The problem
Symptom. A window fabricator pulls four welded corners on an EN 514-style rig and watches one open at 1 450 N, far under the 2 500 N the frame order requires.
What it costs. Failed corners cannot be re-welded once cleaned, so the sash is scrapped and the whole frame order stalls while both parties argue over a plate thermometer.
This guide ties corner strength to plate temperature, heat time and cleaning depth, and gives the four numbers for the weld acceptance clause.
Read the plate thermometer before you blame the PVC
A PVC-U corner is fused by a 250 °C plate held 30 s, then the bead is cleaned. At 220 °C the melt never interdiffuses and the corner peels at 1 450 N; at 250 °C the face tears at 2 980 N, clear of a 2 500 N limit.
| Plate temperature (°C) | Corner strength (N) | Weld line appearance |
|---|---|---|
| 220 (cold) | 1 450 | Peel along the weld plane |
| 235 | 2 100 | Mixed peel and face tear |
| 250 (specified) | 2 980 | Face tears outside the weld |
| 280 (overheated) | 1 900 | Brittle browned weld line |
Table 1 - Weld plate temperature against corner strength
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Send the cross-section drawing, material, cut length and annual volume. We will review die, calibration and tolerance, then return a written tooling and production quotation.
Trace the crack from weld heat to the cleaned corner
Treat the split below as illustrative engineering guidance, not a measured survey. Typically a weak corner traces to a cold plate, short heat time, an over-deep cleaning notch and uneven join pressure, so log the plate thermometer first.
A five-step diagnostic sequence
- Read the plate thermometer.
Log the actual shoe temperature and dwell; 220 °C for 15 s instead of 250 °C for 30 s explains a 1 450 N peel before the compound is blamed. - Inspect the cleaned corner.
Measure the cleaning depth with a depth gauge; 0.6 mm taken off a 2.5 mm wall is a notch that drops strength to about 1 700 N. - Check join pressure.
Confirm 0.35 MPa held through the 30 s join; a swing from 0.25 to 0.45 MPa scatters strength corner to corner. - Pull to one method.
Test corners on an EN 514-style rig at a fixed span and rate; a 2 980 N face tear is a good weld, a 1 450 N peel is a cold one. - Write the weld clause.
Specify 250 °C, 30 s heat and join, 0.2 mm cleaning, 0.35 MPa and a 2 500 N minimum so a cracked corner is a log reading.
The chart below is a typical result from that measurement step. Use it to separate a design limit from a setting that can still be recovered.
Set the weld and cleaning parameters
Four settings hold the weld: a 250 °C plate, 30 s heat and 30 s join at 0.35 MPa, cleaning to 0.2 mm or less, and an EN 514-style test above 2 500 N. At 280 °C the polymer browns and strength falls back to 1 900 N.
| Параметр | Agreed value | What goes wrong outside it |
|---|---|---|
| Weld plate temperature | 250 °C | 220 °C gives only 1 450 N; 280 °C degrades to 1 900 N |
| Heating / joining time | 30 s heat, 30 s join | 15 s heat leaves a cold weld peeling at 1 450 N |
| Corner cleaning depth | 0.2 mm or less | 0.6 mm off a 2.5 mm wall drops strength to 1 700 N |
| EN 514-style corner strength | 2 500 N per corner minimum | Below 2 400 N the frame fails the rig |
Table 2 - Weld and cleaning settings for the corner acceptance test
Prove it with a plate-temperature trial
Run a plate-temperature trial with one profile: weld corners at 220, 235, 250 and 280 °C and pull each on the rig. Strength reads 1 450, 2 100, 2 980 then 1 900 N, so the 2 500 N window sits between roughly 240 and 265 °C.
| Test-rig symptom | Most likely cause | Check that confirms it |
|---|---|---|
| Corner peels apart at low load | Plate too cold or heat time short | Plate log reads 220 °C for 15 s instead of 250 °C / 30 s |
| Weld line brown and brittle | Plate overheated | Contact thermometer reads 280 °C at the shoe |
| Face cracks beside a clean bead | Corner cleaning cut a notch | Depth gauge shows 0.6 mm removed from a 2.5 mm wall |
| Strength varies corner to corner | Uneven join pressure | Pressure gauge swings 0.25 to 0.45 MPa |
Table 3 - Matching the test-rig failure to the weld fault
Write the weld acceptance test into the order
Write the weld test into the order: plate temperature, heat and join times, a cleaning depth of 0.2 mm and an EN 514-style minimum of 2 500 N. Both parties pulling the same rig turn a cracked corner into a force reading, not an opinion.
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Send us the section and the installation conditions. We will review wall balance, tolerance and fixing detail, then return a marked-up drawing with a tooling quotation.
What to send us
- Cross-section drawing with wall thicknesses
- Material and colour
- Installation temperature range
- Fixing method and run length
What a controlled weld saves over a frame contract
A controlled weld protects the schedule. Cleaning 0.6 mm from a 2.5 mm wall leaves 1.9 mm, so bending strength falls by (1.9 / 2.5) squared = 0.58 to about 1 700 N; a 0.2 mm skim keeps 2 980 N and 97% of corners passing.
Before you release this extruded profile
- A 250 °C weld plate with 30 s heat and 30 s join is specified
- Join pressure of 0.35 MPa and its tolerance appear in the process sheet
- Corner cleaning depth is capped at 0.2 mm on the section drawing
- An EN 514-style corner strength minimum of 2 500 N is in the order
- A retained welded corner from the first article is held by both parties
Write the measurement temperature and the tolerance into the drawing; most disputes are really about how the part was measured.
Why did my welded corner crack at 1 450 N?
Most likely a cold plate or short dwell. At 220 °C for 15 s the melt never interdiffuses, so the corner peels at 1 450 N instead of tearing the face at 2 980 N.
What corner strength should I specify?
At least 2 500 N on an EN 514-style rig. A correctly welded 250 °C corner reaches 2 980 N; 220 °C gives 1 450 N and 280 °C degrades to 1 900 N.
Does corner cleaning really reduce strength?
Yes. Cleaning 0.6 mm from a 2.5 mm wall leaves 1.9 mm, and bending strength scales with thickness squared: (1.9 / 2.5) squared = 0.58, so 2 980 N falls to about 1 700 N.
What weld settings hold the window?
A 250 °C plate, 30 s heat, 30 s join at 0.35 MPa and a cleaning skim of 0.2 mm or less. That combination keeps the corner near 2 980 N.
Is a hotter plate better?
No. Strength peaks near 250 °C and falls to 1 900 N by 280 °C as PVC-U browns and embrittles. Step the plate in 15 K stages and pull each corner.
What should the acceptance clause say?
Plate temperature, heat and join times, join pressure, a 0.2 mm cleaning limit, an EN 514-style 2 500 N minimum and a retained first-article corner.
Get a profile that holds its dimension
Send us the profile section, the length and the service conditions. We will review die, calibration and tolerance, then quote tooling and production with a defined measurement method.
To get an accurate quote, send:
- Profile cross-section drawing
- Material and colour
- Cut length and annual volume
- Service temperature and exposure
Why buyers work with Uplastech
ISO 9001, ISO 14001 and ISO 45001 certified. Founded in 2016, with injection moulding lines in Shanghai and an extrusion plant in Jiangsu. Moulding machines from 90T to 1100T and a maximum shot weight of 5100 g. Samples and a written quotation are available on request.
Continue reading
- Extrusion dimensional drift
- Extrusion profile tolerance
- Profile calibration and cooling
- PVC profile weathering and colour
- PVC window profile design
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