Micropulse’s investment in the right equipment delivers orthopedic implants that demand tight part tolerances.
By incorporating manufacturing technology from GF Machining Solutions, medical instruments and implant manufacturer Micropulse Inc. positions itself with competitive pricing, delivery, and efficiency – effectively cutting part processing costs by 30%.
Operating out of a 160,000ft² facility, Micropulse manufactures orthopedic surgical instruments, implants, and case-and-tray, and provides sterile packaging and product logistic services. Throughout the years, medical industry requirements and regulations have continued to dictate tighter part tolerances, demands Micropulse can meet with high-precision machine tools. According to founder, owner, and CEO Brian Emerick, automation, earned trust, and integrity have allowed the shop to provide quality parts, delivered quickly, at competitive prices.
Any delivered product must be traceable to the raw material provider with each manufacturing process tracked by who did what, with which tools and software.
However, the real part tolerance challenge involves the exact geometric positioning of one part feature in relation to the next. Since parts for the human body are never perfectly straight or square in shape, the shop relies on modular workholding systems to clamp a part once and completely machine it. Once such parts are removed, re-clamping them in the exact same positions is nearly impossible.
Micropulse’s high-precision electrical discharge machines (EDMs) and milling machines, as well as its automation, have come from GF Machining Solutions.
Die-sinker-type EDMs at the shop encompass an AgieCharmilles FORM 200 and a RoboForm 350 in a cell with a Mikron HSM 300 high-precision, high-speed milling machine and System 3R WorkMaster robot – the shop’s first foray into automation. Other EDMs at Micropulse include Robofil 240 CC and CUT 200 Sp wire-type EDMs.
The shop continues to delve further into high-precision, high-speed milling with additional Mikron machines, mostly for its EDM graphite electrode production. A few years ago, Micropulse acquired its first linear-motor Mikron mill for machining titanium and has since added six more. Among them are Mikron HSM 400U LPs with 42,000rpm spindles and precise geometric positioning capabilities as well as a pair of Mikron MILL S 400 U machines teamed with a 120-position tool capacity System 3R WorkPartner 1+ robot.
The 5-axis HSM 400U LPs deliver speeds up to 250rpm on their table rotational axes and up to 150rpm on their swivel axes, with B-axis swivel ranges of 220°. The machines use liquid-cooled, linear direct-drive motors with central oil lubrication to reduce friction-induced wear for long-term accuracy and a 1.7g acceleration rate.
The machines’ monobloc bridge designs ensure precision with high levels of rigidity. Polymer granite construction dampens vibration and boosts thermal stability.
Micropulse’s 42,000rpm 5-axis MILL S 400 U machines’ linear axes feature rapid traverse rates as high as 3,937ipm. The machines achieve speeds up to 160rpm in B-axis and 250rpm in C-axis. For unattended operations, the machines include pallet changers with 18 pallets.
“Early on, we realized that we needed high-speed milling capabilities,” says Larry Sutton, general manager at Micropulse Inc. “And we knew that marrying that capability with automation would benefit us greatly.”
In addition to complete process stability, using five automation systems to serve seven machines allows the shop to boost its output. For instance, six employees keep seven machines running across three shifts and throughout weekends in the Implant Division. That division averages about 150 hours of spindle time per week for each machine.
“We aren’t making thousands and thousands of the same parts,” Sutton explains. “Most of our jobs are small batches that entail a lot of changeovers, which can really make cost reduction a challenge.”
Part families typically run in the automated cells in batches of 50 or so, with each operation running about a day-and-a-half before moving on to the next job. For setups, the shop makes some of its own fixtures and uses System 3R modular pallets.
According to Ryan Sims, implant manager at Micropulse, fixtures are the shop’s biggest expense. For greater efficiency, a single-fixture system and automation can provide the necessary speed and flexibility to accommodate part families with minor differences from one group to the next.
“Automation has basically removed setup time from our part-processing equation,” Sims says. “With a conventional machining center, our setups would take four to six hours. Plus, with Mikron machines, we can automatically laser touch off all the tools for a job in less than five minutes.”
Each high-speed Mikron holds up to 66 tools, and because the shop’s jobs typically require 20 or fewer tools, operators will load redundant tooling to keep the machines running when tools wear. The Mikrons also have thermal compensation, so if temperatures vary a few degrees, the machines will automatically adjust to maintain accuracy.
According to Sims, the Mikron Machines’ high spindle speeds and lack of machine vibration help increase tool life, typically 2x to 3x longer.
“For those parts moved over to the Mikrons, we’ve experienced not only longer tool life, but also cycle time reductions, often greater than 25% depending on the part, and reductions in the need for secondary manual finishing operations by 50% in most instances,” Sims says. “Those parts with very small features that require extremely tiny tooling – as small as 0.011" in diameter – have yielded the greatest cycle time reductions. In one instance that amount was more than 60%. Our slower, conventional 12,000rpm machines are unable to feed or move accurately enough for these cutting tools that require precise movement along with high speeds and feeds.”
Such tools are also so small that any type of physical/conventional tool detection system would break them. The shop checks such tools with laser-based systems on its Mikrons that measure diameter and length, and do so within three seconds, according to Sims.
“We are confident that we can continue to compete diligently with offshore competition because of how we can optimize our output,” Emerick adds. “GF Machining Solutions is a big part of that capability because it is a true technology partner and a supplier that wants to keep us successful – not just sell us a machine tool.”
Orthopedic instruments are produced and shipped to customers in batch lots, and during the product’s lifetime, the shop will produce replacements for worn instruments. Implants, on the other hand, are single-use only, which simplifies production planning. For instrument manufacturing, the shop uses general cells for machining and operations. Each cell owns certain part/instrument families and produces related components. For example, one cell produces hip and shoulder broaches. For implants, cells incorporate less equipment variety and are often organized by process, such as milling, turning, or electrical discharge machining (EDM). Part materials can range from stainless steels, titanium, and cobalt chrome to plastics such as PEEK and other medical-grade polymers. Part sizes vary from about 0.078" square to cylindrical-shaped parts up to 12" long. Some of the shop’s tightest tolerances are held on jobs that run lights-out in an automated cell.
Understanding the science of skin can help design engineers select the right adhesives for medical devices.
What do medication patches, diabetic monitors, infusion sets, over-the-counter bandages, and EKG monitors have in common?
They’re all medical devices that stick to skin. And while they can provide life-sustaining and life-changing benefits, successfully adhering these devices to skin can be quite challenging.
The above, in conjunction with age, environment, diet, culture, and overall health, create a unique combination for each person. Other daily routines, grooming habits, and hobbies – such as the use of lotions or powders and moisture from bathing or swimming – add additional wrinkles into the design mix.
Even with these shifting nuances, adhering to skin isn’t impossible. The key to success is understanding the science of skin and how its characteristics interact with adhesive properties.
Unlike a nice bottle of wine, our skin doesn’t get better with age. Each stage of life presents new advantages and/or challenges. As babies, our skin starts out fragile with fewer cell layers. Adolescence brings an increase in oil production and potentially the amount we sweat. The skin of healthy young adults is the most durable, but as we continue to age, our skin becomes drier, thinner, less elastic, and more fragile. Around the age of 55, our epidermis thins and skin loses hyaluronic acid, making it stiffer. These characteristics make it unrealistic to use the same adhesive and expect the same results across the age spectrum. This affects the type of adhesive a project requires. 2 Don’t overstay your welcome Skin can be fickle with how long it will let an object stick to it, especially if that object doesn’t allow skin to function normally.
That’s why it’s important to select the right adhesive and backing to secure the device, along with the device’s housing material. All will help ensure the skin can breathe, flex, and move as needed for as long as the device is required to stay attached.
It’s important to consider the desired wear time when selecting a stick-to-skin adhesive because over-designing could destroy a device’s success. For instance, don’t use an adhesive meant to stay on for two weeks if a device will be removed after five days. The adhesion will be at its maximum value at that time, making removal painful and possibly leading to a medical adhesive related skin injury, or MARSI (see sidebar, pg. 32).
Even aggressive adhesives intended for long-term wear can experience loss of adhesion, edge lift, and eventual failure. Including a skirt – a border of tape extending beyond the footprint of the wearable device – can have a significant impact on the survivability of a device. In one case, a device exhibited 60% to 70% survival throughout 14 days without a skirt and 95% survival when the stick-to-skin tape included a 0.25" extension.
Silicone adhesives, known for their gentleness and consistent adhesive strength, are typically suitable for wear up to five days and can be repositioned. Acrylics, on the other hand, intensify in adhesion after the first day or two of wear and can be optimized for longer term wear.
Sticking devices to skin becomes much more challenging when they come into contact with moisture, but research on experimental tapes look to overcome this. In some recent studies, the tapes’ exposure to moisture was not limited, allowing participants to shower and exercise normally. Exposing experimental tapes to conditions similar to real world use, developers have pushed the boundaries of what’s expected from stick-to-skin technology. Learn more at https://goo.gl/sNCVxd.
Just as not all skin is the same from person to person, not all skin is the same from location to location on the body, so don’t assume adhesives will perform the same way on different parts of the body. An adhesive securing a device to a healthy adult’s chest will have different requirements than one used on a baby’s face. If the location is fragile, using a silicone adhesive might be best. They’re a favorite for being gentle and causing less pain upon removal, as they don’t pull the skin or hair as much. If the location is more robust, acrylic tapes are often the way to go.
The device’s location on the body is not the only important location to note. Keep in mind any special environmental conditions of the end user, such as a baby in a moist incubator, which might impact the needed adhesion requirements.
That’s your skin letting you know that the adhesive you stuck on it isn’t sufficiently breathable. The same thing will happen with wearable medical devices.
The longevity of an adhesive adhering to skin depends on its breathability. Skin, if unable to breathe or release moisture for a length of time, will feel suffocated.
If your project requires a device to stick to skin for more than a few hours, make sure both the adhesive layer and tape backing will allow the wearer’s skin to function as needed. Moisture vapor transmission rate (MVTR) is a function of adhesive and backing that lets engineers know how well the sweat generated underneath will escape. If it isn’t able to do so, sweat will accelerate the failure of the adhesive’s bond to skin. However, MVTRs typically reported for medical tapes aren’t applicable for tapes used beneath most wearables because these devices block moisture from passing directly through the tape in the Z-direction. Nonwoven backings allow some X-Y transmission of moisture, however, and tapes using them may perform better as they can help the moisture escape around the device.
Breathability of the adhesive becomes particularly important in instances when the end user’s skin might become sweaty or wet.
People who wear medical devices stay active and don’t want their device to impede those activities.
These end-use applications make design engineers cringe, but adhesive manufacturers are working toward creating adhesives that maintain breathability and durability when wet (see sidebar).
It’s a challenge, but sticking to skin doesn’t have to feel insurmountable. Engineers should engage adhesives experts with experience designing, testing, researching, and innovating adhesive technology and do so early to have the right support throughout the entire design and development process. Adhesives experts can counsel on the nuances of working with skin, discuss the best medical adhesive options, and provide critical analysis of a project’s needs.
Better understanding the science of skin, and how new adhesive technologies can help make adhering devices to the skin more effective than before, will help produce a more successful product, a happier customer, and a more satisfied user.
About the authors: Diana Eitzman, Ph.D., is director of agile commercialization for 3M’s Critical & Chronic Care Solutions Division and can be reached at email@example.com. Kris Godbey is an applications development specialist in the same 3M division and can be reached at firstname.lastname@example.org.
Skin tears, skin stripping, and tension blisters are common but avoidable examples of medical adhesive-related skin injury (MARSI). MARSI is damage to the skin that may occur when medical adhesives are not selected, applied, and/or removed properly. Learn more at https://engage.3m.com/ preventmarsi.
Electronic pill can relay diagnostic information or release drugs in response to smartphone commands.
Cambridge, Massachusetts – Researchers at MIT, Draper, and Brigham and Women's Hospital have designed an ingestible capsule that can be controlled using Bluetooth wireless technology. The capsule, customizable to deliver drugs, sense environmental conditions, or both, can reside in the stomach for at least one month, transmitting information and responding to instructions from a user's smartphone.
Manufactured using 3D-printing technology, the capsules could be deployed to deliver drugs for a variety of diseases, particularly in cases where drugs must be taken over a long period of time. They could also be designed to sense infections, allergic reactions, or other events, and then release a drug in response.
"Our system could provide closed-loop monitoring and treatment, whereby a signal can help guide the delivery of a drug or tuning the dose of a drug," says Giovanni Traverso, a visiting scientist in MIT's Department of Mechanical Engineering, where he will be joining the faculty in 2019.
These devices could also be used to communicate with other wearable and implantable medical devices, which could pool information to be communicated to the patient's or doctor's smartphone.
"We are excited about this demonstration of 3D printing and of how ingestible technologies can help people through novel devices that facilitate mobile health applications," says Robert Langer, the David H. Koch Institute Professor and a member of MIT's Koch Institute for Integrative Cancer Research.
Langer and Traverso are the senior authors of the study, which appeared in the Dec. 13 issue of Advanced Materials Technologies. Yong Lin Kong, a former MIT postdoc who is now an assistant professor at the University of Utah, is the paper's lead author.
Wireless communicationFor the past several years, Langer, Traverso, and colleagues have been working on a variety of ingestible sensors and drug delivery capsules, which they believe would be useful for long-term delivery of drugs that currently have to be injected. They could also help patients to maintain the strict dosing regimens required for patients with HIV or malaria.
In their latest study, the researchers set out to combine many of the features they had previously developed. In 2016, the researchers designed a star-shaped capsule with six arms that fold up before being encased in a smooth capsule. After being swallowed, the capsule dissolves and the arms expand, allowing the device to lodge in the stomach. Similarly, the new device unfolds into a Y-shape after being swallowed. This enables the device to remain the stomach for about a month, before it breaks into smaller pieces and passes through the digestive tract.
One of these arms includes four small compartments that can be loaded with a variety of drugs. These drugs can be packaged within polymers that allow them to be released gradually over several days. The researchers also anticipate that they could design the compartments to be opened remotely through wireless Bluetooth communication.
The device can also carry sensors that monitor the gastric environment and relay information via a wireless signal. In previous work, the researchers designed sensors that can detect vital signs such as heart rate and breathing rate. In this paper, they demonstrated that the capsule could be used to monitor temperature and relay that information directly to a smartphone within arm's length.
"The limited connection range is a desirable security enhancement," Kong says. "The self-isolation of wireless signal strength within the user's physical space could shield the device from unwanted connections, providing a physical isolation for additional security and privacy protection."
To manufacturing all the complex elements researchers decided to 3D print the capsules, allowing them to incorporate the various components carried by the capsules and build the capsule from alternating layers of stiff and flexible polymers, helping it withstand the acidic environment of the stomach.
"Multi-material 3D printing is a highly versatile manufacturing technology that can create unique multicomponent architectures and functional devices, which cannot be fabricated with conventional manufacturing techniques," Kong says. "We can potentially create customized ingestible electronics where the gastric residence period can be tailored based on a specific medical application, which could lead to a personalized diagnostic and treatment that is widely accessible."
Early responseThe researchers envision that this type of sensor could be used to diagnose early signs of disease and then respond with the appropriate medication. For example, it could be used to monitor certain people at high risk for infection, such as patients who are receiving chemotherapy or immunosuppressive drugs. If infection is detected, the capsule could begin releasing antibiotics. Or, the device could be designed to release antihistamines when it detects an allergic reaction.
"We're really excited about the potential for gastric resident electronics to serve as platforms for mobile health to help patients remotely," Traverso says.
The current version of the device is powered by a small silver oxide battery. However, the researchers are exploring the possibility of replacing the battery with alternative power sources, such as an external antenna or stomach acid.
The researchers are also working on developing other kinds of sensors that could be incorporated into the capsules. In this paper, they tested the temperature sensor in pigs, and they estimate that within about two years, they may be able to start testing ingestible sensors in human patients. They have launched a company that is working on developing the technology for human use.
Other authors of the paper include Xingyu Zou, Caitlin McCandler, Ameya Kirtane, Shen Ning, Jianlin Zhou, Abubakar Abid, Mousa Jafari, Jaimie Rogner, Daniel Minahan, Joy Collins, Shane McDonnell, Cody Cleveland, Taylor Bensel, Siid Tamang, Graham Arrick, Alla Gimbel, Tiffany Hua, Udayan Ghosh, Vance Soares, Nancy Wang, Aniket Wahane, Alison Hayward, Shiyi Zhang, and Brian Smith.
The research was funded by the Bill and Melinda Gates Foundation and the National Institutes of Health through Draper.
Five major suppliers claim approximately 85% of the highly competitive orthopedic device and implant market, with more than 200 other companies vying for the remainder. In light of such intense competition, recent developments allow medical manufacturers to continually seek ways to machine components faster and more cost efficiently. Some of these innovations and strategies include dry coolant and 3D printing, along with advanced cutting tools and high-speed milling.
Orthopedic components are typically machined from bar stock, castings, or forgings, then ground and polished. For hip and knee implants, the most common workpiece material is titanium, with the use of cobalt-chrome alloy increasing. A typical cobalt chrome alloy is similar to CoCr28Mo6, and the titanium alloy Ti6Al4V is most common.
The JH770 end mill is engineered for roughing operations and is available in 4-, 5-, and 6-flute styles, enabling variations of the tool to rough from solid or perform near-net-shape operations.
Because the materials used in orthopedic implants typically generate excessive heat when machined, coolant use is required. However, using traditional coolants is often prohibited or limited to prevent part contamination, where time-consuming and expensive post-machining cleaning processes are needed. In addition, coolant poses environmental issues for employee health, safety, and disposal. An alternative coolant technology uses supercritical carbon dioxide (scCO2) dry-cutting technology. The scCO2 acts as a vehicle to deliver dry and enhanced lubrication to a cutting zone.
Developed by Fusion Coolant Systems, the process can machine parts without oils, emulsions, or synthetics. When carbon dioxide is pressurized above 74 bar (1,070psi) and 31°C, it becomes a supercritical fluid, scCO2. In this state, it fills a container like a gas, but with a density similar to a liquid. When delivered to the cutting zone, scCO2 expands to form dry ice, though it does not create a cryogenic substance like liquid nitrogen. The result is an effective coolant that often outperforms existing systems that incorporate high-pressure water/oil, minimum-quantity lubrication (MQL), liquid CO2, or liquid nitrogen.
Another nontraditional manufacturing technology being applied more often in orthopedic device production is 3D printing. The process uses titanium and cobalt-chromium alloy powders to produce complex, near-net-shape parts. In the medical industry, selective laser melting (SLM) melts the powders to build components layer by layer. The process allows medical manufacturers to generate special part contours and dimensions custom- tailored to individual patients. The process also produces consistent micro-pore surfaces that expedite bonding between the part and living bone.
For finish machining, 3D-printed parts maintain most of their metals’ machining characteristics. However, some parts may need post-printing treatments to relieve uneven stresses generated during processing. In addition, for post-machining, fixturing can sometimes be a challenge due to the parts’ near-net shapes and complex contours.
The metal alloy components of orthopedic implants, such as knees and hips, must possess excellent surface finishes to minimize wear of plastic parts and permit the joint to function for its projected lifetime of 20 years or more. In a knee replacement, the femoral component and tibial tray must be perfectly smooth to protect the plastic bearing insert against wear.
Accordingly, the manufacture of orthopedic components typically requires grinding operations follow the milling process to achieve sufficiently smooth surfaces. However, grinding is time consuming and impacts overall manufacturing efficiency and flexibility. Equally critical, grinding generates high temperatures and stress in components, leading to dimensional errors that can affect the product’s strength and performance.
Grinding can be supported – or in some cases replaced – with the application of advanced cutting tools and high-speed milling strategies. The goal of milling is to achieve a burr-free outside profile and a superior surface finish that delivers the exact required surface quality, integrity, and dimensional accuracy. If post-treatment is required, such as polishing, the time for that task can be minimized because of the defined surface roughness and structure achieved in milling. With high-speed milling, tooling should meet the parallel goals of long, reliable life with maximum productivity.
In a representative application, a cast cobalt-chrome femoral component was finished using a ballnose end mill on a 5-axis milling machine. High-speed copy milling strategies and high-performance end mills eliminated a grinding operation, reducing part cycle time 50% to 11 minutes per part compared with the prior method. The change from grinding to milling of the condyle surface reduced scrap parts. The solid-carbide end mills made from a high-grade carbide micro-grain substrate offered superior toughness compared to standard carbide grades. In addition, a patented high-abrasion-resistant coating maximized tool life. The tools were engineered to provide high metal removal rates and smooth cutting action for superior finishes and minimized polishing time.
The complex contours of orthopedic components often require specific sequences of specialized tools. The tibial tray, for example, can require up to seven separate machining operations including roughing, tray base roughing, tray base finishing, chamfer milling, T-slot undercut machining, wall finishing/chamfering, and undercut deburring. The challenge is to achieve superior surface finishes with minimal manual intervention as well as reliable tool performance with the best combination of productivity, cost, and quality.
Traditionally, performing these multiple operations dictated separate special tools to produce each required contour, dimension, and surface finish. Special tools require design and development time and expense, and due to their low production volume, may have extended lead times and availability constraints. A new approach involves developing standardized tools with flexibility that enables them to be used in a variety of similar orthopedic parts.
Global demographic and economic trends strongly indicate that demand for sophisticated orthopedic components will grow. At the same time, consumer desires and the determination of medical parts manufacturers to differentiate themselves from competitors are promoting development of personalized orthopedic components. Surprisingly, variable part specificity can be achieved with relatively new innovations and strategies that include dry coolant and 3D printing, along with advanced cutting and high-speed milling tools that are less specialized, more flexible, and more cost-efficient than the custom ones previously applied.
Some design details of orthopedic devices differ greatly among manufacturers, but the products share many generic features simply because all human bodies are similar. Although manufacturers traditionally machine parts with custom tools, the wide and productive middle ground for tools that can efficiently machine the generic features in multiple materials meant generating a full suite of custom tools.
Seco engineers analyzed medical component manufacturing processes and employed more than a decade of medical part machining experience to develop a more flexible, standardized range of end mills for machining cobalt chrome and titanium orthopedic components. The new range of tools have performance characteristics allowing them to be applied across a range of parts and materials.
Tool standardization offers multiple benefits. Time is saved by eliminating the design, prototyping, and testing of custom tools. Because the tools are standard, they are also presented in Seco’s catalog and available around the world through the company’s distribution centers. Plus, the tools are produced in high volume to help lower their cost.
This tool range includes nine geometries and 39 items, a selection expanded with different sizes, radii, and dimensions. Tools are engineered to produce features common to various orthopedic components, including knee and hip parts, bone plates, spinal parts, and others. Each of the nine geometries has a specific function or application such as roughing, finishing, T-slot undercutting, or production of fine finishes on complex, contoured parts.
Micro-abrasive blasting works at the micro-scale level so every part of the process impacts surface finish specifications.
Precision sandblasting on a micro-scale – microblasting – suits small parts, delicate materials, and intricate geometries - such as those found in medical manufacturing. A mixture 17.5µm to 350.0µm abrasive and dry air travel at high velocity out of a 0.018" to 0.125" nozzle, producing a focused, controllable abrasive stream. Used to refine and perfect parts as small as a grain of rice to as large as a basketball, microblasting is versatile, precise, and controllable.
Microblasting often alters the finish on a base material by adding texture. A micro-abrasive blaster can deliver texture to a sharp delineation (as precise as 0.007") and create a consistent finish to a specific Ra, often without the need for masking.
(Left) Drug-eluting stent (200x): Peened dimples hold and slowly release a drug to block cell proliferation. (Right) Dental Implant (200x): Sharp jagged cuts create an interlocking surface that readily bonds to bone tissue.
What should be included in a specification when using microblasting to get a textured surface finish? Most stop at abrasive type and blast pressure, but a list this bare risks inconsistent results.
While drawing up a spec, focus on the desired result of the microblasting process, make it a measurable product output, then let the operator or job shop adjust variables to get the specified finish.
Giving a contract manufacturer or job shop a defined target leads to a better understanding of the full scope of the project.
Manufacturers can suggest an abrasive type and a blast pressure, but these two suggestions are just starting points and should remain flexible. Blast pressure is relative to each micro-abrasive blaster, and abrasives come in a range of consistencies depending on the provider.
A part’s surface could need sharp grooves or soft dents. The shape of the abrasive particle and the composition of the targeted surface are the primary variables that determine look.
Particle shape can be grit or bead. Grit abrasives have a sharp, angular profile and produce a matte finish on brittle and ductile base materials. Bead abrasives are spherical and produce a dented or peened finish on ductile materials. Bead abrasives leave a matte finish on brittle materials due to their fracture mechanics and the way brittle materials erode.
To create a permanent, strong, and tight bond between a part’s surface and another surface, a matte finish provided by a grit abrasive, such as aluminum oxide or silicon carbide, is ideal. To create a temporary bond, soft dents created by round, soft media such as glass or ceramic are the best choices.
The specification should cover a combination of amplitude and quantity of features to get consistent, repeatable results. The best measure for amplitude is Ra (2-dimensional) or Sa (3-dimensional). Both express the size of the peaks and valleys created by micro-abrasive blasting. Ra is measured as a line and Sa is measured as a plane.
Abrasive size and type are inputs to the microblasting system. Velocity is driven by the long list of variables under “what’s missing?” Velocity has a linear relationship to Ra value: as velocity increases, so does Ra in a predictable and corresponding trajectory.
Different surface profiles have the same roughness value, so Ra or Sa measurements are limited. They do not measure density or spacing, two variables that matter in texturing.
Including Sdr (developed surface ratio), or the quantity of features in a spec, provides a greater understanding of the part’s surface properties and guarantees a more consistent, repeatable finish. Sdr expresses the amount of additional surface created by the blasting process. Measuring Sdr requires more sophisticated analysis systems with benefits down the line. Comco engineers rely on the Zygo optical profilometer for these measurements.
It is vital to understand how a part’s base material reacts to an abrasive. On brittle materials, surface roughness is generated by using abrasive to take surface material away; while the opposite is true on ductile materials. When an abrasive particle strikes a ductile base it forges a crater on impact and causes a ring of raised material to form around the crater. So, surface texturing does not reduce the overall dimension of a part surface; it may even increase it slightly.
The closer the part surface is to 100% coverage, the higher the Ra and the developed surface – maximizing the opportunities for interaction between bonding surfaces.
100% coverage indicates that the entire surface has been altered, that the peaks on the part surface are all connected with no untouched plateaus. Part makers can measure coverage informally by holding the part up to a light. Reflective surfaces at 100% coverage scatter light uniformly; while a line of light passes across a surface with coverage less than 100%.
A profilometer can also analyze coverage by measuring Sdr and exposing flat regions on a textured surface.
Roughness measurements, be it Ra or Sa, become stable at 100% coverage. Anything less, and measurements can vary significantly. Ra and Sa measurements are not useful without 100% coverage.
Focusing specifications to a look, an Ra, Sa, and/or Sdr value, and 100% coverage requires more investment upfront but reduces output variance. Even if users simply want to turn a shiny finish to a matte or frost clear glass, specifying a desired result sets a pattern for creating consistent and repeatable surface finishes. Setting markers lays a solid foundation early, which eases growth should output need to increase.
In most cases, surface texture is needed to promote adhesion or achieve a specific performance level. Parts that fall into this category must meet strict standards. These parts benefit from the results-oriented approach because it is measurable, repeatable, and easy-to-remember.
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About the author: Colin Weightman is director of technology at Comco Inc. He can be reached at 818.841.5500 or email@example.com.
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