July 1, 2023

Avoiding Common Injection Molding Defects: 7 Tips for Success

Avoiding Common Injection Molding Defects: 7 Tips for Success

Injection molding, though an efficient method of manufacturing high-volume components, demands a high level of technical expertise. Despite the benefits of the injection molding process, there are so many variables at play that even a minor Injection molding defects in the early stages of product development can compromise the product integrity and have serious consequences.

Speed and cost-efficiency of the product development process can get hampered if a Injection molding defects arises and if left unchecked, it can further shorten the lifespan of the product. There can be multiple reasons for injection molding defects, like irregular wall thickness, production errors, quality control issues, poor designing and more. Therefore, adopting a proactive risk-mitigation approach during the product development process is crucial to minimize injection molding defects. 

Common Injection Molding Defects


Flow Lines

Any off-color lines, streaks or patterns that manifest themselves on the surface of a part are called Flow Lines. These lines are caused by variations in the movement of molten plastic during the molding process which causes different solidification rates of the material. Flow lines indicate discrepancies in injection speed and/or pressure, which generally is low. Flow lines also occur when the thermoplastic resin moves through the different parts of the mold with varying wall thicknesses.

To avoid flow lines, it becomes crucial to maintain consistent wall thickness and ensure appropriate lengths of chamfers and fillets. Another tip to minimize the occurrence of flow lines is to strategically place the gate in a thin-walled section of the tool cavity.


Sink Marks

Indentations, depressions or dents on the surface of a molded part, especially on thicker sections, are referred to as sink marks. These are caused by uneven cooling or insufficient material flow. Thicker sections tend to take longer to cool down, which can lead to the inner portions of the part to shrink and contract at a different rate to the outer sections.

Typically, sink marks are a result of insufficient cooling and curing of the plastic inside the mould. Sometimes reducing the thickness of the thickest wall sections can help resolve the issue as it will promote uniform and thorough cooling of the plastic. Decreased pressure within the mould or elevated temperatures at the gate also contribute to the formation of sink marks. Risk of sink marks can be mitigated by ensuring proper rib thickness and wall thickness while designing the part for injection moulding. These efforts will help not only in minimizing sink marks but also to enhance overall strength of the part.

Surface Delamination

When the surface of a part gets separated into thin layers it is called Delamination. These coating-like layers become peelable. Delamination occurs because of the presence of contaminants in the material that do not form a bond with the plastic which further creates localized faults. Delamination can also occur when there is too much reliance on mould release agents.

To prevent delamination and repair the damages, mould temperatures should be increased. Also, optimisation of the mould ejection mechanism is required so there is less dependence on mould release agents as these agents contribute to the rise of delamination. Additionally, thoroughly drying the plastic prior to moulding can help a great deal in preventing delamination.

Weld Lines

When a mould has a hole, two streams of the molten resin come together at the juncture as it passes through the mould’s geometry. As the plastic flows and encloses each side of the hole, the two streams meet to form a weld line, or knit line. Hence, this defect only occurs when any part of the mould geometry has a hole.


The temperature of the flow has to be appropriately controlled, otherwise the two streams do not bond together properly, resulting in a visible weld line. Weld lines compromise the overall strength and durability of the component.


One way to prevent weld lines is by preventing the solidification process from beginning too soon. This can be done by raising the temperature of the molten resin or by increasing the injection speed and pressure. Weld lines can also be prevented by removing partitions in the mould design. Another tip is to take resins with lower viscosity and melting points as these resins are less prone to developing weld lines during injection moulding.


Short Shots

Short shots is a defect where resing doesn’t completely fill the mould cavity, resulting in the production of incomplete and unusable components. Typically, short shots occur because of limited flow within the mould. The flow gets restricted in the mould usually because of narrow or blocked gates, trapped air pockets, or inadequate air pressure. The temperature of the mould and the viscosity of the material used also plays a role in the occurrence of short shots.


To avoid short shots, the temperature of the mould should be raised, optimal material viscosity should be ensured and additional venting should be added in the mould design to facilitate the proper escape of air. These methods collectively help prevent short shots during the injection moulding process.



Warping is an injection moulding defect that occurs because of uneven internal shrinkage during the cooling phase. Warping refers to unintended twists, bends or curves in the part. Warping is primarily caused by inconsistent or non-uniform cooling of the mould. When that happens, a stress is created within the material.


Preventing warping defects in injection moulding means ensuring that parts are given enough time to cool down at a controlled and gradual rate. This is done to minimize the development of internal stresses that damages the piece. Maintaining uniform wall thickness in the mould design facilitates the smooth flow of plastic through the mould cavity in a single direction. The type of material being used also plays a role in the development of warping defects, like materials with semi-crystalline structures are more likely to have warping issues.



Uneven solidification process causes the formation of jetting defects in injection moulding. Visible flow patterns or streaks on the surface of the molded part are called jetting. These flow patterns resemble jets or squiggly lines and they impact the structural integrity and aesthetics of the final product. It also decreases the strength of the part. Jetting occurs when a rapid stream of resin enters the mould and begins to solidify before the cavity has filled completely.


An effective technique to prevent jetting is to position the injection gate in a manner that the material flow runs through the shortest axis of the mould. Decreasing injection pressure is often the best way to ensure that the mould fills gradually. Increasing mould and resin temperature is another way to prevent any jets from setting prematurely.



By understanding and addressing the common injection moulding defects discussed in this article, you can improve the quality of your moulding significantly. If you understand the cause of a defect, you can easily rectify it by implementing preventive measures like optimizing designs, adjusting process parameters and maintaining the moulds. By taking a proactive approach and investing effort in prevention of defects in the initial stages of the injection moulding process will lead to long-term benefits for your manufacturing operations. This way, you can ensure reliability, cost-effectiveness and customer satisfaction of your injection moulding projects.