April 15, 2024

What Are the Common Defects in Plastic Injection Molding and How Can They Be Prevented

A plastic injection molding specialist holding up a plastic part that has a defect.

Plastic injection molding is a common manufacturing process used to produce a wide variety of plastic parts and products. The process involves injecting molten plastic material at high pressure into a mold cavity, where it cools and hardens into the final part shape. Properly designing and operating injection molds is critical for manufacturing high-quality parts efficiently and cost-effectively.

As with any manufacturing process, defects can occur in injection molded parts if equipment or processes are not adequately controlled. Common defects include rough surfaces, voids from improper filling, sink marks from uneven cooling, flash from mold separation issues, and warpages from internal stresses. These defects can lead to functionality issues, poor aesthetics, additional processing requirements, and rejected parts.

Understanding what causes these defects and how to prevent them through proper mold design and process parameter selection is key to achieving high production yields. This knowledge enables more cost-effective production of complex, reliable plastic components. A thorough review of common injection molding defects can provide crucial insight for engineers and operators in the plastics processing industry looking to optimize quality control procedures and produce consistent, functional parts within specifications.

Types of Common Plastic Injection Molding Defects

While injection molding efficiently and precisely produces plastic parts, defects can still occur without proper control. The following outlines critical quality issues seen in injection molding processes, their typical root causes, and general guidelines for preventing their occurrence through design and process adjustments.


One of the most prevalent defects in injection molded parts is flash. Flash refers to thin excess plastic that leaks between the surfaces of the mold halves during injection. This flash solidifies and remains stuck to the finished part once it is ejected from the mold, so it must be removed through additional processing.

Flash is typically caused by inadequate clamping force on the mold, worn mold components, or uneven separation between cavities. Without sufficient clamp force, the high-pressure injection of plastic can cause the mold halves to open slightly. This issue allows the material to seep into these opened gaps, forming thin protrusions of hardened flash on the final part. Over many cycles, flash can erode the molding surfaces as excess material is torn away post-ejection.

Proper mold maintenance and machine operation help prevent flash occurrences. Ensuring adequate clamping force is applied to the mold minimizes the plates’ ability to separate from injection pressure. Tight tolerances between mold components when new are also critical.

Over time, worn platens, sprue bushings, and dynamic seals should be replaced to maintain tight closure between surfaces.

Proper machine parameters should be observed during setup, and factors such as injection rate and pressure should be monitored. With controlled process conditions and vigilant mold maintenance, instances of flash defects can be significantly reduced. Implementing these best practices minimizes undesired flash formation, avoids surfaced damage, and improves overall part quality.

Sink Marks

Another common injection molding defect is the appearance of sink marks—small, localized depressions that form on the surface of parts. These occur when thicker regions cool and solidify slower than surrounding thinner sections. Due to the density differences between amorphous molten plastic and rigid semi-crystalline plastic, localized shrinkage is created as thicker-walled areas cool, causing surface indentations.

Uneven part rigidity and hotter material temperatures in thicker cross-sections cause it to sink. As the plastic flows more readily into thicker areas and maintains heat longer, it continues to shrink and consolidate, pulling away from and deforming the surface. Differential cooling rates thus create an imbalance of shrinkage strains, bending less viscous and hotter material inward.

Preventing sink mark defects requires mitigating uneven cooling effects. Optimizing gate locations during design to ensure balanced filling is critical. The addition of proper venting channels also aids cooling uniformity. Most importantly, strategically placing cooling lines close to thicker features rapidly extracts heat, reducing material viscosity differences between thick and thin areas.

Promoting even solidification minimizes uneven strains, which produce sinks. Careful analysis of thermal gradients using molding simulation software should guide the placement of conformal cooling channels. These proactive design steps drastically improve surface quality by minimizing surface deformation from hot spots.


A third common injection molding quality issue is warpage–the dimensional distortion or twisting of parts after ejection. Unlike sinks and flashes, which are localized surface defects, warpage refers to the overall deviation from the intended shape, compromising function. This issue occurs when asymmetric internal stresses produce uneven shrinking strains as parts cool post-molding.

Unbalanced flow front velocities during cavity filling produce uneven packing densities within the part volume. Along with irregular wall thicknesses or asymmetric geometries, this creates an imbalance of residual stress from the molten state. As sections with greater shrinkage strains solidify, they induce deformations from the intended shape, which magnify as overall rigidity increases. Investing more material or fast cooling thin sections yields warpage toward thick areas upon demolding.

Preventing warpage requires mitigating uneven cooling strains during solidification. A well-designed runner system promotes balanced flow fronts, steadily filling the cavity. Uniform wall thicknesses and radii minimize uneven rigidity/shrinkage between sections. Simulation analysis guides optimum gate numbers and locations, along with cooling channel placements, to ensure even advancement of material fronts.

By maintaining homogeneous temperature and packing densities throughout the cavity as material rheology transforms, internal strain development equalizes, sustaining dimensional integrity from model geometry. Careful mold design coupled with controlled process parameters drastically reduces warpage occurrences.

Burn Marks

Discolored brownish or black spots, known as burn marks, sometimes form on the surface of injection molded parts. These blemishes result from the plastic overheating and partially degrading within the mold. Prolonged exposure of the material to excess temperatures above the polymer’s heat deflection limit causes oxidation reactions, which alter pigmentation and generate burn spots.

Typical causes of polymer burn marks include excess melt temperatures, insufficient cooling capacity, inadequate venting, or extended resident time of the material. Hotter molten plastic increases the conductive heating of mold surfaces. Poor cooling line distribution allows localized hot zones within the cavity or thick features to be filled. Insufficient venting also prevents heated air and gasses from being released during filling, elevating local temperatures. Extended dwell of material while awaiting the next shot maintains polymer at elevated temperatures for too long.

Preventative measures for avoiding burns focus on process adjustments and mold enhancements. Setting melt and mold surface temperatures within material limits is essential for proper filling without degradation. Optimizing cooling channel placements targets hot spot areas identified through thermal analysis.

Sufficient standard or active vent ports aid cooling while allowing trapped gasses to escape. Reducing injection pressures lowers viscous heating while balancing runner diameters decreases the residence time. If defects persist, material formulation changes provide higher heat deflection options. Implementing these processing remedies minimizes the likelihood of polymer degradation, leading to aesthetic burn blemishes.

Weld Lines

Visible seam lines on the surface of parts, known as weld lines, are another injection molding defect to avoid. These form where distinct melt flow fronts converge, and bond as plastic fills the mold cavity. At this interface, where separate flow fronts meet, polymer chains do not properly diffuse across the boundary, creating a localized weak spot.

Weld lines primarily occur due to poor gate positioning, low injection pressure, low mold temperature, or complex part geometries. Restricted flow cross sections can split advancing melt fronts. Once separated, these will rejoin at the end location, fusing polymer chains across layers instead of long strands. This creates a visible seam where orientations and material properties differ from the surrounding area. Defects also arise over multiple shots as residual polymer at flow front intersections degrades.

Strategic gate placement is critical for managing the occurrence of weld seams during design. Single end-gated molds with long flow lengths readily generate welds. Converting these to multi-gated systems promotes continuous front advancement, avoiding separation. Alternating or sequential valve gates help fuse previous material deposited at flow confluences.

Increasing injection fill rates and pressures join molecular strands under shear thinning conditions rather than simply bonding layers. Mold geometries should be simplified whenever possible to prevent flow divergences from producing visible seams. Guided by simulation modeling, proper mold and process adjustments significantly reduce aesthetic and functional concerns from weld line generation.

Voids/Short Shots

The final prevalent injection molding defect is voids or short-shot parts exhibiting incomplete cavity filling. Voids appear as empty pockets or sections on finished parts due to trapped gasses or insufficient penetration of melt into intricate areas. Short shots manifest as missing molded features or areas with dimensions below design tolerances. Both void and short shot occurrence signifies improper plastic flow and packing during the injection process.

Voids and short-shot regions typically form from low injection pressures, clogged gates, inadequate vents, low melt temperatures, or complex geometries restricting flow fronts. Without sufficient pressure, the melt cannot fully displace trapped air pockets, preventing packs into detailed negative features on cores.

Blocked gates also limit material advancement across mold sections, contributing to unfilled zones. Vent depths not reaching part extremities fail to evacuate all air volumes, leaving potential shrinkage voids. Lower melt temperatures increase viscosity resistance, restricting flow into far reaches of the mold. High aspect ratio pins, deep ribs, and thin-walled layouts magnify incomplete fill issues under marginal process conditions due to rapidly cooling flow paths.

Preventing void and short shot defects requires process adjustments tailored to part designs. Applying adequate injection pressures packs material into the extremities of the cavity. Ensuring gates remain unobstructed also guarantees paths for complete section filling without flow divergence. Vent channels should connect to the end of all part projections to allow air evacuation.

Melt and mold wall temperatures balance flow rates before premature freezing restricts advancement into thin extrusions. Simulations guide geometric modifications like radii additions or wall thickness adjustments in problematic locations. Following these guidelines minimizes voids and short shots by promoting comprehensive cavity penetration.

Consult The Experts at MOS Plastics

Understanding common defects in injection-molded parts is key to implementing preventive strategies for optimum quality control. Flash, sink marks, warpages, burn marks, weld lines, and voids/short shots encompass the most prevalent issues seen in production. While causes for each defect vary, most originate from non-optimal process parameters, inadequate mold designs, complex geometries, or poor flow dynamics.

Engineers at MOS Plastics possess this extensive plastics design and processing expertise. By partnering with us early in your injection molding initiative, our insights and quality program experience will set your project up for success. Reach out to our team today.