Trimmer line welds together on the spool when friction heat melts nylon strands into each other. The causes are tight or crossed winding, dry brittle line, cheap round nylon, worn eyelets and bumping on hard ground, and each has a fix.
What welding is
Trimmer line is nylon, and nylon softens at a temperature a spinning trimmer head reaches easily. When the line on the spool is pressed hard against itself and the head is running hot, the strands soften where they touch and fuse. The next time the head is bumped, the line will not feed, and inside the spool the coil has become a solid ring or a lump. That is welding, and it is caused by heat plus pressure. Everything that fixes it either lowers the heat, lowers the pressure between strands, or uses a line that tolerates both.
Cause one: winding too tight or crossed
The spool is the usual culprit. Line wound under heavy tension presses strand on strand, and line wound so that layers cross makes small points of high pressure where two strands cross at an angle. Both are where welds start. Wind the line firmly but not under strain, in flat even layers that run side by side like thread on a bobbin, and never let a turn cross over the layer below. Leave a little space under the cap; an overfilled spool packs the outer turns against the housing. Most heads have arrows showing the winding direction, and winding against them makes the line bind and heat at the eyelets.
Cause two: dry, old line
Nylon absorbs a small amount of water from the air, and that water is what keeps it flexible. Line that has sat for a year in a hot garage or on a store shelf dries out, turns brittle, snaps at the eyelet and, because it is stiffer, presses harder on the spool and welds. The fix is an overnight soak in a bucket of water before winding a new spool. Wipe the surface dry, wind it, and store the rest of the roll in a sealed bag with a damp cloth or in a cool cupboard.
Cause three: cheap round nylon
Bargain round line is plain nylon with the lowest melting point and the least abrasion resistance, and it welds first. Co-polymer lines blend nylon with tougher plastics and tolerate heat better. Twisted lines and square or multi-sided lines present less flat contact between strands on the spool, so there is less area for a weld to form, and they cut cleaner and quieter. For a trimmer that welds, a change of line to a heat-rated co-polymer of the diameter the maker specifies is often the whole cure.
Cause four: heat from the head
The spool gets hot from friction at the eyelets, where the line rubs as the head spins, and from the head itself when it is pushed into fences, walls and bare ground. Worn eyelets with grooves or sharp edges add friction and heat. Running at full throttle with line that has worn short raises head speed and friction while the line does little work. Keep the eyelets smooth and replace them when grooved; most heads sell eyelet kits. Bump for new line before it wears to a stub, trim at partial throttle in light grass, and keep the head off concrete and gravel, which heat it fastest.
Cause five: the wrong diameter or head
Line thicker than the head is designed for packs in tighter and feeds under more friction. Line thinner than specified wears short fast and heats. Use the diameter stamped on the head or printed in the manual. A worn spool with a cracked flange, or a head with a sticky bump mechanism, forces the line to stall and heat even when it is wound well; a new spool or head costs little and often ends the problem.
A winding method that stops welding
Cut the length the manual gives, usually 15 to 20 feet for a single-line head, and soak it overnight. Hook the center in the spool notch and wind both halves in the direction of the arrows, firmly, in flat parallel layers, keeping the two halves in their own channels. Stop with a quarter inch of space under the cap and park the ends in the holding slots. Thread through the eyelets and fit the cap. A spool wound this way, in co-polymer line of the right diameter, with clean eyelets, rarely welds.
When it is not welding
Line that will not feed can also be a bump head with a worn spring or a jammed cap, line that has snapped inside the eyelet, or two halves of line that have crossed channels. If the spool opens and the line is loose and free, the fault is the mechanism, not a weld. Clean the head, check the spring and the cap tabs, and rewind.
How we research
We compare published manufacturer specifications, official manuals, warranty information and recurring owner feedback. Rankings are research-based unless original YardToolLab test evidence is explicitly shown on the page.
FAQs
The line refuses to feed when the head is bumped, and when the spool is opened the strands are fused into a block or stuck in a ring. Pulling the free end does nothing.
Yes. Nylon loses moisture in storage and becomes brittle; an overnight soak restores flexibility so it breaks and welds less. Dry the surface before winding.
Co-polymer and twisted or square lines rated for high temperatures. Plain round nylon from a bargain spool welds most.
Firm and even, in neat layers that never cross, with a little space left under the cap. Tight, crossed winding is the first cause of welding.