Thickness and Curing

Question:

Can the thickness or size of a part affect how long or at what temperature it needs to be cured? (Assuming that the maximum area load is not exceeded.) A. S.

Answer:

Three main physical parameters affect the cure of a part:

1) Physical size vs. the oven opening and length will determine the oven size;

2) Metal type (steel, aluminum, cast iron, zinc diecast, etc.) determines the heat transfer, and part weight will determine the time and temperature required in the oven;

3) Maximum part thickness along with No. 2 can affect the time/temperature, especially if there is a significant difference within a part or group of parts.

Oven design and part loading can also affect the part cure:

4) Airflow within the oven (whether batch, monorail, or square-transfer design) can create hot or cold spots.

5) Significant difference in part weight or maximum metal thickness within any batch of parts can create design challenges. Oven temperatures will over/under shoot and/or parts will be over/under cured. That is, mixing heavy castings and light sheet metal within any short time frame will cause a problem.

6) The number of zones and cooling method can also determine total part cure, coating quality and the heating efficiency

Coating type and quality required could also affect part cure:

7) Each coating has an optimum time/metal temperature cure schedule and a normal overcure without quality loss.

8) Quality effects with part over/under cure vary by coating. Usually coating shrink (loss of film thickness) occurs with overcure and loss of corrosion-resistant properties (salt spray, detergent, etc.) occurs with undercure. Gloss variations can also arise depending on the coating, color, and design gloss level.

Whether you are looking for a new system or troubleshooting an existing system, you should involve your coating supplier and equipment supplier in curing considerations. If you plan on running a variety of part thickness, I would recommend starting with the lightest parts and move towards the heaviest parts. Custom coaters often leave heavy (thick) parts in the oven at the end of the shift to “soak” for a proper cure. Changing line speed can also help, but usually creates other problems. Unscheduled (and some scheduled) line stoppages will probably create cure problems.

Part cure is best tested by “solvent rubs” but this needs to be done on parts that are cooled using a constant method (type solvent, rub media and pressure) by all testing personnel. The use of a traveling thermal couple recorder is an excellent tool to document and communicate with your suppliers. Both coating and equipment suppliers have them to use at your location. These are beneficial in locating equipment problem (hot/cold spots) or chasing quality problems (thickness difference in a part or group of parts, or line speed changes or stoppages). Many users have purchased their own recorders after borrowing one from their suppliers. Costs have dropped significantly during the past few years, and the units are compatible with most PCs. They are also a good tool for monitoring equipment operation and part quality on a routine basis.