Automotive laser blanking

Flexibility, quick responsiveness to customer needs, product improvement, reliability, and higher competitiveness can all be achieved with laser blanking

Michael Bembenek

In the world of traditional sheet metal processing, cutting and blanking are fairly well defined by the purpose of-and easily distinguishable by-the tool used. The production of blanks, or pre-cut pieces of sheet metal used for shaping the final product in the post-cutting operations, is mostly accomplished on fast-acting mechanical presses called blanking presses.

FIGURE 1. Laser cutting chart (6kW CO2 laser, assist gas: nitrogen, material: galvanized steel).

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This press is equipped with a cutting tool (die) that can be cycled at up to 40 strokes per minute. The cost of these presses can range from a few hundred thousand to a few million dollars, depending on the dimensions of the material to be processed.

Blanking is generally the final step in a chain of events such as de-coiling, feeding, washing, and straightening, which are common steps for both traditional and laser blanking. The cutting portion can be done by a blanking press, oscillating sheers (for straight-edged blanks), or laser. Also, the material handling solution is unique to either of these technologies.

While cutting is a general term and blanking falls into a sub-category, laser blanking has some unique properties, requirements, and problems uncommon to other applications of this technology. Moreover, laser blanking referred to in this article implies small and medium volume production leaving very small volume and prototyping as a laser cutting application.

FIGURE 2. Price increase of hot rolled steel 2004/2005.

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Further, automotive laser blanking (ALB) is thought to be a complex blanking solution with the material being fed from the coil, where blanking presses are replaced by laser cutting systems and the material handling system is designed for specific production programs in the automotive manufacturing portfolio. Flexibility and quick responsiveness to car model changes is listed among the most significant features characterized by modern automotive laser blanking.

There are many laser cutting machines on the market offering incredibly high cutting accuracy, beam positioning, and repeatability and they are capable of cutting a variety of materials in a wide range of thicknesses. Some of them are equipped with oscillating dual pallet material changing and scrap removal systems. These machines are not product dedicated, but rather designed to cover the widest possible range of applications.

On the other hand, laser blanking requirements do not call for universality; material thickness used for making the body-in-white is between a fraction of a millimeter to a few millimeters of mostly mild, low carbon steel, which is considered an “easy” material to cut.

Also, the geometry of automotive blanks lacks sharp corners and does not require cutting very small radii, which reduces demand from the motion axis control system.

Because “off-the-shelf” cutters are not designed for specific production needs, using this kind of equipment in a large production environment instead of a specifically designed dedicated system would require a higher investment, including extra dollars for making redundant some features and technical solutions not necessarily useful for the process. Therefore, the cost of such blanking systems would be more difficult to justify, which is a partial reason why automotive laser blanking still remains in the area of talk and speculation.

Laser blanking speed

The key issue associated with laser blanking is speed and the first question often asked is, “How fast can you cut?” The answer is a question of laser blanking economics.

Throughput is the main consideration when it comes to any manufacturing set-up. Cutting speed is a function of material type and material thickness, laser power, beam quality, and assist gas. It is limited by the physics of heat conduction, melt pool dynamics, and melted material removal. This means that increased laser power results in increased cutting speed only up to a certain point.

Cutting speed charts vary slightly from different manufacturers, but it is safe to say that 1.5mm mild grade automotive galvanized steel can be cut with 6KW CO2 laser power at approximately 20 m/min.

Other factors limiting laser cutting speed are: environmental, surface quality, complicated geometry, and straightness of sheet metal. The latter becomes a concern during high-speed cutting over large surfaces, because the mechanical or beam motion system has to assure a constant stand-off of the focusing lens and high rates of change.

The calculation based on the above chart is simple. A fairly common blank used in the automotive industry, a door inner, made of tailor welded pieces with average thickness of 1.5 mm and cutting path of about 6m would take approximately 14s from start to finish using single-head laser cutting set-up. Using multiple cutting heads, modern controls, and a modern material handling system can reduce the cycle time and make efficiency of the laser system comparable to the blanking press.

Hence, applying a dual cutting head set-up to the example, in which two stacking/de-stacking stations are located on both sides of the laser cutting operation, would result in 6.4s cycle time. When we compare this with traditional blanking and include in our calculations the number of batches and time-consuming die changes over the lifetime of the program, we will see that the cycle time of the blanking press would be approximately 6s, just 0.4s less than laser blanking.

Why laser blanking?

The above example addresses only a small part of the issue: speed and cycle time and the result are not overwhelming. This is probably why the response from blank makers and blank users to use laser blanking is always the same: “I don’t see the economics.”

FIGURE 3. Price increase of cold rolled steel 2004/2005Click here to enlarge image

Regardless of cutting speed, comparison with a multi-million dollar blanking press that on a scale of one-to-one (one laser, one press) is totally unfair to lasers, there are other factors supporting the ALB case; worth at least deeper analysis.

Let’s look at some broader aspects of the current car manufacturing landscape to find a justification for ALB, and then analyze certain technical and economical advantages of ALB.

Changes in the automotive industry

Fierce competition for market share drives automobile companies to vehicle manufacturing cost reductions. Capital expenditures, labor cost, and overhead cost reduction are the most targeted areas. Other measures, like benchmarking, transplants technical methodology that includes laser brazing, welding, and a variety of cutting applications. There is also a necessity to develop plant/labor measures and manufacturing processes to improve economics that allow for flexible “niche” manufacturing. ALB is definitely the tool of flexible niche manufacturing and technological advantage.

FIGURE 4. Example of nesting; laser cut B-Pillar

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Steel prices have increased drastically in recent years (see Figures 2 and 3), so material savings have taken a high spot on the priority list. Here ALB offers significant savings as the presented analysis of case 1 and case 2 shows.

Progress in the laser industry

Significant improvements on the technological front have been noted in the last few years. Among these are the improvement of stability and reliability of laser resonators, an overall increase in the quality of laser processing, and new generations of lasers, for example fiber lasers.

Fiber lasers are especially interesting for laser blanking applications. A longer depth of focus (high beam quality) at high power contributes to cutting efficiency and edge quality and it can be beneficial in compensating for material unevenness during high-speed cutting, providing that the design of dynamically adjustable assist gas delivery nozzle will follow.

Also the energy efficiency (wall plug efficiency) of fiber lasers is over 25 percent compared with 10 percent for CO2 lasers and 2 percent for Nd:YAG. Cooling requirements and maintenance of modern fiber lasers are drastically reduced compared to the other technologies, and the overall resonator dimensions are smaller. All this makes fiber lasers an attractive choice for production.

Advantages of laser blanking are:

• Tooling (dies) cost eliminated

• Material savings through nesting and engineering scrap reduction

• Developed (contour) blanks for better forming

• Short order-to-delivery time

• Quicker/less-expensive implementation of engineering changes

• Elimination of expensive die maintenance and transportation

• Elimination of storage space for dies

• Reduction of change-over time

• Possible multiple use of laser resources, like welding offal

• Elimination of slitting and pallet flip-over where applicable

• Higher reliability/flexibility of suppliers, those using ALB

• Increased competitiveness of car manufacturer

Two examples of typical blanks for automotive applications are shown in the accompanying case studies. These two show that there is some merit to the argument that laser blanking can indeed save money. As we see, there are two major components of calculation of savings: material savings and capital cost savings (tooling). Both of them merge into one very important element: piece price saving. The results are different depending on program specifics, but the evaluation of benefits should include all approaches.

Future of laser blanking

It’s hard to imagine that lifting 40 times per minute a few tons of steel installed inside a big, high-energy-consuming press in order to cut a thin piece of sheet metal will be a prevailing technology of the future. Automotive laser blanking will bring benefits to car makers and laser manufacturers when the threshold of willingness, understanding, and cooperation is reached.

The current problem is that the benefits and savings are dispersed between car maker and blank suppliers over different departments, areas of expertise, and accounts, not necessarily communicating with each other for the common goal. Understandingly, this makes the complex evaluation difficult. How do you put a dollar value on flexibility, quick responsiveness to customer needs, product improvement, reliability, higher competitiveness…and so on? It’s not impossible, but it is hardly realistic within the existing scheme of benefits and savings calculations and definitely does not prompt quick actions within the system. An attempt to introduce ALB by a group of companies led by Frank DiPietro six years ago didn’t receive a positive response from the industry for the above-mentioned reasons.

What is needed is a working team of people from the automotive company representing purchasing, engineering, manufacturing, accounting, and product development getting together with blank suppliers willing to build a laser blanking facility to “hammer out” the pros and cons of laser blanking, establishing common ground for a final decision.

Small shop mentality-where every penny counts, decisions are made quickly, and a commitment to every aspect of profit making is a must-seems to be lacking in the big world of manufacturing, at least in some parts of this world.

Acknoweldgement

The author wishes to acknowledge Frank Fenton for contributing to this article.

Michael Bembenek ([email protected]) is an independent laser consultant (www.laserglobalservices.com) and a previous contributor (March 2006) to ILS.

Case 1: Hood

Assuming the calculation data is not far from reality, let’s estimate the savings:

Material Savings:

• Material saving per progression= 2 in.

• Material cost: $1.50 / lb (aluminum @ $1.50/lb)

• Material savings: 0.48 lb/piece

• Annual volume: 290,000 pieces

• Material saving per year: $211,000

• Cost of saved material: $0.72/piece

Tooling savings:

• Removal of blanking dies from the program: $93,000

• Tool savings amortized over 5 years: $0.09/piece

• Total savings per year: $0.72+ $0.09= $0.81/piece

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This example illustrates the difference between a dedicated cut-off die shown on the left and radial laser cut blank on the right. The laser cut blank improves the progression by 2 inches. The produced blank is more draw friendly.