Burn marks and stringing are usually process-control problems, not random defects. Burn marks typically come from trapped air, excessive heat, or too-fast filling, while stringing is usually tied to nozzle drool, excess melt temperature, poor decompression, or moisture in the resin.
For injection molded parts, the difference matters because the fixes are not the same. Burn marks usually point to trapped gas and overheating inside the cavity. Stringing usually points to melt control at the nozzle, sprue, or gate after injection.
What burn marks are
Burn marks are dark brown, black, or rust-colored discolorations on the molded part. They often appear at the end of fill, near trapped air pockets, or in areas where gas cannot escape before the melt compresses it.
The underlying cause is often the diesel effect, where trapped air becomes highly compressed, superheats, and scorches the plastic at the front edge of the flow. That is why burn marks usually point to venting, filling speed, or temperature imbalance.
What usually causes burn marks
The most common processing causes include injection speed that is too high, melt temperature that is too high, excessive back pressure, poor venting, and improper purging between material or color changes.
When the fill is too aggressive, air can be trapped and compressed faster than it can escape. When the melt or barrel temperature is too high, resin can degrade before or during filling. When purging is not done thoroughly, burnt residue from a previous resin or color can carry into the next run.
What stringing is
Stringing is the thin strand of plastic that hangs from the nozzle or sprue area when the mold opens or the machine retracts. It usually shows up as drool or a filament at the gate/nozzle interface rather than as a surface defect on the molded part.
In many cases, stringing indicates that the melt is not freezing off cleanly at the nozzle or gate. That can happen when the nozzle is too hot, decompression is not tuned correctly, or the resin is wet or unstable.
What usually causes stringing
The most common causes are nozzle or front-barrel temperature that is too high, decompression settings that are too low or not tuned to the resin, gate or nozzle geometry that allows drool, wet resin, and worn screw, barrel, or nozzle components that reduce melt control.
Stringing is often small, but it can slow production, create gate-area cleanup, or point to a broader instability in the process window.
The biggest difference between them
Burn marks usually come from trapped gas overheating inside the cavity. Stringing usually comes from melt control problems at the nozzle or gate during or after injection.
That distinction matters because burn marks usually push the team toward better venting, lower heat, or slower fill. Stringing usually pushes the team toward better nozzle temperature control, proper decompression, and cleaner shutoff behavior.
Practical fixes that usually help
For burn marks, improve venting at last-to-fill areas, reduce injection speed if fill is too aggressive, lower melt temperature if the resin is overheating, check for trapped gas pockets and gate placement issues, and purge thoroughly when changing materials or colors.
For stringing, lower the front nozzle temperature if the melt is drooling, tune decompression or suck-back, check gate freeze-off and nozzle tip condition, verify that the resin is dry, and inspect for barrel or screw wear if the problem keeps returning.
Jaycon-style takeaway
At Jaycon, the first question is whether the defect is telling the story of trapped gas or uncontrolled melt. Burn marks usually mean the part or tool is not venting well enough, while stringing usually means the process is holding melt too hot or too loose at the nozzle.
The best prevention is to treat both defects as signs of process imbalance, not just cosmetic noise. When venting, temperature, decompression, and resin condition are controlled correctly, these defects drop quickly and the process becomes more stable.
Need help reviewing a molded part for processing risks, venting issues, or DFM concerns? Upload your CAD and project details at jaycon.com/quote to start a quote and DFM review.