What Masking Devices Do I Use?

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)	Tear Strength	Acid Resistance	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
	Buna Nitrile	EPDM	Flurocarbon (Viton®)	Neoprene
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

Recd=Recommended
Acc=Acceptable
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