Guide
What Does Hygienic Design Mean for Machined Stainless Steel Parts?
Hygienic design is the practice of shaping parts, components and equipment so they can be cleaned easily and completely, which on a machined stainless part comes down to a short list of drawing decisions: internal corner radius, dead-leg length against diameter, exposed threads and crevices, and the finish on faces the product touches. None of these is a material property. Each one is a feature the drawing either specifies or leaves to chance.
By Saola Manufacturing engineering team · Published · Updated
This guide is for food-machinery and process-equipment designers who send a stainless housing, nozzle, fitting or bracket to an overseas machine shop and want it to be cleanable when it comes back. It uses published guidance from EHEDG and Fraunhofer IPA, labels each value with its source, and separates what a CNC shop can hold from what the equipment maker still has to prove. The detailed numeric limits live in standards and member-only guidelines that this guide does not reproduce.
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What does hygienic design mean for a machined part?
It means every wetted or splash surface can be reached by the cleaning method, drains freely, and has nowhere for product to sit. For a machined part that is settled by geometry the drawing controls, not by the grade of steel alone.
Fraunhofer IPA defines hygienic design as the design of parts, components and manufacturing equipment for easy cleaning, and its assessment checklist asks plain questions: are surface characteristics in line with the norms, are screw connections done correctly from a hygiene standpoint, are there dead spaces where liquids could accumulate, can all areas be treated with cleaning and disinfection agents, and can all liquids run off unimpeded (Fraunhofer IPA, hygienic design flyer, 2014). Every one of those questions maps to something a drawing can say. The sections below take them in the order a designer meets them.
The reference most often named for food and beverage equipment is EHEDG Guideline 8, Hygienic Design Principles. EHEDG lists it as a free download after accepting its conditions, and shows it as a fourth edition dated December 2025 that the working group is already updating (EHEDG Guideline 8 page, accessed 2026). Other guidelines for closed equipment, open wet-cleaned equipment, valves, couplings and sensors are listed on the EHEDG guidelines page, and most are members-only (EHEDG guidelines catalog, accessed 2026). Check which one your end customer or auditor actually requires before you fix a number on the drawing.
What internal corner radius should a hygienic part have?
Use rounded internal corners instead of 90 degree ones on product-contact areas; one published illustration marks a corner radius of at least 3 mm as the hygienic example (Fraunhofer IPA, 2014). Treat that as an illustration of the principle, not as a universal limit.
The same flyer shows a sharp corner on a horizontal surface as the hygienic risk and a rounded corner with a sloped surface, marked at least 3 degrees, as the acceptable design. The lesson for a drawing is to remove flat shelves and square pockets where product, water or cleaning agent can sit. Which radius your product needs depends on the customer’s hygiene specification, so ask for it rather than assuming.
On the machining side, an internal radius is produced by the cutter, so the smallest radius a milled pocket can have is set by the smallest cutter that reaches the depth. As typical shop practice, a larger radius lets a stiffer, larger cutter be used and gives a cleaner wall, while a very tight radius at the bottom of a deep pocket forces a long, slim tool. If the design truly needs a small radius deep in a pocket, say so on the drawing and expect the quote to reflect it. Deburring and edge breaks matter too: the flyer’s examples show rounded edges and angles, so a hygienic drawing should state the edge condition instead of leaving the default to the shop.
How long can a dead leg be?
EHEDG’s position paper on easily cleanable couplings says the length of a dead leg in a pipeline must be less than its inner diameter, and, where a sensor protrudes into it, L must not exceed D minus d (EHEDG, January 2024). L is the dead-leg length, D is its inner diameter and d is the diameter of the protruding sensor.
If the ratio of L to D minus d comes out above one for a connection size, the paper says that size is excluded from the sizes considered for EHEDG certification. It also asks for connections that drain by gravity and promote the evacuation of air, and for tank sensors to sit flush with the inside of the tank wall; where a dead leg is technically unavoidable, the cleaning spray must still reach every surface.
This matters for machined parts because a port, a sensor boss or a blind drilled hole on the wetted side is a dead leg the moment it is deeper than it is wide. A designer can use the rule as a drawing check: measure the depth of every side port and blind bore from the main flow surface, compare it with the bore diameter, and either shorten it, open it up or add a note. On a machined manifold or housing, the cheapest time to fix this is at the model stage, because a blind bore that is too deep is not repaired by finishing it better.
How should threads, crevices and seals be handled?
Keep threads out of the product-contact zone, and where a joint cannot be avoided give the seal an axial stop, centering and no more than 0.2 mm of crevice or seal protrusion, which is what EHEDG’s coupling paper names as critical (EHEDG, 2024). A thread in the wetted zone is a helical crevice by definition.
Fraunhofer’s checklist asks the same question from the assembly side: have screw connections been correctly executed from the hygiene point of view and are they easy to clean. As typical design practice, that leads to a few drawing decisions. Move fasteners to the dry side of the wall. Where a threaded stud or blind tapped hole is needed, place it outside the cleaned surface and seal the joint. Use a seal face with a hard stop instead of a free gasket that can squeeze out. Avoid overlapping plates and lap joints that hide a gap, and prefer a continuous weld or a machined single piece to a bolted assembly where the wetted face is involved.
The EHEDG paper also shows that standard hygienic couplings are specified by recognized standards such as DIN 11853 in its 2017 parts for screwed, flanged and clamp connections with O-ring. If a machined part carries one of those connections, cite the standard and the seal on the drawing instead of redrawing the geometry by hand.
What finish should product-contact faces have?
Specify roughness only on the faces the product touches, with the Ra limit taken from the customer’s hygiene specification, and state how it is measured. A single Ra value pasted onto the whole drawing makes every face expensive and does not make any of them cleaner.
This guide does not list a number because the sources opened for it do not state one that is safe to reuse, and published limits differ between standards and between product types. Fraunhofer’s checklist simply asks whether the surface characteristics conform to the recommendations in the relevant norms, which is the right framing: the norm your customer follows sets the number, and the drawing carries it.
Three drawing points travel with the number. First, say whether it applies before or after any polishing or electropolishing, since those steps change it; the electropolishing vs passivation guide explains the difference. Second, name the lay or tool direction where it matters for draining. Third, keep roughness and cosmetic requirements apart: the Ra chart and the Ra versus Rz guide cover how to write and measure the value. Corrosion behavior of wetted stainless surfaces is a related topic covered in the rouging guide.
| Feature | What the sources say | What to write on the drawing |
|---|---|---|
| Internal corner | Fraunhofer illustrates a rounded corner with radius of at least 3 mm and a slope of at least 3 degrees as the hygienic example | Radius on every internal corner; the value from the customer specification |
| Dead leg | EHEDG: length less than inner diameter; L at most D minus d where a sensor protrudes | Depth and diameter of each side port and blind bore, with a check note |
| Seal and crevice | EHEDG: crevice or seal protrusion no greater than 0.2 mm; axial stop; flange centering | The coupling standard, the seal, and the stop geometry |
| Threads | Fraunhofer: screw connections assessed for hygiene and cleanability | Threads on the dry side; sealed where wetted |
| Drainage | EHEDG: self-draining orientation; Fraunhofer: liquids run off unimpeded | Orientation and slope noted on the drawing |
| Surface roughness | No number reused here | Ra limit from the customer specification, faces named |
What can a CNC shop hold, and where does the drawing need a note?
A CNC shop can machine the radii, bore depths, seal seats and faces the drawing defines. It cannot prove hygiene, so the equipment maker still owns cleanability testing and any EHEDG or 3-A evidence.
Saola makes no EHEDG or 3-A claim for machined parts. The useful division of labor is to put each hygienic requirement in a form a shop can inspect. A radius can be checked with a gauge or on a coordinate measuring machine. A dead-leg depth is a depth. A roughness limit is measured with a stylus on the named face. What the shop cannot see from a model is intent, so a short note such as “all internal corners radiused, no blind pockets deeper than their width on the wetted side” is worth more than a general reference to a guideline. Where a requirement is unclear, ask in the RFQ.
Stainless parts of this kind are normally turned and milled from bar or plate. Stainless bar is not among the materials Saola currently lists as running, and passivation and electropolishing are not among the finishes on its surface finishing page, so confirm scope at drawing review. Say in the quote request whether treatment is required after machining, because the finish step changes the final surface and should be on the drawing in sequence. The passivation guide explains what to name.
Frequently asked
Is a bigger corner radius always more hygienic?
A larger radius is generally easier to clean than a sharp corner, but the right value depends on the specification the equipment must meet and on what the machine can reach. Ask for the number from the customer standard instead of choosing the largest one.
Does passivation make a stainless part hygienic?
No. Passivation addresses surface corrosion resistance, not geometry. A part with a dead leg or a crevice stays hard to clean however it is treated afterward.
Where can I get EHEDG Guideline 8?
EHEDG lists Guideline 8, Hygienic Design Principles, as a download that requires accepting its download conditions, and shows a fourth edition dated December 2025 (EHEDG, accessed 2026). Most other guidelines in the catalog are for members.
Which stainless grade is normally used?
The grade follows the process medium and cleaning chemistry, not the hygienic geometry. The 316 vs 316L guide covers the choice most drawings face.
Does a hygienic part have to be machined from one piece?
Not always, but every joint adds a seal or a seam to keep clean. Where a joint is needed, the seal rules in the EHEDG paper cited above apply: an axial stop, centering and no crevice or protrusion beyond the limit it names.
Sources
- Position paper of the EHEDG SubCom Certification: easy cleanable pipe couplings and process connections, version 6, EHEDG, January 2024 (hosted by JUMO)
- Hygienic design, expert assessment flyer, Fraunhofer IPA, 2014
- Guideline 8, Hygienic Design Principles, fourth edition, EHEDG, December 2025 (page accessed 2026)
- EHEDG guidelines catalog, EHEDG, accessed 2026