NEWS

Although great strides have been made in paint shop cleanrooms, there are still major opportunities to improve their cleanliness, especially with improvements in the design and construction of the paint circulation and supply systems . . .

By S. Thomas Boyce and Jan Pitzer
Hosco Livonia, MI

During the past 15 years, the automotive industry has developed a significant number of paint technologies to address the needs of the environment, product appearance and durability. Along with the development of new coatings, most new facilities have incorporated cleanroom environments in which these materials are applied.

The main reason for cleanrooms is to improve the quality of the paint finish by reducing sources of dirt contamination on a wet film. This dirt contamination causes some manufacturers to repaint the entire car rather than making spot or panel repairs. But, by eliminating dirt contamination and the resulting defects, expensive repair procedures can be avoided.

Figure 1

But, what about the other 40% of dirt sources, such as agglomerated paint particles, color carry over during the color change process and paint line cleaning? The newer high-solids and waterborne basecoat materials are much less forgiving than the older low-solids solvent-borne paints. They are more prone to shear degradation, agglomeration, foaming and skimming, all of which can contribute to a greater potential for introducing dirt into the paint circulation system.

Figure 2 .

While there are some continuing refinements of the storage and pumping supply systems, most of the emphasis on the management of dirt has shifted in recent years to the piping system.

Figure 2 shows a typical circulating-to-gun paint circulation system found in many automotive plants. There are many variations of this basic system depending on the OEM customer and geographic location. In this example, a main supply pipe exits the paint mix room and winds around one booth with four spray stations. An automotive plant could have as many as 50-80 spray stations depending on the length and number of spray booths. The size of the supply pipe could be 1.0-2.5 inches depending on the plant.

The first station of a circulating-to-gun system includes a return piping network, which each subsequent station feeds, that eventually carries paint back to the paint mix room. The detail of one drop station is displayed showing a manual spray gun with connecting fittings and hoses, fluid control, mini-filter, fluid quick disconnect and stem.

Figure 3

Spray Station Ball Valves

Normally four ball valves are used per color (two for supply and two for return) at each spray station. These valves are available in a variety of configurations, including threaded, compression fitted or tube stub. The booth wall mounted valves can be either bulkhead mounted or reverse bulkhead mounted depending on whether the customer prefers valve handles inside or outside the spray booth. Generally automatic station valves are mounted outside the booth for safety reasons to facilitate valve closing without interfering with the automation equipment.

Many ball valves used for spray station drops are not specifically designed for paint and contain medium to large size cavities around the ball that can trap paint from previous or existing colors. Paint collects between the Teflon lifesaver seals and the interior of the valve body (see Figure 3). When the ball is rotated some of the sticky or agglomerated paint particles can break free, flow downstream to the applicator and create a defect. These areas are difficult, if not impossible, to clean during flushing or color change processes and represent one of the more significant sources of dirt in a paint system.

Figure 4

The last important consideration regarding valves is to closely match the bore of the valve with the inside diameter of the station drop to prevent any restriction or pressure drop in the system.

Transition Connections

Several types of transition connections are typically used in paint circulation systems. The most common stainless steel tubing connection is the compression style connection. This connection is made using a ferrule and nut mechanical type seal (see Figure 5). This type of connection has two main advantages:

Figure 5

1. The connection normally has a smooth bore internal diameter if the correct tubing wall thickness is used, eliminating internal shelf areas or dead spots. The smooth bore also maintains uniform fluid velocity and laminar flow in straight fitting configurations.
2. The connections can be disassembled for maintenance without significant effort or damage.

The primary drawback of compression fittings is that once the tubing is connected, those fittings can’t be reused in another area.

The next most common type of connection in a paint circulation system is the ball and cone style connection (see Figure 6). These are available in NPS (National Pipe Straight) or BSP (British Straight Pipe). The design incorporates a 30-degree male seat on the side with the swivel nut and a 30-degree female seat on the other side of the connection, much like a JIC or a pipe union connection. These connections are most generally applied inside the spray booth for hose, color changer, regulator and gun connections and around filters and gages with sizes of 0.25 and 0.375 inch. This type of connection has several advantages:

1. The connection has a smooth internal bore diameter if the correct tubing wall thickness is used. The elimination of internal shelf areas or

   Figure 6 - Ball and Cone Design

   dead spots also helps maintain uniform fluid velocity and laminar flow in straight fitting configurations.
2. Ball and cone connections can be rotated and precisely positioned for neat and clean bundling of the paint hoses into and away from the applicator, color changer or bulkhead area.
3. The connections can be broken for maintenance or rearrangement reasons without significant effort or damage.
4. The connection can be reused many times, making rearrangement inexpensive and quick.

There are no known drawbacks of this connection type other than it is normally restricted to 0.25- and 0.375-inch connections. If right-angle hose fittings are used, particular attention should be paid to the internal construction. Many 90-degree fittings have a large dirt pocket in the back of the fitting, which is a major dirt contributor in paint systems (see Figure 7).

Another type of connection is NPT (National Pipe Tapered) or BSPT (British Tapered). Many of these connections have been eliminated in paint circulation systems except at pump, gage and filter joints. There is probably only one advantage to this connection type – it is readily available and well understood from an installation point of view. NPT has three major drawbacks:

1. The assembly of NPT components leaves many internal steps or shelves at the connection area. In addition, internal threads on female connections are exposed and provide many cracks and crevices for the paint to accumulate.

   Figure 7 - Hose Connections

2. During assembly of components, especially with stainless steel, the tapered thread of the fitting will become tight and stop rotating at various and random angles. This is a problem particularly when the installer is working to make racks of lines appear uniform, to have precise locations of ball valve handles or to mount areas around color changers.
3. Also, with stainless steel connections there is a tendency for the two mating surfaces to gall, which damages the threads and makes each mating component unusable for rearrangement or system modification.

Last, is the recent application of sanitary clamp style fittings. These fittings are generally applied around larger line sizes like the compression fittings mentioned earlier. The sanitary clamp style is used around mix tanks, pump and filter areas and in overhead piping mains. It is the only welded fitting listed here. Its benefits and drawbacks are identical to the compression style fittings.

Color Changer and Automation Equipment Cavity-Free Connections

With the increased use of automation for paint application, it is increasingly important for the equipment builder, installer and owner to consider the pros and cons of connector types during the specification development and decision making of the project.

Automatic stations are becoming more compact and smaller due to the limited space inside machines and robots to mount color changers and route paint hoses. The resulting challenge for paint fitting manufacturers is to provide accessible connection points to machines and color changers within smaller areas and to increase line sizes to provide higher flow rates and reduce pressure loss and shear.

Figure 8

1. Paint inlet and outlet points of color changers should remain NPT or BSPT by NSP (M) straight fittings. This allows precise racking or routing of circulation hoses away from the color changer in a neat bundle, since the swivel nut on the NPS (F) hose connector can be positioned at any angle (see Figure 8). One potential pitfall of NPT connections to color change manifolds is the gap, or dirt trap, between the end of the fitting and the base of the manifold. O-rings or Teflon wafers should be used to eliminate these traps and keep the assembly drive cavity free.
2. Hoses, connectors and fittings used for the supply and return circulation lines to color changers should be smooth bore, sweep-type fittings that facilitate laminar flow (see figure 9). These fittings can be 90, 60, 45 or 30 degrees or straight depending on the application requirements. Smooth bore provides consistent fluid stream velocity and eliminates internal steps or shelves where paint can agglomerate. Every effort should be made to closely match internal bores between connecting components. Cavity free specifies that there be no machining steps or pockets (such as drill point pockets, which can occur in angle fittings) on the interior of the fitting body. Laminar flow requirements can eliminate abrupt flow path changes in small fittings that can degrade paint color or luster and can reduce pressure drop at the color changer. This is a major new area of improvement in fittings because circulating systems’ operating

   Figure 9

   pressures are usually driven by the automatic stations and then color change time constraints. Any reduction in pressure loss within these stations provides direct benefit to the higher amounts of maintenance and expense related to seals and energy consumption.
3. Fitting and hose selection for color changer outlets to the applicator has an even more critical performance requirement to ensure optimized color change time, minimize solvent and air use and eliminate color carry-over from one color to the next. One important consideration is the use of FEP or PTFE Teflon as the hose material between the color changer and applicator to further improve performance.

Hose Management and Containment

Proper hose management and containment can have a significant impact on defect-free paint finishes and minimizing reprocessing. Automotive paint circulating

systems generally circulate to the gun, which requires a two-hose arrangement to the spray gun or color changer. One hose supplies the station and the other hose returns the paint to the circulation loop.

It is important to keep in mind that paint hose is specified by its ID since it is critical to maintain paint circulation velocity within the recommended range by the material suppliers. Conversely, plastic tubing is specified by its OD to work properly with push-lok or compression style fittings. Specifying tubing instead of paint hose, while it seems a minor point, can create significant circulation problems that will eventually show up in problems with fluid delivery to the applicators, difficulty maintaining viscosity or paint temperature or formation and discharge of dirt from the system onto filters or parts being sprayed.

Figure 10

There are several methods of cinching twin hoses together that are practiced in the industry (see Figure 10). Some plants simply use masking tape to join hoses together. However, the tape gets sticky when exposed to solvent and picks up overspray. Eventually the dry overspray particles break free, become airborne and get carried to the painted surface creating a defect.

Another approach uses nylon ty-wraps as a substitution for tape. This method is more maintenance free than tape. However, the wraps have a tendency to snag on the booth grates and create motion difficulty for manual spray operators.

Metal spring steel clips are small, compact and low profile along the outside diameter of the hose. They provide a better snag-free alternative to ty-wraps and are much better than tape.

Plastic nylon clips are more compatible with paints and solvents. They are non-conductive for safe use in electrostatic zones. The clip design also separates the hose surface to facilitate full hose surface cleaning, minimizing overspray buildup between the two hoses.

Bonded twin line hose is either continuously or intermittently bonded along its length. This design eliminates the need for clips, tape or wrap to hold the hose set. However, the bond seam along the hose creates a continuous crevice that is difficult to clean. Bonded hose also increases the structural rigidity, which is objectionable to some spray operators.

Figure 11

Monoline coaxial paint hose assemblies (hose within a hose) built in lengths up to 25 ft accomplish the same improvements as the bonded twin line hose, with one big advantage. It has just one hose surface, with no crevices or recesses for housekeeping. It is also the most aesthetically pleasing and one of the most ergonomically superior hose assemblies for use in manual stations.

For both robotic and bell automation zones, bundled hose with an exterior smooth cover provides excellent performance with regard to housekeeping, hose containment and hose life (see Figure 11). Bundled hose includes paint lines and pneumatic signal and trigger lines wrapped with a smooth, solvent-resistant exterior cover.

Advancements in extrusion capabilities and new economies of scale have enabled some suppliers to custom engineer bundled hose in quantities as low as 1,000 ft. Some users of these systems have adopted color-coded pneumatic lines to facilitate quick maintenance response in diagnosing station performance problems. Virtually any existing hose or tube size and material of construction can be assembled into a bundle to solve housekeeping, dirt-in-paint or hose kinking and containment problems using this new technology.

Manual Spray Station Accessories

Improvements in the design of manual spray gun accessory equipment have significantly contributed to reducing defects and reprocessing. No-spit paint quick disconnects, check-valve-style stems and mini-filters mounted on the spray gun are all examples of advancing technology in manual spray zones (see Figure 12).

Figure 12

Ergonomics has also been a driver of new product developments to reduce fatigue for spray operators. Spray gun flow restrictors and quick disconnects are now available in durable composite materials that are 50% lighter and fully compatible with automotive paints and solvents.

Dirt-in-paint reduction is a continuous improvement process. Those automotive manufacturers who are committed to advancing the state-of-the-art cleanroom environments in new facilities have made improving the quality of their paint supply systems a priority.

With the variety of fittings and valves available in the market, OEM companies must take a more aggressive role in requiring turnkey systems integrators and designers to use cavity-free products designed specifically for paint. The benefits are enormous and include reduced dirt-in-paint defects, improved first run capability and reduced repair warranty.

Some automotive manufacturers have even created dirt-in-paint groups within their corporate paint engineering operations and are encouraging each plant to develop an ombudsman for dirt-in-paint reduction.