Parison swell is the reason a die gap is never the same number as the wall thickness you want. The melt leaves the die and immediately expands as its elastic deformation recovers, then stretches again under its own weight before the mould closes. Set the die gap from a drawing and you will chase wall thickness for weeks. Set it from the swell behaviour of the actual material and you will hit the target in a few trials.
The cost of getting this wrong is not measured in scrap alone. A parison that is too heavy at the top and too thin at the base produces a container that passes a visual check and fails a drop test. A parison that sags between the die and the mould produces a part that is thick where it does not need material and thin exactly where the handle or the corner needs it. Both problems are usually blamed on the mould, when the cause sits upstream at the die.
Sailwin has delivered extrusion blow molding machines covering containers from 0.5 L to 1000 L into 500+ installations across 60+ countries over 15+ years, manufacturing under ISO 9001:2015 with CE marking. This guide separates what swell actually is from what is often assumed about it, then gives a tuning sequence you can run on the machine with the material already in the hopper.
Key Takeaways
- Die gap and wall thickness are different numbers separated by two effects: elastic recovery at the die exit, and drawdown caused by the weight of the hanging parison. Both must be accounted for before the first mould trial.
- Melt temperature moves swell more than most operators expect. Raising temperature reduces melt viscosity and changes recovery behaviour, so a temperature change made for a different reason will move your wall thickness as a side effect.
- Change the die gap to change average thickness; change the wall thickness programme to change distribution. Sailwin machines let both be set independently, and mould changes are designed to complete in under 30 minutes.
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1. What Parison Swell Actually Is
Polymer melt is elastic as well as viscous. Inside the die it is compressed and sheared; on exit that stored energy is released and the parison expands in both diameter and wall thickness. This is die swell, and it is a property of the material and the flow conditions, not a fault in the machine. Swell ratio is simply the measured parison dimension divided by the corresponding die dimension at the same point.
On top of that comes drawdown. Between the die exit and the moment the mould closes, the parison stretches under its own weight. The longer the parison and the heavier the shot, the greater the drawdown, and the more the top of the parison thins relative to the base. Swell and drawdown act in sequence and partly oppose each other, which is exactly why parison dimensions must be measured on the machine rather than calculated from a table.
| Variable | Effect on swell | Effect on drawdown | How to confirm it |
|---|---|---|---|
| Die gap | Sets the absolute wall thickness leaving the die, not the swell ratio itself | Greater die gap means a heavier parison and therefore faster drawdown | Measure parison wall at the die exit and again just before mould close |
| Melt temperature | Higher temperature reduces elasticity and lowers swell | Lower viscosity increases drawdown | Hold the profile steady and change one zone at a time, then re-measure |
| Die land length | A longer land allows more stress relaxation inside the die and reduces swell | Almost no direct effect | Compare swell ratio between two die sets on the same material |
| Extrusion rate | Higher shear rate generally increases swell | Faster extrusion shortens hanging time and reduces drawdown | Run the same die at two output rates and compare wall profiles |
| Material grade and structure | Molecular weight distribution is the dominant material factor | Density affects parison weight for the same volume | Establish a swell figure per grade and keep it with the mould record |
2. From Swell Ratio to a Starting Die Gap
A practical starting point is to divide the target parison wall thickness by the measured swell ratio for that grade, then trim the result with the drawdown you expect over the hanging time. Industry-typical swell ratios for common blow molding grades sit in a relatively narrow band, but the band is wide enough that a grade change alone can move wall thickness noticeably. Treat published ratios as a starting point and measure your own.
Sailwin machines are built with the die set and the wall thickness programme as separate adjustments, which matters here. The die gap sets the average; the programmer sets how that average is distributed over the length of the parison. Trying to fix a distribution problem with the die gap is the single most common cause of a long tuning loop, because every change to the gap shifts the whole profile instead of the section that needed attention.
Change one variable at a time and measure the parison before the mould closes. Swell and drawdown interact, so a two-variable adjustment leaves you unable to tell which change produced the result.
3. A Tuning Sequence That Converges
Work in this order. Each step depends on the previous one being stable, and skipping ahead is what turns a two-hour job into a two-week one.
- Establish the melt temperature profile and leave it alone. Record the set points before you touch the die, because temperature is the variable most often changed for an unrelated reason and then blamed for a wall thickness shift.
- Set the die gap for average wall thickness. Measure the parison at the die exit and just before mould close. The difference between those two readings is your drawdown, and it should be consistent from shot to shot.
- Programme the wall thickness distribution last. Only after the average is correct should the profile points be adjusted, because the programmer redistributes material rather than creating it.
- Verify on the finished part, not on the parison. Section the container at the points that matter for the product — base, shoulder, handle, corner — and compare against the target rather than against the previous job.
- Record the settings with the mould. Sailwin machines use Siemens or Mitsubishi PLCs with FESTO blow valves, SMC cylinders, Schneider electricals and ABB components, and the PLC monitors 40+ parameters in real time — so a validated recipe can be stored and recalled instead of rediscovered at the next changeover.
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4. Case Study: A Blow Molded Handleware Container With a Thin Corner
A packaging producer running a blow molded handleware container had a recurring thin corner that failed a drop test, while the base of the same container was consistently heavier than the drawing specified.
- A thin corner failed drop testing while the base was heavy, so material was present in the wrong place rather than missing overall
- Wall thickness had been chased by adjusting the die gap, which shifted the entire profile instead of the corner
- Melt temperature had been raised earlier to improve surface finish, without re-checking wall thickness afterwards
- Average wall thickness and distribution separated into two adjustments: die gap for the average, wall thickness programme for the corner and shoulder
- Parison measured at the die exit and again just before mould close, so drawdown over the hanging time was quantified rather than assumed
- Melt temperature profile frozen for the duration of the trial, then re-validated once the profile was correct
- Validated recipe stored in the PLC so the next changeover recalls the settings instead of rebuilding them
- Material moved from the base into the corner by programming distribution rather than by opening the die gap wider
- Drop test performance was addressed at the parison stage, where the correction costs trial time instead of a rejected batch
- Changeover no longer resets the work, because the validated recipe is stored with the mould rather than held by the operator’s notes
Scenario based on a Sailwin customer project; site-specific figures available on request during engineering review.
Frequently Asked Questions
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Related Reading:
• Extrusion Blow Molding Machines — 0.5 L to 1000 L
• Jerrycan Blow Molding Machines for industrial packs
• EBM vs ISBM: Which Process Fits Your Bottle?
• PET Stretch Blow Molding Machines — the alternative route
• Injection Molding Machines for closures and preforms
• Filling Machines downstream of the blow moulder
