Trimmer line melts and fuses inside the head from heat buildup. Learn the causes and simple habits that stop line from welding together.
Trimmer line welding together inside the head is a frustrating failure that usually shows up as a jam: the head stops feeding, or line comes out in a stiff fused clump instead of smooth individual strands. It happens when friction and heat inside the spool housing get high enough to soften the plastic line just enough for adjacent wraps to fuse.
What actually causes the welding
Trimmer line is a thermoplastic, and like most plastics it softens well before it reaches a true melting point. Inside a spinning trimmer head, line under tension can generate meaningful friction against itself, especially where wraps cross or rub during feeding. Combine that friction with a hot day, extended high-RPM operation, and a spool packed tightly enough that wraps are pressed hard against each other, and the plastic can soften enough at the contact points to fuse when it cools back down between uses.
Overfilled spools are the most common trigger
An overfilled spool is the single biggest contributor to welding, because tightly packed wraps have far more surface contact and far less room to dissipate the heat generated during feeding. When reloading a spool, wind line only to the fill line molded into most spool designs, and keep wraps flat and even rather than crossed or bunched to one side. A spool wound correctly and not overfilled runs cooler and rarely reaches the point where welding occurs, even during long, hot sessions.
Extended high-RPM runtime without a break
Running a trimmer at full throttle continuously for long stretches, particularly in hot weather, raises the temperature inside the head housing more than intermittent use does. Commercial users covering large properties in a single pass are more prone to this than homeowners doing a quick edge job, simply due to sustained runtime. Taking short breaks during long sessions, especially on hot days, gives the head a chance to shed heat before it accumulates enough to soften the line.
Line diameter and quality
Thinner line has less mass to absorb heat before reaching a softening point, so it can be somewhat more prone to welding under the same conditions compared to thicker line, though thick line is not immune if the spool is overfilled or run hard enough. Line quality also plays a role: less consistent manufacturing can produce line with a lower resistance to heat than premium options. If you are also dealing with frequent breakage in addition to welding, confirm you are using the correct diameter for your machine using our trimmer line sizing guide, since a mismatch can contribute to both problems.
How storage conditions make welding worse
Line that has already dried out slightly from poor storage is more prone to sticking and fusing under heat than fresh, properly conditioned line. Dry, brittle line generates more friction as it feeds, compounding whatever heat is already building inside the head. Our guide on storing trimmer line so it stays flexible explains how to keep spare spools in condition that resists both breaking and welding.
Fixing a spool that has already welded
If a spool has partially welded, the fused section usually needs to be cut out and discarded rather than pried apart, since forcing fused line back into shape often leaves weak points that will break again quickly. If the welding is limited to a small section near the outer wraps, the remaining unaffected line can sometimes be salvaged. For a spool welded through multiple layers, it is generally faster and more reliable to strip the head and reload fresh line correctly, following the fill-line guidance above.
Preventing repeat problems
The combination that most reliably prevents welding is a correctly filled spool, line stored in conditions that keep it flexible, and short breaks during extended high-heat sessions. If welding continues even after adjusting these habits, an underpowered or worn head straining to spin line may be generating more friction than a healthy head would; our trimmers and edgers buying guide can help you compare current models if the head itself needs replacing.
Inspecting a head after a welding incident
After clearing a welded spool, check the eyelets and internal spool cavity for residue or slight deformation before loading fresh line, since a housing that has run hot enough to weld line once may have minor internal changes that make repeat welding more likely under the same conditions. Wiping away any melted residue and confirming the spool still seats and spins freely reduces the chance of an immediate repeat failure with the next load of line.
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FAQs
Line welds together when friction and heat inside the head, usually from an overfilled spool, prolonged high-RPM running, or line spinning against itself under tension, is enough to soften the plastic and fuse adjacent wraps.
Welded line typically jams and will not feed at all, or feeds in a stiff fused clump instead of smooth individual strands. Opening the spool often shows a section where wraps have visibly melted into a single mass.
A small welded section can sometimes be cut away and the remaining good line reused, but heavily fused spools are usually faster and safer to discard and reload with fresh line rather than trying to separate melted strands.
Extended idling with the head not spinning does not typically weld line, since welding is driven by friction and heat from motion under load, not by heat from the engine or motor itself sitting nearby.
Thicker line has more mass to absorb heat before reaching a melting point, so it is somewhat more resistant to welding under similar conditions, but overfilling or prolonged hard use can still weld thick line.