July 19, 2012 A Penton Media Publication Tune in to EngineeringTV.com BASICS OF BALL SPLINES, page 50 RESOLUTION AND ACCURACY IN 3D PRINTING, page 56 FASTER MOLDING WITH HIGH-FLOW POLYCARBONATES, page 72 BETTER WAYS TO ASSEMBLE PEEK PARTS, page 80 Tips & Traps in GD&T Page 68 RS# 101 Motion Control Products HDR Series Starts at $135 Gearboxes Worm Gear Speed Reducers Ratios Available from 5:1 to 60:1 Gear Box Sizes from 1.33 to 3.25" Universally Interchangeable Design for OEM Replacement Double Bearings Used on Both Shaft Ends Visit omega.com/hdr General Purpose AC Motors, 56C Flange, Single Phase, Fractional and Integral HP General Purpose AC Motors, 56C Frame, Three-Phase, Fractional and Integral HP OMT Series Starts at $125 Permanent Magnet DC Motors OMPM-DC Starts at $ 200 OMAT Series Starts at $115 Visit omega.com/omt_series Visit omega.com/omat_series Visit omega.com/ompm-dc omega.com ® Νο ® RS# 102 © COPYRIGHT 2012 OMEGA ENGINEERING, INC ALL RIGHTS RESERVED VOLUME 84 ISSUE 10 JULY 19, 2012 MEDICAL INDUSTRY FOCUS Authored by: Lonny Wolgemuth Senior Medical Market Specialist Specialty Coating Systems Inc Indianapolis, Ind Edited by Stephen J Mraz stephen.mraz@penton.com Resources: Specialty Coating Systems Inc., scscoatings com For more information on parylene coatings, scan this code or go to: http://machinedesign com/article/coatingadhesion-promoterspecialty-coatingsystems-0609 Many medical devices and their electronics need protection from moisture, chemical contamination, electrical charges, and body fluids Otherwise, patients and healthcare providers may be put at risk One way biomedical engineers provide this protection is by encapsulating devices in a conformal coating, one made of a dielectric, or poor conductor of electricity, such as silicone, acrylic, urethane, or epoxy But one of the best materials for this purpose is parylene Parylene basics Parylene is the generic name for a series of organic polymers — poly(para-xylylene) polymers — used as coatings They are polycrystalline and linear in nature, optically clear, and colorless Parylene coatings have useful dielectric and barrier properties and are chemically inert Three different types give engineers a range of dielectric and other properties from which to choose The coatings contain no fillers, stabilizers, solvents, catalysts, or plasticizers, so they are not subject to any leaching, outgassing, Protecting MEDICAL ELECTRONICS with parylene FEATURES This biocompatible conformal-coating material protects medical devices against fluids, chemicals, and stray electrical charges rotary 50 Exploring ball splines Rotary ball splines can produce submillimeter accuracy However, the choice of angular-contact or crossed-roller bearings plays a critical role Parylene coatings are optically clear and little to change the look or dimensions of the underlying component or part, as demonstrated in this photo of two circuit boards The top one is coated with parylene, the bottom one isn’t 20 MACHINE DESIGN.com JULY 19, 2012 or extraction issues Parylene coatings are also compatible and stable in the presence of bodily fluids and tissues, critical factors in the medical-device industry Parylene provides dryfilm lubricity with coefficients of friction similar to that of PTFE (Teflon), and dielectric strengths up to 7,000 V at a mil (25 microns) of coating thickness No other material can be applied as thinly as parylene and provide the same levels of protection Par ylene withstands all common sterilization methods — steam, ethylene oxide, electron beam, hydrogen peroxide plasma, and gamma radiation It can be applied to most vacuum-stable materials, including plastics, metals, ceramics, fabrics, paper, and even granular materials For example, parylene coatings could be applied to microspheres or moisture-absorbent powders Parylene can be selectively removed with plasma, lasers, or strong abrasion, for instance, to repair devices Parylene is not soluble in harsh detergents and chemicals; in fact, it protects components from such chemicals Parylene is not a “hard” coating, so excessive abrasion will remove it However, most components coated with parylene not abrade or rub against other parts If an application does include abrasive contact, it is not a good candidate for parylene Parylene has been successfully used on implantable electronics such as cardiac-assist devices, neurostimulators, cochlear implants, and pacemakers It’s also been used to coat nonelectronic medical devices as well, including coronary and cerebral stents, and catheters sublimating it directly to a vapor, and then heated again until the dimer cracks into a monomeric vapor This vapor flows into an ambient-temperature deposition chamber kept at a medium vacuum (0.1 torr) where it spontaneously polymerizes onto all surfaces, forming an ultrathin, uniform film No curing or additional steps are required The size of the coating chamber may be an issue if Continued on page 22 A short history of parylene Parylene deposition Parylene coatings are applied using vapor-deposition polymerization (VDP) in a vacuum chamber at room temperature Film deposition actually takes place on the molecular level, with the coating literally growing one molecule at a time This lets parylene penetrate and coat small cracks, crevices, and openings, and protect even hidden surfaces in areas where other coating methods such as sprays and brushes cannot reach Coating thickness is uniform, even on irregular surfaces And VDP is a clean, self-contained process that uses no additional chemicals Parylene is deposited as a vapor, so it surrounds the target and perfectly follows its contours, literally encapsulating it Parylene coatings are ultrathin and pinhole-free The only raw material used in the coating process is known as dimer Technicians place the powdered double-molecule dimer into the vaporizing chamber at one end of the coating machine The dimer is heated, In 1947, Michael Szwarc was pursuing his academic career in physical chemistry at the Univ of Manchester, England His interest in the strength of individual chemical bonds led him to investigate a class of aliphatic carbon-hydrogen bonds in which the carbon was directly attached to a benzene ring While doing so, he heated gases of the simplest compounds having both benzene and carbon — toluene and the xylenes — to high temperatures He monitored both the decomposition products and rates of decomposition as a function of temperature With p-xylene only, a tan-colored deposit formed in the cooler reaches of his glassware The material has been described as a thin, flimsy, tube-shaped mass, “the skin of a small snake.” Szwarc correctly deduced that this film had been formed by polymerizing reaction products of the p-xylene, called p-xylylene He also noticed the new polymer’s physical properties and chemical inertness This serendipitous polymerization was the world’s first vapor deposited poly(paraxylyene) Today its purer colorless form is called parylene N A few years later, William Franklin Gorham at Union Carbide Corp continued the research on parylene By 1967, this work led to the availability of a new polymeric coating “Parylenes” was the term used to describe both a new family of polymers and the vacuum-deposition process for applying them In fact, Union Carbide developed over 20 types of parylene, but only three were deemed commercially viable JULY 19, 2012 20 MACHINE DESIGN.com Protecting medical electronics with parylene Parylene coatings give medical devices the dielectric strength to resist stray voltages as well as fluids and gases for designing 64 Options the best hinge Hinges with spring pins instead of solid pins can lower costs and extend life CAD/CAM/CAE/FEA Accuracy in SPC leads 68 When engineers astray ADDITIVE The quest for minimizing process variations can lead to parts that are needlessly expensive go with the flow 72 Polycarbonates A new class of high-flow polycarbonate copolymers meets or exceeds the physical and mechanical properties of traditional materials, and provides better molding characteristics assembly techniques 80 Advanced for high-performance plastics Engineers switching from metal to plastic need to revisit how they assemble the parts MACHINE DESIGN.com JULY 19, 2012 21 MANUFACTURING A common mistake is to confuse high resolution with accuracy In the realm of additive manufacturing, what exactly are the distinctions between accuracy, repeatability, and resolution? Accuracy describes how closely a manufacturing machine’s output conforms to a tolerance within a specified dimensional range Repeatability captures the equipment’s capability to produce consistent output, time after time And resolution refers to the smallest measurement the machine can reproduce These concepts are second nature to designers and manufacturing engineers Yet, there is a widely held misconception about these measurement concepts as they relate to additive manufacturing Over the years, some engineers have slid into using resolution attributes — such as layer thickness or dots per inch — as a careless shorthand term for accuracy 56 MACHINE DESIGN.com Accuracy in additive manufacturing The color maps reveal that the top part — built on a high-resolution 3D printer — is not as accurate as the bottom part, built using a lower-resolution setting on an FDM system Purples and reds reveal that the high-res printer produced deviations of ±0.020 in or more Greens reveal the FDM-built part is mostly in the ±0.005-in range Photo courtesy of T A Grimm & Associates Inc A common mistake is to confuse high resolution with accuracy JULY 19, 2012 56 Access our Reader Service Web site to quickly find and request information on the products and services found in the pages of MACHINE DESIGN www.machinedesign.com/rsc Get a big bang for your buck We squeezed the features of our popular TFT 6-inch C-more Micro into a 4-inch package for even more value! 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