Until recently the only way to put precision features on brittle substrates was with conventional machining and cutting techniques. But a new process borrows a few ideas from photolithography to handle such tasks efficiently. The method is called Photo-Machining Process (PMP) and comes from Ikonics Corp., Duluth, Minn. It is said to be an economical alternative to machining designs and patterns into hard, brittle substrates and roughening/texturing to promote adhesive bonding between adjoining parts. It can also reportedly scribe or mark surfaces for labels or add nomenclatures and logos with pinpoint accuracy.
PMP combines an ultraviolet, light-sensitive photoresist with abrasive etching or sandblasting to precisely machine parts. The process will not entirely displace conventional machining, but can help lower part costs through reduced setup and machine time and because it does not demand skilled (and thus more highly paid) operators.
The key to PMP is a family of ultraviolet light-sensitive photoresist films that sport resolutions to 50 m (0.002 in.). One of the films is a derivative of technology exclusively licensed from DuPont. A second is a patent-pending technology-from Ikonics.
Photoresist films have been widely used for years in the electronics and decorative etching industries. The films Ikonics uses survive aggressive abrasive etching. Ikonics Type I film is a high-impact photoresist designed for deeper etching of high density materials including alumina and silicon carbide. It must be developed in water after ultraviolet light exposure, like creating a stencil on the part to be machined.
A Type II Film is a low-impact photoresist for lighter etching and selective material removal. It develops without water. Areas exposedto UV become brittle (turning-blue) and etch away under abrasive impacts.
Brittle materials such as ceramics are notoriously difficult to machine. They often chip or crack with conventional cutting tools and wear out costly tools rather quickly. In contrast, PMP offers an easier and faster alternative to most other machining methods. PMP benefits vary depending on the type of machining needed. They include detailed patterns and shapes such as channels, surface pads, cavities, and surface texture.
PMP can also deeply etch lowdensity materials and remove large amounts of material quickly. It can quickly form array patterns of holes in thin wafer materials as well as permanently engrave hard-to-mark parts.
PMP can texture surfaces to prepare them for subsequent coatings or to promote adhesive bonds. And it can selectively remove coatings from surfaces for a variety of applications.
PMP can be faster than operations otherwise done with ultrasonic machining, rotary tooling/end milling, EDM, lasers, and grinding equipment.
PMP also doesn't have the inherent limitations of ultrasonic milling on large and asymmetrical work pieces. Its other advantages include little if any need for special tooling fixtures or jigs. It uses no cutting tools so there is nothing to wear out or replace. It generates little heat so there's no need for coolants. There are fewer tendencies for "chipped" edges and no pitting or stray markings. PMP parts need no subsequent deburring and can be engraved on their interior or contoured surfaces.
Any design that can be created on a computer can be photomachined. PMP starts by first printing on a transparent film with a standard inkjet or laser printer. This becomes the working tool or phototool for the masking operation.
After photoresist exposure to UV light through the phototool and development, parts are etched in a closed chamber by means of compressed air loaded with a fine abrasive, such as silicon carbide. The process resembles sandblasting but, coupled with a photoresist, reportedly produces parts with a precision equal to conventional machining, but at a lower cost and improved productivity.
Abrasive etching is a dry, lowtemperature process that puts little stress on parts as it quickly removes large amounts of material from most hard, brittle surfaces. The photoresist mask selectively resists or repels the abrasive to protect the covered part of the surface.
Material removal rate depends on the density of the surface material being etched. Etch depth is determined by the type of material, the photoresist, and the time in the etcher. An automated etching system can precisely control etch depth to within 0.001 in. (25 m). Manual etching can easily control depth to about 0.005 in.
Once the etching operation completes, parts go into a warm water bath that easily removes the photoresist. It is just water, no chemicals. The Ikonics photoresists are biodegradable and safe for municipal waste systems. Depending on production needs, each step in the process can be automated to boost throughput and ensure part-to-part (etch depth) consistency.
The photoresist comes in rolls and easily laminates to surfaces. Multiple parts can be exposed to UV light simultaneously. Type I photoresists, can sit on a conveyor for their water development.
As with all machining operations, there are limits to what PMP can do. Although metals and plastics can be surface engraved or marked, PMP cannot machine them deeply because of their inherently ductile nature. PMP does work well, however, in selectively removing thin metal-coated layers from most materials such as metal-encapsulated ceramics.
With deeper PMP etching abrasives build-up so the sides of the cavity will etch less than the center. This tapers the sidewalls and gives the cavity bottom a radius.