The extrusion of PVC and PE pipes for various applications is achieved by using a 'head' consisting of a bowl, spider, torpedo, compression chamber and pin arranged as shown in Fig 1 The individual parts are shown in Figure 2.

Different sized heads are used for producing pipes of different diameters and grades; ie., for sewage, stormwater and pressure pipes, etc. The problem generally encountered with the head is in the spider and bowl area, where the chamber diameter increases after leaving the extruder screw area.

This increase in chamber diameter results in a decrease in flow rate through the bowl area, but an increased flow rate again as the torpedo splits the flow as shown in Fig1. In the case of unstabilised PVC (among other extrudates), there is a tendency for the slow moving material to stick to the surface (hard chrome coated) of the bowl and spider, leading to "burning" of the material due to overheating.

This burnt material appears in the final product as a discoloured streak, which is unsightly, has inferior mechanical properties and is unfit for sale.

This burning also causes the breakdown of the PVC, with one of the by-products being hydrochloric acid. As the chrome coating is porous, the acid penetrates through to the metal substrate, damaging it and undermining the chrome coating, causing debonding and allowing it to flake and be pulled off by the flow of extrudate.

The use of unstabilised PVC is desirable, as additives (such as stabilisers) to the feed material increase the cost of the product. Therefore, profitability would be increased if these additives could be removed with no detrimental effect on the final product. This is now possible, using C-Ramic's C-Ramachrome process to seal the porosity in the chrome plate, providing increased protection for the chrome plated extrusion components.

Hard Chrome Plating, although often used for wear and corrosion protection, has an inherent weakness - it has a microcrack network, fissures and porosity, sufficient to allow corrosive fluids to penetrate to the substrate/chrome interface, attacking it and causing debonding of the plating.

The debonded flakes of chromium metal still retain their hardness and if caught up in the product will behave as large grinding particles scoring and scouring the surface that it was expected to protect. C-Ramic's
C-Ramachrome is designed to penetrate and seal the chrome plate with ceramic, preventing penetration by fluids and the consequent debonding leading to product damage. Scanning electron microscope pictures, in conjunction with energy dispersive chemical analysis (EDAX) of surfaces treated with C-Ramachrome shows the complete filling of the porosity and further corrosion studies in a salt water environment confirmed the sealant nature of C-ramachrome.

Abrasive wear is also a problem on the spider, which has a hard chrome coating and is regularly ground back to the tool steel substrate and then re-chromed. The PVC is compressed greatly on entry into the spider and most wear occurs here. Debonding of the hard chrome coating from the hydrochloric acid attack opens up the whole substrate material to wear and corrosion, which has to be repaired.

The usual repair cycle has been to either weld the die and machine to original size and polish, or re-chrome - which sometimes can present difficulties, due to hydrochloric acid being resident in the parent metal. When production is interrupted (due to inferior product production) and the head is disassembled, the parts are cleaned to remove impacted/stuck plastic. This is normally carried out using a Scotch Brite-type pad or cleaning wheel on a rotary grinder - which contributes significantly to accelerated wear around the areas of small radii, more than on flat areas.

The result of this cleaning process tends to allow lifting and removal of the hard chrome coating by the flowing material and progressive exposure of the substrate material, which has a much higher wear rate than the chrome and which increases burning of the extrudant (as adhesiveness between the substrate and the melt is much higher than for the hard chrome).
C-Ramachrome is applied to an existing hard chrome coating to achieve total sealing of the coating by filling the microcrack network, eliminating porosity and rendering the coating impermeable.

This causes no measurable increase in thickness of the coating. This coating is now of the CERMET class of materials, or a metal-ceramic composite. A spider coated with C-Ramachrome is still in full operation many years after re-coating, shows no signs of severe wear and is continually producing 'A' grade product. Normal cleaning practises have been applied to this spider. The C-Ramachrome retains its metallic character but does not suffer from the same problem of debonding and product damage.

If you require advice on how to optimise your extrusion process, contact the experts at C-ramic.