Clamping Force in Extrusion Blow Molding

Thick flash on the parting line, a weld that splits when the container is dropped, and a mould that needs new pinch-off inserts every few months are usually described as three separate problems. In extrusion blow molding they are frequently one problem: the clamp is not holding the mould halves together with the force the part actually requires.

The difficulty is that clamp force rarely fails loudly. A machine with marginal clamping still produces containers, so the defect is absorbed as extra flash that the trimmer removes, or as a lower drop-test pass rate that is attributed to the material. Meanwhile the mould suffers — parting lines wear, pinch-off edges round over, and the clearance that should seal the flash is gradually destroyed. Replacing a mould costs far more than specifying the right clamp force at the outset.

Sailwin builds extrusion blow molding machines from 0.5 L containers up to 1000 L industrial parts, with more than 15 years of experience, 500+ machines delivered to 60+ countries and CE plus ISO 9001:2015 certification. This article sets out what the clamp actually has to do, how to size it from the part rather than from a machine catalogue, and how to read clamp problems from the finished part.

1. What the Clamp Actually Has to Do in Extrusion Blow Molding

It helps to separate the clamp’s responsibilities, because only one of them involves the blowing pressure directly.

• Resisting the opening force created by blowing pressure. Inflating the parison pushes the mould halves apart across the projected area. This is the classic sizing calculation, and in extrusion blow molding the pressure is modest.
• Squeezing the parison flat at the parting line to form a sealed flash. This is the duty that consumes most of the available force. The clamp must compress two molten walls down to a thin, controlled flash thickness along the whole perimeter, and it must do so while the material is still hot enough to fuse.
• Holding the mould halves in register. Large moulds for containers of 100 L and above are heavy and long. Any deflection or shift at the parting line shows up immediately as an offset flash line and a step on the finished part.
• Surviving the mechanics of the carriage. The mould halves are carried on platens that move, and on large machines the inertia of the moving assembly is part of the structural load, not just the process load.

This is why comparing clamp force numbers between an extrusion blow molding machine and an injection molding machine is meaningless. A typical industrial blow molding process runs with blowing pressure in the region of 3 bar — an industry-typical figure, and a different order of magnitude from the 25–40 bar blowing pressure Sailwin specifies for PET stretch blow molding machines. At that pressure, holding the mould shut is the easy part and sealing the flash is the hard part.

2. Sizing the Clamp: The Arithmetic

The starting calculation is straightforward. What matters is what you put into it.

Three inputs are commonly underestimated.

First, the flash area. On a container with handles, a large neck or a non-symmetric body, the flash that must be sealed runs right around the complex outline of the part. That perimeter can be considerably longer than the perimeter of a simple cylinder of the same volume, and it is the perimeter that the clamp has to squeeze shut.

Second, the real blowing pressure at the mould. Pressure measured at the regulator and pressure acting on the parison are not the same number. Restriction in the blow pin, the valve and the air path reduces the effective pressure, but if the machine is set higher to compensate, that higher value is what the clamp sees.

Third, temperature. A colder parison is stiffer and needs more force to flatten at the parting line. Processes that run at the lower end of the material’s blowing window to save cooling time increase the clamp requirement without changing the arithmetic on paper.

A worked illustration. Take a 200 L drum with a body approximately 590 mm in diameter and 880 mm tall. Seen in the closing direction the projected area is roughly 5,192 cm ². At a blowing pressure of 3 bar — about 3.06 kgf/cm² — the opening force is approximately 15,900 kgf, or roughly 156 kN. Adding allowance for flash sealing, mould mass and parison stiffness brings the practical requirement to a multiple of that figure. The purpose of the calculation is not to produce a single number but to show how much of the requirement comes from the part dimensions and how little from the blowing pressure — which is exactly why clamp selection in extrusion blow molding is driven by flash sealing and mould rigidity rather than by pressure alone. Substitute your own drum dimensions and mould layout to run the same estimate.

If a mould is showing thick flash along part of the parting line and normal flash along the rest, the clamp is not the first thing to check — the mould faces are. Uneven flash means the mould halves are not making contact evenly, which can come from worn parting-line lands, distorted platens or a mould that was never matched to the platen surface. Increasing clamp force to close a gap caused by geometry will wear the mould faster, not fix the seal.

3. How Clamp Problems Show Up in the Finished Container

The table below separates the symptoms of too little force from the symptoms of too much, because the corrective action runs in opposite directions.

4. Clamp Control Features Worth Specifying

• Two-stage closing profile. A fast approach followed by a slower, force-limited close protects the parison and the pinch-off edges. Single-speed closing forces a compromise between cycle time and mould wear.
• Force-limited mould protection. Detects an obstruction or a mis-placed parison and stops the close before the mould is damaged. On a large mould the cost of that protection is a small fraction of the cost of the repair it prevents.
• Programmable clamp force per mould recipe. Different containers need different force. Storing it in the recipe rather than relying on an operator’s memory removes a whole class of shift-to-shift variation.
• Blow valve and cylinder quality. Sailwin machines use FESTO combined blow valves and SMC cylinders, with Siemens or Mitsubishi PLC control monitoring 40+ parameters in real time, so clamp and blow timing stay repeatable rather than drifting between shifts.
• Mould change speed. Sailwin machines are designed for mould changes in under 30 minutes. Clamp platen design, hydraulic connections and mould clamping hardware decide whether that is realistic for the moulds you actually run.

5. Sailwin Case Study: Thick Flash Traced to the Wrong Fix

Scenario based on a Sailwin customer project; final configuration is confirmed against your container drawing during engineering review.

6. Matching Clamp Force to Your Container Programme

Sailwin’s extrusion blow molding range covers 0.5 L to 1000 L, with machines for small technical parts through to large industrial containers and multilayer barrier structures. Relevant models for clamp-sensitive work include SW-S30L for containers up to 30 L at 600 per hour, SW-S60L at 450 per hour, SW-S80L at 360 per hour, SW-S160L at 300 per hour, SW-S260L for containers up to 250 L and SW-S1000L for parts up to 1000 L at 250 per hour. All-electric models are available in the SW-60, SW-70, SW-80 and SW-90 series.

The range processes PE, PP, ABS, EVA, PC and PA, including multilayer PE+PA+EVOH structures where the barrier layer changes the stiffness of the parison and therefore the force needed to seal it. Deliveries run on a 30–45 day lead time, extended to 45–60 days for custom builds, with full-load FAT testing before shipment, 3–7 days on-site installation and commissioning, common wear parts shipped within 48 hours, 7×24 remote support and a 2-year whole-machine warranty.

When you request a quotation, send the container drawing, the material and the annual volume. The clamp calculation follows from those three inputs, and so does the answer to whether the flash perimeter of your part, rather than its body, is what really sets the machine you need.

Frequently Asked Questions