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Pre-treatment tunnels are a series of stations spraying a variety of chemicals. Some chemicals such as the phosphates used for anti-rust conversion coating, are specifically used because they react with metals. The whole point of the phosphate spray is to cause a reaction on the surface of the metal being sprayed to form a corrosion-resistant film. Spraying phosphates through metal nozzles is thus problematic. There are metals that are resistant to the actions of phosphate but they tend to be more expensive and as pre-treatment plants run at temperatures below 100 degrees, plastics such as glass filled poly-propolyene tend to be the more cost effective choice of material.
The SNP Uni-Spray quick change system is manufactured in glass filled polypropylene, PVC, CPVC or PVDF. These materials give excellent chemical resistance, are hard wearing and relatively inexpensive. This makes them all ideal for use in pre-treatment tunnels. Indeed the SNP Uni-Spray system is regarded as the market leading system worldwide in this niche segment. A table of their relative properties is given below.
Material
Temp rating
Chemical compatibility
Corrosion resistance
Strength
Cost
PVC
60oC
High
Good
Low
CPVC
100oC
Medium
Glass filled poly-propeleyne
70oC
PVDF
120oC
Excellent
Surface finishing key applications
Spray challenges for surface finishing