Alexei Volkov
Plastic is a versatile material that can be moulded into various shapes and products. However, the manufacturing process can introduce flaws, such as burn marks, blistering, delamination, flashing, and sink marks. These flaws not only affect the appearance of the plastic but can also impact its strength and functionality. Understanding and addressing these issues are crucial to ensuring the quality and safety of plastic products. This topic will explore the various defects that can occur during plastic production and the measures that manufacturers can take to prevent them.
| Characteristics | Values |
|---|---|
| Excess material on blow-molded/injection-molded items | Flash or spew |
| Plastic tree model parts came on | Sprue |
| Delamination | Thin layers on the surface of a molded part that are easily separated or peeled off the underlying material |
| Burn marks | Black or rust-colored discoloration on an edge or surface of a molded plastic part |
| Bubbles | Large or small bubbles formed by trapped air |
| Volume expansion | An area on the surface that has expanded more than its surrounding area |
| Blistering | Thin, film-like layers that elevate the surface of the product |
| Vacuum void | Shrinkage in the interior of a plastic part |
| Meld lines | Formed when two or more plastic streams meet at different temperatures |
| Warpage | Injection parts that are inconsistent with the requirements of the design drawings and present visible deformations |
| Short shot | Incomplete compartments in plastic shelves of a display or missing prongs on a plastic fork |
| Flow lines | Caused by variations in the cooling speed of the material as it flows in different directions throughout the mold |
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What You'll Learn
- Burn marks: discolouration, usually dark reddish-brown or black streaks, caused by trapped air
- Flow lines: caused by variations in the cooling speed of the material
- Flash: excess plastic that escapes from the mould, cools, and remains attached to the product
- Delamination: thin layers on the surface of a moulded part peeling off the underlying material
- Blistering: a thin, film-like layer that elevates the surface of the product
Burn marks: discolouration, usually dark reddish-brown or black streaks, caused by trapped air
Burn marks are discolourations, typically dark reddish-brown or black streaks, on the surface of the final moulded part. They are caused by trapped air in the cavities of the injection mould. During the injection phase, the air trapped inside the cavity mould is highly pressurised and becomes superheated, scorching the plastic. This scorching can turn black parts into a reddish-brown or white parts into a deep brown. The dark colour is carbon residue from the burned front edge of the flowing plastic.
Burn marks are a common issue in plastic injection moulding. They are caused by air or other gases trapped within the mould, which cause overheating and discolouration. Proper venting is required to release these gases and stabilise the temperature. This will depend on the proper number and width of channels and a slower, steady injection.
To prevent burn marks, it is important to ensure adequate ventilation in the mould to allow trapped air to escape. This can be achieved by enlarging the gas vents and gates, as well as shortening the mould cycle time to prevent trapped air and resin from overheating. Additionally, reducing the injection speed can help limit the risk of trapping air inside the mould.
It is also important to consider the size and location of the vents in relation to the tool and material used. While burn marks generally do not affect part integrity, excessive burning can lead to degradation of the plastic. Therefore, it is crucial to take preventative measures to avoid this issue during the injection moulding process.
Burn marks are just one of several common defects that can occur during plastic injection moulding. Other issues include flashing, delamination, sink marks, short shots, and flow lines. Each of these defects has its own unique characteristics and causes, and it is important for manufacturers to be aware of them to ensure the production of flawless parts.
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Flow lines: caused by variations in the cooling speed of the material
Plastic injection molding is a fast and efficient method for creating high-quality plastic parts. However, it is not without its flaws. One such flaw is flow lines, which are caused by variations in the cooling speed of the material.
Flow lines are a common issue in plastic injection molding, appearing as wavy lines, patterns, circles, or ripples on the surface of the molded part. These lines are usually most noticeable near the gate of the mold, where the material enters the cavity. They are the result of non-uniform cooling of the molten plastic within the mold. When the molten material reaches a cavity wall, it begins to cool and harden, while the inner area of the molten stream continues to move, causing a ripple effect.
Differences in wall thickness can also contribute to flow lines. Thinner areas of the mold cavity will cool faster than thicker areas, resulting in uneven cooling and the formation of flow lines. Additionally, the placement and type of gate can play a role. Ideally, gates should be positioned to promote even flow, especially in thicker areas, to prevent certain parts of the material from cooling faster than others.
The presence of flow lines indicates an uneven flow of material, which can reduce the aesthetic qualities of the final product. While flow lines do not affect the structural integrity of the part, most manufacturers and customers prefer to avoid them. To tackle flow lines, adjustments can be made to the mold design and injection molding process, such as increasing injection speed, pressure, and material temperature to ensure the material fills the mold before cooling.
It is important to note that flow lines can provide valuable insights into the behavior of material flow. By observing the direction and pattern of flow lines, manufacturers can gain a better understanding of how the material has moved and cooled within the mold, which can be used to optimize the injection molding process and prevent future defects.
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Flash: excess plastic that escapes from the mould, cools, and remains attached to the product
The correct term for excess plastic that escapes from the mould, cools, and remains attached to the product is "flash". Flash is a common defect in plastic injection moulding and is often found near the parting line or the ejector pins. Inadequate clamping force, worn moulds, or excessive injection pressure can cause flashing. To prevent flashing, it is important to ensure that the moulds are in good shape and that clamping pressure is increased.
Flashing is not a desirable phenomenon in injection moulding production as it can lead to die joint damage if not cleaned immediately. This type of damage will cause more evident flashing in future products, so it is crucial to look out for flashing marks and remove them promptly. When the mould cavity fills up, the flow of plastic may be too quick, causing air to become trapped and forming bubbles in the material.
There are several other common defects in plastic injection moulding, including burn marks, flow lines, delamination, short shots, blistering, and weld lines. Burn marks are discolourations, typically dark reddish-brown or black streaks, caused by trapped air in the cavities of the injection mould becoming superheated and scorching the plastic. Flow lines are caused by variations in the cooling speed of the material as it flows in different directions throughout the mould, resulting in wave patterns on the surface.
Delamination refers to thin layers on the surface of a moulded part that separate or peel off the underlying material, reducing the strength of the component. Short shots are caused by the mould shot falling short of filling the mould, resulting in incomplete or deficient products. Blistering is when a thin, film-like layer elevates the surface of the product, and weld lines form when two or more plastic streams meet at different temperatures but do not mix correctly.
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Delamination: thin layers on the surface of a moulded part peeling off the underlying material
Delamination is a defect that occurs when thin layers on the surface of a moulded part peel off the underlying material. It is characterised by a flaking surface layer, similar to what is commonly found on flake mica. Delamination is considered a serious defect as it reduces the strength of the component, which can be dangerous if the part is intended for safety-critical use.
The most common cause of delamination is contamination of the resin pellets or other base materials with a foreign material. This results in flaky separation as the two materials cannot properly bond with each other. For example, combining a common base plastic like acrylonitrile butadiene styrene (ABS) with an incompatible plastic such as polypropylene (PP) can lead to delamination.
To prevent delamination, it is important to ensure that the resin pellets or base materials are not contaminated with foreign substances. Proper quality control measures should be implemented to detect and eliminate any potential contaminants before the moulding process. Additionally, care should be taken to select compatible materials that can properly bond with each other during the moulding process.
Other common defects in plastic injection moulding include burn marks, flow lines, flashing, short shots, and vacuum voids. Burn marks are discolourations, typically dark reddish-brown or black streaks, caused by trapped air in the mould cavity during the injection phase. Flow lines are caused by variations in the cooling speed of the material as it flows in different directions throughout the mould, resulting in visible patterns. Flashing occurs when plastic escapes from the mould and cools, remaining attached to the part. Short shots refer to incomplete filling of the mould, resulting in deficient products. Vacuum voids are caused by shrinkage in the interior of a plastic part, often in areas where the plastic is thick or where multiple channels come together.
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Blistering: a thin, film-like layer that elevates the surface of the product
Blistering is a common issue in plastic production, characterised by a thin, film-like layer that elevates the surface of the product. This phenomenon occurs due to several factors, primarily related to the cooling process and the presence of air.
Firstly, blistering can be caused by uneven cooling or shrinking of the plastic during production. This often happens in areas where the plastic is very thick or where multiple channels come together to form a rib or wall. The uneven cooling results in a thin, raised layer on the surface of the plastic product.
Another cause of blistering is trapped air within the plastic. When the mould cavity fills with plastic too quickly, air can become trapped and form bubbles. These bubbles can then expand and cause a thin, film-like layer to elevate the surface of the plastic. This issue is similar to vacuum voids, which are caused by uneven cooling and shrinking in the interior of a plastic part, but vacuum voids do not involve the presence of air.
To prevent blistering, manufacturers can take several measures. Firstly, ensuring proper venting to release trapped gases and stabilise the temperature is crucial. This involves adjusting the number and width of channels and controlling the injection speed and pressure. Additionally, manufacturers can increase the temperature of the molten plastic and the mould to prevent premature cooling and allow for a smoother fill.
It is important to address blistering as it can affect the integrity and appearance of the final product. While blistering may not always cause functional issues, it can lead to cosmetic defects that are undesirable. Therefore, understanding the causes of blistering and implementing preventive measures are essential to ensuring high-quality plastic products.
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Frequently asked questions
Those extra plastic flaws are called "flash".
Flash is plastic that escapes from the mold, cools, and remains attached to the part. It is often found near the parting line or the ejector pins.
To prevent flash, ensure that molds are in good shape and increase clamping pressure.
