Technical Resources Area
Technical Resources Area |
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Polyethylene coating materials form a thick "wrap-around" coating that is highly resistant to moisture.
No phosphating or chromating is required before application of a polyethylene coating. However the metal should be free from dirt and grease.
Refrigerator shelves have been coated in polyethylene, without chemical pre-treatment in Europe and the Far East for 35 years with no reported incidence of corrosion.
Typical pre-heat times can vary from 2 to 4 minutes, depending on the set pre-heat temperature and the diameter of the wires. If the pre-heat temperature is increased, then the pre-heat time can be reduced.
Typical oven pre-heat temperatures vary between 350 and 400 șC (or 300 and 360 șC for NG30). If the pre-heat temperature is too high there is a risk of degradation of the polyethylene - apparent by "yellowing" either during the coating process or later in service!
If the pre-heat temperature is too low "pin holing" may result, which may result in the need to slow down the line or increase the post-heat time.
In this respect NG30 is unique in that lower temperatures can still be used without the risk of pinholes.
Optimum pre-heat conditions (time and temperature) are determined by the diameter of the wires employed in the shelf design and, in particular, the difference between the thickest and thinnest wires.
A dip time of 3 to 5 seconds is recommended. If the dip time is too long, the coating is likely to be too thick. If the dip time is too short, pinholes or inadequate coverage will result.
Pin holing can also often result from insufficient movement of the wire item in the fluidised bed. It is usually better to move the items in the fluidising powder than to move the fluidising bed around the item.
Pin holing can also result from dipping items to only just below the surface of the fluidising powder. In this area of the bed the density of the powder particles is lower and it is therefore advised that items be dipped to a minimum of 100 mm below the surface of the powder.
Except in a few cases (and when using NG10 or NG30) a post heat is necessary because of the varying thickness of the wires used in the design of the shelf. Sufficiently high pre-heat temperatures to obtain a smooth surface finish on thin wires without post-heating may lead to degradation of the polymer coating on the thicker wires.
Typical post-heat times are 1 to 2 minutes at 170 to 200șC.
The optimum post-heat conditions (time and temperature) are the minimum necessary to obtain a smooth finish on the thinnest wire.
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