What Masking Devices Do I Use?

Types of masking materials to use in plating baths, curing ovens and more...

Ever since I tried to paint both sides of some shelves at the same time, I have never really enjoyed painting. I needed some way to hold the painted object up so I could paint both sides. Painters, platers and processors face this problem every day. One-pass all-side coating is the most efficient process, but cannot be accomplished all the time because of the unusual shapes of objects.

For most applications, the product can be hung from racks, placed in trays or loaded on a conveyor. However, when painting spheres, cylinders, tubes and other unusual shapes, sometimes a more singular solution is required. Expanding a plug into the hole can be the trick that solves two problems. The first and most obvious is that the plug seals the hole, preventing the coating from entering the item and saving coating material. The second feature is that a properly designed plug can be used to hold the coated object from hooks or hangers.

Plating applications face similar problems since there are usually some access or drain holes that must be sealed.

To obtain the proper finish, all plugs and seals need to be compatible with the coating materials. This means the sealing device cannot leach any material into the bath. All seals meant for reuse cannot be destroyed by the process, and, therefore, must be compatible with the temperature and bath.

For painting and coating applications, most reusable sealing devices withstand any commercial paint or powder. However the drying and curing processes require that the rubber sealing portion withstand temperatures in excess of 400°F. In addition, for applications where the seals will be cleaned, it is important that the seal resist acidic cleaners.

For plating applications, the rubber seal needs to be compatible with the applied bath material. Table I presents selections for designing a sealing device compatible with your process.

Table II provides guides that are general recommendations and should be tested prior to starting production. All processes should be tested using disposable samples. The preferred hardware for sealing devices is stainless steel. However, small savings can be realized using zinc-plated steel.

After selecting the proper material, the requirements for a closure mechanism need to be addressed. For pressure seals, a hex nut or handle should be used to ensure adequate closing pressure is applied. For repetitive processes, a snap-action cam can be used to allow quick insertion and removal. The variety of closure styles is unlimited and usually designed right on the plant floor.

When selecting a sealing device, all parameters must be reviewed prior to selecting the material. The best material may not be discovered until after several trials. For example, while silicone is an excellent high-temperature material if used in a threaded hole, it may split frequently. By examining the sealing materials beforehand, one can avoid calamities on the production line. PF

TABLE I -- Painting and Curing Processes
Sealing Material Max/Min
Temp (°F)
Compression Set
Neoprene 250/-65 Good Fair Very Good
Silicone 550/-135 Poor Poor Very Good
Buna Nitrile 300/-75 Fair Fair Very Good
Flurosilicone 550/-90 Fair Fair Very Good
EPDM 300/-60 Good Fair Good
TABLE II -- Plating and Coating Processes
Plating Material Recommended Seals
EPDM Flurocarbon
Chrome - Chromiums
Chromic Acid
  Acc Recd  
Nickel - Boric Acid/
Nickel Chloride or
Nickel Sulfamate or
Nickel Fluoborate
Acc Recd    
Acid Chloride Zinc -
Ammonium Chloride or
Potassium Cloride
Acc     Recd
Alkaline - Non-Cyanide Zinc
Zinc Oxide or
Sodium Hydroxide
  Recd   Acc
Cyanide Zinc   Recd   Acc
Chromic Anodize   Acc Recd  
Sulfuric Anodize   Recd   Acc

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