Omar Khalid
Stress marks on plastic are an inherent result of plastics manufacturing processes and occur when a molten polymer is cooled and shaped. These marks are usually white in colour and are caused by residual stress, which can be induced by inconsistent temperatures, flow rates, material contaminants, mechanical loading, thermal cycling during transport and storage, inadequate or non-uniform annealing, and sharp corners or protrusions in the design. While it is impossible to fix stressed plastic, the marks can be covered up with paint or filler.
| Characteristics | Values |
|---|---|
| Cause of stress marks | Residual stress |
| Occurrence | An inherent result of plastics manufacturing processes |
| Occurs when | Molten polymer is cooled and shaped |
| Colour | White |
| Other causes | Bending, inconsistent temperature, mechanical loading, thermal cycling during transport and storage, inadequate or non-uniform annealing, and sharp corners or protrusions designs |
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What You'll Learn
Residual stress from the manufacturing process
Residual stress is an inherent result of plastics manufacturing processes, particularly plastic injection moulding. This occurs when molten polymer is cooled and shaped, causing stress marks on the surface of the plastic. The molten polymer is shaped by being injected under high pressure into a mould, which is then released. This process can cause stress marks to appear on the plastic, which can be a problem leading to field failures and leaving manufacturers exposed to product-liability claims.
There are several factors that can contribute to residual stress during the manufacturing process. One of the main causes is differential cooling rates, where different parts of the plastic cool at different rates, causing inconsistent temperatures and flow rates. This can lead to cracking and crazing, especially under normal load conditions, which is usually evidence of excessive residual stress.
Another factor is non-uniform annealing, which can cause field failures in medical, pharmaceutical, and cosmetic blister packaging. This refers to the process of heating and cooling the plastic to relieve internal stresses and improve its strength and ductility. If this process is not uniform, it can lead to residual stress and distortion in the final product.
Additionally, mechanical loading, sharp corners or protrusions in the design, and thermal cycling during transport and storage can also contribute to residual stress. Thermal cycling refers to the changes in temperature that the plastic undergoes during transportation and storage, which can cause distortion and cracking if the residual stress is excessive.
To avoid stress marks and the negative consequences of residual stress, manufacturers can take several steps. One method is to reduce the duration of holding pressure during the injection moulding process, which has been found to be effective in reducing cracks and marks on precision injection plastic parts. Additionally, adjustments in the process, material, or part design may be made once the stress profile within a plastic part is known. This can help eliminate or reduce residual stress to acceptable levels.
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Heat and stress causing whitening
Plastic is susceptible to stress marks, which are actual damage to the plastic itself. These marks are often white and are caused by residual stress, which is an inherent result of the plastics manufacturing processes. Residual stress occurs when molten polymer is cooled and shaped during plastic injection moulding.
When polymers experience stress, one of the most common reactions is a colour change, turning white at the point of stress. This whitening occurs due to either heat (molecular energy) or stress. Simply bending a plastic item can cause molecular changes that lead to whitening. This phenomenon can be observed when bending a plastic rod or tube, such as a Wiffle bat.
Optical birefringence testing examines how a material refracts light and helps determine whether stress factors caused the whitening in a plastic sample. Another method to identify the cause of whitening is differential scanning calorimetry (DSC), which involves heating and cooling a material to extreme temperatures to understand its thermal transitions.
To avoid stress marks on plastic, it is crucial to use high-quality polymer powder and maintain proper temperature, pressure, and time control during manufacturing. Additionally, reducing the duration of holding pressure during the injection moulding process can help minimise the occurrence of stress marks.
While it is challenging to fix stressed plastic, there are some potential solutions. One approach is to use plastic cement sparingly to melt and re-bond the stressed plastic. However, this method must be done carefully to avoid creating holes or uneven surfaces. Alternatively, the stress marks can be covered up with paint or a matte coat to blend them in and make them less noticeable.
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Distortion from thermal cycles
Plastic is susceptible to deformation when exposed to high temperatures, especially when subjected to a load or force. This phenomenon, known as "creep," occurs gradually over time. The impact of temperature on plastic is an important consideration in various applications, including safety, structural, and functional components in medical, transportation, and industrial fields.
Thermoplastic materials, in particular, have a heat distortion temperature (HDT) of less than 500 degrees Fahrenheit. While HDT provides a comparative specification of how materials respond to test conditions, it offers limited insight into the long-term effects of continuous high-temperature exposure on their physical, mechanical, thermal, and electrical properties.
When plastic is subjected to increasing temperatures, its stiffness (flexural modulus) decreases. This loss of stiffness and the resulting material distortion (heat deflection) are critical factors to consider when addressing temperature requirements in projects involving plastic. Additionally, plastic expands as its temperature rises due to its coefficient of thermal expansion (CTE).
This thermal expansion can become problematic when plastic is used in conjunction with other materials, such as metal, that have different expansion rates. If the dimensional change during thermal expansion is hindered, excessive tensile, shear, or compressive stress loads can be induced in the plastic, leading to unexpected distortions. Therefore, it is essential to select plastic thermoforming materials with appropriate temperature properties for the specific operating environment.
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Cracking under pressure
Plastic can be placed under a lot of pressure, and sometimes it cracks. The technical term for this is "residual stress", which occurs when molten polymer is cooled and shaped during the plastic injection moulding process. The stress mark itself occurs when the plastic is cooling and being shaped.
The refractive index of plastic changes when it is under stress, introducing optical distortion. This can be harmful to the surface appearance of products such as bottles, windows, plastic lenses, and monitor screens. This distortion can also affect the performance of products, such as compact discs.
Differential cooling rates, inconsistent temperatures, or flow rates, material contaminants, mechanical loading, thermal cycling during transport and storage, inadequate or non-uniform annealing, and sharp corners or protrusion designs can all cause cracking. The cracks themselves can be almost invisible to the naked eye, but they can cause the plastic to fail.
The cracks can be observed as lots of micro-cracks on the product surface. This is a sure sign that the plastic has been under too much pressure.
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Creep and changes in mechanical properties
Creep, also known as cold flow, is a deformation mechanism that affects solid materials, including plastics. It is characterised by the slow and continuous deformation of a material under persistent mechanical stress and high temperatures. This phenomenon is particularly prominent in plastics due to their viscoelastic properties, which cause them to behave like both an elastic solid and a viscous liquid simultaneously. The rate of deformation depends on the material's properties, exposure time, temperature, and applied structural load.
In the context of plastics, creep occurs when the material is subjected to stress near its melting point. The temperature range at which creep occurs varies depending on the specific plastic, with some plastics creeping at room temperature or even below freezing, while others require much higher temperatures. For example, lead may creep at room temperature, whereas tungsten requires temperatures in the thousands of degrees.
The sensitivity of plastics to temperature changes and mechanical stress makes creep a significant concern in various applications, such as pipes, structural parts, and sealing elements. As the temperature rises, the molecular structure of plastics becomes more mobile, allowing the material to deform more easily. This deformation can lead to elongation, sagging, or warping of the plastic. Additionally, creep can cause product issues and impact the mechanical properties of the plastic, affecting its load-bearing capacity and structural integrity.
To manage creep in plastic materials, several techniques can be employed:
- Material upgrading: Reinforced plastics, such as fiberglass-reinforced nylon, can enhance the material's ability to withstand stress. Adding fibres like glass or carbon to the polymer changes its mechanical properties and makes it more difficult for polymeric chains to slide past each other.
- Load-sharing approach: By reducing stress at specific points in a component, the effects of creep can be mitigated. Engineers can avoid sharp corners and incorporate gradual transitions between geometries to distribute stress more evenly and reduce the occurrence of stress raisers, which are areas with a high density of applied force.
- Strong support structures: Integrating ribs or gussets into the design can enhance the overall structural integrity and reduce the vulnerability of the plastic component to stress.
- Even stress distribution: Using fillets, radii, and gradual transitions in high-stress regions can ensure uniform stress distribution, preventing faster crawling or deformation in specific areas.
It is important to note that creep in plastics can be managed but not entirely eliminated. By understanding the factors that influence creep and implementing appropriate design and material selection strategies, engineers can minimise its impact and ensure the safe and reliable operation of plastic parts, especially in high-stress and high-temperature environments.
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Frequently asked questions
Stress marks are damage marks that occur on plastic when it is bent or broken.
Stress marks are caused by residual stress, which occurs when a molten polymer is cooled and shaped during the plastic injection moulding process.
Optical birefringence testing can be used to determine whether stress factors have caused whitening in plastic.
You can try to reduce the appearance of stress marks by scratching at them with your fingernail or covering them up with a matte coat and paint. However, it is difficult to completely remove stress marks from plastic.
To prevent stress marks, you can reduce the duration of holding pressure during the plastic injection process. Using a very sharp set of nippers when cutting plastic can also help to lessen the chances of causing stress marks.
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