Why PVC is welded rather than glued
PVC window frames and doors are joined by fusion welding, not by adhesive or mechanical fasteners. The reason is that PVC is a thermoplastic with a sharp melting range, and a correctly made weld reaches 70 – 90 % of the parent material's strength — which no adhesive achieves on PVC at a competitive cost.
The process is simple to describe and hard to control: two mitred ends are held against a heated plate until the material softens, the plate is withdrawn, the ends are pushed together, and the joint is held under pressure until it freezes. Everything about the joint's strength comes from four numbers, and all four are on the machine's controller.
The four numbers: plate temperature 240 – 260 °C, heating time (set by the wall thickness), changeover time under 4 seconds, and welding pressure 2.5 – 4 bar. Changeover time is the one that operators get wrong, because it is a mechanical action rather than a setting, and a slow operator produces weak corners that look identical to strong ones.
The welding cycle, step by step
| Шаг | What happens | Typical value | What goes wrong |
|---|---|---|---|
| 1. Clamp | Profile is held in the welding heads | Clamping pressure 4 – 6 bar | Too little: profile shifts, corner offset |
| 2. Heat | Plate contacts both ends, material melts | 240 – 260 °C, 20 – 45 s | Too cold or too short: no fusion. Too hot: degraded material |
| 3. Preheat / melt bead | A bead forms on each end | Bead 2 – 4 mm | Bead too small: insufficient melt depth |
| 4. Changeover | Plate withdraws, ends meet | < 4 s | Too slow: melt skins over, weld is a cold joint |
| 5. Weld | Ends pressed together | 2.5 – 4 bar, 20 – 35 s | Too little pressure: weak. Too much: profile collapses |
| 6. Cool | Joint held under pressure | Until the bead is rigid | Released early: joint opens as it cools |
Table 1 — The welding cycle. Step 4 is the one that is not a setting and therefore the one that fails.
The heating time is a function of wall thickness, not of the profile's overall size. The melt front has to penetrate the full wall on both faces, and PVC conducts heat slowly. A rough rule is 1 second of heating per 0.1 mm of wall thickness at 250 °C, plus a few seconds to establish contact — so a 3 mm wall needs about 30 seconds, and a 2.5 mm wall about 25.
Changeover time is the quality killer. When the plate withdraws, the molten surface starts to oxidise and skin over within about two seconds. If the ends do not meet before that skin forms, the weld is a butt joint between two solid skins rather than a fusion of two melts. It looks fine, it machines fine, and it fails at 30 % of the expected corner strength. On a manual machine with an operator moving a heavy frame, this is exactly what happens at the end of a shift.
Corner strength: what to expect
| Test | Метод | Typical result | Acceptance |
|---|---|---|---|
| Corner tensile | Pull the corner apart in tension | 2,500 – 4,500 N | ≥ 3,000 N for a standard sash |
| Corner bending | Three-point bend on the corner | — | No cracking at the weld |
| Weld seam inspection | Section the corner and examine | Full fusion across the wall | No visible interface line |
| Drop test | Drop the assembled frame | — | No corner failure |
| Water tightness | Spray test on the assembled window | — | No ingress at the corner |
Table 2 — Corner tests. The sectioned corner is the only one that shows why a joint is weak.
The sectioned corner is worth the effort because it distinguishes the two failure modes. A joint with a visible interface line across the wall was welded with too short a heating time or too slow a changeover — the melt depth was insufficient. A joint with a porous, chalky zone was overheated and the PVC degraded. Same low strength, opposite fixes.
Cleaning and machining
| Operation | Метод | Tolerance / result |
|---|---|---|
| Bead removal, top and bottom | Fixed or floating cutter | Flush within 0.1 mm |
| Bead removal, outside corner | Rotating cutter or router | Radius must match the profile |
| Corner cleaning (welded corner) | Multi-axis CNC cleaning machine | Removes the weld bead on all faces |
| V-notch drainage | Router, 3 – 5 mm | Required for water egress |
| Drain slots | Drill or punch | Aligned with the frame chamber |
| Hardware holes | Drill or punch | ±0.2 mm |
| Surface cleaning | Solvent wipe or aqueous | No aggressive solvent on stressed PVC |
Table 3 — Post-weld operations. The corner cleaning machine is the largest capital item after the welder.
Corner cleaning is where most fabricators lose money without noticing. A four-head cleaning machine costs €60,000 – €150,000 and removes the bead from both faces and the outer corner in one cycle. Doing it by hand takes 40 – 90 seconds per corner and leaves a surface that is visibly different from the extruded face — which customers see on a white window in daylight.
Weld defects and their causes
| Дефект | Внешний вид | Причина | Fix |
|---|---|---|---|
| Weak corner | Fails the tensile test | Changeover too slow, or heating time too short | Speed the changeover; add 3 – 5 s heating |
| Visible weld line after cleaning | A faint line across the face | Insufficient melt depth | Longer heating; check plate temperature |
| Yellow / brown weld | Discoloured joint | Plate too hot, or long contact time | Lower plate to 245 °C; reduce heating time |
| Porous, chalky weld | Crumbly material at the joint | Overheating — PVC degraded | Lower temperature; check the plate for hot spots |
| Corner offset | The two profiles do not align | Clamping pressure too low, or profile out of tolerance | Increase clamping; check profile straightness |
| Collapsed wall | Dent at the weld | Weld pressure too high | Reduce to 2.5 – 3 bar |
| Joint opens later | Gap appears after weeks | Released before cooling, or high residual stress in the profile | Extend the cooling hold; test the profile's heat reversion |
| Bead tear-out | Chunk missing at the corner | Cleaning cutter too dull or too deep | Sharpen or replace; set the depth |
Table 4 — Weld defects. "Joint opens later" is usually an extrusion problem, not a welding problem.
That last row is worth dwelling on. A corner that opens after a few weeks is often blamed on the welder, and often the welder is fine. If the profile left the extrusion line with high frozen-in stress, it will slowly revert — and a welded corner is the only place where that reversion can express itself as a visible gap. The test is EN 478 heat reversion on the raw profile: under 2 % is good, 4 – 6 % means the extrusion line is running the profile too cold or cooling it asymmetrically.
Myth: "a stronger weld needs more pressure." Weld strength comes from melt depth and speed of changeover, not from pressure. Excess pressure squeezes the melt out of the joint, collapses the chamber walls and produces a corner that looks tight and is thin. The pressure is there to ensure contact, not to force fusion.
The rest of the fabrication sequence
| Шаг | Equipment | Примечания |
|---|---|---|
| Cutting to length | Double-mitre saw | ±0.5 mm; the weld allowance is built into the cut |
| Steel insertion | Manual or automatic | Continuous through corners; fix with screws |
| Drainage and hardware machining | CNC machining centre | Before welding — much easier than after |
| Welding | Single or four-head welder | Four-head for volume; single-head for large frames |
| Corner cleaning | Cleaning machine | Both faces plus the outer radius |
| Gasket installation | Manual or roller | Must be continuous at the corners |
| Glazing | Setting blocks, beads | Blocks at the quarter points, not the corners |
| Hardware | Manual | Adjust after glazing, not before |
| Final test | Air/water test rig | Sample basis, not 100 % |
Table 5 — The fabrication sequence. Machining before welding is the ordering that saves the most time.
The ordering matters. Drilling a drain slot into an assembled frame means working around a corner that is already welded and cleaned; doing it on a straight length of profile takes a fraction of the time and leaves a cleaner edge. Fabricators who machine after welding usually do it because their machine cannot handle a long length — which is a layout problem, not a process one.
Choosing a welder
| Тип | Capacity | Стоимость | Лучшее для |
|---|---|---|---|
| Single-head, manual | 1 corner per cycle | €8,000 – €20,000 | Small shops, large frames, prototypes |
| Two-head | 2 corners | €20,000 – €45,000 | Medium volume, long frames |
| Four-head | 4 corners, complete frame | €45,000 – €120,000 | Window production at volume |
| Four-head with cleaning | Weld plus clean in one cell | €120,000 – €250,000 | Automated lines |
| CNC welding centre | Weld, clean, machine | €250,000+ | High-volume, few variants |
Table 6 — Welder types. A four-head machine is only economic when the frame sizes are consistent.
The constraint on a four-head welder is frame size range. Each head has a limited travel, so a machine set up for standard windows cannot weld a 2.5 m patio door without changeover. That is why most fabricators run a four-head for standard product and a single-head for the large and bespoke work — and why "we can weld anything" from a supplier usually means they have both.
Frequently asked questions
What is the correct welding plate temperature?
240 – 260 °C for standard rigid PVC. Below 235 °C the melt depth is insufficient and the joint is weak; above 265 °C the PVC starts to degrade and the weld becomes porous. The plate should be checked with a contact thermometer, not with the machine's display, because plate temperature drifts as the plates foul.
Why do my corners open after a few weeks?
Two candidates. If the joint was released before it cooled, the weld was never fully fused. If the profile has high residual stress, it reverts slowly and the corner is where that shows. Test the raw profile for heat reversion (EN 478) — over 2 % means the extrusion line is the problem, not the welder.
Can PVC be solvent welded instead?
Yes, for non-structural joints and for repairs. Solvent cement gives a good bond on PVC but it does not reach the strength of a fusion weld, and it leaves a visible joint. Windows and doors are always fusion welded; solvent welding is used for trim, boxes and site repairs.
How long should the weld be held before release?
Until the bead is rigid to the touch — typically 20 – 35 seconds for a standard window profile, longer for a thick door section. Releasing early is a false economy: the joint opens by a fraction of a millimetre, the cleaning machine then cuts into a joint that is still moving, and the corner fails inspection a week later.
Does the steel need to be in place before welding?
Usually yes, because the corner connector is screwed through the steel. Welding a profile with the steel already inside is normal; the steel does not interfere with the weld because it sits in the chamber, not at the joint face. Check that the steel does not protrude into the weld zone — that is a common cause of a corner that will not close.
What is the weld allowance?
The amount added to the cut length to allow for material consumed in the weld bead. It is typically 2 – 3 mm per end for a standard profile, and it is built into the cutting list rather than set on the saw. If the finished frame is undersized, the allowance is wrong, not the saw.
Can different colour profiles be welded together?
Yes, if they are the same base formulation. Welding white to a laminated profile works, but the weld bead removal exposes the core colour, which is visible on a laminated profile. That is why laminated windows usually have the laminate applied after welding, or use a through-coloured profile.
How strong is a welded corner compared with the profile?
70 – 90 % of the parent material's strength when the weld is made correctly. That is strong enough that a corner almost never fails before the surrounding profile does. When a corner fails, it is nearly always a process fault rather than a design one.
Fabricating PVC windows and fighting corner strength?
Tell us the profile, the welder and the failure mode. We will tell you whether the problem is the weld parameters, the machine or the profile's residual stress — and which is cheapest to fix.
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