Making surface quality a priority
July 26, 2001
Designers and manufacturers of progressive or stage tooling are faced with a project involving production of metal stampings that will be post-processed with painting, powder coating, or plating operations.
From time to time designers and manufacturers of progressive or stage tooling are faced with a project involving production of metal stampings that will be post-processed with painting, powder coating, or plating operations. Customer requirements for postprocess finish quality often are critical in such situations.
Steel tooling can be very unforgiving to surface quality. By following simple design guidelines, tooling designers and manufacturers can substantially increase the likelihood of success.
A thorough advanced product quality planning (APQP) or design review can determine the best tool design. The review process is important from a manufacturing standpoint when determining the best method of cutting and forming; it is equally important when considering how the tooling will affect the critical surfaces of the part.
The tool designer must have a thorough understanding of customer requirements:
• Are the surfaces in question class one or class two?
• How many imperfections are allowed, and how far apart must they be?
• What candlepower of light will be used for inspection?
• At what distance will the part be viewed and for how long?
• Will functional holes be coated or plugged?
These are just some examples of the numerous criteria customers apply to the finished product of coated or plated stampings.
With a proper understanding of the process, a tool designer can create features in the stamping to make downstream processes more efficient or cost-effective. The designer can supplement knowledge of customer requirements by consulting the operators or suppliers who will perform the processes.
Suppliers, whether internal or external, should be allowed to suggest design features. For example, the addition of hook-hanging holes can allow the paint shop or plating house to process the stampings more effectively. Suspension options allow the stamping to be fixtured, which improves quality and reduces defects by eliminating handling and permitting a better angle for processing.
In addition, for hems or Dutch bends, designers can time the tooling to leave the materials slightly open. This allows lubricants to drain that may become trapped between the two materials. Trapped lubricants can seep out during postprocessing, a detrimental circumstance known as bleedout.
Of course, incorporating such features is at the discretion of the customer. However, customers are more likely to allow such features if they are educated about their potential benefits.
Tool design strategies and methods constantly are evolving. Although there are a variety of approaches, the following methods seem to produce the best results at a minimal cost:
1. Part orientation—Parts that will be coated should be placed with the largest surface facing up, which eliminates or reduces the possibility that tooling components will drag across or come into contact with the main surface. For example, parts such as computer covers that require cosmetic stamping can be positioned to eliminate contact marks on critical surfaces from die lifters and rails. Otherwise, lifters will create longitudinal marks, visible through paint, along the entire surface of the part during the progressive feed cycle.
2. Lifters or rails—Particularly vulnerable parts can benefit if the plunger-type lifter or lifting rail design uses a material such as Delron®, a substance similar to hard nylon. Such materials are less likely to mar the surface of parts. A protective contact surface can be helpful when, for example, part specifications such as burr direction require the main surface of the part to be face down.
Delron can be used just at the steel lifter's contact points, or the entire unit can be made of it. A lifter made entirely of Delron, with a steel retention washer, will not damage the die if it comes out of the tool during operation. The lifter will be crushed with minimal effect on the tool.
3. Hole locations—Wherever possible, through-holes, such as stripper bolts or screw holes, should be avoided in critical areas. Given the size and complexity of today's progressive tooling, strippers usually employ extreme amounts of pressure. This pressure is translated onto the part, and burnishing may occur at the construction holes.
4. Stripper plate and die steel sections—These components should be sectioned along areas of scrap or in noncritical areas of the part. If stripper plates are sectioned in critical areas, a kink or burnishing might occur. This common defect is difficult to detect in an uncoated part, but it becomes strikingly clear after coating.
5. Hardened stripper inserts—Tooling components such as stripper plates and die blocks are becoming increasingly large. With the addition of CNC machining and huge wire electrical discharge machining (EDM), there are few reasons to create a tool of small die sections. This can result in stripper surfaces that are 30 by 60 inches or larger.
Common causes of stamping imperfections are nicks, dings, and slug marks in the plates' faces. To keep costs low and performance high, stripper faces should be inserted with a 1¼4-inch-thick plate. The stripper can be made of any machine steel, but it should have the durability of hardened metal to ensure a clean, smooth contact surface.
6. Direction of surface grinding—In finish surface grinding (the final phase of tool construction), the direction of the grind is important. Plates, blocks, and inserts always should be ground in the same direction of the grain of the raw material used in the die. With a progressive die, plates and blocks should be finish-ground left to right, allowing imperfections in the grinding to be camouflaged by the material grain.
Attention to grind direction is especially critical if the tooling is being bottomed for flatness or coining. If the grinding is perpendicular to the grain marks of the material, it is almost certain to be detected after coating. Tool designers should, therefore, specify grind direction and surface root mean source (RMS) average (a measurement of surface finish).
7. Rocker forming—Rocker forming has been around for many years and is a helpful method for forming precoated or surface-critical products. One of its drawbacks, though, is that often it leaves a strike or bite mark where the rocker contacts the steel and subsequently pivots.
Designers can overcome this problem by designing a stripper plate to cover the entire form up to the bend line. In this design, the stripper is machined down to 1/4 inch thick and 3/4 inch back from the form, and the rocker is mounted above it. The machined step, instead of the rocker, becomes the contact point, and the rocker performance is not affected.
Proper planning is of the utmost importance in tooling design. A designer's top priority is to have a complete understanding of customer requirements and specifications.
Once the customer's requirements are understood, the designer should, in turn, understand the processors' requirements. The people who understand the process best (painters, platers, and powder coaters) are the best resources for information on what to avoid or what to do to make the process run more smoothly.
Combining these resources effectively will improve efficiency and overall quality of the finished part, and conducting an initial design review with everyone involved in the process will ensure the designer is on the right track before production begins.