Increasing the capacity of a Rabit 2/3 injection molding machine?

[Tenaja]

Thanks for the detailed help. I believe you are suggesting that it would be more effective to find a machine with a larger capacity than my little 2/3, keeping it for the smaller jobs. I do love tinkering though and 'fixing' the machine so it does a bit more could be a fun activity.

Trying to squeeze two dozen clowns into a Smart ForTwo might be fun, and it might be possible, but the car will not get far. Even if you can get it to drive, the first bump you hit will collapse the suspension. Similarly, you might be able to increase the capacity of your Rabit to 4x its design rating, but it was not designed for greater forces, so it won't last long.

Is the maximum surface area one square inch or one cubic inch? Slight difference...

Think about the "units" of pressure the machine is rated for... pounds per square inch. The injection pressure with a 10mm plunger is roughly 20,000 psi. On a part that has one square inch, that is 10 tons. The force keeping the mold closed is 5 tons. Not going to happen. A larger plunger has less force, so the 15mm plunger only puts out 8520 psi, which is about 4.3 tons... enough for 1 square inch, if everything is absolutely perfect. A larger plunger will have even less force.

And when I say 1 square inch, that is "projected area," or the area at your parting line. That is the amount of area where the plastic tries to press your mold halves apart. Holes in the part are subtracted from the total area, since there are no forces pushing the mold apart where there is no plastic. On the part, walls that are perpendicular to the parting line do not try to push the mold open, they try to push the side of the mold out, so they don't really count when sizing the clamping force.

As an example, one part I am looking at making is roughly 2" X 3/8" and is hollowed out, and has a small post. Weight is around 4gm (my postage meter doesn't handle small weights well). Made of Nylon. Are you suggesting that this can be done using two shots? How would one do that?

If you replace the controller, you can set it up so the plunger goes in to its max and immediately withdraws and plunges again. It must be fast, or the first shot will freeze off before the second gets there. It will not be an easy setup. You will need to use the Mold Closed sensor to detect when the plunger is withdrawn, and immediately turn around. It won't work with materials that freeze quickly. With soft sticky rubber pellets I had to use an air cylinder to pack the barrel (they were making a bridge, and would not fall)...you might have to do this so you are not waiting on gravity to fill it between shots.

Just remember, this trick will not increase your clamping force. It will only fill up a deep cavity that has an acceptably sized area at the parting line.

I did try tapping the 10mm bolt once loosened but no joy. Perhaps it needs to be heated up to 200C first - that suggestion was in the manual too.

Where is the FTP archive here? I'd be interested in seeing what I may have that may be missing.

After much googling to find the pesky link, I realized I downloaded it from your link!

[JRR]

Thanks for sending me your file, I have now edited my copy incorporating your additional pages and some duplicate pages that have some good handwritten notes. If anyone wants a copy please feel free to download it from here.

[Tenaja]

Considering the higher quality of the scans of my manual, you might consider migrating your extra pages into my file, instead...

The schematic on pg 47 with the timing diagram on pg 48 is something I added. When I got my machine, it had two timers on it that were supposed to cycle the ejector pin, but they did not work. I replaced them, and also added that circuit so the ejector pin would not run continuously when the machine stopped.

[JRR]

It turns out my machine is perhaps ten years older than the one covered in the manual. Most of the mechanical functions are identical, however the controller for my vintage machine is a digital logic (TTL logic - date about 1982/1983) based system, whereas the manual describes a machine that uses a 6809 CPU and a LCD type of display. My machine has simpler LED displays that show just numeric values (Temperature, number of cycles, etc.).
A quick question for those of you with CPU controlled machines - have you backed up the EPROM? The EPROM is a 2764 and can be easily read and stored or copied as a backup - highly recommended!!!
I would like to find the manual on the earlier machine like what I have, but I suspect this is a bit of a wild goose chase. My serial number is P300 by the way - I wonder if that is for Prototype? That may be wishful thinking though on my part.
My next project will be to get this controlled by an Arduino like device so the parameters can be set more easily. The suggestion was raised that I use more than one injection stroke in order to make products of greater than 2 grams, this may not be too difficult to accomplish using the Arduino or other controller.
I do see that the schematics show a company named CIRRUS REYNOLDS LTD. drew up the schematics, and perhaps they designed the electronics. I will see if they still exist (don't seem to) and have records going back that far. I would prefer to interface my controller to the existing electronics rather than make everything more from scratch. Of course I first will test the device using the controller I have to see if it performs correctly and learn how to use it. Should be a bit of fun!

[Tenaja]

If you are going to update, I would recommend a Raspberry Pi over the Arduino. More flexibility, fewer restrictions, and in the grand scheme of things, the same price.

Does your control look like this?
https://www.youtube.com/watch?v=BpyDre_IuVo

The machine is simple enough, I don't really see much benefit of my 6809 controller vs what I saw in that video. They just took two timers (mold closed time & mold open time), and a temperature control, and replaced them with a cpu.

The biggest benefit to going with a home-brew or PLC control is an easier implementation of the ejector time-out timer setup that I tacked on. In your case, you have a compelling reason to try...you might be able to get two shots to work. No guarantees, though. Unless you are doing it for entertainment, you are probably better off finding a larger machine.

What volumes are you trying to run? I have a few small machines that have much more capacity than the Rabit.

[JRR]

I've played with the Arduino and understand it, although I did buy a Raspberry Pi to try out and may as well try that. I was indeed looking for more control over the press cycles, in the hopes of making parts larger than the 2 gm ones the machine can currently run off.

I am actually only looking to do runs of a few hundred parts at most for the unit. This machine is overkill for that sort of thing, but the runs are probably too much for hand operated machines.

That video IS of my machine (same blemishes on the rods that show about 55 seconds in), although it was sold to the person I bought it from and they kept the water cooling machine.

What other machines are out there that may be more suited for parts up to about five square inches with runs of only a few hundred? I really bought the Rabit on a whim, so while I am interested in using it, I am also open to considering other options - like selling it to buy machines closer to what I need.

[Tenaja]

For just a few hundred, something like the Morgan Press is well suited. Sure, you have to stand there and pull parts, but setup time is so much faster, and the molds are super simple.