Diagnosing and Preventing 10 Common Injection Molding Defects

Injection molding is an efficient, high-volume manufacturing process for producing plastic parts. However, small variations in process parameters can result in defects. Recognizing common flaws and their causes allows technicians to troubleshoot issues quickly. This guide examines frequent injection molding defects, how to identify them, and strategies to prevent them.

Detecting Flash in Molded Parts

Flash refers to excess material leaking along mold parting lines and seams. It is typically caused by the high injection pressure forcing molten plastic to escape through small gaps in the mold tooling. Worn or damaged mold components can also create gaps around the shut-offs for material to leak through.

flash of injection molding

Adjusting the injection pressure and flow rate to the optimal settings can help prevent flash. Improving the mold clamping force and alignment ensures the tool halves close tightly without gaps.

Replacing any worn mold components around the cavities eliminates potential openings for flash. Proper lubrication of the mold is also important to allow clean separation after the material has solidified.

How Short Shots Reveal Low Fill

short shot

Short shots are incomplete molded parts where the plastic melt has failed to entirely fill the mold cavity during injection. Short shots commonly occur when the injection pressure is too low and there is insufficient force filling the tool.

Another potential cause is low melt temperature that leads to premature freezing and interruption of the fill pattern. Obstructions within the mold runner system like debris blocking a gate can also impede complete cavity filling.

Typical solutions for short shot issues include increasing the injection fill pressure to improve the mold pack, raising barrel and melt temperatures to avoid early freeze off, and clearing any blockages within the runners or gates.

Adjusting the fill time and flow rate settings can also help ensure the plastic has sufficient time to completely fill every detail of the mold tool before freezing.

Sink Marks and Shrinkage Causing Depressions

Sink marks describe spots on the surface of an injection molded part that have slightly sunk in or depressed. This happens as the outer sections of the part make contact with the cool mold surfaces and solidify rapidly.

However, the interior core areas remain molten longer and shrink later on in the cycle after the outer layers have already set. This shrinkage in the core pulls on the solid outer sections, causing deformations and sinks. Thicker regions of a part are more prone to sink marks because their interior mass requires more time to cool and shrink.

Solutions for reducing sink marks include using faster mold cooling to increase the solidification rate of the core and minimize variation between sections. Materials with less shrinkage properties can also be selected to avoid pulls. Strategic mold design modifications like coring out heavy walls speeds external cooling of thick areas.

Warpage from Uneven Shrinkage Rates

Similarly to sink marks, warpage in molded parts refers to an overall distortion from uneven shrinkage rates as the material cools. As some sections like corners and extremities shrink more quickly than other areas, it creates internal stresses that lead to part bending or twisting out of shape.

Strategic changes to wall thickness and gating locations can promote more uniform cooling across sections and reduce shrinkage gradients. Clamping pressure can also be increased to hold parts tightly in place as they cool, resisting any warping out of the mold.

Post-molding stress relieving and annealing processes help remove and minimize the internal stresses from uneven cooling rates that cause warpage and distortion.

Burn Marks from Excessive Temperatures

Burn marks or black streaking on the surface of molded parts are indicative of material degradation from excessive exposure to heat. Keeping the plastic resin melt at overly high temperatures in the barrel for extended periods can cause burning, charring, and discoloration. Similarly, very high nozzle temperatures as material is injected into the mold can overheat and break down polymers at the flow front. Following recommendations from the resin supplier for optimal barrel and nozzle temperatures helps prevent burning defects. Closely monitoring the condition of the plastic material is also important to avoid contamination from degraded resin sitting too long in the barrel as well as overheating during processing.

Flow Lines Revealing Filling Pattern Issues

Flow lines, also known as weld lines, are streaks or patterns visible on the surfaces of injection molded parts. They reveal the progression of the plastic melt flow front as material filled the mold tool. Potential causes of flow lines are low injection speed and fill velocity that allows the flow front to cool prematurely, low melt temperature that increases viscosity and interrupts flow, or inadequate venting that traps gases and hinders flow.

Solutions for minimizing flow lines focus on process adjustments to smooth out filling. Increasing fill speed provides more momentum for the flowing material. Raising barrel and melt temperatures keeps the plastic more fluid as it travels through the mold.

Adding extra vents or modifying vent locations prevents trapped gases from interrupting flow and cooling the leading edges. Introducing flow leaders and enhanced contours into the mold tool itself can also help divert and reconverge flow fronts to minimize visible weld lines.

Jetting Weld Lines

Jetting defect describes severe flow lines that appear as pronounced weld line markings from the fast-moving plastic melt flow rebounding and folding back on itself. Jetting most often occurs on wide flat surfaces without obstructions when the high velocity flow impacts a slower moving front and rebounds rather than combining smoothly. Solutions for reducing jetting focus on slowing and smoothing the filling pattern. Lower cavity injection pressure prevents the abrupt collisions. Higher melt temperatures keep the flow more fluid. Slower fill times allow the material to merge more gradually. Altering gate locations introduces the flow into sections in a more controlled manner. Adding flow leaders or contours encourages the flow to gradually converge rather than jetting upon impact.

Gas Burn

Gas Burn

Gas burn causes brownish streaks and discoloration in molded parts along the flow path front. This defect occurs when trapped gases such as air, water vapor, or volatiles in the melt become compressed and combust, partially burning the plastic material in spots.

Typical causes are inadequate drying of hygroscopic resin allowing moisture absorption, high melt temperature degrading material, regrind contamination, or poor venting.

Solutions for preventing gas burn defects include both removing the gas source and improving venting. Monitoring material moisture content and maintaining levels below the threshold through proper resin drying eliminates a primary gas source. Keeping melt temperatures and nozzle heats at supplier recommended minimums also avoids material degradation.

Contaminated regrind must be kept isolated from virgin resin. Additional vents incorporated at optimal locations provide an escape path for trapped gases to prevent burning discolorations.

Interpreting Weld Lines

Weld lines in injection molded components refer to the interface where two separate melt flow fronts converged within the mold cavity before freezing. Much like flow lines, prominent weld lines indicate issues with internal knit lines and merging of the filling pattern flows.

Optimizing process settings such as fill speed, pressure, melt temperature, and venting locations helps reduce weld lines by smoothing the convergence of flow fronts. Altering gate positions or adding additional gates changes how the cavity is filled and can eliminate problem knit lines.

Streamlining part geometries to avoid drastic flow obstructions and variations in wall thickness also enhances flow front merging. In some cases, a small witness line may still remain at flow interfaces even with process optimization.

Importance of Machine

Maintenance While adjusting process settings can resolve many injection molding defects and flaws, component wear and regular maintenance of the equipment is also critical for consistency.

Normal fatigue of common wear parts like nozzle tips, check valves, seals, and clamp mechanism components can degrade performance and quality over time.

Preventative maintenance procedures and scheduled replacement of standard consumable tooling components ensures machines remain capable of producing high quality parts throughout long production runs.

In summary, recognizing injection molding defects points to flaws in the overall process. Addressing these common defects through parameter adjustments, tooling fixes, resin changes, and maintenance improves part quality and consistency.