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

**JRR** · 12-04-2016

Is it at all possible to increase the capacity of the injection for the Rabit? The machine that I have (model 2/3) handles up to 2 grams of material which is not enough for my purposes. I didn't realize that the 2/3 only handled 2Grams as there is very little information on this machine online, and the original manufacturer appears to have moved on to new projects (and was bought out). Have to wonder if the 2/3 refers to the volume of the shot or the tonnage of the press. (Does the HY50 handle 50 grams or press to 50 tonnes?)

I have scanned the relatively complete manual that I got with the machine - which shows the mechanical operation of the machine along with schematics and designs for the molds - you can download it here...(6mb)

The beauty of this machine is the very short sprue (virtually non-existent in this design) and the fast cycle time (5 to 10 seconds) - it would be really nice if I could get the shot capacity up to 6 to 8 grams, but I suspect I am dreaming. If it is possible then the machine would be quite useful to my shop for reproducing small plastic parts in volume.

Nice other features of the machine are the various plastics it will handle right up to nylon.

Not much activity on injection molding here (hobbiest, and small shop), hope to see more!

**Tenaja** · 04-05-2016

I have two machines, one with a 10mm plunger, and a 15mm plunger. Increasing the shot size depends on the size of plunger you have. They came in three sizes, 10 (1.5cc), 12(2cc), and the largest at 15mm(2.5cc).

If you pull the barrel, the plunger slides right out. The plunger fits into a matching, removable sleeve (IIRC, roughly 75-100mm long), so they are the only two parts which require changing. You might even be able to fit in a set larger than 15mm, but your injection pressure will be reduced, and you will still have to mind the surface area so you don't blow through the parting line.

**JRR** · 05-05-2016

So, as far as you can tell the machine can't go over about 2.5 gram shot? That is unfortunate as I was really hoping to get a bigger shot out of the unit - looking for about 6 to 8 grams in fact. I have to wonder if there is any room for a redesign of the clamping or the injection pressure by either increasing the size of the air cylinder or making a latching clamp action on the mold so the injection pressure can be increased. As you say, if the injection pressure rises the liquid plastic may weep out at the mold lines otherwise.
I have a photocopy of a photocopy of the original manual for the CPU controlled version of the 2/3 (email me if you want a PDF copy - or can I put it up on this site somewhere?) and while it talks about how to remove the injection chamber I can't get the pin out from the underside of the securing part that is accessed by removing a 10mm bolt from the side of the frame just below the hopper. The instructions say to drive out the pin using a 4mm rod inserted into the hole, but I whacked that a couple of times and the pin did not move and I don't really like the idea of hitting it harder until I am sure that is the correct spot to be pounding.I will put up some photos later today of the parts I am speaking of.

**Tenaja** · 05-05-2016

Using standard components, yes, 2.5gm is the max. You could stretch it to 3gm, maybe more, with a custom plunger and sleeve. I think 6-8 might be expecting too much. The real limit will be the clamping pressure vs your part surface area. If you install a custom PLC (heck, a raspberry pi would do nicely), you can "double-shot" your parts to increase your volume, but again, the limit will be the surface area.

So, what is the surface area of your part? The rule of thumb is around 3 tons per square inch, depending on the resin you may require more (up to 5) or less (2-2.5 tons). Since a new machine is limited to 5 tons, you can't really make a part much larger than around 1sq". I have made a 1" square with 1/8" radii in the corners successfully, and I have also managed to get it to blow the mold apart slightly. Maybe I did not have enough preload on my mold (see below), but that gives you an idea of max size.

The clamp already has a toggle action, with leverage, etc. It's a pretty complicated system, with quite a few parts considering the size of the machine. It's because the one air cylinder both closes the mold and also injects the plastic, and it does it in a mechanical sequence. There is no physical way to push material without closing the mold. As far as increasing the clamping tonnage, the levers in the linkage will be one limiting factor (like a fuse), and the tie rods will be another limit. The way it's designed, with full clamp force, the tie rods already stretch .018". If you want to double that, you'd be stretching almost a full millimeter, and I doubt it will last long like that.

Moving on, the rod holding in the barrel has a screw in one side; you have to loosen and remove the screw. It's like a pinch bolt, IIRC, so the screw pulls the rod (which has a flat taper on one side of it) into the bottom of the barrel, thereby pushing the barrel up. Once you loosen the screw, you can push on it on the screw side (maybe tap the screw after one turn loose), and the tapered rod will pop out the other side.

I already have a manual, and I also downloaded a PDF from this forum; it had a few bits of information that mine did not. If your version has more information, then I'd appreciate it, otherwise I'm set.

I have a sales brochure, and that lists the specs of the plungers.

**JRR** · 06-05-2016

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.

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

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?

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.

Thanks again for your advice!

**Tenaja** · 06-05-2016

Originally Posted by JRR

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** · 06-05-2016

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** · 06-05-2016

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** · 10-05-2016

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** · 11-05-2016

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.