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View Full Version : See, this is why SERVO MOTORS trump Hydraulics.



Liger Zero
06-23-2010, 05:09 PM
Working with my friend who has the rubber molding machine.

I'm fine-tuning his CNC deflasher when all of a sudden the machine starts making this horrific sound and hot sticky EPDM comes gushing out of the mold, the PARTING LINE of the mold!

Turns out the clamp-cylinder lost pressure as it was injecting, forcing the mold open. Some forensic engineering determined that one of the check-valves in the Clamp Drive Signal Stack on the manifold failed.

Not cracked... not stuck... but a small rubber o-ring failed, looked like it had been chewed.


...less than three cents worth of seal caused four hours of shutdown, wasted $70 worth of custom-blend rubber and that doesn't count the cost of the part... turns out you "can't" just get the seal you have to replace the whole valve! (we'll see about that I have connections at an o-ring supply house.)



Me... I'll stick with my ELECTRIC SERVO driven mechanical-toggle clamp molding press. Can't part that mold once the toggle locks... no sir. :) On the other hand when the servo dies, it's going to set me back about $3,500. :D

uncle pete
06-23-2010, 05:19 PM
L.Z.
LOL, While I'd agree with your assesment I run one piece of equipment at work that has 2 V-16 Cummins engines in it that do nothing but run hydraulic pumps @ 5000 psi and rad cooling fans. Just can't see servos being quite up to the task.

Pete

Liger Zero
06-23-2010, 05:23 PM
L.Z.
LOL, While I'd agree with your assesment I run one piece of equipment at work that has 2 V-16 Cummins engines in it that do nothing but run hydraulic pumps @ 5000 psi and rad cooling fans. Just can't see servos being quite up to the task.

Pete

Indeed. For high-pressure applications hydraulics rock. It just astounds me that a tiny little seal caused a 600-ton clamp to fail like that.

...says the man who replaces fuses in his CNC with automotive fuses. :D

Black_Moons
06-23-2010, 05:24 PM
Why on earth would you need something as complex as a servo to close a mold?

Could'nt it be as simple as an off the shelf worm drive motor that turns a shaft or nut to close the mold? maybe several of them for distrabuted clamping?

Liger Zero
06-23-2010, 05:37 PM
It is a Fanuc servo motor attached to a Fanuc control. It gives astounding repeatablity and speed control when you deal with multi-plate molds.

Servo drives a screw which actuates the toggle, that moves the platen.

Also have a servo for the eject function.

Servo driven inject... the servo drives the plate which pushes the screw forward. Again, astounding repeatability compared to the "other way."

Is it worth it? Yes. Almost all new molding machines are servo driven. No oil circulating during "dead times" between cycles = massive energy savings, and I mentioned the repeatability.

uncle pete
06-23-2010, 05:44 PM
L/Z,
The machine I mentioned before, (a hydraulic mining shovel) Shut down on me due to a 2" long piece of rubber coolant tubeing. About $1300 an hr. to operate this machine. 2 HRS. to replace $1.50 worth of hose. So I can definately sympathise with a dirt cheap part costing huge $$$ due to a simple failure. In your case the servos would be far more durable and a lot less failure prone. Plus no huge clean up when a hydraulic system decides to puke it's guts out.

Pete

TGTool
06-23-2010, 06:02 PM
Well, lots of ways to skin the proverbial cat. I've seen molding presses that had a hydraulic cylinder operate a toggle link so when it was closed it couldn't back up. Horses for courses.

And in terms of control, there was an NC machine way back in the days run with hydraulics (no lead screw) so it's also capable of precision movement if designed for it.

Good luck with the repair. I really hate the "you'll have to buy the whole assembly" business.

Liger Zero
06-23-2010, 06:26 PM
Well, lots of ways to skin the proverbial cat. I've seen molding presses that had a hydraulic cylinder operate a toggle link so when it was closed it couldn't back up. Horses for courses.

That's what I learned on.

The whole "giant ram driving a platen" thing I don't "get" as there is too much float... you need epic-power/pressure to hold it closed... and some of the hydromechancial setups that Battenfeld and others have designed over the years... only a repair-parts salesman could love them.

amateur
06-23-2010, 07:43 PM
How large is the piece being molded ?

Rustybolt
06-23-2010, 07:44 PM
And in terms of control, there was an NC machine way back in the days run with hydraulics (no lead screw) so it's also capable of precision movement if designed for it.

Moog Hydropint.Proportional hydraulics if I remember right. Very rugged. Point to point only. .001 repeatability.

Liger Zero
06-23-2010, 08:06 PM
How large is the piece being molded ?

Roughly 14 inches long, there are six of them. 600 tons is "just" enough clamp for this job, if it were my choice I would run it on a 800 ton or a 1000 ton press. :rolleyes: Not my invention, not my thought process, not my money.

Of course I wish I could afford a press that big. So easy to spend hypothetical money. :D

wierdscience
06-23-2010, 08:18 PM
Did you guys see an evidence of where the damage happened to the O-ring?

If there aren't any burrs or roll marks to blame look for a seal sucking air on the return stoke somewhere.On hydraulic systems over 3,000 psi disfused air can percipitate out and cut O-rings like cheese,looks just like a Rat has been chewing on them.

The local plastic compounding plant had a screen changer seal fail and let a bunch (1500lbs) of the hot sticky stuff ooze out on the press frame,control conduits,the bar grating deck,through the deck and onto the service panel below.Once it cooled it was quite a powertool driven chore to remove all the spillage:D

Liger Zero
06-23-2010, 08:31 PM
Did you guys see an evidence of where the damage happened to the O-ring?

If there aren't any burrs or roll marks to blame look for a seal sucking air on the return stoke somewhere.On hydraulic systems over 3,000 psi disfused air can percipitate out and cut O-rings like cheese,looks just like a Rat has been chewing on them.




He believes it was "installed wrong" by his previous Helper. I checked the inside of the bore where it goes with a scope that I have, it looked fine.

There are two sharp points inside where the ports were milled. I've encountered these before in other manifolds... it is possible that the other Helper installed it in such a way the seal got nicked.

ON THE OTHER HAND... the press does dribble and drip from certain places it is possible there is air in the system.



The local plastic compounding plant had a screen changer seal fail and let a bunch (1500lbs) of the hot sticky stuff ooze out on the press frame,control conduits,the bar grating deck,through the deck and onto the service panel below.Once it cooled it was quite a powertool driven chore to remove all the spillage:D


Nozzle on my press retracts so I can put different tips/extensions/accessories on it. Also for "sprue break mode" and for purging. All the way forward it presses against the sprue bushing.

When I got it, it was driven by an electric motor. Vibration would cause something to change position, relaxing the seal against the bushing causing plastic to leak during injection.

Left it in Auto spitting out parts that dropped into a bin, came back had a GIANT PLASTIC BOOGER made of "cold" nylon.

Had to take the recip-saw to it, lost the first two heater bands. Took the gear box and drive motor apart, replace the worn gears and I'm good to go.