Perfecting Your Product: How to Prevent the Most Common Defects in Plastic

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Like most processes, plastic injection molding is not without its challenges. Producing injection-molded parts requires both attention to detail and technical expertise to prevent defects and other costly mistakes that can equate to significant losses and delays for manufacturers.
This post will examine some of the most common plastic defects that occur in the molding process, as well as practical tips and strategies to avoid them.

The 10 Deadly Sins of Injection Molding and Keys to Prevention

Outlined below is a list of the top 10 design flaws and defects that commonly occur in plastic injection molding, along with some solutions for addressing these areas.

1. Flow Lines – Flow lines refer simply to lines or streaks that appear in the part. These defects are often off-color and are the result of variations in the speed of the plastic’s flow within the mold. Varying degrees of wall thickness in the mold and slow injection speeds can also cause flow lines; as the plastic sets at different speeds in these scenarios. To prevent flow lines, manufacturers should increase injection speeds and pressure; identify areas where changes in wall thickness occur and round-off the corners, and position the gate on the thinnest walls within the tool cavity.

2. Sink Marks – Small craters or depressions in thicker parts of the molded piece caused by shrinkage in the inner areas of the piece are known as sink marks. Sink marks are typically caused by insufficient cooling and curing of the plastic. Low pressure in the cavity or high temperatures at the gate are also common causes. Sink marks can be prevented by decreasing the mold temperature and increasing the holding pressure, as well as extending the holding time. These preventative steps help ensure the piece can properly cool and cure. This can often be avoided early on by making adjustments to part design.

3. Vacuum Voids – Vacuum voids occur when air is trapped within a part. This defect occurs when layers of the plastic solidify unevenly. Casting a part with a two-piece mold that is not properly aligned can also cause vacuum voids. This issue can be avoided by utilizing a less viscous plastic; increasing the holding pressure and holding time; and correctly aligning mold pieces.

4. Surface Delamination – Aptly named, surface delamination occurs when a contaminant material is present; creating thin, excess surface layers on the part. Naturally, contaminants cannot bond with plastic, thus the foreign materials separate. While surface delamination is unsightly, it is also indicative of weaknesses within the part. The presence of the contaminant within the plastic represents a fault in the piece.

Increasing the mold temperature, pre-drying the plastic prior to molding, proper material handling, and working to smooth corners and angles within the mold to minimize dramatic changes in melt flow can all help prevent surface delamination.

5. Weld Lines – Weld lines are the points at which molten plastic meet when entering the various parts of the mold. Weld lines occur when the materials fail to bond properly or when the molten plastic partially solidifies. Increasing both temperature and injection speed can help minimize the risk of weld lines. Using material to a plastic with lower viscosity or a lower melting point can also help prevent this defect.

6. Short Shot – A short shot occurs where a molding shot does not fully fill the mold cavity. The result is an area in which there is no plastic; creating an incomplete and unusable part. Plastics that are excessively viscous, improper calibration of the shot, and poor gas venting can all contribute to short shots. In order to avoid short shots, manufacturers should carefully select materials – plastics that are less viscous and flow more easily will better fill difficult areas of the mold. The flow of the material can also be aided by increasing the mold or melt temperature. Careful mold design that accounts for adequate venting will help prevent gas from becoming trapped in the mold.

7. Warping – An undesired twisted or bent shape due to uneven shrinkage in the piece, warping is caused by inconsistent and uneven cooling of the material. To prevent warping, manufacturers should allow for adequate cooling time and design molds with consistent wall thicknesses to help encourage a uniform flow of the plastic. Care should also be taken to avoid materials that are susceptible to warping, such as semi-crystalline compounds, when they aren’t necessary for the part’s application.

8. Burn Marks – Burn marks are areas of discoloration and degradation of material resulting from excessive heat or high injection speeds. Trapped air and overheating are also known causes of burn marks. Operating at the appropriate injection speeds, properly venting and degassing, and reducing temperatures can all help prevent burn marks.

9. Jetting – Jetting occurs when improper injection speeds prevent the molten plastic from adhering to the mold. When this occurs, the solidified plastic includes waves or folds of the jet stream. Low melt temperatures and high viscosity are key contributing factors to jetting. By increasing mold and melt temperatures, as well as the gate size, jetting can be effectively avoided.

10. Flash – Molten plastic that leaves the mold cavity and remains attached to the finished product after cooling is known as flash. Materials typically escape through the parting line or ejector pin and can be caused by a mold that is not clamped together tightly enough – often older, malfunctioning molds. Excessive injection pressure can also be a factor. To help prevent flash, manufacturers should:

  • Increase the clamp pressure to ensure the mold stays shut
  • Properly maintain molds and repair those that are no longer in proper working order
  • Use good molding practices related to factors such as injection speed and gas venting


With the potential pitfalls that exist in injection molding, sound processes and practical knowledge are key to prevention. To ensure quality and accuracy in your injection molding processes, rely on New Berlin Plastics. From mold flow analysis and mold engineering to material management, New Berlin Plastics’ services and solutions have proven capable of delivering quality outcomes for an array of clients.

To learn more, contact us or check out some of our case studies at

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