Common Injection Molding Defects that Customers Should Know About

The most common injection molding flaws

Plastic injection molding requires a great deal of experience and knowledge to handle. Many factors related to the mold, machine, molding process, and material properties come into play in the process, and if not handled properly, a number of injection molding defects may occur.

When you start a plastic molding project with your mold maker, it is important to have them verify what kind of injection molding defects are likely to occur. The mold maker may think that some minor defects are acceptable, or they may not even consider them from the beginning, which can cause problems when the injection mold is being manufactured.

Participating in DFM and mold flow analysis is a good way to achieve this.

The main types of injection molding defects are:

1. Sink marks, 2. Flashes, 3. Gate blush, 4. Weld line, 5. Voids and bubbles, 6. Silver Streak. 7. short shots

Sink marks

Sink marks are unwanted surface depressions on the surface of injection-molded plastic parts, they usually occur in the thick-walled area or the area with uneven wall thicknesses. 

sink mark on an ABS part
sink mark caused by a rib

The main impact is aesthetical, and shiny surfaces are more sensitive to sink marks. If you look at the below image, the 2 parts are exactly the same, but the painted part (the one on the right) has more noticeable sink marks.

shiny surface is more sensitive to sink marks

Anyway,  in some extreme cases, it will also affect the dimensions and thus the functionality of the parts. As you can see the sink mark in the white sample is quite bad, the only solution for this particular case is to change material (the black one on the picture).

sink mark on a thick wall part

Sinks marks are often the consequence of poor design. Here are 3 typical designs that will cause sink marks:

  1. The rib is too thick compared to the outside walls.

Poor design: the rib is the same thickness as the wall, or even thicker.

rib too thick will cause sink mark quite noticeable

Improved design: make the rib smaller than 0.75x of the wall thickness, this will alleviate the sink mark.

less thick rib will alleviate the sink mark

2. Too uneven wall thickness will cause sink mark in the conjunction area.

Uneven wall thickness will cause sink mark

Try to core out the thick area (so it also has thinner walls), or if this is not possible, add a slope between the thick and then area, so the sink will not be so steep and noticeable.

3. Simply the wall is too thick.

wall too thick will cause sink marks

Always try to avoid thick walls in plastic parts design.

Cause: When the molten plastic flows into the cavity, it starts to cool and solidify. At the same time, it shrinks (gets smaller in size). In the thick-wall area, the inside material cools and hardens later than the surface. If the inside shrinkage is not compensated with the new flow of material (after the gate freeze), then it will pull the surface inwards and forms a depression. If the surface is too hard, voids will be formed inside.


  • Optimize part design (please see above);
  • Increase the packing pressure and time;
  • Lower the mold and molten plastic temperature;
  • Put the gate in the thick-walled section and enlarge the gate size, so as to allow better material compensation in the cooling and solidifying process;
  • Conduct mold flow analysis before starting making the mold.
  • Change the material which is less sensitive to shrinkage.

Flashes or too noticeable parting line

Flash are irregular excessive material (usually in the form of thin-film) on the plastic parts that escape from the mold cavity. They usually appear along the parting line or any other place where there is a gap between moving parts inside the mold.

flashes on an ABS part
Slight flash
too noticeable parting line
Too noticeable parting line

Flashes have an unpleasing appearance, it causes the edge to be unclean.

unclean edge caused by flash
Unclean edge
improved part with less flash

Cause: An injection mold consists of an upper mold and lower mold when they are pressed together form the enclosed cavity. However there may be a slight gap between them due to inaccuracy of machining, or because of the inside pressure overcome the clamping force. At the same time, there are moving components like sliders, ejector pins in the mold, they all have slight gaps around them. The gap is totally allowed and even helpful for the gas vent. However, when the gap is greater than the limit of the viscosity allows of the molten material, it will leak into the gap and form the flashes. The impact of flash is mostly aesthetical.


  • Increase the precision of mold manufacturing to have smaller and consistent gaps in moving components;
  • Use a bigger injection machine (with a higher tonnage) so as to get greater clamping force;
  • Lower the packing pressure and temperature;
  • Change to higher viscosity material (less flowability).

Gate blush, or sometimes simply called as blush, is the scar-like, cloudy discoloration near the gate area. It has an inconsistent appearance than the other area. As the name suggests, many of them have a dull appearance, however, some may be mixed with brighter (more glossy) area, and they usually appear in the pattern of circular or fan-shaped, depending on the gate type and flow direction of molten resin inside of the cavity.

gate blush on a black nylon part
gate blush on a white ABS part
circular gate blush behind a direct gate

People may think of the word “gate scar” or “gate marks”, if they do not know the exact name, but the gate scar means another thing, so gate blush is a better name.

Cause: Gate blush is mainly caused by incorrect designs of gate type, size, and location. You should avoid a drastic change of flow direction near the gate, since this will produce excessive shear stress around this area. However the gate design is usually a compromise of all considerations, there are many other limits, but the mold design must keep this into consideration in the mold design.

Other reasons are too high injection filling speed, improper mold temperature, and the properties of the chosen resin.

The too high viscosity of the resin (or poorer flowability), which will also lead to higher shearing stress.


  • Slow down the injection filling speed.
  • Adjust the mold temperature and nozzle/cylinder temperature/pressure.
  • Widen the injection gate a bit.
  • Change the material to lower viscosity and higher shearing strength.
  • Change the gate type and location. The gate should be located in a thick-walled area instead of the thin-walled area. Try to avoid a drastic change of flow direction inside the mold.
mold flow animation corner connector

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Weld lines, or knit lines, are hair-like thin lines (sometimes comes with discoloration) on plastic parts, usually on one side of holes (or similar features). They can be straight or curved, but they are quite regular in shape.

Some weld lines are quite noticeable.

weld line on a black glossy surface
weld line with discoloration

Some defects are very minor and can only be seen at certain angles of reflection.

Sometimes the weld lines can be covered by painting or plating.

weld line covered by plastic painting
The weld line can be covered by painting on plastic parts.

Weld lines are very common in plastic parts and can hardly be completely avoided. They are acceptable if controlled at a minimal level. We just need to define to what level the weld lines are acceptable for a plastic part.

Cause: When molten plastic is injected into the mold cavity, they will be forced to be divided into 2 flows when meeting an obstacle like a hole (which is a post inside of the mold), then the separated flows will meet at its downstream, or on the other end of the hole. During this process, the molten plastic gets partially solidified, which will cause the flows cannot merge so well together.

When this partial solidification becomes worse, discoloration will happen.

An animation to show the form of weld lines:

The forming of welding line in plastic molding
The weld line predicted by Mold Flaw Analysis
The actual weldline on real parts


  • Increase the mold temperature, use a mold heater (there will be a bit of cost increase).
  • Increase the mold and cylinder temperature.
  • Increase the filling speed.
  • Change the plastic material that is less sensitive to weld lines.
  • Optimize the mold design.

Vacuum voids and air bubbles

Vacuum voids and air bubbles are similar in appearance and can be often be confused with each other. They are both bubbles inside of a plastic injection molded part. 

For transparent parts the bubbles are an aesthetic or functional issue, for example, for LED lenses, even the smallest bubbles are not acceptable. At the same time, bubbles can sometimes weaken the mechanical strength, thus we also need to check if the weakening will cause any problem for their specific application.

However, sometimes when strength is not a concern, the voids and bubbles is not a big issue for opaque plastic parts.

Vacuum voids caused by thick-wall structure
Vacuum voids
air bubbles in first few shots of trial production
Air bubbles

We can do a heat test to check what kind of bubbles they are, use a heat air gun to heat and soften the plastic, if the bubbles grow bigger, then they are air bubbles, if they collapse (or become smaller), then they are vacuum voids. 

Although they are quite alike, the reasons and countermeasures are different:

1. Vacuum voids

Cause: The mechanism for the formation of vacuum voids are quite similar to sink marks. The outside material gets cooled and solidified first, while the inside material still continues to cool down and shrink, at first it will get compensated with the new material flow from the gate. After the gate freezes,  there will be no material compensation anymore, and if the outside material is too hard to be pulled inwards, then the voids will form.


  • Optimize part design, make the walls thinner if possible
  • Increase the packing pressure and time;
  • Lower the mold and molten plastic temperature;
  • Widen the gate. Locate the gate in the thick-walled section so as to allow better material compensation;
  • Change the material which is less sensitive to shrinkage.

2. Air bubbles

Cause: When the injection filling speed is too high, or the plastic part has very thin walls, or sometimes when the mold vent is not adequate, it will cause the air to be trapped inside of the plastic part and form the air bubbles.


  • Slow down the filling speed,so as to avoid air trap in the mold;
  • Improve the vent of the mold.
  • Dry the material completely before injection molding;
  • Make sure the cylinder screw work properly not to mix air in the molten plastic;
  • Do not overheat the plastic in the tank or stay too long, this may cause decomposition of resin and form gas.
plastic mold for a helical gear

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Silver streak / splay

Silver streaks or splays are clusters of silver lines on the surface of plastic parts, the lines are usually arranged in the way of flow direction.

Silver streak will affect the performance of material as well as the appearance, so it should be paid attention to.

silver streak on a PC plastic part

Cause: Silver streaks are usually caused by moisture of the material, but they can also come from gas trapped inside of the mold, or gas generated from decomposed resin caused by overheating or degraded material.


  • Dry the material completely;
  • Increase the mold temperature;
  • Improve cooling of the whole mold to prevent partial overheat.
  • Improve the vent of the mold;
  • Slow down the filling speed.
  • Increase the gate size or location.

Short shots are the inadequate filling of the molten material to the mold cavity. When the part is taken out of the mold, it has some missing pieces, often at the far end of the injection gate.

short shots, molding defects 1
short shots, molding defects 2


  • The wall or rib is too thin and too tall compared to the overall part dimensions, which results in high flowing resistance for the molten material;
  • The material is too viscous or dense;
  • Inadequate venting, causing the air to be trapped in the mold;
  • Injection pressure too low;
  • The mold temperature is too low, which caused the material to solidify before it fills;
    The gate is too small or it is not in the proper position.


  • Redesign the part to increase the wall thickness and reduce the height or ribs;
  • Change the material with lower viscosity;
  • Improve venting of the mold;
  • Increase the injection pressure, but there is a side effect if the pressure is too high;
  • Use a mold heater.


These are the most common injection molding defects that we have dealt with in our earlier jobs. If you are the customer, you need to verify which flaws can not be accepted and be addressed beforehand so the injection molding manufacturer understands your quality requirements clearly. In order to avoid translation misunderstanding, when you work with overseas manufacturers, it will be better to use pictures to explain each defect.

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