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How do they polish ejecter pins

E

edwin dirnbeck

Hot Rolled
Joined
Oct 24, 2013
Location
st,louis mo
Die cast and plastic injection molds use ejecter pins to to eject the finished part from the mold.They come in many sizes. A typical one is 3/8 diameter and 16 inches long.This would have a head on one end bout 9/16 diameter by 3/8 long.They are tool steel hardened and ground to about 40 rc. The O.D.Is nitride hardened to almost glass hardness ,a few thousandths deep.The long skinny outside diameter is SOMEHOW POLISHED IN A LEGHTWISE MANNER TO A SUPER SLICK FINISH.I am working on a product that needs this lengthwise polishing. HOW DO THEY DO IT. Edwin Dirnbeck
 
edwin dirnbeck said:
Die cast and plastic injection molds use ejecter pins to to eject the finished part from the mold.They come in many sizes. A typical one is 3/8 diameter and 16 inches long.This would have a head on one end bout 9/16 diameter by 3/8 long.They are tool steel hardened and ground to about 40 rc. The O.D.Is nitride hardened to almost glass hardness ,a few thousandths deep.The long skinny outside diameter is SOMEHOW POLISHED IN A LEGHTWISE MANNER TO A SUPER SLICK FINISH.I am working on a product that needs this lengthwise polishing. HOW DO THEY DO IT. Edwin Dirnbeck
Click to expand...

You mean typical for your application? I wouldn't say what you described is "typical" as they come in all sizes and lengths. But back to your question, I don't know how they polish them (they do this on punches for dies too), but I am not sure what the polishing would do? From most of what I have seen, the ejector pin is inside the mold cavity so as long as it has a 'good' finish there wouldn't be a need to polish it as it only (not always, but from my experience) contacts the part on the end. Now core pins, or unscrewing elements inside the molded part may be a different story.
 
I am going to venture an educated guess and say it is probably a multi step vibratory tumbling process. I have made some pretty shiny parts that way.
 
If you are talking about the complete length of the pin minus the head, I doubt if they are draw polished. Most likely electropolished during mass production. No need for length wise polishing as there is at least .001" slip fit in the mold base. Draw polishing is normally used for core pins that core out plastic or metal and are polished in the direction of mold pull.

Paul
 
Hi All:
Just for curiosity's sake I grabbed a couple of DME and National pins I have in stock and checked them under the microscope.
The DME pins are centerless ground and the grind marks are clearly visible.
The National pins are old old stock, and they show centerless grind marks too, but they look to have been electropolished as Paul says.

Super premium pins such as are used for very tight clearance applications like molding nylon Santoprene or LIM rubbers may well be lapped; I don't have any around right now, but I know the Royals we used to use in a toolroom I worked in many moons ago were dead nuts accurate within a tenth over their whole length.

Even then, there were applications where we had to wire the holes, then lap them and then select the best fit pin for each hole and mark it to only run in that hole.
The running clearances were microns, and everything had to be just right or the ejection system would pick up and jam and it would be an expensive mess to fix.
LIM molds were the worst...fussy as hell because you had to be able to pull a vacuum in the cavity, but I digress...

Back to the OP's inquiry, if you are seeing evidence of draw polishing on the pins you're referencing, they could either be lapping marks or wear marks.
I've never looked at a high accuracy pin under the scope so I don't know.

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining
 
I stand corrected

implmex said:
Hi All:
Just for curiosity's sake I grabbed a couple of DME and National pins I have in stock and checked them under the microscope.
The DME pins are centerless ground and the grind marks are clearly visible.
The National pins are old old stock, and they show centerless grind marks too, but they look to have been electropolished as Paul says.

Super premium pins such as are used for very tight clearance applications like molding nylon Santoprene or LIM rubbers may well be lapped; I don't have any around right now, but I know the Royals we used to use in a toolroom I worked in many moons ago were dead nuts accurate within a tenth over their whole length.

Even then, there were applications where we had to wire the holes, then lap them and then select the best fit pin for each hole and mark it to only run in that hole.
The running clearances were microns, and everything had to be just right or the ejection system would pick up and jam and it would be an expensive mess to fix.
LIM molds were the worst...fussy as hell because you had to be able to pull a vacuum in the cavity, but I digress...

Back to the OP's inquiry, if you are seeing evidence of draw polishing on the pins you're referencing, they could either be lapping marks or wear marks.
I've never looked at a high accuracy pin under the scope so I don't know.

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining
Click to expand...
Marcus, thank you for taking the time to look with a microscope. I don't have a microscope, but I did dig up an old hi power eyeloop.Yes I could clearly see the cylindrical grinder marks.I have a handheld 1x 42 belt grinder that I use on the lathe. I will need to get super fine belts. Thanks to all that replied.Edwin Dirnbeck
 
many parts are lapped with a thin lapping compound and rotated and moved back and forth lengthwise then they are cleaned of lapping compound. glass lenses are called ground but its a type of lapping compound and methods of moving lenses when lapping can be complex. some laps are split and can be expended or collapsed to different sizes
.
be aware lapping compound out of the can is very thick and will not magically flow into tightly spaced clearances. it often needs to be thinned with thin oil or mineral spirits or a mix
 
If they have been nitrided, they are probably run thru the QPQ process. That is Quenched, polished, and quenched again in a liquide nitride. There's some heat involved, I'm not up to speed on it. Theres a place I used to use near me called Houston Unlimited, I don't know if they still operate under that name or not. There are others out there that provide this process.

Quench Polish Quench QPQ Hardening | Engineers Edge | www.engineersedge.com

Ken
 
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