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Best metal finish for plastic extrusion vacuum calibrator : Cadmium vs. Nickel vs. Chrome?

(-----) January 25, 2008

Summary: We are currently researching the best surface finish of a vacuum calibration sizing unit. In the plastic extrusion process the hot semi molten extruded profile is pulled through a hollow cavity, which by cold water and vacuum the part is cooled and held dimensionally to shape as it passes through the sizer. Typically the exterior walls of the extruded profile make contact with the interior walls of the sizer as it passes through. The contact can cause drag. To compensate for the drag or coefficient of friction the material compounder will add lubricant to the plastic formula such as waxes to reduce the friction. In addition to the lubricants the sizer has slits where cooling water passes through and creates a water layer between the exterior wall of the extruded profile and interior wall of the sizer, whereas the water acts a lubricant layer. The sizing units are made of high grade stainless steel. The internal walls are highly polished to allow minimal drag between sizer and part.
Problem: While all precautions are taken to minimize drag (high polished surfaces, lubricants, etc.) there are still incidents where drag is present. This is evident by the phenomenon known as chatter. Our goal is to eliminate chatter. Rather than load the plastic formula with more lubricants we would like to find the best metal finish of the sizing unit to minimize drag. When too much lubricant is added to the plastic formula it tends to bleed out and build up on the metal surfaces thereby creating more drag and increasing the problem. We have been looking into Cadmium plating, Nickel plating, Chrome plating, anodizing or just sticking with the highly polished SS. All have advantages and disadvantages. Since our tooling is stainless steel we are not concerned about corrosion protection. We are mostly interested in the best metal finish that provides good lubricity with the least amount of friction and material build up for this application and in turn is the best option for maximum wear and heat transfer. Any insight would be greatly appreciated!

Stephen Hoffmann
New Product Development - Franklin, Ohio

January 25, 2008

If it's not too steep for you (I don't know the surface area and frequency of plating), I'd think about silver plating.

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

First of two simultaneous responses -- January 26, 2008

Maybe you should also consider composite electroless nickel (EN).
EN+Teflon or EN+Moly disulfide if temperature is an issue with Teflon. Both are hard, corrosion resistant and lubrious.

Guillermo Marrufo
Monterrey, NL, Mexico

Second of two simultaneous responses -- January 27, 2008

Hello Mr. Hoffmann
In my opinion cadmium would be useless of its softness and chemical activity. Please consider electroless nickel. All my experiences with extrusion of plastics have finished with electroless nickel. Electroless nickel could be also applied in its composited form as nickel-PTFE or similar. You can consider nickel alloys as well (Ni-Mo, Ni-W). They are used for glass extrusion.

- Warsaw, Poland

February 5, 2008

Hello Mr. Mooney,
What are the advantages in going with silver plating? Also what is the durability in using silver plating? The surface area will vary with each profile we run. The smallest surface area for each sizer (there are 4 sizers per profile) is approximately 180 square inches. The largest is approximately 360 square inches. The line rate is around 5 to 6 feet per minute. (I know some readers who are familiar with profile extrusion are wondering why such a slow line rate? The answer is it is cellular PVC foam). The line will be running 24/5 - 260 days a year. So with that in mind we need a plating or finish that can withstand long exposure.

Stephen Hoffmann
New Product Development - Franklin, Ohio

February 5, 2008

Thank you to all who responded on the information regarding Electroless Nickel. Any direction as to where I might find out more information on Electroless Nickel and the process would be appreciated?

Stephen Hoffmann
New Product Development - Franklin, Ohio

February 18, 2008

Silver is soft, offers good lubricity, is corrosion resistant, and is often used specifically to prevent galling between stainless steel and aluminum components. However, as a mold surface, Electroless Nickel is certainly better as suggested by Guillermo and Janusz; I may have read the question too quickly.

To learn more about electroless nickel consult a supplier of the process chemistry (see the Chemicals directory) or a shop that applies it (see the Jobshops directory) or consult a book on it (I'd recommend Mallory and Hajdu's "Electroless Plating".

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

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