Cleaning tooling used in Plastics Manufacture - The cleaning process itself is very simple, using the heat transferred from the fluidized bath to the tools to degrade the plastic residue. It should be noted that Fluidization is not abrasive to any tools or parts you put into the bath. Objects to be cleaned, supported in a wire basket for ease of handling, are placed in the fluidized bath, operating at a temperature between 450°C (842°F) and 550°C (1020°F) depending on the polymer, for approximately 20 to 30 minutes. Actual burn-off times and temperatures vary depending upon the weight, complexity and material composition of the item to be cleaned in addition to the quantity of residue to be removed. During the first two thirds of the total time, the plastic is reduced to a tar mix state. In this phase all the initially combustible products of the plastic leave the fluidized bath as fumes, ducted away by means of appropriate fume extraction. In the last third of immersion time, the tar mix state is reduced to carbon which either burns away or remains loosely bound to the tool. In the latter case it can be blown away or brushed off without causing damage, after the part is removed from the fluidized bath.
The clean item can then be put to one side, preferably on a steel plate, to cool before refitting on a machine or returned to the tool store. In some cases, particularly with dies from blow molding machines using PVC, a further operation of polishing with a soft cloth may be required. The aluminum oxide fluidizing medium is not degradable but will need to be replenished due to loss from spillage or entrainment in the exhaust. In the particular case of PVC, chlorinated hydrocarbons remain in the fluidized bath after burn-off and this dictates special maintenance procedures given below.

Heat Treatment and thermal analysis - The large working area allows metal tubing to be coiled up for use in change of state analysis of gases and liquids and vessels can be submerged for reactive chemistry work. Heat treatment and calibration of any device type that needs to be fully immersed into a dry heated environment. Using one of the optional baskets a large number of devices can be tested at one time. The excellent heat transfer of the aluminum oxide make Fluidized baths much more energy efficient than conventional ovens and far safer than salt baths. 

MAINTENANCE FOR Techne® INDUSTRIAL FLUIDIZED BATHS

The aluminum oxide, not being degradable, will only require replacement when losses occur due to attrition, spillage or contamination with inert pigments, filler or acidic byproducts from the burn-off process. Note that the aluminum oxide pulled out of the bath through the exhaust duct can be captured for reuse by the incorporation of a "cyclone" option (Techne® Model CN-100).

On at least daily intervals, the bed should be cleaned of floating residues by means of a wire mesh hand scoop. This procedure removes carbon char which impairs fluidization and acts as an absorbent. More importantly, it can also remove un-charred plastic and so reduce the quantity of fumes produced and the time of processing.

The optional air-line filter into the bed is self-draining. However, it should be kept in good condition by inspection at two week intervals and by cleaning the bowl and washing or replacing the filter element as necessary. With exceptionally dirty or wet air supplies this frequency may have to be increased. Fe water or water vapor in the air supply is a notorious source for the production of hydrochloric acid in the bed when PVC is burned off. In addition, oil vapors in the air supply which reach the fluidizing plate are carbonized within the pores of the plate, quickly causing blockage and consequent poor fluidization.

All articles should be completely cleaned and removed from the bed before shut down. Corrosion of processed parts could be seriously increased if they are left immersed overnight. Furthermore, residual plastic, instead of being burned off in a fluidized state, could percolate down through a static bed and settle on the porous plate causing blockage and poor fluidization.When parts are removed from the bed, they should be allowed to cool in the air and, while still warm, wiped with on oily cloth to prevent rusting. If the bath is left unused for long periods of time, empty the aluminum oxide and store it in a separate container. Keep the inside of the bath clean and dry.

SPECIAL MAINTENANCE PROCEDURES FOR Techne® INDUSTRIAL FLUIDIZED BATHS
WHEN BURNING OFF PVC OR OTHER HALOGENATED POLYMERS

Burning off PVC (polyvinyl chloride) in a fluidized bath offers one of the most severe conditions of operation. Hydrogen chloride (HCl) liberated on the breakdown of PVC is absorbed by the bed medium creating an acidic environment within the bed. This happens especially when the bed also absorbs water from the atmosphere or when the fluidizing air is cold. HCl is extremely corrosive, especially when it is aerated and wet. In addition, in water it produces chloride ions which, even in neutral or alkaline solutions, promote corrosion and rusting in steel. Witness, for example, the corrosive nature of sea water and calcium chloride road de-icer.

Consequently, fluidizing beds used for burning off PVC require strict supervision to minimize corrosion of the bed itself and of parts cleaned in it, especially if these are of un-coated steel. The purpose of most of the recommended maintenance procedures is aimed at keeping the bed medium clean, free-flowing, free of gums, acids, agglomerates, partly decomposed plastic, char and larger particles. These cleaning processes have the additional benefit of ensuring good fluidization and thus good heat transfer throughout the bed and through immersed parts. This, in turn, reduces burn-off time, uneven heating of parts and thus distortion, increases heater life by eliminating localized hot-spots and makes cleaning easier on a regular basis.

The following procedures are essential when PVC is burned off on a regular basis,
but they can also be followed profitably by users of other plastics.

The bed should be completely emptied at monthly intervals and visually inspected for signs of corrosion. Examination should include the walls of the inner cylinder, the porous fluidizing plate, the thermocouple sheath and the loading baskets with particular emphasis on exposed weld lines. Serious corrosion should be dealt with immediately by improving maintenance procedures or by replacing the inner container before holes appear and cause failure of the heating element and corrosion in more inaccessible parts.

The fluidizing media should be screened by passing through a 50 to 70 mesh sieve on at least monthly intervals to remove foreign bodies, agglomerated gummy material and, periodically, be completely changed for a new charge of aluminum oxide.

During shutdown overnight or over the weekend, the temperature should be reduced to 212°F (100°C) to ensure that the moisture from the atmosphere is not condensed into the bed to create a hydrochloric acid solution. Fluidizing air may be turned off in these circumstances but, when it is practical to do so, it is preferred that it should be continued. For extended shutdowns exceeding two days, the medium should be removed and the inside of the bath wiped out with a rag wetted with a 15% washing soda solution (sodium carbonate).