A necessary part of the process for handling low level waste in the Nuclear Industry is drying.
This is usually done in large vertical drum tanks (total weight about 10 tonnes) which need lowering into place over the special heater rigs.
60mm thick PTFE plate was chosen for the contact pad between the drum and the heater rig in this application. This was because of its temperature capabilities and for its load bearing abilities, as well as its very low coefficient of friction. The load bearing and low friction characteristics, allowed the unit to slide into position without sticking or jamming.
PTFE is also used in Bridge Bearings (yes modern bridges often have bearings) as a means of allowing the bridge to expand and contract. You may have seen the “saw tooth” joints in elevated sections of motorway. At the foot of the elevated section or bridge you will often find bespoke pad bearings which both support the bridge and allow it to move during expansion and contraction depending on ambient temperature.
As a ring section was needed for this PTFE support pad to match the base profile of the drum which contains the nuclear waste, we suggested that in order to save material wastage and money on this project, that annular sections rather than a complete ring were used. This reduced material wastage considerably, with a correspondingly significant reduction in cost to our customer.
In order to create the profile needed, a special cutter was commissioned which included the correct cutting angles for PTFE. Once we had skimmed the sheet to thickness, in order to meet the tolerance required, we carefully designed the toolpathing, along with pocketing to allow clearance for the cutter during the arc needed to machine the shape. The outline was created using multiple passes as there was a lot of material to remove.
Once the top detail was finished, we had to set about creating the detail on the underside. This was done by making a custom jig to ensure perfect positioning of the annular section. Once placed upside down in this jig, the reverse detail of the blind slots was machined using a second program.
Because of our extremely large vacuum pump (Rietschle Vacfox 500) we are to put whole sheets of material on the bed (3000 x 1500) of our Multicam MG router, enabling us to machine multiple components on the same program, meaning much shorter cycle times, with subsequent cost savings for our customers.