Solve These Injection Molding Plastic Defects to Achieve Optimal Quality

  • 2022.10.27
  • Flying Tiger

Making injection-molded prototypes takes science, skill, and high-quality machinery. Technical knowledge, expertise, and attention to detail are vital in preventing common errors that can be costly for your business. 

Some of the typical injection molding defects include:

  • Weld Line
  • Flow Line
  • Knit Line
  • Sink Mark
  • Splay Marks
  • Gas Marks
  • Short Shots
  • Flash
  • Plastic Warpage

Most of these errors are due to a lack of experience and technical knowledge or improper use of equipment. Here we will look at the steps you can take to achieve high-quality, defect-free prototypes to save valuable time, money, and materials.

Injection Molding Surface Defects

Weld Line

Weld lines occur when the molten material has converged after splitting into multiple directions in the mold. These weld lines have a hair-like appearance and occur due to weak material bonding, lowering the strength of the prototype where these lines occur.

To prevent injection molding defects such as weld lines, technicians can:

  • Raise the temperature of the mold or molten plastic. In this case, a mold temperature controller can help.
  • Increase the molten plastic injection speed
  • Redesign the mold to eliminate partitions
  • Use materials with low melting points

Flow Line

Flow lines appear wavy, often a darker color than the rest of the prototype. These lines occur near the entry points of the mold where the molten material flows through. These injection molding defects do not cause structural damage to the prototype but can be unsightly for consumers. This defect occurs when cooling speeds variate as the material flows into the mold or if the injection speed is too slow.

To prevent flow lines, technicians can:

  • Increase the injection speed
  • Round the corners of the mold to regulate flow speed
  • Increase nozzle diameter
  • Extend the time the plastic pellets are being melted 

Knit Line 

Knit lines in plastic injection molding occur when two plastic flows meet within the mold and solidify simultaneously, creating a joining “knit line” at the point of contact. Knit lines can appear as faint marks or have a similar look to a crack in the plastic. These lines can reduce the functionality of parts due to an increased chance of breakage.

Note that some plastics are more prone to knit lines than others.  Usually, materials with lower flow rates such as ABS are prone to knit lines. And to fix this, consider elevating the mold temperature with a mold temperature controller.

Other solutions to troubleshoot knit lines include:

  • Use molds with thicker walls
  • Move gates away from critical areas
  • Use high-quality materials with high filler content and long fibers

Improving Molded Appearance

Sink Mark

Sink marks are small recessions in the prototypes that can appear as hollow points or recesses in the material. These injection molding defects occur due to the inner part of a molded component shrinking, pulling material from the outside inward. Common causes include materials cooling too rapidly or slowly, creating a shrinkage that pulls the material inwards.

To prevent sink marks, technicians can:

  • Increase holding pressure
  • Increase cooling time or lower the mold temperature with a water-cooling chiller
  • Design the mold to have thinner walls

Silver Streaks

Silver streaks are splattered lines on the plastic’s surface, appearing as silver streaks. These streaks occur along the direction of melt flow. Silver streaks are caused by the rapid start of injection. When the air in the mold cannot discharge quickly enough, the result is silver streaks on the surface of the plastic, which can reduce the strength of the prototype. Eliminating gasses within the mold can solve this problem. 

To prevent silver streaks, technicians can:

  • The resin contains too much moisture for production, adding, and upgrading a dehumidifying dryer.
  • Lower the melt temperature. 
  • Create a larger gate and change its position
  • Improve the mold exhaust 
  • Execute resin drying.

Splay Marks

Splay marks are wavy patterns that typically occur on smooth finished plastics. These injection molding defects occur due to the first melt into the mold cooling too quickly while the hot melted plastic is forced into the front to push the initial melt forward.

To prevent splay marks, technicians can:

  • Increase injection speed
  • Increase holding times
  • Increase the size of the gate and change its position
  • Increase injection pressure

Gas Mark

Gas mark defects appear as small bubbles of gas in the plastic prototypes. These defects occur when gas interference during the filling process. If gasses do not discharge before filling the mold, gas will mix with the molten plastic, creating internal gas pockets.

To prevent gas marks, technicians can:

  • The resin contains too much moisture for production, adding, and upgrading a dehumidifying dryer
  • Reduce injection molding pressure and injection speed
  • Use the correct molding equipment
  • Ensure gas exhaust functions smoothly

Checking against injection molding defects.

Deformation of Injection Molded Parts

Short Shots

Short shots are described in the name when the molding shot falls short, and the prototype results in a cavity with no plastic. In addition, the finished product looks incomplete due to the molten plastic not filling the mold correctly. These injection molding defects create faulty products, so it is essential to avoid this.

To prevent short shots, technicians can:


Flash is a defect caused by improper mold clamping, allowing the molten plastic to escape the mold. This results in an extrusion of plastic attached to the prototype. Molds that have exceeded their lifespan can create these injection molding defects. It can also be caused by excessive injection pressure, which damages the mold.

To prevent flash, technicians can:

  • Increase the clamp pressure
  • Ensure the mold is maintained, cleaned, and replaced when necessary
  • Use optimal molding techniques for injection speed and pressure

Plastic Warpage

Warping occurs when there is uneven shrinkage, forcing the plastic to warp and contour out of its desired shape. The result is a twisted, bent, and uneven finished product. Warping is caused by improper cooling techniques and variations in cooling speeds, which stress the plastic.

To prevent plastic warpage, technicians can:

  • Ensure the cooling time is long enough or use a water-cooling chiller
  • Design the mold with thick walls
  • Select materials that are less likely to shrink or deform 

Your Best Bet: Mold Temperature Machines, Chillers, or Dehumidifiers

When combating these common injection molding defects, it is essential to use high-quality, industry-standard equipment. And our solutions can help you troubleshoot in 3 aspects:

  • Mold Temperature Controllers. With consistent high mold temperatures, you can achieve better melt fluidity and have better control over the processes. This eliminates the chance of multiple defects, including weld lines, knit lines, and short shots. 
  • Dehumidifiers and Dryers. Remove any trace of water vapor or moisture prior to processing. This is a critical step in preventing defects on the finished product, such as silver streaks and gas marks.
  • Water Cooling Chiller. Industrial plastic chillers efficiently cool down hot melt and maintain stable temperatures throughout production, especially when the ideal mold temperature sits below 25°C, by adopting a chiller, common injection molding defects like sink marks, silver streaks, and plastic warpage.

When deciding on machinery for your injection plastic processes, consult the industry leaders Flying Tiger for comprehensive solutions. Discover high-quality professional solutions for your needs and browse our range of products here.

Article Update: 2022.11.22
Share this post:

We use cookies to ensure you the best experience on our website. When you browse the website you agree to our use of cookies. Learn more