Professional Manufacturer for Plastic Injection Molding

9 Common Plastic Injection Molding Defects In Plastic Parts Manufacturing

Views: 39     Author: China Mold Factory     Publish Time: 2020-06-07      Origin:

You could be faced with expensive errors whenever you are doing plastic injection molding. Quality problems regarding injection molded items can range from little surface defects to other more serious malfunctions that are capable of disrupting the performance and safety of the products. Some of the problems can arise from the molding process, tooling design, material use, or all of them.

But just like most industrial challenges, knowing the defect itself has already given you half of the help you would need to produce top-quality items. As someone who wants to manufacture or buy injection molded items, knowing these defects will aid you in reducing your total cost of production. Let us run through them one after the other.

China plastic injection molding manufacturer 5

Flow Lines

Flow lines may be seen as ring-shaped bands close to the mold’s entry point. Flow lines are not to reduce the integrity of the item per say, but they can be very ugly as to cause the rejection of the product eventually. Flow lines can be experienced when the material flowing in the mold is cooled at various temperatures. Wall thickness difference can equally cause a variation in cooling which can attribute to flow lines. For instance, molten plastics cool within a short space of time, so when the cooling is delayed for whatever reasons, flow lines begin to appear.

Burn Marks

These are rust or black colored discoloration that usually make up the edge of a molded plastic part. Burnt marks may not be too much of a problem, except when the plastic has been burnt to the extent of degradation. The biggest reason for this is due to trapped air or resins, giving rise to overheating inside the mold cavity with the injection process. Extreme injection speeds of the used material mostly lead to the overheating that occasions burnt marks. You can reduce melt as well as mold temperature to checkmate burnt marks.


This is the product of an injection molded part shrinking in a disproportionate manner. Just the way wood can warp when it does not dry up uniformly, plastics as well as other materials are subject to warping when their cooling is disproportionately done. The non-uniform shrinkage adds more pressure the molded part. This is most obvious in a part that is designed to lay flat when put on a surface but could not because it had developed gaps due to warping. One of the factors that could result to this problem is when cooling happens too fast.

Vacuum voids

These are enclosed air bubbles which can be seen on the body of a molded item. This defect is usually seen by quality control experts as something that is not too serious. But in a case where there is too many voids in a molded component, then that could become a problem. A major cause of this defect is when insufficient air pressure is used to force air molecules down the mold cavity. In other instances, it could be that the material closest to the walls of the mold has cooled before expected, which causes the material to become hardened.

Sink marks

These are insignificant depressions or recessions in a molded part that is supposed to be flat. This is possible when the component’s inner part shrinks, causing the outer part to be drawn inwardly. Sink marks are almost like vacuum voids with the major difference being that the material in the mold takes more than enough time to cool. Sink marks can be seen in thicker areas of an item. You can reduce this by increasing time and holding pressure to let the material approach the part surface where it can cool. You can also increase the cooling time so as to bring about a reduction in shrinkage.

Weld Lines

This is visible in a molded part’s surface where a molten material converges after splitting into two direction or more within a mold. It could be born from a material bonding that is weak, which reduces how strong the part is. Two fronts are expected to have a particular temperature when they are cooling. Else it will lead to partial solidification and will not bond well where they converge, giving rise to weld lines. A common approach to reduce weld lines is to prevent part solidifications. Another way is to have the mold redesigned to eliminate any partitions.


This is a deformation type that can happen in a molded item where a jet of molten material is initially forced into the mold cavity, and it now begins to solidify before filling the cavity. The flow pattern occasioned by jetting can weaken molded item. The major reason for this defect is extreme injection pressure. Anytime molten polymer has to be injected via a little gate at extremely high pressure, it mostly squirts through the gate instead of having the mold cavity filled. To prevent jetting in molded items, you can reduce injection pressure to stop any rapid squirting of such material via the mold cavity.


Color streaking or discoloration often happens when intended color of a molded item is not achieved. And in most cases the unusual color can be seen in a separate area on the molded component. While this defect may have nothing to do with the strength of the component, it certainly ruins its appearance. Leftover resins from the mold’ nozzle due to a production from the past could be responsible for this ugly sighting. Incorrect masterbatch mixing or the coloring agent’s poor thermal ability also could be responsible.

China mold factory 35


If you’ve witnessed thin layers of a molded item coming off or separated from the material underneath, then you probably had already seen the molding defect known as Delamination. This defect is particularly occasioned by a surface area that is flaking. Experts consider this as a severe defect since it is capable of compromising the strength of the molded item. A major reason for is due to the resin pellet’s decontamination. Too much moisture atop the material as a result of inadequate drying before using can equally lead to delamination.

For more about plastic injection molding,you can pay a visit to Yiannopoulos at


Yiannopoulos provides one-stop rapid prototyping services & low volume injection molding manufacturing services. With years of manufacturing experience, our experts offer the best solution for all your rapid manufacturing needs.


   Phone: +86-752-6682869
   Mobile: +86-13352636504
   Email:
   LongGang Village, LongXi Town, BoLuo County,HuiZhou City, GuangDong Province, China


Injection Molding


About Us

Copyright  2012 - 2020 Yiannopoulos Plastic Injection Molding | All Rights Reserved sitemap