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Pioneering Lasers for Rapid Tooling

 

Using laser power to machine molds and dies


By Mark Holthaus
Manager of Advanced Technologies
Ethicon Endo-Surgery Inc.
Cincinnati, OH


As an established leader in single-patient-use surgical instruments for less-invasive procedures, as well as traditional surgical instruments, our company is constantly on the lookout for ways to rapidly design and produce metal tooling for plastic injection-molded components used for the development of new products. We are well-versed in rapid prototyping for designing and evaluating new products, as we own an array of rapid prototyping equipment, covering stereolithography, fused-deposition modeling, and selective laser sintering.

In 1999, we established a formal rapid-tooling strategy to review the current state of the industry for plastic injection-molded components. We also proposed initiatives to address both rapid response and cost-effectiveness in providing tools for the components we produce. As we saw it, the key objective of the tooling strategy was to "provide molded components rapidly from desired engineering materials and from processes that would be like those used in production."

This led us to investigate the well-known tool-manufacturing technologies first: high-speed CNC machining and high-surface-finish EDM. We also extended the search to nontraditional production methods including laser machining. We first became aware of laser machining in the early 1990s as a possible technology that might revolutionize the machining industry. By 1997, our company was acting as a "user-consultant," working with Deckel Maho Gildemeister (Schaumburg, IL) and helping them further the concept of laser machining and develop their initial model, the DML (for Deckel Maho Lasertec) 40. In 1999, we took ownership of the first DML 40 in the US, which we began using in early 2000.

Of all the rapid-tooling technologies we researched, laser machining was identified with the following advantages:

  • Simple approach
  • Progressive technology
  • Least intrusive on the R&D design process
  • Easily complemented by other technologies
  • Functions equally well for prototypes or production
  • Directly CAD-driven

What is laser machining? First things first, laser machining is not laser cutting. What laser machining does is use ultra-fast laser pulses of very short duration to remove material in layers from the surface of the workpiece. Machining depths per layer are from 1 to 5µm.

The laser is a Q-switched Nd:YAG. The Q-switch is an electro-optical interrupter located between the crystal YAG rod and one of two mirrors positioned at either end. The Q-switch controls the discharge of amplified light energy in the form of a pulsed laser beam. This pulse transfers so much energy to the workpiece that the majority of the material the beam strikes is directly vaporized (a fractional amount is melted), without transferring heat flow to the surrounding area. Vaporized/molten material is extracted by a vacuum system.

The laser beam can move within an area of 70 X 70 mm through a deflection system using the two mirrors. This does not mean laser machining is limited to tools of this size. The DML 40 has a 300 X 400 mm table size, and machining can be set up in 70 X 70 mm blocks.

Removal rates depend on the material being machined. For steel, laser machining removes an average of 4 - 6 mm3/min; for aluminum, up to 20 mm3/min; for graphite, up to 25 mm3/min; for ceramics such as silicon nitride, silicon carbide, and aluminum oxide, up to 20 mm3/min; and for brass, bronze, or copper, up to 10 mm3/min. Remember, too, that lasers neither dull nor break, meaning machining-related tool wear and breakage for these applications is a thing of the past for us.

Laser machining is not programmed like traditional cutting/milling equipment requiring speeds and feeds, but instead is driven directly from digital CAD data, mainly STL files. We have seen our necessary skill sets changing, bringing CAD knowledge more to the forefront as an important component.

The DML 40's operating software, LaserSoft 3-D, automatically generates all necessary NC information from the incoming CAD file. The DML 40 comes standard with a network card, and the manufacturer recommends transferring 3-D CAD data in STL format to the machine's control via a network connection. CAD data transferred this way are displayed as 3-D graphics on the operator screen. The operator determines the machining zero point, then the machine program for all relevant operations is automatically generated. Once the program has been started, the workpiece is finished without operator input. A built-in touch probe can be used for in-process measuring and correction for machining depth and other parameters at regular intervals.

Subsequently, our process flow for laser machining for a component mold goes like this:

  • Generate core and cavity detail in 3-D CAD.
  • Generate electrode detail in 3-D CAD for laser machining process.
  • Generate STL file for electrode detail.
  • Prepare STL file for laser machine.
  • Slice STL file for laser machine.
  • Set up insert block in machine.
  • Laser-machine detail in insert block.
  • Clean insert by electro-polishing or micro-bead blasting.
  • Post-machine parting line or other features not laser machined by EDM
  • Finish polishing of machined detail by traditional methods.

What have we learned through this process? Most importantly, if you want to take advantage of laser machining, you must consider the laser machining process during component design. For example, in core/cavity design, for direct machining of deep ribs, you must consider the cone angle of the laser beam when the cavity walls are vertical. One of the few limiting factors is that the conical shape of the laser beam can leave a draft angle of up to 15º. A subsequent machine, the DML 40SI, compensates for this by making the laser lens adjustable. In most core/cavity cases, geometry is not affected by the process, provided the laser beam does not clip the opposite side of the cavity.

The component CAD model must be a valid solid model. Keeping the solid-model geometry as clean and straightforward as possible enhances the success of laser machining. Consult your production molder or tool designer for parting-line recommendations when creating the component design. Providing geometry that is parting-line-friendly also will enhance the process.

What does it mean to provide process-friendly design requirements? Our requirements for direct-machining on the DML 40SI are as follows: 

  • Minimum draft on all external part surfaces is 0º.
  • Minimum draft on all internal part surfaces is 0º.
  • Free-flowing surfaces are allowed and encouraged, as long as they are part of the component solid model and take parting-line requirements into consideration.
  • Minimum draft on all internal ribs is 15º.
  • Minimum recommended internal radius is 0.003" (0.0762 mm). If you design your part with square corners, the laser machining process will generate an approximate 0.003" internal radius.

Consideration of a technology like laser machining during the design process can have a significant impact on time and cost. For example, we designed the tool for a component in solid-modeling software where we could laser-machine the cavity detail and CNC-machine the core detail. Overall, we saved a total of 11 steps (the traditional production process would have required 21 steps versus 10 using laser machining), and we cut production time from 3.5 days to one, netting up to a 70% reduction in time to produce the component. Designing a tool to use the laser machining process exclusively saves even more time. Our example was a tool for molding our company logo, including text. Traditionally, this would have involved 31 steps, taking five days to produce. Laser machining cut the steps down to 18 and total production time to six hours, netting over a 90% reduction in time.

 

This article was first published in the January 2002 edition of Manufacturing Engineering magazine. 


Published Date : 1/1/2002

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