Melt Degradation in the Barrel: Causes and Fixes

Melt degradation is thermal or hydrolytic breakdown of the material while it sits in the barrel, showing up as black specks, streaks, a color shift, or reduced mechanical properties in the finished part. The most common causes are the material spending too long in the barrel relative to the shot size, melt temperature set higher than the material needs, or residual moisture causing hydrolytic breakdown that looks similar but has a different fix.

TL;DR - check these first:

• Compare shot size against barrel capacity - a small shot in an oversized barrel means long residence time
• Check actual melt temperature against the material's data sheet range, not just the barrel zone setpoints
• Rule out residual moisture with a dew-point/dryer check before changing anything on the machine
• Look for degradation that gets worse after a stoppage or slow cycle, which points to residence time
• Check screw speed and back pressure for excess shear heating beyond what the barrel heaters are set to

Is excessive residence time in the barrel the cause?

Material that sits in the barrel far longer than the shot itself requires keeps absorbing heat and staying molten, and the longer it's held at melt temperature, the more it degrades - independent of what the temperature setpoint actually is.

How to confirm: Calculate the ratio of shot volume to barrel capacity, and check whether degradation is worse after a stoppage, a slow cycle, or a changeover - all situations where residence time stretches out.

Fix: Match barrel size to the shot rather than always running a large barrel for flexibility; on machines with a full-electric drive, consistent, repeatable screw recovery keeps residence time predictable shot to shot instead of drifting with cycle variation.

Is melt temperature set higher than the material needs?

Every material has a processing window from its data sheet; running noticeably above the upper end accelerates thermal breakdown, particularly when combined with time spent waiting in the barrel.

How to confirm: Compare the actual melt temperature (measured, not just the barrel zone setpoints) against the material's recommended range, and check whether degradation reduces as temperature is lowered toward that range.

Fix: Set barrel zone temperatures within the material's data sheet range, and confirm actual melt temperature with a probe rather than assuming the barrel zones reflect it exactly.

Is residual moisture causing hydrolytic rather than thermal degradation?

Hygroscopic materials that haven't been dried to the correct dew point break down through hydrolysis in the barrel, producing symptoms - splay, brittleness, reduced properties - that can look like thermal degradation but need a different fix.

How to confirm: Check the dryer's actual dew point and drying time against the material's requirement rather than assuming the dryer is working correctly; see Dryer dew point too high for the full diagnostic.

Fix: Correct drying first - dew point, temperature and residence time in the dryer - before adjusting barrel temperature or screw settings, since a moisture-driven problem won't respond to a temperature change on the molding machine.

Cause overview

Perotec Systems supplies Japan Steel Works (JSW) full-electric injection molding machines with consistent, repeatable screw recovery, and Plastic Systems dryers for correct dew point control. See also Dryer dew point too high for related moisture diagnostics. Contact us with your material, cycle time and shot size relative to barrel capacity - we help determine whether the cause is residence time, temperature or moisture.