December 2012 / Volume 64 / Issue 12|
The write stuff
By Susan Woods, Contributing Editor
| The pros and cons of marking parts with laser, micropercussion and scribing technologies.
With the increasing need for traceability, marking operations have become essential at all levels of metal, plastic and ceramic parts manufacturing. There are many ways to permanently mark those parts, each with advantages and limitations, depending on the application. Three such permanent marking technologies are laser marking, dot peen (micropercussion) and scribing.
The choice depends on various parameters, such as part volume, material and shape; surface treatment; post-marking processes; required cycle times; and total cost. Laser marking, dot peen and scribing machines are often run as dedicated workstations, such as in the medical industry, where part batches can be small. However, marking operations are more commonly integrated in high-production manufacturing environments.
“That is where [laser system] class qualification really comes into play,” said Alexandre Boffi, business applications manager for Technifor Inc., Duluth, Ga., which offers laser marking, dot peen and scribing equipment. “A Class 1 system meets all the safety regulations as dictated by the CDRH [Center for Device and Radiological Health]. It is completely sealed and safe, and can be put on the shop floor as is. A Class 4 system needs to be integrated into a production line for safety reasons.” (The classifications pertain to safety. Class 2 and Class 3 lasers are not used for marking, as they are typically in the visible spectrum.)
Laser marking offers the advantages of high processing speeds, consistent high quality, intricate marking and ease of traceability.
One of the biggest advantages is versatility. Lasers can mark the part by removing material or by applying heat to produce a contrasting color change in the material.
“Laser marking is material dependent,” said Peter Bickel, general manager for SIC Marking USA, Allison Park, Pa., which offers laser marking, dot peen and scribing machines. “There are wavelengths of light that can actually create a colored mark, but just on certain materials.”
Courtesy SIC Marking
Courtesy SIC Marking
This contrasting mark capability is often used on medical devices that require a nondisrupted surface. “There are cases where laser is the only way you can mark something and medical implants is one of the biggest,” Bickel said. “It would be very easy to dot peen these parts, but the FDA doesn’t allow any surface deformation.”
Applications where laser marking removes material include parts for harsh environments. “You need some depth to the mark if abrasion is going to occur on the surface of the part, such as with bearings and auto parts,” said Brian Hilliker, sales manager Americas for FOBA North America, Boxborough, Mass., a maker of laser marking and laser engraving equipment. Parts that will be painted also require marks with depth, he added.
Lasers typically create a mark by removing material via ablation (partial removal of layered material, exposing a base material) or engraving (removal of material, resulting in a depression). “With ablation, the mark is 10µm or less deep,” said Mark Boyle, laser product engineer, Miyachi Unitek Corp., Monrovia, Calif., which manufactures laser marking equipment. “Engraving goes 50µm to 100µm deep, and deep engraving can be hundreds of microns deep.”
Deep laser engraving is for parts that will be subjected to significant wear and for mold relief. “Engraving typically is defined by the creation of 3-D structures,” Hilliker said. “Deep engraving is used for tools and dies in the minting industry and tool and mold construction. Three-dimensional surface structures are possible, as are frostings, textures, visual effects and safety features on engraved stamps or markings and edge letterings on coins and implants.”
Marking applications that require depth might be better served with a micropercussion or scribing solution. “If you want to make a really deep mark, laser marking is not preferred,” Bickel said. “The laser can do it but it will take a long time. If you have to go deep, depending on the application, you might be better off using dot peen than trying to burn in a laser mark. The dot peen takes maybe 15 seconds whereas the laser cycle could take 15 minutes.”
Different laser wavelengths provide different marking capabilities and each wavelength may address a different material. The laser choices for marking metal, ceramics and some plastics are typically infrared (IR) wavelengths. This can come from ytterbium-doped fiber, neodymium-doped yttrium orthovanadate (Nd:YVO4, or vanadate) or neodymium-doped yttrium aluminum garnet (Nd:YAG) diode-pumped lasers. For all three types of lasers, the wavelength range is from 1,060nm to 1,070nm, depending on the manufacturer.
CO2 lasers are also used for parts marking but typically plastic, paper, wood and glass ones. For metal parts, these lasers are less suitable because of the small absorption at their long wavelength—10,604nm.
Several variables go into the decision to use a fiber laser or a diode-pumped laser, according to Technifor’s Boffi. These include the mark quality and material being marked.
“One of the key differences between the diode-pumped and the fiber-pumped technologies is the quality of the beam,” he continued. Fiber lasers emit longer pulses with lower pulse energies and peak power. Diode-pumped lasers have higher pulse repetition rates and peak power. “Therefore, the diode has much better beam quality, which can lead to a better quality mark on a small scale—a small mark,” he said. “It also allows you to control the laser a little better for materials that can be somewhat tricky to mark, such as some polymer compositions and plastics.”
Courtesy of FOBA
However, a fiber laser can accomplish the same mark in less time than a diode-pumped laser. Fiber-pumped systems typically have larger beam diameters than diode-pumped systems and the beam can be dispersed over a larger area. This allows the same mark to be completed in less time, on average.
For marking gold and other precious metals, which have low absorption in the IR wavelength, short wavelengths are essential. To achieve shorter wavelengths, one must use the green vanadate or YAG laser frequency doubled, providing a 532nm wavelength, or the UV vanadate or YAG laser frequency tripled, providing a 355nm wavelength.
As with gold, quite often with plastic the IR wavelength (1,060nm to 1,070nm) is not as absorbent. “So you are challenged to find a wavelength that is more absorbent and makes a better mark,” FOBA’s Hilliker said. “That is when people go to green lasers. And people are also moving to 355nm for UV, especially for medical parts. Marking with UV on steel provides very good corrosion resistance.”
Another distinguishing factor between fiber lasers and diode-pumped lasers is diode-pumped lasers have an array of diodes with a lifetime of 10,000 to maybe 20,000 hours that can cost $10,000 to replace, noted Miyachi Unitek’s Boyle. “The fiber diode has a lifetime of around 100,000 hours so there is no need to replace a diode pack every few years.”
The Shape of Things
A flat surface is the easiest to mark and most parts have at least one flat surface area. For curved parts, rotary devices are available, and, for parts with steps, a programmable Z-axis is offered.
“There are some cases where you have to mark on a slope or far along a curved surface,” Boyle said. “Each laser lens has a depth of focus that defines how far along that curved surface it is possible to mark. If the difference between the height of the part and where you want to mark is more than 4mm, you’ll probably want to look into rotating the part or changing the laser lens focus to mark at that depth.”
Thin-walled parts are probably best marked with lasers, depending on the industry being served. Micropercussion disrupts the part surface and scribing removes material. Also, scribing places more force on a part than laser marking and dot peen equipment in the relative X and Y directions, so the parts must be securely fixtured.
Courtesy of Miyachi Unitek
The place of dot peen technology in the industry is as a high-speed, cost-effective marking system. Compared to laser marking, the cost of dot peening is low. Dot peen equipment ranges from $6,000 to $18,000 and laser marking equipment starts at $20,000 and can go up to $100,000, according to SIC Marking’s Bickel.
Dot peen technology is widely used to mark automotive, aerospace, oil and gas, agricultural and construction machinery and electronic parts.
Dot peening works by striking a carbide stylus assembly against the part’s surface. The mark is a succession of dots where each dot is created by the impact of the stylus on the surface.
A host of materials, from plastics to hard steel, can be dot peened using a range of styli. “The carbide stylus allows us to mark on harder materials,” Boffi said. “With plastics, we might use a smaller stylus, so less force is exerted on the part, or sharpen the stylus to a more acute angle, so characters have more definition.”
Courtesy of FOBA
Most major industrial plastics, such as ABS and PVS, can take the impact of dot peening. Some plastics, however, may not have enough memory to hold a dot peen mark and others might be too brittle for the impact of the stylus. Also, ceramics are typically not marked with dot peen or scribing because the material is too brittle.
Dot peening is faster than scribing and can be faster overall than laser marking. While the laser marking speed is faster, the safety parameters necessary with laser marking may increase its cycle time.
Scribing technology is used across numerous industries, including automotive, oil and gas and electronics. It also can be used for plastics and metals. It has two key purposes: to meet plant-noise level limitations and to create intricate marks.
The mark is produced by a carbide or diamond stylus that penetrates and moves material as it is dragged along the surface, leaving raised material. “Diamond styli are not used with micropercussion because the diamond is held onto the stylus with adhesive,” Boffi said. “If you used micropercussion, it would just crack it right off. With scribing, there is no repeated impact.”
The diamond stylus allows marks on harder materials. A carbide stylus, whether scribing or dot peening, is typically capable of marking materials as hard as 62 HRC, whereas a diamond stylus can mark up to around 65 HRC.
Scribing is selected mostly for low-noise applications. For example, marking a huge tube could resonate with a dot peen machine, whereas a scribing machine could mark it with little noise. “One of the biggest applications is automotive emissions control, such as tail pipes, mufflers and catalytic converters,” Bickel said. “Those manufacturers tend to buy scribing machines.”
Scribing’s ability to make deep marks, depending on the material, is also beneficial. “Scribing can achieve depths greater than laser marking and, in most cases, greater than micropercussion,” Boffi said.
Scribing, however, is not typically used to produce a 2-D data matrix code like dot peen or laser marking. “The step up and down to precisely position the dots in the code would be too time-consuming to mark like that,” Bickel said. “And you probably wouldn’t get the same quality you might with dot peen or laser.” Some manufacturers, however, do offer data matrix marking capability with their scribing technology.
No matter what type of parts one needs marked, “You don’t want to buy the equipment, you want to buy the process,” FOBA’s Hilliker said. “Every marking application is driven by three factors: speed, quality and cost.” CTE
About the Author: Susan Woods is a contributing editor for CTE. Contact her at (224) 225-6120 or firstname.lastname@example.org.
Courtesy of 2L
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