Which materials and surface finishes make sense for hygienic powder processes in order to minimise build-up?
In hygienic powder processes, corrosion-resistant, readily cleanable materials and smooth, hygienically compliant surfaces are the most important levers for minimising build-up, bridging and cross-contamination. In practice, the combination of austenitic stainless steel, a defined surface roughness and a hygienic design of the components has proved itself above all.
Suitable materials
- 1.4404 (AISI 316L) is the standard material for product-contact parts in food, pharmaceutical and fine chemical plants, because it offers high corrosion resistance, good cleanability and robust resistance to typical CIP/SIP media.
- 1.4435 is used where higher demands are placed on corrosion resistance and the quality of the passive layer, for instance with more aggressive media or stricter pharmaceutical applications.
- Duplex and super duplex steels can make sense where high mechanical loads or chloride-containing environments are present in addition.
- Nickel-based alloys such as Hastelloy come into consideration with strongly corrosive powders or particularly aggressive cleaning media.
- Titanium is used in special pharmaceutical applications where low mass, high corrosion resistance or particular process requirements are relevant.
For seals, linings and components with direct product contact, PTFE, modified PTFE, PEEK, PFA or UHMW-PE are frequently used, since these materials are chemically inert and have a low surface energy. Elastomers such as EPDM, FKM or VMQ should be selected in a standard-compliant form for food and pharmaceutical applications.
Sensible surface finishes
- Ra ≤ 0.8 µm counts as the customary standard for hygienic applications.
- Ra ≤ 0.4 µm is usually the better choice for pharmaceutical processes and strongly adhering powders.
- Ra ≤ 0.2 µm is used in aseptic applications or with particularly critical products.
The lower the roughness, the fewer microfine depressions are available for particle adhesion, mechanical interlocking and residues. What is decisive here is not only the numerical value but also the quality of the grinding and the avoidance of scores, edges and machining marks.
Surface treatment
- Mechanical grinding and polishing reduce the micro-roughness and improve cleanability.
- Electropolishing is particularly effective, because it removes peaks in the micro-profile, smooths the surface and improves the passive layer.
- Pickling and passivating are important after welding, in order to remove heat tint, foreign metals and corrosion nuclei.
- Special non-stick coatings can lower the surface energy further, but must be expressly suitable and demonstrably approved for the food or pharmaceutical contact concerned.
Constructional requirements
Even the best surface works reliably only where the construction is hygienically compliant:
- Dead-space-free geometries and readily emptiable shapes
- Internal radii of at least 3 mm, often larger in sensitive applications
- Fully welded, flush-ground and passivated seams
- No crevices, overlaps or sharp edges in the product space
- Suitable sealing systems that create no cavities on the product side
A practical recommendation
For most hygienic powder processes, 1.4404 / AISI 316L with a surface quality of Ra ≤ 0.8 µm – or rather Ra ≤ 0.4 µm with more demanding or strongly adhering powders – is the technically most sensible basis. Where build-up is particularly critical, electropolishing markedly improves cleanability in addition. In combination with a dead-space-free hygienic design, product loss, cleaning effort and the risk of contamination can be effectively reduced.
How amixon® solves surface finish and material selection for hygienic powder processes
Surfaces: ground, low in adhesion, cleanable
At amixon®, the mixing chamber and mixing tool are welded free of crevices and ground smooth, without scratches, pores or undercuts in which product residues could adhere or biofilms could form. The surface finish is specified according to the application, from industrially ground through to highly polished designs for GMP-critical processes such as pharmaceuticals or infant formula.
Since every amixon® apparatus is a one-off in accordance with the URS, the required Ra value is established and documented project-specifically. Smooth, homogeneous surfaces reduce build-up directly and, together with a precise addition of liquids, support clean, reproducible process behaviour even during moistening or granulation steps.
A range of materials for every product chemistry
As a certified welding company with European, Japanese, Korean and American qualifications, amixon® processes, depending on the application, mild steel, wear-resistant materials, austenitic stainless steels, duplex stainless steels as well as highly corrosion-resistant special alloys such as Alloy 59, Hastelloy-C22 or nickel. Abrasive, corrosive, moist or chemically demanding products can thereby be catered for appropriately in each case.
For strongly abrasive goods, hardened tools and ceramic plasma coatings are additionally available as wear protection. This not only increases the service life but also secures a reproducible surface quality over many years of operation.
Construction and cleanability belong together
At amixon®, the surface finish never takes effect in isolation but always together with freedom from dead space and constructional accessibility. The mixing tool is mounted exclusively at the top, product-contact areas are of dead-space-free design, and the discharge fittings as well as the Clever-Cut® inspection doors with an OmgaSeal® seal support reliable cleaning and inspection.
The residual discharge of 99.997 per cent and better with free-flowing goods reduces the quantity of residue that has to be cleaned at all. On request, the mixers meet relevant standards such as EHEDG, the FDA hygiene guidelines and the 3-A Sanitary Standards, which is a decisive advantage particularly for hygienically sensitive powder processes.
Available surface qualities
The surface roughness of the product-contact parts can be selected project-specifically, typically with Ra values of 0.8 µm, 0.6 µm, 0.4 µm or 0.2 µm. Beyond this, amixon® can lap to a mirror finish and electropolish where particularly high demands are placed on cleanability, freedom from build-up or visual quality.
Manufacture in Paderborn as the quality foundation
amixon® develops and manufactures exclusively at the Paderborn works with a high depth of manufacture and all components from Germany. As a certified welding company with European, Japanese, Korean and American qualifications, amixon® designs every apparatus as a one-off on the basis of the operator's URS, and quality control remains entirely in-house without gaps.
This manufacturing autonomy also secures long-term supply, since every component can be reproduced even decades later. For regulated environments, amixon® additionally supplies the plant in qualifiable form and assists on request with DQ, IQ and OQ.
Qualifiable and documented
Documentation and execution follow, on request, EU GMP and FDA 21 CFR Part 11. On request, the mixers meet the relevant standards such as EHEDG, the FDA hygiene guidelines, the 3-A Sanitary Standards, USDA, GMP, ATEX and ASME and are integrated into the operator's validation concept from the URS through to commissioning.
Verification in the amixon® pilot plant
amixon® checks in advance in the pilot plant, with 35 test units, whether the specific formulation delivers the expected results. Further pilot plants are available in Japan, India, Thailand, China, South Korea and the USA. Trials with the original product under real conditions are carried out, evaluated and documented jointly and serve as a robust basis for decision-making before the investment.