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mistake in this chapter. Please find it. Do not look on the next page.

VDI What does this mean in surface finish terms?

Q. Our company is a supplier to the automotive industry worldwide. We are a plastic injection moulding company in the North East of the UK and manufacture our own mould tools. For surface finishes on our tooling we have used VDI comparator blocks. Can anybody tell me what the initials "VDI" stand for?

Graeme Hutton
Plastic Injection Molding & Tools - Stockton on Tees, Cleveland, UK

Q. I also want to know about the VDI finish and its conversion in Ra value. Is it EDM related term if yes then how is it controlled?

Waiting for your positive response,

designing - Udaipur, Rajasthan, India

Q. Same questions as previously posted by others: a formal comparison of VDI to Ra? information on VDI - what is it?

A. Hi. VDI = Verein Deutscher Ingenieure, the Society of German Engineers. There is a VDI scale for surface finishes, which is widely but not exclusively used in the EDM field. My lack of German has hampered my search for a cross reference chart or a description of what the VDI scale is about, but hopefully a reader will help.

Ted Mooney , P.E.
Striving to live Aloha - Pine Beach, New Jersey

A. I had the same problem with VDI grades. The solution came from Rubert & co. England. They offer EDM finish scale to VDI 3400 standard. It comes with comparison table Ra values. For more information:

Jaakko Havola
Medical equipment - Helsinki, Finland

Ed. note: Update April 2013 -- The referenced link no longer works, and it appears that that website no longer includes any such charts.

A. I have the same problem to find a correspondence between VDI and Ra, and after some search I made this comparison table :

David Bourdin
- Bobigny, France

Q. We have the VDI 3400.39 grain call out on a print for a part we are to supply. Can anyone tell me the depth of this grain and the draft required to eject the part with this grain?

Sabrina Arsenault
- Barrie, Ontario, Canada

A. Bayer Plastics has some good Information about surfaces and injection molding available on their web- Site.

According to a (German) Document the VDI surfaces correspond to the following Ra / Rz Values and require the following drafting angles:

DA-PA = Drafting Angle for Polyamide
DA-PC = Drafting Angle for Polycarbonate
DA-ABS = Drafting Angle for Acrilnitrile-Butadiene-Styrol

glass reinforced materials require more drafting

Lorenz Wiedemar
- Langendorf, Switzerland

But be careful: The table with the VDI related draft angles Mr. Lorenz Wiedemar provided (he says it's from BAYER plastics) is possibly not right!

STANDEX International communicates that their VDI 36 equivalent (MT 11540) requires minimum 3,5 even for PA !!
So just ask your surface supplier BEFORE being fooled by this table (like me).

Simon Mueller
- Fuerth, Bavaria, Germany

I have never found any surface standards for which "conversion" or "substitution" is actually possible. Rather, it is like comparing people's height vs. weight, or torque vs. horsepower -- one parameter tends to "track" the other, but cannot actually be converted. We see this in Ra vs Rz where the "conversion factor" must be low in one direction to be relatively safe and much higher in the other direction to be relatively safe because they are simply not the same thing.

My unfortunate lack of German prevents me from understanding VDI standards and how they translate to other systems, but it would greatly surprise me if there is an actual conversion rather than "usually roughly equivalent to somewhere around . . ." :-)

Ted Mooney , P.E.
Striving to live Aloha - Pine Beach, New Jersey

A. Nr is the number of roughness or the class, using VDI d

Try with the equivalence table.

Aitor Ruiz
- Basq Country (north of Spain)

A. VDI Table
In the following, you can determine the roughness for your application according to VDI table.

Qijin Wu
- Nanjing Jiansu Province, P.R.China

Q. Besides surface finishing of VDI standards, there is another standard SPL-2, would like to know the relation of VDI versus SPL-2, how to convert SPL-2 into VDI?

Q. Hello, is there anybody who knows how is possible to compare VDI scale to SPI scale?

Paolo Aprile
Datasensor - Bologna, Italy

Q. Hi, can anyone help me with VDI 27 (RA 2.24) finish? I need to know if it's like mirror polished or just EDM textured finish.
I am trying to order plastic molds and need Mirror finish polish for better clarity of molded parts, my mold maker says that VDI 27 is just EDM textured finish.
Any documented help on this will be highly appreciated.

A. VDI is a textured finish usually from spark eroding, for polished finish you should work in microns, lower the number higher the polish.

Daniel David Wood
- Blyth England

We are a Rubber parts manufacturing company, one of our German customers has sent us a drawing where on one surface they have marked "surface erodes - VDI Cl.27". I wanted to understand whether this mean that the surface should be rougher than VDI Cl.27 or smoother. Our normal surface is polished, would that be accepted in this case or not? Thank you.

Darshan Shah
rubber industries - Mumbai, India

A. VDI, while I am less aware of the call-out, is an EDM finish loosely related to the roughness of the resultant finish. SPE/SPI is a modified surface finish ranging from D-3 to A-1. D surfaces are media blasted. C surfaces are stone polished, B surfaces are paper abraded. A surfaces are diamond polished.
This page has the complete definitions for SPI finishes.

Russ Tartock
- Crawfordsville, Indiana, USA

A. See chart I obtained, with SPI, VDI and Ra Rz, in my flicker:

Andrew Brown
Medical Devices - Swansea, Wales


Plating Shop Manager [Salisbury, MD]
Junior Quality Engineer [Santa Clara, CA]
Plating Process Engineer [Wenatchee, WA]

Disclaimer: It's not possible to fully diagnose a finishing problem or the hazards of an operation via these pages. All information presented is for general reference and does not represent a professional opinion nor the policy of an author's employer. The internet is largely anonymous & unvetted some names may be fictitious and some recommendations might be harmful.

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A Polyurethane Alternative

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