Injection Molding Defects

Top-10 Injection Molding Defects And How To Fix Them

Making injection molded model is both an art and a science. High levels of technical expertise and attention to article are required to bar small mistakes from costing companies big money when it comes to manufacturing of narrative parts.

Design shortcomings we will discuss include:

  • Flow Lines

  • Sink Marks

  • Vacuum Voids

  • Surface DE lamination

  • Weld Lines

  • Short Shots

  • Warping

  • Burn Marks

  • Jetting

  • Flash

Flow Lines

Flow line defects are create by the changeable speed at which the glowing plastic flows as it changes direction through the curves or bends inside the mold tool. They also appear when the plastic flows through sections with changeable wall thickness, or when the injection speed is too low create the plastic to crystallize at different speeds.


Sink Marks

Sink marks are generally generate when the cooling time or the cooling mechanism is deficient for the plastic to fully cool and cure while in the mold. They can also be begin by inadequate pressure in the cavity, or by an extreme temperature at the gate.

Vacuum Voids

Vacuum voids are often create by uneven crystallization between the surface and the inner sections of the precursor. This can be irritate when the holding pressure is deficient to condense the glowing plastic in the mold.

Surface DE lamination

Dry the plastic accordingly before molding.Boost the mold temperature.

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

Focus more on the ejection structure in the mold design to reduce or blow the confidence on mold release agents.

Weld Lines

Weld lines are create by the defective bonding of two or more flow fronts when there is partial calcification of the glowing plastic.

Boost the temperature of the mold or glowing plastic.

Boost the injection speed.

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

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

Short Shot

Short shots can be caused by a number of things. Erroneous analysis of the shot or plasticizing quantity can result in the plastic material actuality deficient to fill the crater. If the plastic is too syrupy, it may crystallize before fully controlling all the cavities and result in a short shot.


Warping is usually create by capricious cooling of the mold material. Different cooling rates in different parts of the mold create the plastic to cool variously and thus create internal weight. These weight, 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 appear in contact with the mold walls, it is expeditiously cooled and the viscosity is expanded. The material that flows through behind that viscous plastic pushes the viscous plastic further, leaving scrape marks on the surface of the accomplished 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 grant the plastic to bleed through. The use of molds that have outpace their oldness will be worn out and commit to the circumstance of flash. Furthermore, enormous injection pressure may force the plastic out through the avenue of least defiance.

About Zaighum Shah 57 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.