The problem
Symptom. A contractor fits a second delivery of grey PVC trim beside the first and sees a visible difference, measured at ΔE*ab 2.6 under D65 against a contractual limit of 1.0.
What it costs. The mismatch is only visible once both batches are on the same elevation, by which point the first batch is installed and the whole run has to be replaced to keep one face uniform.
This guide ties the shift to melt temperature and masterbatch dose, and gives the four controls that settle a colour dispute before installation.
- 1. Agree the number before you compare the parts
- 2. Trace the shift back to zone 3 and the dosing unit
- 3. Set the colour controls that hold between batches
- 4. Prove it with a melt-temperature trial
- 5. Write the master and the geometry into the order
- 6. What one retained standard saves over a project
Agree the number before you compare the parts
Colour is judged as ΔE*ab against a retained master under D65 at 10°. Batch one read L* 72.4, a* -1.2, b* +3.1; batch two read 73.8, -1.5, +0.9, giving ΔE 2.6 where the order allowed 1.0, and the b* drop shows degraded yellow pigment.
| Zone 3 melt temp (°C) | ΔE*ab vs master | Batches passing ΔE ≤ 1.0 (%) |
|---|---|---|
| 185 (specified) | 0.6 | 96 |
| 190 | 1.2 | 82 |
| 195 | 1.9 | 48 |
| 200 (drift) | 2.6 | 18 |
Table 1 - Melt temperature against colour difference and first-pass yield
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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 shift back to zone 3 and the dosing unit
Typically, the shift traced to melt temperature drift, masterbatch dose error, regrind dilution, a missing retained master and metamerism under site lighting. Check the doser first: a 0.15% short dose of a 2.00% masterbatch moves L* by about 1.5 units.
A five-step diagnostic sequence
- Agree the master first.
Retain an approved sample and measure both batches against it under D65/10°; without a master, ΔE 2.6 is only an opinion. - Read the zone 3 log.
Compare the melt temperature of both batches; a rise from 188 °C to 200 °C degrades organic pigment and drops b*. - Check the gravimetric doser.
Confirm 2.00 ± 0.05% masterbatch; a 0.15% short dose moves L* by about 1.5 units. - Rule out regrind and lighting.
Verify 0% regrind on the visible face and compare ΔE under both D65 and the site lighting to exclude metamerism. - Run the melt-temperature trial.
Hold 185, 190, 195 and 200 °C and measure ΔE; the 0.6 to 2.6 rise proves the drift, not the compound.
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 colour controls that hold between batches
Four controls hold the colour: ΔE*ab of 1.0 or less under D65/10°, masterbatch dosing at 2.00 ± 0.05%, zone 3 melt at 185 to 190 °C, and no regrind on the visible face. Above 198 °C the organic pigment degrades and b* falls.
| 매개변수 | Typical target | What goes wrong outside it |
|---|---|---|
| Colour difference ΔE*ab (D65/10°) | 1.0 or less against master | Above 2.0 the trim is visibly different on one elevation |
| Masterbatch dosing | 2.00 ± 0.05 % | A 0.15 % short dose shifts L* by about 1.5 units |
| Zone 3 melt temperature | 185-190 °C | Above 198 °C organic pigment degrades and b* falls |
| Regrind on the visible face | 0 % | 10 % regrind dilutes pigment and lifts ΔE by 0.8 |
Table 2 - Colour controls that hold a grey PVC trim between batches
Prove it with a melt-temperature trial
Run a melt-temperature trial with one compound: hold zone 3 at 185, 190, 195 and 200 °C and measure ΔE against the master. It reads 0.6, 1.2, 1.9 then 2.6, while the share of batches passing ΔE 1.0 falls from 96% to 18%.
| Field symptom | Most likely cause | Check that confirms it |
|---|---|---|
| Second batch yellower / bluer | Melt temperature drift | Zone 3 log reads 200 °C instead of 188 °C |
| Whole batch lighter | Masterbatch dose short | Gravimetric doser log shows 1.85 % not 2.00 % |
| Match in the factory, not on site | Metamerism under different lighting | ΔE changes between D65 and shop fluorescent |
| Shift grows with regrind | Regrind diluting the pigment | Regrind ratio logged at 10 % on the visible face |
Table 3 - Matching the colour symptom to the process drift
Write the master and the geometry into the order
Write the master and the geometry into the order: a retained approved sample, ΔE*ab under D65/10°, the dose tolerance and conditioning temperature. Both parties measuring the same master turn a visible mismatch into a 2.6 reading, not an argument about the lighting.
Need the section checked before tooling?
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 one retained standard saves over a project
One retained standard protects the whole project. A facade takes 4 000 m of trim over several batches; holding zone 3 at 188 °C keeps ΔE near 0.6, while a 12 °C drift shades a whole delivery before it is placed beside batch one.
Before you release this extruded profile
- A retained approved master sample is held by both parties
- ΔE*ab limit and the D65/10° measurement geometry are specified
- Masterbatch dose and tolerance are written into the order
- Zone 3 melt temperature and its upper limit are stated
- Regrind is excluded from the visible face by specification
Write the measurement temperature and the tolerance into the drawing; most disputes are really about how the part was measured.
Why does my second batch of grey trim look different?
Most often the melt ran hot. At 200 °C instead of 188 °C the organic yellow pigment degrades, b* falls and ΔE*ab rises from 0.6 to 2.6 against a limit of 1.0.
How is colour difference properly measured?
As ΔE*ab against a retained master using a spectrophotometer under D65 at 10°. Visual judgement under shop lighting is unreliable because metamerism can make two matching parts look different.
What ΔE limit should I specify?
1.0 or less for a visible architectural trim. Above 2.0 the difference is obvious on one elevation, which is why a 2.6 reading usually means replacing the whole installed run.
Does masterbatch dosing matter that much?
Yes. 2.00 ± 0.05% is the working tolerance. A 0.15% short dose shifts L* by about 1.5 units, which is already beyond most colour limits.
Can I use regrind on the visible face?
No. Ten per cent regrind dilutes the pigment and lifts ΔE by about 0.8 on its own, before any temperature drift is added. Keep regrind to the core or a hidden face.
What should the acceptance clause include?
A retained master, the ΔE limit under D65/10°, dose tolerance, zone 3 temperature, and no regrind on the visible face. Both parties then measure the same numbers.
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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