Technical Resources Area
Technical Resources Area |
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When pinholes occur on wirework items, they will tend to appear either on the underside of the wire interstices (wire joints and welds), or at the very top of the coated item. They will occur: -
1.1) If the powder can bridge across a gap at the interstices.
The powder can melt to form a "bridge" between the wires, but on cooling shrinkage can break the "bridge" and form a pinhole.
In this case air-cooling may give fewer pinholes than water quenching.
Alternatively, during the post-heating stage, any air trapped beneath the bridge can expand creating a bubble. This bubble can burst to form a pinhole. If this is the case the lowest possible post-heat temperature should be used.
1.2) If any item is dipped in a fluidised bed without any horizontal or vertical movement air can become trapped.
This can occur on flat surfaces as well as at the wire interstices. A well-designed automatic fluidised bed coating system (e.g. Harrier Autocoat) will have this ability to move the items in the fluid bed.
1.3) When there are a series of large diameter parallel wires close together, "shadowing" can occur.
"Shadowing" is the result of one wire preventing powder from reaching another. This is generally overcome by the movement of the item in the fluid bed as discussed in 1.2) above.
1.4) When the fluidising density is inadequate.
This form of pin holing is apparent by the fact that it is more noticeable along the wires and at the joins at the top of the bed. It can be overcome by either adding more powder to the bed, by dipping to a minimum of 100mm below the powder surface, or by movement of the item in the fluidised bed.
2.1) Increase pre-heat temperature
In general, higher pre-heat temperatures give fewer pinholes, due to the reduction of the melt viscosity. However, too high a pre-heat temperature may result in polymer degradation and subsequent "yellowing" of the coating.
2.2) Increase dipping time
Longer dipping times will reduce pinhole formation but will also increase the thickness of the coating. The thickness of the coating may be critical to the design characteristics of the item - for instance if the shelf must fit into a runner.
2.3) Move the item in the fluidised bed
Movement of the wire item in the fluidised bed will always reduce pinholes. The Plascoat Harrier Autocoat is fitted with such functionality.
2.4) Pre-heat the fluidised bed
In some factories pinholes are worse at the start of a shift when the powder is cold. A higher fluidised bed temperature will result in less heat loss during the dipping process and therefore better "flow-out" of the powder. A bed temperature of 40 or 50șC is common in most automatic lines. However if the bed gets too hot, the powder can melt to form a "crust" on the top of the powder or on the sides of the bed nearest to the oven.
In addition if the temperature of the bed becomes too high, fluidisation can be affected, which, in turn, can cause pinholes.
2.5) Introduce post-heating
Many coating machines are fitted with a post-heat facility including the Plascoat Harrier Autocoat. The use of a post-heat oven will reduce the occurrence of pinholes, give a smoother finish and enable output rates to be increased. In addition, pre-heat temperatures can be lowered, thereby reducing the likelihood of degradation.
2.6) Optimise post-heat temperature
The lowest possible post-heat temperature to ensure an acceptable surface finish should be used to avoid degradation. Reduction of the post-heat temperature will reduce the likelihood of pinholes forming from "bubbles", but increasing the post-heat temperature will help the polymer to flow out. The optimum is best determined during production trials.
2.7) Allow the coated item to cool in air
In general, air-cooling gives fewer pinholes than water quenching. Rapid cooling in water can cause shrinkage of the coating and any "bridges" that have formed between wires may break to form a pinhole. However air-cooling is rarely acceptable on automatic lines where speed is essential. The optimum for polyethylene is to allow the item to cool in air for 20-30 seconds before water quenching.
2.8) Ensure the fluidised bed is topped up with powder
Pinholes can form at the top of the item where the density of the powder is at its lowest. In order to keep the density of the powder in the bed as consistent as possible, the bed must be regularly "topped up".
2.9) Dip the item to a minimum of 100mm from the surface of the powder
The density of powder in the top 100mm of a fluidising bed is always the lowest. If pinholes occur along the top edge of the item, increase the dipping depth and keep the bed well "topped up".
2.10) Change to Plascoat LDPE,
NG10 and
NG30
All Plascoat polyethylene powders are designed to coat easily without pinholes. However Plascoat NG30 is a new concept in polyethylene powders and has been specifically designed to coat at lower temperatures and with shorter dip times, without the risk of pinholes.
3.1) "Weld Splatter"
Any sharp pieces of weld may penetrate through the coating and give a pinhole. Whilst this is rare with modern wire manufacturing plants, it is a benefit of LDPE coatings that the coverage over welds is superior to that offered by thermoset powder paints.
3.2) Weld Design
If welds can be used to fill any potential gaps or air traps, the possibility of pinholes can be minimised.
3.3) Avoid use of significantly different wire thicknesses
If a wire item is manufactured from wires of significantly different thickness, then the processing window is narrowed and it is more difficult to achieve an acceptable coating. If this is unavoidable, it is recommended that the thinner wires are horizontal during dipping.
3.4) Avoid very thick wires
Wires of 5mm diameter or more can present particular problems. Air entrapment at the welds is more common with larger wires.
3.5) Avoid narrow gaps between wires
If the wires are too close together there is a greater likelihood of "shadowing" during the coating process. Plascoat recommend a minimum distance of 10mm between parallel wires.
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