Two sizes of mills are used to blend ingredients for paint. Their PTFE bellows have vulcanized seams to resist the aromatic solvents inside the mixing chambers.
Bellows seals accommodate a 44-in. travel as these shafts are raised and lowered during operation.
Polytetrafluoroethylene (PTFE) shaft seal bellows with vulcanized seams are withstanding damage from aromatic solvents on mills that blend paint ingredients at Rust-Oleum Corp.'s Wisconsin plant. The bellows have been in service for almost a year under conditions that caused previous seals to fail within days.
The bellows seal on this single-shaft mixer is located within part of the machine's housing.
At the plant, two sizes of mills are used to blend various ingredients for solvent-based paint products. In the mills, either one or two shafts extend down through the housing into a sealed chamber where they drive a mixing impeller. Depending on the viscosity of the material, the shafts are moved up or down to change the location of the vortex in the ingredients and achieve homogeneity. Robert Johnson, engineering and maintenance manager, says temperatures typically average 140° to 160°F, and the bellows area is permeated by a steamy, solvent-laden vapor. To contain the solvent fumes, the mixing chambers are kept at a slight positive pressure of about two inches of water, and a sealed system with an inert nitrogen atmosphere is maintained. Because the mills also incorporate a control system that shuts them down if the oxygen level rises above 3 percent, sealing the shafts is critical to maintaining the inert atmosphere.
An overall view of the single-shaft mill shows the feed chamber and piping used to load pigments and solvents into the mill's sealed system. The bellows seal (arrow) helps to contain solvent fumes and maintain inert atmosphere.
Each shaft is sealed where it enters the housing with a bellows that can accommodate the travel of the shaft as it is moved during the process. The corrugated bellows originally supplied with the mixers lasted only a short time before they had to be replaced because aromatic solvents such as xylene softened the elastomer-coated material and allowed the bellows to collapse. Replacement bellows made by stitching together rings of elastomer-coated nylon fabric fared no better because the solvents dissolved the stitches and were absorbed into the material. Johnson says, "We would see weeping right away, and within a couple of shifts we were babysitting them on every batch." He notes, "As a bellows failed, it would get soft and collapse, so we had to pull it back up and avoid raising or lowering the mill. Then, it took more time to make the paint because we couldn't set the right height for the viscosity of the batch." Failure of a bellows made it necessary to disassemble the mill to remove and replace the bellows. This caused a significant loss in production and incurred unnecessary expense as well, according to Johnson.
Bellows are made to resist solvents by vulcanizing rings of PTFE material in a specialized Vulca-Seal process.
To solve the problem, the company worked with A & A Manufacturing Co. Inc., New Berlin, WI, a specialist in the design and manufacture of protective bellows. Johnson says A & A supplied samples that he tested to determine their resistance to typical solvents used in the process. As a result, the bellows were made of PTFE material using a specialized version of the company's Gortite Vulca-Seal process. This is an original A&A development, designed specifically for demanding applications where the seams inherent to a sewn bellow are not acceptable. In the process, the sections are cut separately and then joined by vulcanizing, resulting in a "seamless" cover.
The mills use two sizes of bellows. The larger size, used on the twin-shaft machines, has a 6.75 in. ID and 12 in. OD with an extended length of 44 in. and a retracted length of 2 in. The smaller size, used on the single-shaft machines, measures 12 in. ID by 18 in. OD with an extended length of 22 in. and a retracted length of 1 in.
Johnson reports that the new bellows have been in service for about a year without experiencing any failures or requiring any maintenance. He says the plant runs generally 24 hours per day, usually five days a week but six days a week during the plant's busy season. "We're running them an average of 16 to 18 hours a day."
More information is available by contacting A & A Mfg. Co. Inc., 2300 S. Calhoun Rd., New Berlin, WI 53151, calling 262-786-1500, or sending an e-mail to [email protected].