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Jaakko Fagerlund
06-05-2012, 01:52 AM
At the moment I'm building a small hand operated plastic injection machine and it is going to be similar in design&size to what Dave Galomb sells at injectionmolder.net.

However, I was thinking of the nozzle design. Yes one can easily just drill a hole and be happy with it, but does anyone have any good ideas for a drip free nozzle? Haven't seen anything in erson, only know that in some molds I've done we used a nozzle that had a small pin in it that closed it, but don't know how that worked inside.

Was thinking of a ball and a spring first (like a check valve), but I'm sure the spring will die from the heat (250 Celsius) and/or the melted plastic solidifies around it somehow without enough heat from the band heaters...or does it?

Would like to hear what you have in mind :) After I'm done with the CAD I can send some pictures, the construction is dead simple and the body of the machine weighs only about 5 kg, made out of steel beams.

Wirecutter
06-05-2012, 12:58 PM
Jaakko -

I've operated a 30-ton injection molding machines for plastic, and I've used what are called "anti-drool" nozzles.

The nozzle is constructed as you suspected. There is a pin that closes the hole in the end of the nozzle, and the pin is held in place by a spring. The pressure of the molten plastic will overcome the spring pressure (easily) when the ram moves forward during a shot. The nozzle, of course, won't let anything through until all the plastic inside is completely liquid.

I believe the nozzle can be designed such that, although the spring is exposed to the molten plastic, it is not in the primary path. It's been a few years, and I don't recall if the nozzles I've used were designed like that. I don't think they were. Oh, and a spring made from the right metal can easily handle injection molding temperatures.

I used anti-drool nozzles when working with DuPont Rynite. The variant used had a melting temp over 500F, but it went from solid to "just like water" very quickly, so it would drool from the nozzle of the machine. We tried using anti-drool nozzles on the machine, but the turbulence caused by the nozzle made bubbles form in the part. In the end, I removed the anti-drool and used a regular nozzle, brought down the temperature of the ram's "screw chamber" (I forgot the correct name) as low as I could, then pre-heated the mold as much as I could with the available equipment. I was able to get about 25 shots per hour if memory serves. The results were very good, and the drooling wasn't bad at all. I just had to keep the machine moving, and break the solidified drool from the nozzle about every 12th shot. (If I didn't clean the drool from time to time, the nozzle wouldn't properly seat up to the mold and leaks would develop.)

-Mark

rowbare
06-05-2012, 01:35 PM
I found these on Google Patents. I am sure there are more:

These have expired
http://www.google.com/patents/US4299791
http://www.google.com/patents/US3934626
This one is recent
http://www.google.com/patents/US7824175

In the latter one the spring seems to sit in the molten plastic. I wonder what kind of spring material they use.

bob

Jaakko Fagerlund
06-05-2012, 05:02 PM
Okay, thanks for the information and the links to the patents, clarified some things and gave new ideas :)

Have to check the Lesj÷fors spring catalogue to see how much their high temp springs withstand, as simple ball & spring would be easy to make.

duckman
06-05-2012, 07:12 PM
When I worked in the mold building business we made some special molds for Sony, the molten plastic had a blowing agent in it so if pressure was not kept on the molten plastic it would expand, our injection nozzle had a 1/4 turn valve in it , air operated, mold would close, machine would start to inject plastic and the valve would open, after injection valve would close, then the neat part, there was a movable cavity which would with draw about 3/4" allowing the plastic to expand, so you end up with rigid plastic molded part with a foam filled core. Really neat and I got to deliver the 7,000# mold to Logan airport for shipment to Japan, got to Logan and they couldn't unload me it took 2 fork trucks and slings to lift it up enough for me to drive out from under it, they were then able to pick it up and load it in to a shipping container. Boy some of the things I remember.

Jaakko Fagerlund
06-06-2012, 03:29 AM
As soon as I get some mill time at work off-hours, I'll do a quick test injector like the Gingery injector design. Just need to go and buy some 10 mm round bar as I'm all out, buteverything else I have: 2 x 250 W cartridge heaters, screw mounted thermocouple, PID controller, a length of 50 mm round bar for the injection cylinder, length of 16 mm drawn round bar for the injection ram and a couple of screws to put it all together.

I already made two nozzles. Both are attached with an M20x1 fine thread, so I can swap them in testing. One nozzle has just a 3 mm hole in the end and the other has a small ball bearing and a spring I found to test out the "drip-free" idea.

Though, have to see how runny the plastics are that I have, PP, PET and PE and also some PA6 with added 30% GF.

Mcruff
06-06-2012, 09:32 AM
Our molding machines at work use a hydraulic shut off nozzle with a cylinder about the size of a small energy drink can. Most small machine nozzle tips run what we call torpedo tips in them, they are a small pointed tip with 3 holes drilled in them that allow plastic thru under pressure but when the pressure stops they freeze off just enough to hinder flow. This torpedo goes inside the actual nozzle body below the radius ram tip.

Jaakko Fagerlund
06-06-2012, 01:24 PM
Our molding machines at work use a hydraulic shut off nozzle with a cylinder about the size of a small energy drink can. Most small machine nozzle tips run what we call torpedo tips in them, they are a small pointed tip with 3 holes drilled in them that allow plastic thru under pressure but when the pressure stops they freeze off just enough to hinder flow. This torpedo goes inside the actual nozzle body below the radius ram tip.
Hah, have been cussing at those damn torpedoes at work with hot runner manifold systems used in molds, as the torpedoes are carbide and damn fragile and still needs quite a lot of torqueing on the nut to keep them in place. One piece of crud in there and you hear a faint "knack" inside :mad:

Seems that the mill is available tonight, after work it's time to make some parts :)