This fact sheet series is produced by the Minnesota Association of Metal Finishers & Minnesota Technical Assistance Program for metal fabricators and their platers.
Holes, Seams, Threads, Recesses and Tubing Assemblies: Improve Drainage to Minimize Contamination
Fact Sheet #6
Recessed areas on a part can accumulate process oils and fluids. During cleaning and plating steps, substantial amounts of trapped fluid can leak out and contaminate baths. Or worse, as the part contracts and expands as it changes temperature, trapped fluids can seep over clean surfaces. Contaminated surfaces prevent plating from bonding to the metal surface. The worst contributors are narrow, blind holes and lapped sheet metal seams. Finishing shops are well equipped to remove surface films of most machining fluids. But unless alerted or the fluids are obvious, most metal finishing job shops assume no pools of fluid contamination remain. Many problems can be avoided by eliminating these troublesome features during product design.
Improved Part Design Reduces Fabrication and Plating Costs
An original equipment manufacturer (OEM) contracted with a sheet metal fabricator to form and assemble a family of parts constructed from lightweight steel. The parts had numerous bends, with spot welds on lapped and folded seams. The fabricator brought the parts to a plater to have an electroless nickel finish applied to improve corrosion and wear resistance. The family of parts represented over $100,000 in plating sales annually.
After a trial run, the plater quoted a cost 25% higher than the fabricator had budgeted. The quote was accepted because other platers processing these parts had experienced 70% reject rates, with costly delays and increased costs. The primary reason for the high reject rates was the spotty appearance of parts.
Because appearance had become the critical criteria for acceptable parts, it was necessary to duplicate exact inspection procedures at the manufacturer, fabricator and plater.
Visual inspection of critical surfaces was done at arms length, without tilting or rotating the part, and under identical lighting. Using these standard procedures, all parties were able to compare incoming defects against rejects to establish accountability.
After keeping meticulous records, three causes of rejects were identified which led to:
- Reducing oil use on a straightline sander;
- Designing more efficient packaging that was reusable and more protective; and
- Running smaller plating lots to speed drying and minimize water stains.
With the process in control, rejects were reduced to 5%.
Even though product quality was now good and parts were delivered on schedule, plating costs were still significantly higher than the OEM had budgeted. Recessed areas, spot-welded seams, and the absence of racking holes contributed to difficult parts processing and, ultimately, to the cost.
At a meeting of all three companies, inspection and process time documentation was invaluable in demonstrating how part design increased the cost of plating. The OEM decided design changes were necessary to cut costs:
- Racking holes were positioned to promote drainage and to protect critical surfaces.
- Airways were created in recessed areas.
- Drain holes were added, reducing cycle time. On one part, two additional holes (3/16 inch I.D. [inside diameter]) cut drain time in half, reducing cycle time proportionately.
- Endtabs replaced full length seams. This greatly reduced the volume of fluid trapped in seams, which leaked during subsequent operations.
In the end, the plater was able to reduce its plating charges by 10% over the next two years. In addition, relocating the spot welds to the end tabs allowed sheet metal fabricating steps to be combined. This reduced manufacturing costs by more than enough to bring the overall project within budget. With improved part design, quality was reliable, delivery schedules improved, labor was reduced and profits increased for all three companies.
Problem Sources and Solutions
In this case, the source of the problem was small amounts of process fluids remaining in seams. A number of design features--such as blind holes, small diameter holes, threads, surface texture, porous metal structures and recesses in general--can cause process fluids to be carried into later processing steps.
In the following section the most troublesome of these features will be addressed and a number of possible solutions proposed. Work with your metal finisher to decide how and in whose facility to implement a solution. Start evaluating at the design stage.
WELDED TUBULAR FRAMES
Enclosed areas in these products are generally assumed to be completely sealed, isolating exterior from interior surfaces. In most cases, pinhole leaks are present, created by air escaping as the part is heated during welding. Even if no contaminants are present when the product leaves the welding station, parts undergo expanding and contracting cycles in many finishing operations. This first sucks process chemicals into the tube interiors, and then bleeds them out, contaminating surfaces being plated or finished.
- Specify drain holes at the top and bottom of each cavity. Use hole diameters consistent with the volume of the cavity: a minimum of 3/16 inch for small assemblies; 3/8 inch minimum for 2-inch tubing; 1/2 inch minimum for 4-inch and larger tubing. Hole size is important to produce drain times that are in seconds rather than minutes. If needed, these holes are easily sealed with rubber plugs for subsequent operations or with Allen-head bolts for a permanent seal.
- V-notch tube ends prior to welding as an alternative to drilling holes.
- Add racking holes so assemblies are oriented to promote drainage. Try to combine racking holes with some of the drain holes.
- Check with the plater during the design phase to assure that tanks are sufficiently large to allow the desired racking orientation.
- Add holes to provide pressure relief during welding to reduce weld defects.
If plating is required inside holes, plating thickness cannot be guaranteed at depths greater than the length of the hole diameter without special procedures. Blind holes that open downward trap air, preventing plating solutions from entering. In addition, holes collect and retain cutting fluids which tend to drain out slowly when exposed to heat and strong plating chemicals. This contaminates product surfaces, showing up as drip marks and circles around holes, and also contaminates plating baths. Threads, surface roughness, surface texture, grooves or keyways increase the difficulty of dealing with soils in holes.
- Specify a through hole or a bleed tap where possible so air can get behind any residual liquid. Shaking or trying to drain liquid plugs from deep narrow holes creates a vacuum behind the plug, which works to prevent drainage. Bleed taps break this vacuum to allow drainage.
- Specify minimum hole diameters of greater than 0.125 inch (1/8 inch) for smooth bores and greater than 0.1875 inch (3/16 inch) for threaded bores if at all possible. Larger is better for finishing operations.
- Specify hole depth no greater than necessary for the minimum strength requirement. Ideally the depth to diameter ratio is less than two to one (2:1). Remember that deep holes cost in terms of fabricating time and tool wear, in addition to creating processing difficulties.
- Add chamfers to threaded holes to: 1) promote drainage; 2) avoid over plating the first thread; and 3) protect threads from damage.
- Apply cutting fluids by drip, mist or through the tool to minimize the machine fluids present. Avoid flood application. Use air or vacuum to remove chips.
- Use low viscosity, low surface tension fluids that will drain easily.
- Avoid silicone-based fluids and high sulfur content fluids.
- Specify that holes be plugged or masked.
- Bore and thread holes after parts have been plated. This requires careful fixturing and handling of parts to avoid damaging plated surfaces. There are cases where reduced inspection requirements have more than offset the added costs. For parts fabricated in clean rooms, boring after plating gives more control over the amount and type of soils likely to be present. Remember, the insides of small holes generally do not get well plated without special and expensive procedures.
- Store parts so fluids drain completely away. Avoid situations where parts on the bottom of a tray are submerged in drained fluids.
If holes are unavoidable, remove contaminants as soon as possible.
- Fabricate a fixture to direct compressed air, vacuum or cleaning fluids into each hole. Ideally, such a fixture would be part of the machine tool so no extra steps are needed.
- Manually blow out residual liquids. Compressed air nozzles are commonly used. An alternative is a vacuum gun that operates off compressed air combined with a filter or capture device. One available model combines vacuum with a compressed air blast. The vacuum gun alternative also reduces oil mists in the workplace and the related facility cleaning needs.
- Have the metal finishers manually blow fluids out of recesses. Generally, finishers will not be able to set up fixtures for more automated removal of bulk liquid contaminants.
SHEET METAL SEAMS
Folded or lapped spot-welded seams form thin recesses which draw substantial volumes of liquid into them due to capillary forces. When exposed to heat, which expands the liquid and air volumes, or to harsh chemicals, this residual liquid bleeds out contaminating part surfaces and baths.
- Eliminate lapped seams where possible. Use butt welds or reduce the number of pieces in an assembly.
- Construct lapped seams with a gap of greater than 0.002 inch so liquids can be removed, or make the seam tight with a gap less than 0.0001 inch so liquids will not bleed.
- Minimize the lapped seam area by reducing its length or width. Use tabs or end tabs in place of full-length seams and locate spot welds there.
- Minimize the number of spot welds and create a dimple at them to minimize the area of tight clearance.
- Use evaporative stamping lubricants that will not remain on the parts. Carefully evaluate and avoid lubricants that contain a silicone oil.
- Use the minimum volume of lubricant needed for quality forming and adequate tool life.
- Apply lubricant to the tool wear points rather than the part to minimize the part surface area coated.
- Apply the lubricant precisely where metal on metal movement and stress will occur.
- Dry parts in an oven to force residual liquids to evaporate.
- Direct a high pressure spray of cleaning solution at the seam to force oils out. Dry with high pressure air to displace liquids. Or use vacuum drying to aid evaporation. Hot air drying can be done, but a long drying time is generally required.
Get it Plated Right Series
- Introduction: Cleaning and Design for Plating
- Fact Sheet #1: Cleaning Processes - Capabilities and Limitations
- Fact Sheet #2: Particulate Contamination on Part Surfaces - Eliminate to Reduce Plating Rejects
- Fact Sheet #3: Selecting Materials for Plated Parts
- Fact Sheet #4: Dried on Process Fluids and Fluid Combinations
- Fact Sheet #5: High Temperature Processing Burns on Soils
- Fact Sheet #6: Holes, Seams, Threads, Recesses and Tubing Assemblies
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