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jeremy13
10-26-2013, 12:27 PM
I finished up the machining of one of my injection molds. Now it’s time to cut the ejector pins to proper length. These bad boys are hard!! A file only polishes the surface and flattens out the file. So my plan is to cut them with an abrasive cut off wheel and grind to length. I thinking of making an aluminum block with a hole for the pin and clamping system and a key that will fit in my tool grinder guide slot. And just slide back and forth tell I get the right length. Anything wrong with this? On hind sight I won’t be so quick to order pins and wait tell the mold is finished. Then order the pins pre-cut to length.
http://i1224.photobucket.com/albums/ee374/Jeremy_Hanak/null_zps594d498f.jpg (http://s1224.photobucket.com/user/Jeremy_Hanak/media/null_zps594d498f.jpg.html)
Still needs some polishing

Jaakko Fagerlund
10-26-2013, 12:34 PM
Yup, ejector pins are nitrided so file and usual cutting tools don't work. Abrasive cut-off is done at work with a tool grinder that has a 1 mm thick cut-off wheel mounted. It has a V-block with a adjustable back stop. Normal procedure is to mark the approximate length with a marker on the pin, set it so the mark protrudes from the V-block, then cutoff with a little excess. Measure the pin lenght, calculate needed removal amount to get to proper length, dial it in and cut it. Done.

Edit to add: If we need a good surface and exact length (the cutoff wheel gets to +-0.05 mm length as the cutoff machine has some mechanical issues), we use our surface grinder to finish them in a V-block.

mc_n_g
10-26-2013, 06:21 PM
Assemble the mold with the ejectory pins and mount it in you ejection machine. Measure, and or mark with marker, the length of pin sticking out when at full ejection cycle. This may or may not vary depending upon your machine. Use cut off wheels to reduce ejector pins to a reasonable size.
Remove everything and grind the pins or a surface grinder or use you cut off wheel and check constantly.
You must grind them to your final dimension (length).
There are several pictures of ejector pin grinding fixtures on the web. They look like V block as described above.
Do not order precut length. You might not be right and then you will need to order them again.

Zero_Divide
10-26-2013, 06:37 PM
Here at work we just cut them off on a pedeststsl mounted cut-off disk.
Thsts it.

I actually turned them as well. Carbide tooling is the way to go.
Once you break through the skin they are pretty soft actually.

Jaakko Fagerlund
10-26-2013, 07:46 PM
I actually turned them as well. Carbide tooling is the way to go.
Once you break through the skin they are pretty soft actually.
The base material sure machines nicely, but the nitrided layer will chip off from the edge of the cut. This is not good if that said edge is part of the moulding cavity, as it will leave an unwanted nudge of plastic material to the part.

KiddZimaHater
10-26-2013, 08:08 PM
We had a snazzy fixture for grinding ejector pins.
It was a tall vee-block, standing up, about 6 inches tall, with recesses in it spaced 1/2 inches apart for the pin head to set in.
The pin was held by a sliding u-type clamp.
I loved that gadget.
It was similar to this picture. But without all the extra stuff.
-
http://www.rockford-industrial.com/Agricultural-/Rockford-/West-/Toolmaker-knee-with-adjustable-vblock-and-2-clamps-wow-provided_image-11.jpg

jeremy13
10-26-2013, 09:27 PM
Well looks like Iím on the right track. I have a chines tool grinder. A nice surface grinder is on the wish list. Kidd I like the fixture Iíll do something similar just laying down. Put in some elbow grease this evening.
http://i1224.photobucket.com/albums/ee374/Jeremy_Hanak/null_zps35bd1e33.jpg (http://s1224.photobucket.com/user/Jeremy_Hanak/media/null_zps35bd1e33.jpg.html)

jeremy13
10-26-2013, 09:55 PM
This is a nice one not $300 nice http://www.choicemold.com/pinvise.asp

WhatTheFlux!
10-26-2013, 10:38 PM
Perhaps some kind of shape-charge would work.

jeremy13
10-26-2013, 10:59 PM
I was thinking of a small donut shaped cutting charge. Like a liner shape charge just rolled up. But I would need an injection mold for that too.;)

jeremy13
10-28-2013, 08:30 PM
This is what I came up with for more precise grinding of the pin after I cut it with the angel grinder. It’s a little flexible out at the micrometer but will work just fine.
http://i1224.photobucket.com/albums/ee374/Jeremy_Hanak/null_zps3df5b476.jpg (http://s1224.photobucket.com/user/Jeremy_Hanak/media/null_zps3df5b476.jpg.html)

Jaakko Fagerlund
10-28-2013, 11:18 PM
Attaboy :) Looks good enough, knowing you are not doing day in day out operations with it.

mc_n_g
10-29-2013, 07:33 PM
Great adaptation! Hope it all works out for you.
Now all you need is for the die to fill, pack and eject properly on the first shot!
You are well on your way.

Grinding 'angels' is the devils work!!! LOL;)

boslab
10-29-2013, 07:49 PM
We had to section and cut hardened stuff quite often in the lab i used to work, including ejectors, ball bearings even carbide tips!, metallurgical cutting wheels are really cheap and handy, leave a good surface and with coolant dont do much to the heat treatment,
http://www.kemet-met.co.uk/products/cutting/aluminium-oxide-cut-off-wheel
Worth a look
Mark

jeremy13
10-30-2013, 11:07 AM
I have one more question how much room do I leave around the ejector plate holes that hold the ejector pin head? Right now I have them almost press fit .0000 clearance and there not lining up with the holes in the mold. Do I give myself a few thou clearance to allow the pins to float into correct aliment then bolt on the back plate of the ejection plate? Its hard to explain what I'm thinking of doing but ill try.

I'm going to take my B plate with the ejector holes in it and turn it upside down. Put some plastic spacers under it so the ejection pins will go all the way threw. Set the ejection pin plate on top of back of B mold. No springs or guides in place. Drop all the ejector pins in threw the ejector plate and mold plate. Set the ejector pin retainer plate on top of ejector plate and bolt in place. this should hold the pins in proper alignment with the B mold plate. Then raise the ejector plate add the springs and guide pins. I should be ready to inject.

The grinding fixture for the pins worked out perfect. I ruff cut the pins with a angel grinder and cut off wheel. Took the shortest one and ground the rest down to mach. Then ground them down in .005 increments. I made the pins .001 shorter than needed. This will be on the inside of the cap and wont be seen. I just hope the ejectors don't just push a hole threw the cap. A little extra plastic on top of the ejector wont hurt.

Jaakko Fagerlund
10-30-2013, 11:19 AM
I have one more question how much room do I leave around the ejector plate holes that hold the ejector pin head? Right now I have them almost press fit .0000 clearance and there not lining up with the holes in the mold. Do I give myself a few thou clearance to allow the pins to float into correct aliment then bolt on the back plate of the ejection plate? Its hard to explain what I'm thinking of doing but ill try.
0.5-1.0 mm bigger holes and 0.1-0.3 mm deeper pockets than the heads. General rule is that you don't use ejector pins as guides. If guidance is needed, then separate guides on the back side of the mold OR you put 4 10-20 mm ejector pins outside the mold area to act as guides and to force the ejection plate back when the mold is closed. These pins are usually ground 0.1-0.3 mm shorter than the "base length", the length from shear plane to the ejection plate.

jeremy13
10-30-2013, 11:54 AM
I cant thank Y'all enough for all the help. It's hard to get help from a part of industry that is some what elusive.

Jaakkoo I'll throw the ejector plate back in the CNC and enlarge the holes.

mc_n_g
10-30-2013, 12:08 PM
The key is that the back of the plate where teh ejector pins touch is at the same reference point all the time. The pins will get 'pushed' back slightly from the pressure of teh plastic. But you still need to retract the pins from the mold. I like to use slotted clamp-style to wrap around the pin head. These can be a little loose but must be secured with screws/bolts.
The best I can describe it is like a bolt slot on the side of a vise with the back cut to the depth of the pin head (plus a few thousands) and secured with screws to the ejection plate.
Another option is a sndwich plate with loose holes for the ejection pins to go through. The back half of the plate is cut/slotted to the depth of the ejection pin heads. These two plates are joined with screws and places in the mold. sometimes I use long shoulder bolds to attach this to the back of the mold. The shoulder bolts allow the plate to slide along a path and can be retracted to a set position. If needed springs can be mounted on the shoulder bolts to move the plate back.
You have to have some sort of mechanism or springs to retract the pins at the end of the operation to reset the pins. I know you know this but I don't know how your mold or machine is set up.

jeremy13
10-30-2013, 12:29 PM
The sandwich plate is what I'm doing along with the shoulder bolts and return springs on the shoulder bolts. I even put bronze bushings in the ejector plate for the shoulder bolts to ride on. This is a MUD mold, for my application I think this is the best for me.

Jaakko Fagerlund
10-30-2013, 01:00 PM
The sandwich plate is what I'm doing along with the shoulder bolts and return springs on the shoulder bolts. I even put bronze bushings in the ejector plate for the shoulder bolts to ride on. This is a MUD mold, for my application I think this is the best for me.
Sounds good :) Though I don't like springs as they are prone to break, but then again you are not banging these products out 24/7/365 I think. That is why I suggested the forced return pins that will push the ejection plate (the sandwich) in to the back position when the mold is closed. But ejection then requires either an ejection cylinder in the machine itself or some such scheme. If the machine doesn't have ejection cylinder, then the mold has to have springs to push the ejectors out.

jeremy13
10-30-2013, 03:22 PM
According to the PCS catalog chart I'm at 40% deflection on the springs witch is average life of the spring. I would think runs of 100 at a time is more like it as far as cycles one a week. My machine does have hydraulic ejector plate.

Jaakko Fagerlund
10-30-2013, 03:31 PM
According to the PCS catalog chart I'm at 40% deflection on the springs witch is average life of the spring. I would think runs of 100 at a time is more like it as far as cycles one a week. My machine does have hydraulic ejector plate.
Ah, no worries then :) The springs are cheap, would keep a set of extras on the shelf for JIC.