould at a lower temperature, the material near the cavity wall cools and solidifies rapidly, while the core solidifies more slowly. This can result in a temperature gradient within the product, leading to residual thermal stresses.
In addition to flow rate and cooling time, material choice is also important. Different types of polymers have unique material properties, and they can be affected by post-production heating or cooling during shipping and transportation. For example, crystalline and non-crystalline polymers exhibit different tensile strength characteristics depending on the location of the gate and the direction of mould filling.
To manage residual stress and minimise stress marks, it is crucial to consider proper part design. This includes incorporating rounded corners, simplicity in design, and draft. Additionally, reducing the holding pressure duration can help mitigate stress marks. While residual stress cannot be completely eliminated, effective management and monitoring can ensure that it does not compromise the quality or safety of the final product.
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Stress marks occur when the molten polymer is cooled and shaped
Stress marks on plastic are actual damage to the plastic itself. They are caused by residual stress, which is an inherent result of plastics manufacturing processes. This residual stress occurs when the molten polymer is cooled and shaped.
The cooling and shaping of molten polymers can introduce directional stresses in oriented polymers that lead to undesirable changes. For instance, an extruded tube or sheet may show decreased performance in the unstretched direction. If the plastic is processed at too low a temperature, it can shrink in the machine direction and expand in the cross-machine direction.
Differential cooling rates and non-uniform temperatures can cause stress marks to appear on plastic. These marks are a sign of damage and can lead to field failures, impacting the product's quality and performance. High levels of stress can lower the product's impact strength and contribute to environmental stress cracking, especially at high or cycling temperatures.
In some cases, residual stress is intentionally produced to achieve desirable characteristics. For example, in oriented PET films and bottles, residual stress can be used to create specific shapes or properties. However, in most cases, residual stress is an invisible problem that can cause issues such as cracking and distortion.
While it is challenging to fix stressed plastic, there are ways to cover up or reduce the appearance of stress marks. Painting over the marks, using a matte coat, or applying plastic cement to melt and re-bond the stressed plastic can help blend and minimize the marks' visibility.
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Stress marks can be purposely formed to obtain desirable characteristics
Stress marks, also known as white marks, on plastic are actual damage to the material itself. They occur when a molten polymer is cooled and shaped during the plastic injection moulding process. While these marks are undesirable in most cases and can lead to product failure, residual stress is sometimes purposely formed to achieve certain characteristics in products like oriented PET films and bottles.
Residual stress is an inherent result of the plastic injection moulding process. In some cases, it is leveraged to obtain desirable characteristics in products. However, in most cases, residual stress can be an invisible problem that leads to field failures.
One of the issues caused by residual stress in plastic parts is cracking under normal load conditions. This indicates the presence of internal residual stress, which can cause micro-cracks on the product's surface. Distortion is another problem that occurs when plastic parts with excessive residual stress undergo post-processing thermal cycles such as sterilization, ultrasonic welding, or heat sealing. Additionally, products exposed to varying ambient temperatures during transportation or storage are also susceptible to distortion.
While it is challenging to fix stressed plastic completely, there are ways to mitigate the appearance of stress marks. One way is to paint over the marks to blend them in and make them less noticeable. Applying a matte coat can also help in this regard. To prevent stress marks, it is recommended to leave a large nub off the gate and then remove the excess plastic. Using sharp nippers during the cutting process can also reduce the chances of causing stress marks.
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Ways to improve plastic stress marks include modifying product and mould design
Plastic stress marks are actual damage to the plastic itself, manifesting as shiny or whitish surface marks. They are caused by internal stress, which can be attributed to orientation stress and cooling shrinkage stress. Orientation stress results in irregular stress marks near the gate, while a difference in wall thickness arises from a combination of orientation and shrinkage stress.
To improve plastic stress marks by modifying product and mould design, consider the following:
- Injection speed should be moderate—variable speed injection is recommended, and filling should stop when speed gradually decreases.
- Holding time should be reduced to prevent an increase in the shearing effect of the plastic melt, which leads to greater elastic deformation and the freezing of more orientation stresses.
- Injection pressure and pressure holding time should be adjusted to ensure that the residual pressure in the mould is close to atmospheric pressure when the mould is opened, thereby reducing internal stress during demoulding.
- Mould temperature can be increased to improve filling and reduce packing pressure, or decreased to make the textured surface shiny, reducing the visibility of stress marks.
- To reduce internal stress and improve the performance of parts, a heat treatment method can be employed. This involves placing parts into liquid media such as water, glycerin, mineral oil, ethylene glycol, or liquid paraffin, or using an air circulation oven to heat the parts to a specified temperature. The parts are then allowed to stay at this temperature for a set period before being slowly cooled to room temperature.
- Holding pressure and time should be reduced to prevent the formation of thickness difference stress marks, ejector pin and angled pin stress marks, and irregular stress marks near the gate.
- The overall wall thickness should be considered—if it is too thin or the wall thickness of the flowing end is too thick, it will cause high filling and holding pressure. In this case, either add rubber or reduce wall thickness at the flowing end.
- Avoid poor wall thickness—if necessary, differentiate it.
- Ensure the rib of the male mould is not too large to prevent rib stress marks in the female mould.
- Set up reasonable gates—the gate should not be too small or too few, and they should be evenly distributed.
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Frequently asked questions
Stress marks on plastic are white or shiny marks that appear on the surface of plastic parts due to internal stress. They are caused by the cooling and shaping of molten polymers during the plastic injection molding process.
Yes, stress marks indicate actual damage to the plastic itself. These marks are caused by residual stress, which can lead to potential problems and field failures.
To prevent stress marks on plastic, it is important to reduce the holding pressure and holding time during the manufacturing process. Using high-quality polymer powder and proper temperature control can also help avoid stress marks.
While it is difficult to completely fix stressed plastic, there are ways to minimize the appearance of stress marks. One method is to paint over the marks or use a matte coat to blend them in and make them less noticeable.
There are several types of stress marks on plastic, including thimble stress marks, insert stress marks, and wall thickness difference stress marks. These marks can occur due to orientation stress, shrinkage stress, or a combination of both.
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