Injection Molding Defects

Top-10 Injection Molding Defects And How To Fix Them

Making injection molded prototypes is both an art and a science. High levels of technical expertise and attention to detail are required to prevent small mistakes from costing companies big money when it comes to mass-production of novel parts.

Design shortcomings we will discuss include:

  • Flow Lines

  • Sink Marks

  • Vacuum Voids

  • Surface Delamination

  • Weld Lines

  • Short Shots

  • Warping

  • Burn Marks

  • Jetting

  • Flash

Flow Lines

Flow line defects are caused by the varying speed at which the molten plastic flows as it changes direction through the contours and bends inside the mold tool. They also occur when the plastic flows through sections with varying wall thickness, or when the injection speed is too low causing the plastic to solidify at different speeds.

Sink Marks

Sink marks are often caused when the cooling time or the cooling mechanism is insufficient for the plastic to fully cool and cure while in the mold. They can also be caused by inadequate pressure in the cavity, or by an excessive temperature at the gate.

Vacuum Voids

Vacuum voids are often caused by uneven solidification between the surface and the inner sections of the prototype. This can be aggravated when the holding pressure is insufficient to condense the molten plastic in the mold.

Surface Delamination

Pre-dry the plastic properly before molding.

Increase the mold temperature.

Smooth out the corners and sharp turns in the mold design to avoid sudden changes in melt flow.

Focus more on the ejection mechanism in the mold design to reduce or eliminate the dependence on mold release agents.

Weld Lines

Weld lines are caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic.

Raise the temperature of the mold or molten plastic.

Increase the injection speed.

Adjust the design for the flow pattern to be a single source flow.

Switch to a less viscous plastic or one with a lower melting temperature

Short Shot

Short shots can be caused by a number of things. Incorrect calibration of the shot or plasticizing capacities can result in the plastic material being inadequate to fill the cavities. If the plastic is too viscous, it may solidify before fully occupying all the cavities and result in a short shot.


Warping is usually caused by non-uniform cooling of the mold material. Different cooling rates in different parts of the mold cause the plastic to cool differently and thus create internal stresses. These stresses, when released, lead to warping.

Burn Marks

Burn marks are caused either by the degradation of the plastic material due to excessive heating or by injection speeds that are too fast. Burn marks can also be caused by the overheating of trapped air, which etches the surface of the molded part.


Jetting occurs mostly when the melt temperature is too low and the viscosity of the molten plastic becomes too high, thereby increasing the resistance of its flow through the mold. When the plastic comes in contact with the mold walls, it is rapidly cooled and the viscosity is increased. The material that flows through behind that viscous plastic pushes the viscous plastic further, leaving scrape marks on the surface of the finished product.


Flash can occur when the mold is not clamped together with enough force (a force strong enough to withstand the opposing forces generated by the molten plastic flowing through the mold), which allows the plastic to seep through. The use of molds that have exceeded their lifespan will be worn out and contribute to the possibility of flash. Additionally, excessive injection pressure may force the plastic out through the route of least resistance.

About Zaighum Shah 56 Articles
Zaighum Shah is a mechanical engineer having more than 20 years of experience. Zaighum is specializing in product development in Sugar Mill industries. Zaighum has gone through all phases of mechanical engineering and it’s practical implementation. Zaighum has been solving most complex problems, designing new systems and improving existing models and systems.