Testing & measurement for 3D prototyping

To serve the aerospace market, executives at 3D printing and rapid prototyping company Star Prototype say putting a priority on quality is critical. The company has invested heavily in testing equipment for raw materials and finished products including:

• Optical emissions spectrometer (OES) – For inspecting incoming metals to ensure the right mix of alloys
• Surfometer – To measure roughness to ensure precise surface finishes
• Faro laser scanner – For fast, accurate scans of surfaces for comparison to customer-supplied CAD files

More than 20 employees use the quality assurance control equipment to ensure production effectiveness and submit inspection reports on production.

Star Prototype Founder Gordon Styles explains why testing and measurement are crucial to aerospace manufacturing.

Aerospace Manufacturing and Design: What type of parts does Star Prototype make?

Gordon Styles (GS): Most of our plastic injection mold tools are small- to medium-size, typically up to 1,000-ton press size (450mm x 450mm x 350mm). CNC machined parts can be as large as 1,000mm x 400mm x 400mm. We make prototypes and small manufacturing runs of intricate parts and assemblies, as well as finished tooling for plastic injection molding, vacuum casting, and pressure die casting. The majority of our parts are intended to be fully-finished, functional parts when they leave the factory or else display models/prototypes which look and feel like full production items.

AM&D: What processes does Star Prototype typically use?

GS: Our main services are CNC machining of raw material into finished or semi- finished parts; machining of raw material into fully-finished mold tools and vacuum casting molds; 3D metal printing with post-machining; and finishing services (painting, sanding, bead blasting, polishing, anodizing, plating, etc.)

AM&D: What metal and plastic raw materials does Star use?

GS: Aluminum, mild and stainless steels, titanium, magnesium, brass, copper, zinc, hardened tool steels (NAK80, H13, P20), and all commercially available stocks of plastic resins: nylon, ABS, acetal, polypropylene, PVC, etc. Other materials specified by customers can also be procured for manufacturing.

AM&D: How important is testing raw materials?

GS: It is essential to test raw materials to ensure a customer is getting exactly what was requested. We have optical emission spectrometers, X-ray fluorescent guns, and Raman scattering analyzers – among others – to fully qualify all incoming metal and plastic material to customer specifications.

AM&D: How do pre- and post-production tests differ?

GS: For pre-production, we are concerned with chemical and metallurgical analysis of raw material and hardness testing. Post-production may require the same testing in addition to all dimensional measurements and surface qualifications: color, scratches, texture, reflectivity, transparency, and other cosmetic defects are the primary examples.

AM&D: How does the optical emission spectrometer (OES) work? How accurate is it?

GS: We have an OES from Oxford Instruments, Model PMI Master Pro, which provides a highly accurate reference for metals. A high-voltage spark vaporizes a small amount of metal which then emits a characteristic band of light. The wavelengths of light and their intensities are then measured and compared to a library of known material types. Individual constituents and their relative concentrations, provide precise identification of the material. This mildly destructive test leaves a small scorch/witness mark on the metal.

We also have an X-ray fluorescent (XRF) gun from Oxford Instruments, Model X-MET 7000, which conducts a non-destructive test for a wide variety of materials in solid, liquid, or powder form. We use it for both metals and plastics. In the latter case, it can detect if the material contains any elements that may be banned in some countries, such as those specified by the Restrictions on Hazardous Substances (RoHS) regulations in Europe. It uses a high-energy X-ray which excites the target material and forces it to emit a characteristic X-ray in return which identifies it. However, the XRF gun cannot tell us precisely what the material is, only what it contains.

The Raman spectroscopy PolyMax gun from Chemlogix-TSI is a tester for plastic only. A monochromatic laser light is shined upon the test subject which causes the molecules to vibrate at characteristic frequencies. This frequency is picked up by the detector and compared to a database of known material types. This test is useful not only for telling us the material constituency but also for identifying and confirming exact material formulations.

AM&D: What sort of errors are found in alloy mixes? How often?

GS: With metals, the most common error detected is fake stainless steel. In some cases, 304 stainless steel will be switched for 201 stainless steel. Both are non-magnetic, therefore difficult to detect with a magnet, but 201 will rust very easily. When we first started testing this, we found that two-thirds of the so-called 304 stainless was in fact 201. Interestingly, 201 is almost unheard of outside of China, although it is creeping into the supply chain all over the world.

AM&D: How does the surfometer work? What is its level of accuracy?

GS: When using a Mitutoyo SJ-201R Profilometer, a stylus is dragged across the surface of the test specimen, which creates a piezoelectric pulse that is measured and recorded. A profile of this pattern is measured to produce an Ra number, which is the mathematical average of the peaks and valleys. This test is used for qualifying both cosmetic and mechanical fitness for use and is accurate to within a few microns.

AM&D: How does the surfometer compare to laser measurements?

GS: The surfometer/profilometer allows us to measure the surface finish – it is a micro measurement of the part surface. The Faro laser arm is non-contact, using reflected laser light to measure the macro 3D shape of a component. We can use the results from the Faro laser scanner to compare to the original 3D CAD model and get a colored deviation map of the part showing us the areas that are out of tolerance.

AM&D: What quality standards does Star use for additively manufactured products?

GS: For metallurgy, the parts must conform to international standards for density, composition, and hardness. We have all the equipment needed to make these tests. The main concern with additive manufacturing is that the raw sizes of parts are not as accurate as machining. Therefore, combining additive and subtractive processes is a common strategy. You can build a metal part with direct metal laser melting (DMLM) with some material left on, and then CNC machine important dimensions. Together, additive and subtractive manufacturing provides the best of both worlds.

AM&D: How does attention to quality in prototyping show up in final products?

GS: One of the key outcomes of the great attention to detail and the skill that we use in our factory is that the prototype parts we make look just like production parts. Most prototypes made around the world are a sad reflection of the final product, but our customers enjoy having prototypes that can pass for high-quality manufactured products. This helps customers to sell their concepts to potential investors and intrigue customers to pre-order products.

ChemLogix-TSF

www.tsi.com

Faro Technologies

www.faro.com

Mitutoyo America Corp.

www.mitutoyo.com

Oxford Instruments

www.oxford-instruments.com

Renishaw

www.renishaw.com

Star Prototype

www.star-prototype.com

About the author: Eric Brothers is senior editor of Aerospace Manufacturing and Design. He can be reached at ebrothers@gie.net or 216.393.0228.