Injection Molding Troubleshooting Guide

Injection molding is a highly efficient method of producing plastic components on a large scale, with consistent quality and low per-unit costs. It is widely used in various manufacturing sectors due to its precision engineering, repeatability, and scalability, which have transformed production lines worldwide.

Over the course of production, manufacturers often encounter various issues that can end up halting operations abruptly. These issues can lead to significant delays and increased costs, disrupting timelines and budgets. This is why it’s critical to resolve these technical hiccups swiftly and efficiently in order to maintain productivity and profitability.

In this blog, we’ve compiled a list of some of the most frequently encountered challenges in injection molding to help diagnose these issues quickly and prevent costly delays.

Why Regular Machine Maintenance is Important

Injection molding requires regular maintenance to ensure that the mold and machine function efficiently and produce high-quality parts. A well-maintained mold is crucial for consistent part quality and quicker cycle times, ultimately contributing to increased productivity. Machine maintenance is equally crucial since it addresses issues that can disrupt production and compromise efficiency.

By conducting routine checks and preventative measures, manufacturers can prevent problems that would otherwise require extensive troubleshooting, reducing downtime and production delays. Proactively caring for molds and machines ensures the integrity of the products and optimizes operational performance.

The long-term benefits of regular maintenance include enhanced reliability, reduced repair costs, and uninterrupted production schedules, all of which highlight its significance in sustaining the quality and efficiency of injection molding processes.

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Common Issues in Injection Molding

Below, you’ll find some of the most common issues that lead to product defects during injection molding:

Problem	Cause	Solution
Warpage – refers to the undesired deformation or distortion that can cause the parts to bend, fold, twist, or bow, leading to complications during the final assembly of the end product.	Under Packing	To better pack out the parts, increase pressures and times.
	High Melt Temperature	To minimize shrinkage, decrease melt temperature and increase cooling time.


	Gate Freeze-Off	To increase the amount of time there is to fill and pack, consider increasing gate size or decreasing land length.
	Gate Freeze-Off
	Unbalanced Runner	Ensure that all cavities fill at the same rate by adjusting the volumes of the runner system.
	Non-Uniform Cooling	Check the line layout and measure the temperature and water throughput of each half.
	Over Packing	To reduce molded-in stress, back off the second-stage time and pressure.
Sink Marks – slight depressions or indents on the surface of molded parts .	Insufficient Feed	Increase shot size and/or decrease cushion to push more resin into the cavity. Ensure consistent cushion size.
	Insufficient Pressure	Increase the screw forward time under pressures to ensure gate freeze-off is obtained.
	Cold Mold	Slow the cooling/increase mold temp so that the outer skin and the interior of the part shrink at the same rate.
	High Melt Temperature	Lower the melt temperature to, in effect, lower the amount of heat that needs to be pulled from the part. This will help reduce differential cooling and shrinkage.
	Runner & Gate Size	Increase the runner and gate sizes to get best pressure transfer to the part in the cavity of the mold.
	Poor Part Design	Use nominal wall design principles with standing features at 50% of nominal wall thickness.
Bubbles and Voids – trapped air or insufficient material creates hollow areas within a molded part. Bubbles typically surface as spherical cavities, whereas voids are larger, irregularly shaped cavities found within the core.	Low Pressure	Can be resolved by increasing injection, hold and pack pressures and times. Part weight should also be monitored until it plateaus to ensure complete cavity filling through gate freeze-off.
	Insufficient Volume	Can be addressed by increasing shot size and/or lowering the cushion to allow more rein volume into the cavity.
	Low Mold Temperature	Increase mold temperature to slow down the cooling of the skin, making the material inside less prone to pulling away from itself.
	High Melt Temperature	Decrease melt temperature to ensure minimal shrinkage and faster curing of the part.
	Thick Wall Sections	Design the part with uniform wall thickness, ensuring ribs are on 50% of nominal wall thickness.
Burn Marks – characterized by black or rust-colored discolorations on the final plastic product.	High Injection Velocity	Lower the injection speed to allow trapped air in the mold to escape and reduce the shear heating of the resin.
	Insufficient Venting	Increasing the size, number and/or changing the location of the vents.
	Excessive Residence Time	Reduce the amount of time the resin stays in the barrel by speeding up the cycle or decreasing the barrel size.
	Excessive Melt Temperature	Lower the barrel heaters, screw speed, and the back pressure to reduce resin temperature.
	Damaged Heater Band	Damaged heater bands can be checked and replaced if necessary to prevent overriding or burnout.
	Dead Zones	Inspect the resin’s path to ensure there are no dead zones present to prevent collecting and degrading material that breaks free on injection.
Color Inconsistencies – pigment distribution in molten plastic is uneven, leading to variations in hue or intensity across different parts of a molded item.	Contamination	Purge out the injection barrel until the resin runs clean.
	Burning	Lower the melt temperature, screw speed, and/or back pressure. Injection velocity can also be decreased.
	Long Residence Time	Reduce the cycle time. Use a small-volume barrel. You can also use a reverse temperature profile.
	Not Enough Heat Soak	Movieto a larger barrel volume to aid the dispersion of additives and reduce severe shear heating when the shot capacity is too small.
	Heater Band Override	Verify the proper screw design and raise the rear zone temperatures to lower the viscosity of the resin and shear effects.