When an extrusion blow molding line produces containers that are heavy in one area and thin in another, the investigation usually starts at the mould and the process recipe. It should start at the die head. The die head forms the parison — the molten tube that the mould closes around — and every wall thickness decision in the finished container is a decision the die head already made before the mould ever closed.
The cost of ignoring this is not an occasional defect. It is a permanent weight penalty: a container that weighs more than its specification because the wall was thickened to cover the thinnest point, an output figure that cannot be reached because the cycle is set by the slowest-cooling section, and a weld line in a handle root that only shows up in service. None of it is fixed by adjusting the mould or turning a process dial.
Sailwin builds extrusion blow molding machines covering 0.5 L to 1000 L containers, processing PE, PP, ABS, EVA, PC and PA including multilayer PE + PA + EVOH structures, under ISO 9001:2015 with CE marking. This guide explains what a die head does, the three adjustments that decide parison quality, and how to match the head type to the container you are making.
Key Takeaways
- Die gap and land length decide the parison, and the parison decides the wall: these are two geometric features of the head, and changing them affects every downstream setting. Specify them against the container, not by trial and error after installation.
- Head type follows container size, not preference: a spider-type head suits small and medium containers, a crosshead solves an extruder layout problem, and an accumulator head is what large parts need to control parison sag.
- Weld lines are a die head signature: wherever melt streams rejoin around a support structure, a weld line exists. The design question is where it ends up — not whether it exists.
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1. What the Die Head Actually Does
The die head takes melt from the extruder, distributes it into an even annular flow, and extrudes it downward as a parison of controlled diameter and wall thickness. Simple to describe, difficult to execute. The melt arrives as a concentrated stream and must leave as a symmetrical ring, with the same temperature and the same velocity at every point around the circumference. Anywhere that condition is not met, the parison is uneven, and the container made from it will be uneven in exactly the same place.
This is why the die head is the component that determines whether a wall thickness problem is fixable. If the head produces an even parison, the mould and the process recipe can be tuned to shape it. If the head produces an uneven parison, no amount of mould or process adjustment removes the unevenness — it only redistributes it.
2. Die Gap, Land Length and Parison Swell: The Three Adjustments
Three geometric and rheological factors control the parison, and they interact. Adjusting one without understanding the other two is how a wall thickness problem becomes a permanent feature of the process. Parison swell is the expansion of the polymer as it leaves the die, caused by the elastic recovery of the melt; it means the parison is never the same diameter as the die gap that produced it.
| Factor | What it controls | Symptom when it is wrong |
|---|---|---|
| Die gap | The annular opening through which melt exits; the primary control on parison wall thickness | Too narrow, and melt fracture or surface roughness appears; too wide, and the parison sags and the container gains weight |
| Land length | The parallel flow section at the die exit; controls melt relaxation and surface quality | Too short, and surface defects and unstable diameter follow; too long, and pressure drop and melt temperature rise |
| Parison swell | How far the parison expands after leaving the die, based on shear rate, temperature and material | Unpredicted diameter and weight, made worse by changing material or output without re-verifying the head |
| Wall thickness programming | Varying the die gap while the parison is extruding, so wall thickness follows the container’s demands | Unprogrammed parison means the heaviest section of the container sets the weight of every unit |
Wall thickness programming is where most weight is saved. A container rarely needs the same wall from top to bottom, and a parison that is uniform is a parison that is too thick somewhere. Every gram removed from every container is a saving repeated for the life of the mould.
3. Spider, Crosshead and Accumulator: Choosing by Container
Head type is decided by container size, material and machine layout — not by what is available. Each design solves a specific problem and introduces a specific compromise, and knowing which compromise you are accepting is the point of specifying a head properly.
| Head type | Best suited to | Design compromise to manage |
|---|---|---|
| Spider-type head | Continuous extrusion of small and medium containers, where output is steady and the parison is short | The spider legs split the melt, so weld lines form where the streams rejoin; leg count, profile and position have to be engineered against the container |
| Crosshead | Where the extruder cannot sit in line with the head and melt must be turned through an angle | Turning the flow introduces asymmetry, so the internal flow channels have to be balanced to deliver an even ring at the exit |
| Accumulator head | Large containers and technical parts, where the parison would sag if extruded slowly | Melt is stored and pushed out rapidly, so the container is formed while the parison is still hot and uniform — at the cost of a more complex head |
| Multi-layer head | Containers needing a barrier or a view strip, where two or more materials must form one parison | Layer thickness control and interlayer stability become the critical variables, and each material needs its own process window |
For scale reference, Sailwin’s published EBM range covers 0.5 L to 1000 L, with verified output figures such as SW-S30L at 600 pcs/h for containers up to 30 L, SW-S60L at 450 pcs/h, SW-S80L at 360 pcs/h and SW-S1000L at 250 pcs/h for containers up to 1000 L. As container size increases, the parison gets longer and heavier, and the argument for accumulator technology gets stronger.
4. Co-Extrusion Heads for Barrier Containers
A multilayer head allows several materials to be combined into one parison, which is how a container gets a barrier without becoming expensive throughout. Sailwin EBM machines support multilayer PE + PA + EVOH structures, and the die head is where that structure is created: each layer is distributed as its own annular flow and they are joined before the parison leaves the head.
Barrier work raises the specification standard in three places. Each material needs its own temperature window, so the head needs independent temperature control zones. Layer thickness has to be controlled and verifiable, because barrier performance depends on the thin layer being continuous. And the sequence of layers matters for adhesion and for how the container behaves on recycling.
The same multi-layer logic produces a view strip on a monolayer container: a defined section of the parison is formed from clear material while the rest carries colour. It is a die design decision, so the strip geometry has to be specified against the fill volumes the container will actually use — specifying it after the tool is built means chasing the result with process adjustment that never fully stabilises.
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5. Case Study: Stabilising Parison Weight on a 12 L Container
A container manufacturer running 12 L HDPE containers on an SW-S12L class machine needed to bring unit weight under control without losing impact performance at the handle.
- Containers consistently running above the weight target, because the thinnest section dictated how thick the whole parison had to be
- Visible surface irregularity on part of the circumference, traced to the parison rather than to the mould
- Handle performance sensitive to any reduction in wall thickness near the root
- Die gap and land length reviewed against the container and the material, with the surface irregularity addressed at the head rather than through temperature compensation
- Wall thickness programming set to follow the container profile, so material is placed where the handle and base need it instead of uniformly
- Process window recorded per product, with the PLC monitoring 40+ parameters so any drift away from the validated setting is visible
- Full-load FAT before shipment, 3–7 day on-site commissioning, wear parts shipped within 48 hours and 7×24 remote support
- Surface quality resolved at source — the irregularity disappeared once the head geometry was corrected, rather than being masked by process settings
- Weight brought under control through programming, with material allocated to the handle and base instead of being spread evenly
- A repeatable process window recorded per product, so the validated setting survives operator changes, shift changes and mould changes
Scenario based on a Sailwin customer project; site-specific figures available on request during engineering review.
Frequently Asked Questions
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