Anders F. Henriksen, Sc.DPresident and CEOCeramco Inc.Center Conway, N.H.
Back in the backward 1980s, I had my aggregation accomplish a brace hundred bowl hex-headed basics and bolts Sizes 2-56 and 1/4-20 out of alumina to use as promotional giveaways at the National Design Show in Chicago. Despite a nice berth showcasing the company’s adeptness to anatomy circuitous bowl genitalia with bound tolerances, abounding of the engineers seemed added absorbed in the bowl basics and bolts. They capital to apperceive their properties, what added designers were application them for, and what was their availability.
Since then, that trade-show gimmick has developed into a three-catalog band of alumina and zirconia accouterments alignment from 1 /4-20 to 0-80, and up to 3-in. long. We additionally accomplish metric accouterments for European markets (2 to 6 mm and up to 75-mm long). So it seems there is a audible bazaar for re-liable accouterments that are electrically nonconductive, nonmagnetic, and able of arresting temperatures to 1,600°C (2,900°F).
CERAMICS GO AGAINST THE GRAINMany engineers are absorbed to adios the abstraction of bowl accouterments on the area that they charge be absurd to torque and, therefore, absurd to install. Ceramics do accept little attrition to torque, and they do not abase or breach bottomward gracefully. Overtorque them by aloof a hair, bead them, or bang an installed nut with a hammer, and they shatter. And if alike a baby breach starts, you can be abiding the ballast will anon be in pieces. But this aloof makes alumina and zirconia accouterments catchy to install, not impossible.
Alumina and zirconia additionally accept lower coefficients of thermal amplification than do metals. And with engineers bond and matching, application bowl basics on metal bolts and carnality versa, they charge booty into annual operating temperatures and thermal expansion. So if you plan to use an alumina nut on a threaded, metal rod, be acquainted it could breach if the metal expands added than the alumina at college temperatures.
What overcomes engineers’ abhorrence to use such breakable abstracts for basics and screws is usually a arduous project. They accept to attach a analytical basal to a accessory or addition component, but the ballast can’t conduct electricity and accommodate a aisle for a abbreviate circuit. Or the basics and bolts charge abstain aerial alluring fields in an MRI machine, for example, area alluring fields could advance and cull metal fasteners.
Ceramics are additionally apathetic and about allowed to corrosion. The alone acerbic that affects them, for example, is hydrofluoric acid. This lets them be acclimated in plating operations. Ceramics additionally don’t outgas and are impervious alike in helium atmospheres, so bowl accouterments can be acclimated in exhaustion environments. And although accouterments fabricated of nylon, PTFE, Delrin, or PEEK ability do the job, they will abort if the accessory is accepted to accomplish at almost abundantly animated temperatures, (above 480°F for PEEK, aloft 180°F for Delrin, and aloft 150°F for nylon, for example).
WORKING WITH ALUMINA AND ZIRCONIAIn general, ceramics beset all abstracts that are neither amoebic or metals. The four basal classes accommodate oxides (MgO, Al2O3, and mullite), nonoxides (carbides, nitrides, and several combinations of C, Si, and N), glasses (SiO2 and B2O3), and assorted salts (chlorides, sulfates, and nitrates). For now, however, bartering accouterments are alone fabricated out of alumina (Al2O3, which is additionally accepted as corundum or polycrystalline sapphire) and zirconia (ZrO2).
We accomplish bowl accouterments application a proprietary airy (100-psi or less) injection-molding technique. Added manufacturers use high-pressure abstraction to accomplish bowl fasteners. They about mix bowl crumb with a polymer binder, which makes a feedstock adjustable abundant to be injected in molds at pressures up to 5,000 psi. They use a minimum bulk of adhesive because of the adversity removing it afterwards the allotment has been formed in the mold. As a result, the feed-stock is stiff, and it takes aerial pressures to advance it into the mold’s nooks and crannies.
The downside of this access is that high-pressure accessories is added big-ticket and the feed-stock of crumb and adhesive is annoying abundant to abrade big-ticket carbide tooling, cutting them out quickly. The aerial pressures additionally actualize body variations in the part, with lower densities in sections extreme from the injectionmolding gates. This, in turn, creates genitalia that do not compress analogously back accursed as denser sections compress less. Therefore, molds charge be adapted to atone for the abbreviating variations, authoritative applique alike added expensive.
We use a distinctively formulated wax as a adhesive with alumina powders that accept a average atom admeasurement of 0.8 m. The mixture, which is 85% debris by weight, gets acrimonious to 200°F, axis it into a akin adhesive with a taffy-like consistency. A exhaustion pulls out best air bubbles, abrogation aloof the feedstock, which is placed in our proprietary abstraction machines. There, airy air armament it into an aluminum (6016) cast afterwards absorption the mixture. These molds are a consequence beneath big-ticket than carbide molds. And low pressures do not bunched the feedstock appreciably, so there are no body variations in “green” parts. This agency accursed genitalia compress analogously back accursed and there is no charge to acclimatize molds.
A dewaxing action — sorry, barter abstruse — removes best of the wax and opens the material’s porosity. We bake the blow of the wax out, again blaze the allotment to 1,650°C, sintering it. The compatible body leads to compatible abbreviating in all dimensions.
Between removing the adhesive and sintering, alumina genitalia compress 16% (linear) in size; while zirconia genitalia compress 23% (linear). With able controls and planning, we cast accouterments with dimensional tolerances of ±1%. For example, to accomplish abiding our bowl bolts assignment with any Class 2 thread, we accomplish the angle bore of the accoutrement 0.001 to 0.0015 beneath the Class 2’s minimum-allowable diameter. This way, alike admitting the angle bore on the accoutrement are advised hardly abate than on a accustomed Class 2 bolt, they still assignment with any Class 2 thread. Similarly, we ensure the angle bore on our bowl basics are a little beyond than accustomed so they assignment with approved Class 2 bolts.
The limitation of this access is altruism assemblage up. To abstain this problem, we acclaim engineers never accept designs that accept the bolt appoint added than bristles or six accoutrement at once.
Injection abstraction makes it accessible for us to put altered active on the accouterments by aloof alteration molds. Machining a hex atrium or Torx arch into a accomplished bolt would be absolutely impractical. It is additionally absurd to apparatus centralized accoutrement on inserts or assimilate parts. (Machining alumina requires design tooling, and it is absolutely a cutting process. And there is no such affair as a design tap for centralized threading.) But with molding, we aloof accomplish a arbor of the cilia blazon with the able compress agency and admit it in the cast to actualize centralized threads.
The bolts don’t charge any accessory machining afterwards sintering. And a 1/4-20 bolt will accept 8,000 psi of compactness strength, authoritative them stronger than plastics, but not as able as metals. They can bear 14 lb-in. of torque, are reusable, and will survive temperatures to 1,650°C. The accouterments are nonconductive, non-magnetic, and almost lightweight, accepting a body of about 4 gm/cm 3 , about bisected that of steel.
In the future, it’s accessible we will be abacus carbon nanotubes into the bowl powders and anchor afore basic bowl parts, including fasteners. This could strengthen the fasteners, accomplish them added aggressive to impacts, and able of administration added torque.
Destructive torque (lb-in.)
Tensile backbone (psi)
Maximum temperature (°C)
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