- 1. Why hollow profiles exist
- 2. The families of hollow profile
- 3. How the die makes a hollow: spider versus bridge
- 4. Weld lines: what they cost and how to live with them
- 5. Wall balance: the arithmetic of even flow
- 6. Cooling the inside of a hollow profile
- 7. Design rules for hollow profiles
- 8. A worked comparison: solid versus hollow
- 9. Inspection and acceptance of hollow profiles
- 10. Frequently asked questions
- 11. Designing a hollow section? Send the drawing before you cut the die
Why hollow profiles exist
A hollow profile is a stiffness decision. Bending stiffness rises with the cube of depth, but weight rises linearly. So moving material away from the neutral axis — into the walls of a hollow box rather than the middle of a solid one — buys stiffness at almost no weight cost.
That is why window frames, pipe, siding, decking, furniture profiles, automotive trim and cable trunking are all hollow. A 60 × 60 mm hollow box with 2 mm walls is roughly as stiff in bending as a solid 22 mm bar of the same material, at about a fifth of the weight.
| Section | Material area | Relative bending stiffness | Stiffness per kg |
|---|---|---|---|
| Solid 20 × 20 mm | 400 mm² | 1.0× | 1.0× |
| Solid 40 × 40 mm | 1,600 mm² | 8.0× | 2.0× |
| Hollow 60 × 60, 2 mm wall | 464 mm² | 7.4× | 6.4× |
| Hollow 60 × 60, 3 mm wall | 684 mm² | 10.4× | 6.1× |
| Hollow 100 × 40, 2 mm wall | 552 mm² | 8.6× | 6.2× |
| Hollow 60 × 60, 2 mm + 1 internal wall | 580 mm² | 8.1× | 5.6× |
Relative to a solid 20 × 20 mm section, same material. The hollow sections deliver 3 – 6× more stiffness per kilogram than a solid section of similar stiffness.
But every hollow section brings three problems that a solid section does not have: the die needs a core support that leaves a weld line, the calibrator has to hold an inside and an outside at once, and the internal walls cool more slowly than the external ones. Those three problems are the whole engineering of hollow profile extrusion.
The families of hollow profile
| Type | Description | Examples | Difficulty |
|---|---|---|---|
| Single chamber tube | One closed void, round or rectangular | Pipe, conduit, square tube | Low |
| Multi-chamber | Two or more voids separated by internal walls | Window profiles, door frames | Medium – high |
| Open hollow (C, U, J) | Not closed — a bent section | Trim, channels, guides | Low – medium |
| Thin-wall large hollow | Large void, thin walls | Siding, panels, trunking | High |
| Hollow with inserts | Metal or foam inside the void | Reinforced window profiles | High — post-extrusion |
| Co-extruded hollow | Different material on outside or as a seal | Window gaskets, two-colour trim | Very high |
Difficulty roughly tracks the number of internal walls. Each one adds a core, a weld line, and a cooling problem.
How the die makes a hollow: spider versus bridge
To form a void, the die needs a core floating in the middle of the flow. Something has to hold that core, and whatever holds it interrupts the melt. The interruption is the weld line.
Spider die
Three to six legs support the core, crossing the flow channel radially. Melt splits around each leg, and the streams recombine downstream of the spider. The recombination happens under pressure and at temperature, so the weld heals partially — but not completely.
Spider dies are cheap, compact and easy to clean. They dominate PVC pipe and simple hollow profiles.
Bridge die
The core is supported by a bridge (or "torpedo" support) that sits upstream, and the melt flows over the bridge and recombines in a large chamber before the die orifice. The chamber gives the weld time, pressure and temperature to heal properly.
Bridge dies are larger, more expensive, and harder to clean. They are used whenever weld strength matters — engineering resins, pressure parts, structural profiles.
| Parameter | Spider die | Bridge die |
|---|---|---|
| Number of weld lines | Equal to the number of legs (3 – 6) | One, or a small number |
| Weld strength | 50 – 80 % of parent | 80 – 95 % of parent |
| Die length | Short | 30 – 60 % longer |
| Die cost | Base | +30 – 60 % |
| Minimum void size | Small voids possible | Void must be large enough to pass the bridge |
| Pressure drop | Lower | Higher — needs more head pressure |
| Cleanout | Easy | Harder — hidden volume |
| Best for | PVC, PE, PP tube; non-structural hollows | PA, PC, POM, ABS; pressure and structural parts |
A 4-leg spider die leaves four weld lines 90° apart. On a visible profile that means four visible lines unless they are placed on hidden faces.
The design decision that matters most: on a visible profile, the weld line position must be specified at design stage, before the die is cut. On a spider die the welds sit at the leg positions, and the die maker can rotate the spider so the welds land on a hidden face or a corner. That is free at design stage and impossible afterwards.
Weld lines: what they cost and how to live with them
A weld line is where two melt fronts met and joined. The join is weaker than the parent material for three reasons: the molecules at the interface have less time to entangle, the interface is cooler than the bulk, and any contamination or air at the interface stays there.
| Factor | Effect on weld strength | How to improve |
|---|---|---|
| Melt temperature | Higher temperature = stronger weld | Raise die temperature 10 – 20 °C locally |
| Pressure at the weld | Higher pressure = stronger weld | Longer land downstream of the spider; more head pressure |
| Distance from spider to land | Longer = stronger | Design the die with a longer recombination chamber |
| Number of legs | Fewer = fewer welds | 3 legs instead of 4 where geometry allows |
| Material | Amorphous polymers weld better than crystalline | ABS and PVC weld well; PP and PE less well |
| Filler content | Fillers reduce weld strength sharply | Glass-filled profiles can lose 50 – 70 % at a weld |
| Moisture | Moisture at the interface creates voids | Dry the material |
Weld strength is normally reported as a percentage of the parent material's tensile strength, measured on a specimen cut across the weld. It is worth testing on any structural hollow profile rather than assuming.
Practical ways to live with a weld line when it cannot be eliminated:
- Move it. Rotate the spider so the weld sits on a corner or a hidden face.
- Design the load path around it. If the profile is loaded in bending rather than tension, a weld at the neutral axis matters far less than one at the extreme fibre.
- Increase local wall thickness at the weld. A 10 – 15 % thicker wall at the weld location recovers most of the lost strength without adding much weight.
- Test it. Specify a weld-strength requirement and test a sectioned specimen, rather than discovering the weakness in a field failure.
Wall balance: the arithmetic of even flow
In a hollow profile, every wall is fed from the same die. If the walls have different thicknesses, they have different flow resistance, and the thick wall wins. The result is a profile where the thick wall is at nominal and the thin internal walls are starved.
The rule is simple and often ignored: the land length on each wall should be proportional to that wall's thickness. A 3 mm outer wall and a 1.5 mm internal wall need roughly a 2:1 land ratio, not a constant land.
| Wall thickness | Land length (10× rule) | Relative flow resistance |
|---|---|---|
| 1.0 mm | 10 mm | High — needs the shortest land |
| 1.5 mm | 15 mm | — |
| 2.0 mm | 20 mm | — |
| 3.0 mm | 30 mm | Low — needs the longest land |
| 4.0 mm | 40 mm | Lowest |
Resistance in a slot scales roughly with thickness cubed, so a 3 mm wall passes about 3.4× the flow of a 2 mm wall at the same land length. The land has to make up that difference.
Two design moves that make balance much easier:
- Keep all walls within a 1:2 thickness ratio. A profile with 2 mm outer walls and 1.2 mm internal walls balances reasonably. One with 4 mm and 1 mm does not, and no die will fix it.
- Keep internal walls at least 1.2 mm. Below that, the wall is difficult to fill, difficult to cool, and the calibrator cannot reach it. Many window profile standards set a 1.2 – 1.5 mm minimum for this reason.
Cooling the inside of a hollow profile
This is the hardest part of hollow profile extrusion, and the one that most often limits line speed.
The outer surface is in contact with a water-cooled sleeve. The inner surfaces are in contact with… air. Air is a poor heat transfer medium, so the internal walls cool much more slowly, and they are still soft when the outer walls are rigid.
Consequences:
- The profile collapses or distorts internally when the haul-off pulls on it, because the internal walls cannot resist.
- Internal walls sag under their own weight while hot, producing a profile with a thickened bottom internal wall and a thinned top one.
- Differential shrinkage between the hot inside and cool outside produces internal stress, which shows up as bow, twist or cracking later.
| Technique | How it works | Effectiveness | Cost |
|---|---|---|---|
| Internal air pressure (air blow) | Pressurised air inside the void keeps the walls out against the sleeve | High — the standard solution | Low (compressor + seals) |
| Internal water spray / mist | Water injected into the void through a lance | Very high | Medium — needs a lance and drainage |
| Longer cooling tank | More time for the heat to conduct out | Moderate | High — floor space |
| Cold mandrel / internal calibrator | A cooled metal core inside the void | Very high | High — complex, wear-prone |
| Reduce line speed | Simply more time in the tank | Effective but expensive | Lost output |
| Thinner internal walls | Less heat to remove | Helps cooling, hurts strength | — |
Internal air pressure is the default because it solves two problems at once: it holds the walls out against the sleeve and it prevents collapse. Typical pressure is 0.02 – 0.08 MPa, sealed at the die end and at a plug downstream.
Design rules for hollow profiles
| Rule | Reason | Consequence of breaking it |
|---|---|---|
| All walls within 1:2 thickness ratio | Balanced flow | Thin walls starve; thick walls run heavy |
| Internal walls ≥ 1.2 mm | Fill, cooling, calibration | Unfilled or collapsed internal walls |
| Internal corners radius ≥ 0.5 × wall | Flow at the corner; stress | Corner void, cracking under load |
| Void width ≥ 3 × wall thickness | The core must fit and be supported | Core too thin — breaks or blocks flow |
| Weld line on a hidden face or corner | Cosmetic and strength | Visible line; weak point in the wrong place |
| No closed void smaller than 8 × 8 mm | Cooling and calibration cannot reach it | Void distorts; dimension uncontrollable |
| Symmetry where possible | Even cooling, even shrinkage | Twist and bow |
| Drain or vent path for any sealed void | Water ingress, pressure equalisation | Condensation, trapped water, bulging |
The last rule is often forgotten: a hollow profile used outdoors that seals a void completely will collect condensation and, in freezing conditions, split. A small vent hole or a designed drain is cheaper than a warranty claim.
A worked comparison: solid versus hollow
A furniture rail, 900 mm long, loaded at the centre with 300 N, maximum deflection allowed 3 mm.
| Option | Section | Mass per metre | Predicted deflection | Verdict |
|---|---|---|---|---|
| Solid PVC bar | 30 × 20 mm | 0.72 kg/m | 4.8 mm | Fails |
| Solid PVC bar | 40 × 25 mm | 1.20 kg/m | 1.9 mm | Passes, heavy |
| Hollow 40 × 25, 2 mm wall | Rectangular tube | 0.36 kg/m | 2.6 mm | Passes, half the weight |
| Hollow 40 × 25, 2 mm + 1 internal wall | Two chambers | 0.42 kg/m | 2.1 mm | Passes, better margin |
| Hollow 50 × 25, 2 mm wall | Rectangular tube | 0.42 kg/m | 1.7 mm | Passes, best margin |
Illustrative values for rigid PVC at 3 GPa modulus. The hollow 40 × 25 at 0.36 kg/m does the job of a 1.20 kg/m solid — a 70 % material saving on the same function.
The commercial conclusion is the one that drives most hollow profile projects: the hollow section is not chosen because it is cheaper to tool — it usually costs more to tool — but because it removes 50 – 70 % of the material cost from a part that is bought by the metre.
Inspection and acceptance of hollow profiles
A hollow profile cannot be checked with a caliper on the outside alone. The dimensions that matter are often internal, and the defects that matter are often invisible from outside.
| Check | Method | Frequency |
|---|---|---|
| External dimensions | Caliper, profile gauge, optical scanner | Every setup, then per shift |
| Internal dimensions | Section a sample, measure on a profile projector | First article, then per batch |
| Wall thickness | Ultrasonic gauge on accessible walls; section for internal | Per shift |
| Weld line position and strength | Visual; tensile test on sectioned specimen | First article and on material change |
| Straightness / bow | Flat table and feeler gauge, or laser | Per batch |
| Internal collapse | Section, or light transmission on thin walls | First article and after any cooling change |
| Mass per metre | Weigh 1 m | Per shift — the fastest indicator of drift |
Mass per metre is the most under-used check on a profile line. It catches wall-thickness drift, die wear and haul-off changes in one number, in ten seconds, without any measuring instrument.
Frequently asked questions
What is a hollow profile in extrusion?
A profile with one or more closed voids running along its length — pipe and tube are the simplest, window and door profiles the most complex. The voids remove material from the neutral axis, so the section is much stiffer per kilogram than a solid one.
Why do hollow profiles have weld lines?
The die needs a core to form the void, and the core must be held by something — spider legs or a bridge. The melt splits around that support and rejoins downstream, and the joint is a weld line. Its strength is typically 50 – 95 % of the parent material depending on die design and material.
How strong is a weld line?
On a spider die, 50 – 80 % of parent tensile strength. On a bridge die, 80 – 95 %. Glass-filled materials can lose 50 – 70 % at a weld regardless of die type. It should be tested, not assumed, on any structural part.
How do I stop a hollow profile collapsing?
Internal air pressure — typically 0.02 – 0.08 MPa — inside the void, sealed at both ends. It holds the walls out against the calibrating sleeve and stops them sagging while hot. On large or thin-wall sections, an internal water spray is also used.
What is the minimum wall thickness for a hollow profile?
About 1.2 mm for internal walls and 1.5 mm for external walls in PVC. Below that the walls are difficult to fill, almost impossible to cool from inside, and the calibrator cannot hold them. Thin-wall large hollows down to 0.8 mm exist but need specialised lines and much slower speeds.
Can a hollow profile be co-extruded?
Yes, and it is common — a rigid PVC core with a flexible TPE seal leg, or a virgin outer skin over a recycled core. Co-extruded hollow profiles are the hardest category to tool because the flow balance has to work for two materials at once.
Why does my hollow profile twist?
Uneven cooling around the section, or an asymmetric wall layout. The side that cools first shrinks first. Internal walls cool more slowly than external ones, so a profile with internal walls on one side only will twist unless cooling is deliberately unbalanced to compensate.
How do I reinforce a hollow profile?
Three ways: a metal insert slid into the void after extrusion (standard for window profiles), a thicker wall on the load-bearing face, or a change of material. Inserts give the highest stiffness gain but add a secondary operation and a corrosion consideration.
Can hollow profiles be bent after extrusion?
Yes, but the void collapses unless it is supported. The usual methods are filling the void with sand or a low-melt alloy before bending, or bending hot with an internal mandrel. Tight radii on multi-chamber profiles are difficult and often need a purpose-made bending fixture.
What tolerance can a hollow profile hold?
Typically ±0.2 – 0.5 mm on external dimensions and ±0.3 – 0.8 mm on internal ones, for a profile up to about 100 mm. Larger sections and multi-chamber profiles are worse. Tighter than ±0.2 mm generally needs post-extrusion machining, not extrusion alone.
Designing a hollow section? Send the drawing before you cut the die
We review wall balance, void size, weld line position and cooling feasibility, and tell you which changes make the profile runnable. DWG, DXF or a dimensioned PDF is enough to start.
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