The problem
Symptom. During commissioning a buyer logs 24 °C water at the top of a 6 m cooling tank and 14 °C at the bottom, and the extruded PVC profile still reads 45 °C at the tank exit.
What it costs. The line cannot be signed off, and the resulting length scatter and post-extrusion warp are blamed on the die while the tank design is the real gap.
This guide links stratification to recirculation flow and baffles, and gives the four measurements that settle a commissioning dispute.
Measure the tank at three depths before you blame the die
Warm water rises, so an unbaffled 6 m tank forms a stagnant top layer. At 100 L/min we logged 24 °C at the surface against 14 °C at the bottom, a 10 K split, and the profile left at 45 °C rather than 32 °C.
| Recirculation flow (L/min) | Top-bottom split (K) | Profile exit temp (°C) |
|---|---|---|
| 100 (stagnant) | 10.0 | 45 |
| 250 | 5.0 | 37 |
| 325 | 2.8 | 34 |
| 400 (specified) | 1.2 | 32 |
Table 1 - Recirculation flow against stratification and exit temperature
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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 warm layer to the flow design
Typically, the split traced to a stagnant top layer, an undersized chiller, blocked nozzles, a poor inlet header and a short tank. One dead leg can stratify the tank within 20 minutes of start-up.
A five-step diagnostic sequence
- Log three depths first.
Measure water temperature at the surface, mid-depth and bottom after 20 minutes; a 10 K split is the fault, not the die. - Check recirculation flow.
Read the flowmeter at the header; 100 L/min instead of 400 L/min explains both the split and the 45 °C exit. - Inspect nozzles and baffles.
Find blocked spray rows and confirm baffle spacing of 1.5 m or less; one dead leg stratifies the tank quickly. - Verify chiller capacity.
Log inlet temperature through a full shift; an undersized chiller lets it climb from 14 °C to 22 °C. - Run the flow trial.
Step 100, 250, 325 and 400 L/min and log three depths; the 10.0 to 1.2 K fall proves the tank design.
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 water parameters that quench evenly
Four settings give an even quench: a top-to-bottom difference under 3 K, water velocity of 0.25-0.4 m/s, an exit temperature of 30-35 °C, and baffles at 1.5 m spacing or less. Below 0.1 m/s a warm boundary layer clings to the profile.
| Parameter | Typical target | What goes wrong outside it |
|---|---|---|
| Top / bottom water difference, 6 m tank | 3 K or less | A 10 K split sets differential shrink and a permanent bow |
| Water velocity over the profile | 0.25-0.4 m/s | Below 0.1 m/s a warm boundary layer clings to the surface |
| Profile temperature at tank exit | 30-35 °C | Above 42 °C the part keeps shrinking after the saw |
| Baffle / header spacing | 1.5 m or less | Unbaffled tanks stratify within 20 minutes of start-up |
Table 2 - Tank settings that quench a rigid PVC profile evenly
Prove it with a recirculation-flow trial
Run a recirculation-flow trial at fixed output: set 100, 250, 325 and 400 L/min and log three depths. The split falls 10.0, 5.0, 2.8 then 1.2 K while exit temperature drops 45 to 32 °C, clearing the 3 K limit at 325 L/min.
| Field symptom | Most likely cause | Check that confirms it |
|---|---|---|
| Top water warm, bottom cold | Stagnant layer, no baffles | Three-depth log shows a 10 K split |
| Length scatter after start-up | Profile leaving the tank hot | Exit probe reads 45 °C instead of 32 °C |
| One face of the profile shinier | Blocked spray nozzle row | Nozzle pressure reads zero on that header |
| Split worsens through the shift | Undersized chiller | Inlet temperature climbs from 14 °C to 22 °C |
Table 3 - Matching the tank symptom to the flow fault
Write the tank acceptance into the commissioning order
Write the tank acceptance into the commissioning order: a 3 K limit measured at three depths, minimum recirculation flow and exit temperature at rated output. One shared ramp turns a 10 K dispute into a flow figure, not a new die.
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 an even quench saves over a year
An even quench protects downstream length. At 2.4 m/min the line makes about 8 km a day; 400 L/min holds a 1.2 K split and 32 °C exit, while a blocked header adds ±6 mm of cut scatter before the saw is checked.
Before you release this extruded profile
- A 3 K top-to-bottom limit measured at three depths is specified
- Minimum recirculation flow in L/min appears in the commissioning order
- Profile exit temperature at rated output is written into the sign-off
- Baffle or header spacing of 1.5 m or less is required
- Chiller capacity and inlet temperature are stated for a full shift
Write the measurement temperature and the tolerance into the drawing; most disputes are really about how the part was measured.
Why is my cooling tank 24 °C at the top and 14 °C at the bottom?
Warm water rises and an unbaffled tank has no mixing. At 100 L/min we logged a 10 K split; at 400 L/min with baffles it fell to 1.2 K.
Does the water temperature really affect the profile?
Yes. A 10 K split quenches the upper face more slowly, so the part leaves at 45 °C instead of 32 °C and keeps shrinking after the saw, adding about ±6 mm of cut-length scatter.
What top-to-bottom difference should I allow?
3 K or less. Above that the differential shrink can set a permanent bow. Measure at three depths after 20 minutes of running, not at the inlet alone.
How much recirculation flow does a 6 m tank need?
In our trial 325 L/min cleared the 3 K limit and 400 L/min held 1.2 K, with water velocity around 0.3 m/s over the profile. Below 0.1 m/s a warm boundary layer clings to the surface.
Will a bigger chiller fix stratification?
It helps the heat load but not the mixing. Blocked nozzles, a poor inlet header and missing baffles are the leading causes in typical cases; fix the distribution first.
What should commissioning sign-off include?
A 3 K limit at three depths, minimum flow, exit temperature at rated output, baffle spacing and chiller capacity. Then a 10 K dispute is settled with a flowmeter reading.
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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