Mike Thompson, Southwest Polymer Supplies outlines the company’s expertise in extrusion tubing
Based in Torquay, Devon, Southwest Polymer Supplies (UK) specialises in the import of thermoplastic and rubber processing equipment from laboratory to full-scale manufacturing. The company also supplies a range of equipment from injection moulding machines to single and twin screw extruders.
I’ve worked for various companies including Amcor Flexible's, Alcan, Petlon Polymers, Lorient Polyproducts and Millmerran. Within these companies I fulfilled a variety of positions and roles from operator through to development manager and more lately polymer engineer for Millmerran. In this previous role as polymer engineer, I developed an extrudable, ceramifying intumescent which has certification for fire doors from 30 minute up to two hours.
Extrusion is ideally suited to the production of continuous lengths of tubing. The successful manufacturing of tubing depends greatly on good die design and the correct setup of the extruder which should include the correct screw design to optimise output dependent upon the polymer being processed. Generally this is not as straightforward as it may appear since the interaction effects of post extrusion swelling (die swell) and the complex flow characteristics of viscoelastic fluids (rheology) need to be balanced. Much research has been written around the viscoelastic flow and its complexities.
The majority of profile dies (including annular dies) are designed from the experience of the tool designer/maker. This experience is now being supplemented by computer aided design packages that take out into account the material’s rheological properties. By performing complex mathematics these packages that can help produce a die that negates the majority of guesswork from the design. Thus adding ‘value’ to the business by reducing the cost of die trialing and bringing the cost of the whole project down.
Tubing is made from a die with an annular die exit. Dies for tubing products are predominately made from in-line dies (also called spider dies) shown opposite. Although it must be noted that other dies including cross-head and co-extrusion are available
The key elements of an in-line die are:
Housing: Mounts onto the end of the extruder, providing a circular passage through which the melt flows; this supports the mandrel and retaining ring.
Mandrel (Torpedo): suspended in the centre of the circular passage in the die body with metal bridges called spiders (typically three are used). One spider allows for passage of air into the centre of the torpedo and is streamlined to avoid flow stagnation, and supports the die pin.
Die pin: mates with the torpedo to provide a stream- lined sizing to the final inner diameter of the melt tube leaving the die. An air hole runs through it to allow the passage of air through the die body to the interior of the melt tube. A slight positive air pressure may be used to keep the inner diameter of the tubular extrudate from collapsing on exiting the die.
Die land: This forms the outer diameter of the tubular extrudate, held in place with a retaining ring and position adjusted with centering bolts. The die land can be changed to create a tube of a different diameter or wall thickness while keeping the original die pin.
Heater band: Closely fitted to the housing (and for larger pipes to the exposed portion of the die pin) to ensure that the die is held at a temperature close to the required temperature of the melt.
Flange for extruder attachment: Tapered flange to permit alignment and attachment to extruder with split locking collars.