The problem
Symptom. PC housings show silver splay on 30 % of a run after the desiccant dryer dew point drifts from -40 C to -10 C, lifting resin moisture from 200 ppm to 650 ppm.
What it costs. Splayed PC parts fail visual inspection and the dried resin lot is suspect, so a batch is scrapped while moulder and buyer argue over whether the dryer or the resin supplier owns the moisture.
This guide links dew point to resin moisture and splay, sets the drying figures for PC and PA, and writes a dryer verification clause into the supply contract.
What a dew-point drift looks like on the part
Splay is hydrolysed PC, not a flow mark. A dryer dew point drifting from -40 C to -10 C carries about twenty times the water vapour, lifting resin moisture from 200 to 650 ppm and marking 30 % of parts.
| Observed result | Most likely cause | First check on the line |
|---|---|---|
| Silver splay on 30 % of PC parts | Dew point drifted to -10 C | Measure dew point at the hopper inlet |
| Moisture reads 650 ppm | Saturated desiccant bed | Check the regeneration heater and log |
| Splay with normal 200 ppm moisture | Shear or a hot runner fault | Look at injection speed and gate size |
| Splay returns after a long idle | Hopper hose leak or no airflow | Check airflow and hose seals |
Table 1 - Reading a splay fault before blaming the mould
Need a fixed price for this moulded part?
Send the 2D drawing, resin grade and annual volume. Our engineers return a DFM review, a material recommendation and a written quotation you can compare line by line.
Separate wet resin from a moulding fault
Measure dew point at the hopper inlet with a portable meter and take a Karl Fischer moisture sample from the same lot. A -10 C reading with 650 ppm is the dryer; normal 200 ppm with splay points at shear.
A five-step diagnostic sequence
- Measure dew point at the hopper inlet.
A drift from -40 C to -10 C raises the air's water load about twentyfold and explains the splay. - Take a Karl Fischer moisture sample.
Confirm 200 ppm target against 650 ppm; normal moisture with splay means look at shear, not the dryer. - Check the desiccant bed and regeneration.
A saturated bed or a failed 200 C heater lets dew point climb; log regeneration per batch. - Verify the drying time and temperature.
Hold PC at 120 C for 4 h and PA6 at 80 C for 6 h with measured airflow. - Re-dry and re-mould a short trial.
Quarantine the lot, re-dry under -40 C dew point and confirm moisture under 200 ppm.
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.
Drying figures for the supply contract
Put the figures in the supply contract: PC at 120 C for 4 h, PA6 at 80 C for 6 h, a hopper-inlet dew point of -30 C or below, and resin moisture at most 200 ppm by Karl Fischer per lot.
| Parameter | Typical target | What goes wrong outside it |
|---|---|---|
| Hopper-inlet dew point | -30 C or below, target -40 C | At -10 C moisture reaches 650 ppm |
| PC drying condition | 120 C for 4 h | Short drying leaves splay on the part |
| PA6 drying condition | 80 C for 6 h | PA6 hydrolyses above 0.2 % moisture |
| Resin moisture, Karl Fischer | At most 200 ppm for PC | 650 ppm hydrolyses PC at melt temperature |
Table 2 - Drying parameters for the supply contract
Desiccant bed and airflow that hold dew point
A saturated desiccant bed, a failed 200 C regeneration heater or a leaky hopper hose all push dew point up. At -10 C the air holds roughly 2.3 g per cubic metre of water against 0.12 at -40 C.
| Decision | Effect on moisture | Trade-off |
|---|---|---|
| Regenerate or replace the desiccant | Returns dew point to -40 C | A regeneration cycle or new bed |
| Re-dry PC at 120 C for 4 h | Brings moisture under 200 ppm | 4 h of dryer time per lot |
| Verify dew point at the hopper inlet | Catches drift before moulding | A calibrated portable meter |
| Raise melt temperature to hide splay | Masks the silver streaks | Risks more hydrolysis and brittleness |
Table 3 - Decisions ranked by batch cost
Write dryer verification into the order
Agree verification before the run: a calibrated dew-point meter at the hopper inlet, a Karl Fischer result under 200 ppm, a drying record of 120 C for 4 h, and a desiccant regeneration log per batch.
Not sure whether the fault is in the tool or the process?
Send us the drawing, the process window and a few sample parts. We will tell you which change fixes the fault before you commit to a tool modification.
What to send us
- Part drawing with the affected dimension
- Current process settings
- Sample parts from a good and a bad batch
- Annual volume
Recovery actions ranked by batch cost
For a splayed batch, quarantine the lot and re-dry at 120 C for 4 h under a verified -40 C dew point. Re-mould a short trial; if moisture stays above 200 ppm, replace the desiccant before the next run.
Before you release this injection moulding job
- A hopper-inlet dew point of -30 C or below is written into the supply contract
- PC drying at 120 C for 4 h and PA6 at 80 C for 6 h are specified
- A maximum resin moisture of 200 ppm by Karl Fischer is agreed per lot
- A calibrated dew-point meter and a desiccant regeneration log are required
- First-article approval includes a moisture result and a drying record for the resin lot
Attach this list to the RFQ. Most moulding faults are decided at the drawing stage.
Why does splay appear when the dryer is running?
The dryer runs but its dew point drifted from -40 C to -10 C. That air carries about twenty times more water vapour, lifting PC moisture from 200 to 650 ppm.
How do we confirm it is the dryer?
Measure dew point at the hopper inlet and take a Karl Fischer sample. A -10 C reading with 650 ppm is the dryer; 200 ppm with splay points at shear.
What drying figures belong in the contract?
PC at 120 C for 4 h, PA6 at 80 C for 6 h, hopper-inlet dew point -30 C or below, and resin moisture at most 200 ppm by Karl Fischer.
Why is dew point better than dryer temperature?
Hot air can still be wet. Dew point measures the water actually in the air: at -40 C about 0.12 g per cubic metre, at -10 C about 2.3.
Can a hotter melt hide the splay?
It masks the streaks but drives more hydrolysis, so parts turn brittle. Re-dry the resin under a verified -40 C dew point instead.
Who owns a splayed batch?
Put it in the supply contract. Name the dew point, drying time, 200 ppm moisture limit and a verification record so the lot is judged by a meter reading.
Get a mouldable part and a fixed quotation
Send us the part file and the failure history. Our engineers will return a DFM review, a material recommendation and a fixed quotation with a defined first-article acceptance test.
To get an accurate quote, send:
- 3D file or 2D part drawing
- Resin grade and any regrind limit
- Annual volume and target unit price
- The failed sample or defect photos
Request a DFM review and quote
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.
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