CNC driven plastic injection machine

[aadams]

I have been toying with the idea to make a cnc machine that will inject or pour melted plastic into molds (small molds, cavities less than 1" wide by 10" long by 1" deep). I have several rough plans on the cnc machine, and really like the one posted on this site, however I do not know how the injection piece will work into the system.

I will typically be pouring 3 colors in each cavity in 3 passes (in controlled amounts along the path). I have looked at several stepper driven metering pumps that look as if they'll do the job but I am not sure how this will work as most controller boards seem to take up to 4 axis drivers. In this scenario the x and y axis will be guiding the pour nozzles, while I need 3 more controllable motors turning 3 seperate pumps.

I am fairly new to robotic automation, and am doing this as a hobby/learning project. Please advise as to any solutions you may have or guidance you may give. Is there a way to control 5-6 axis without spending 40k? or 1k for that matter! Also, if someone has done something similar I would love to hear about it.

Thanks,
-Abram

[dwwright]

You could add a second parallel port to your computer and hookup another controller for the additional motors. Most software will let you choose which parallel port each axis is connected to.

[aadams]

Thanks! Now on the subject of controllers, motors and drivers, what brand/model would you suggest for this application? The machine will be approx 2'x3' Gantry style (would like to if possible be able to convert to a mill to mill aluminum molds, but that isn't top priority). I am planing on using stepper motors for the pumps and probably for the xy as well. My goal is to spend as little as possible without sacrificing too much quality...I guess that is everyones goal...

And yet another question regarding the plastic dispensing piece. I plan on using the stepper driven pumps, but would it be more accurate to build a screw injection pump (keeping in mind the small cavity size)? Does anyone have suggestions on a heater for the lines that will carry the material (length approx 5')?

Thanks again!

[Evodyne]

Hi. Interesting project. What kind of temperature do you need to maintain on the line? There are different electrical heat tapes available, but what range of temperatures you can expect I don't know.
Another thought would be a steam trace-steam has a lot of heat-5 times more than water at 212 degrees! They use it a lot in industry to keep pipes from freezing in the cold. Just copper tubing tied along the length of the plastic tube.
The cool thing about using a screw pump is that it can be a simple on/off unit. Several of the CNC packages allow control of "extra" digital or bit outputs.
What about no pump at all? It occurred to me you would want to purge the lines between colors. How? Blow them out with air. Then it struck me that you could used compressed air to push your molten plastic through the lines.
Control would be via a two-way solenoid valve that would allow pressure to be applied when activated, then vent all pressure off when deactivated. Just some things to think about.

[aadams]

Thanks for the input! The material is to be maintained at 350F. I like the compressed air idea, but would this be easy to controll small predictable amounts? Most of the molds I use are open molds that the plastic is hand poured into in up to three passes, each pass consisting of a different color and amount (cavity size is typically the size of a ink pen). The method of pouring/injecting must pour slowly as to not splatter in the open faced molds.

Also, the plastic I use sets up really fast, within 30 seconds in a ink pen sized cavity it is already solid enough to remove, though not good as it is still hot enough to damage quality. I am looking at a heating device to keep the molds warm long enough to make 3 passes on 30 or so cavities, I may still have to keep it down to just a handfull at a time.

My thoughts are to have 3 stepper contolled pump, screw, air (or whatever will work on a budget) driven dispense nozzles, each pouring a different color. These could all be mounted on the would be "z axis" of the Gantry style CNC machine, and controlled via G-code (offsetting each by the distance of the nozzle(s) in front of it). Each pass would dispense different quantities of plastic, in possibly different locations of the mold.

I would have to purge the lines after the entire run is completed, which I am still scratching my head about, but I like your idea of compressed air to do that.

Originally Posted by Evodyne There are different electrical heat tapes available, but what range of temperatures you can expect I don't know. Another thought would be a steam trace-steam has a lot of heat-5 times more than water at 212 degrees!

The "hoppers" in this system will be similar to crockpots with temp controll and a line to the pump from the bottom and from the pump it would travel up to 5' (probably closer to 3') to it's final destination in the mold. Would I need something like the steam trace for keeping the plastic at 350F or would the heat tapes work (where to find electrical heat tapes?).

Thanks again for all your wisdom (and please keep it comin!)

[jimini]

Have you considered having your 3 pots of molten plastic stationary and moving your molds to each individual pot and triggering a valve.
Then move the next mold (on a X Y axis) into position and repeat, should be possible I would think. That way 3 axis would be plenty, X and Y to move into position and Z to trigger molten plastic, from a stationary position.
With this setup you would not need a moving gantry machine, but rather a fixed Z axis, and a moveable X and Y.
Just a thought......

[aadams]

Hi Jimini, thanks for the input!

This is an idea I have actually pondered, and am still considering. In fact my initial sketches were of a machine where the molds were on a rotary table (x axis) and the dispensing nozzle was on a y axis arm. How would I be able to control which of the three valves to open? The plastic I use at 350F is similar in consistency as corn syrup, so the idea of an open/close valve may work, but would it be possible to predictably pour measured amounts at different points in the toolpath?

I'm not absolutely convinced, but I think I will still need 5 axis controllers, one for each plastic source and two for the movement. The pots will remain stationary with a hose line from the pots to the dispensing nozzle.

Another hope of mine is to be able to "retool" this with a z axis mill spindle to mill its own molds. This is the main reason I've been leaning towards the Gantry style, not to mention the many resources, plans, and expertise this site and others provide on such a machine.

[onthemarc]

I'm with Jimini
Keep it simple, the fewer pumps, long hoses,stepper motors the better.
I say use a fixed gantry with Y axis moving your three hot pots right above your moving X table with solenoid valves on the bottom of your pots and gravity feed, easy to heat up easy to clean few things to go wrong. Now you just need 2 axis and three on off signals.

onthemarc

[ynneb]

If the process is to be exactly the same all the time, then CNC so to speak may not be the answer. You could use a PLC and normal motors that are just on and off devices.
You could fit either limit switches or beam break switches to the different parts of the machine. You can then program the machine to do whatever function you like once it hits each limit/beam switch.
This method would eliminate the need for a computer to drive it, and also expensive software,steppers,driver cards,etc.
While I have never used a PLC b4 I know what they can do and it sounds like this is more what you need.

EDIT: A Plc is basically a computer with lots of inputs to fit sensors to. Like switches etc. And then it has lots of output relays or electronic switches. You program it to turn on and off swithches when its sensor switches are activated. Does that make sence?

Have a read of this, and tell us if this is more what you need http://www.plcs.net/chapters/whatis1.htm

You mentioned less than a 1000 bucks. Well here is a PLC for 50 US bucks that has 6 inputs and 6 outputs. Possibly you will need more than 6 inputs, but this will give you an idea http://cgi.ebay.com/ws/eBayISAPI.dll...874231929&rd=1
You could use windscreen wiper motors from a car for your drives. You could build a machine for next to nothing. Just so long as it is the same routine all the time.

[Evodyne]

Your getting some good input-this is why I like this site so much. If we go off of the "less is more" principle, then I like the idea of moving the molds under a stationary spout. That's how they fill the torpedo cars with molten steel in the mills: a train moves the next car under a runner. This whole assembly is under the furnace, one level down. They basically poke a hole in the side of the furnace and the stuff runs down a runner, through a hole in the floor, and into the car below. Simple. Nothing to clog.
I'm pretty sure you would be hating life if your plastic hardened inside a pump or the lines-so do away with them. You could put solenoid valves at the bottom of your heated pots, that would give you the precise metering you are looking for. Get the right kind and the body will handle the heat just fine.
You could either use gravity feed or use air pressure (adjustable to adjust flow rate).
Finally, don't get confused by axis control: you only need that for speed/position control. You don't need it to turn a 12v or 24 v motor on or off-a single bit can handle that. Same for solenoid control: a single bit driving a transistor driving a relay and you can turn hundreds of amps on and off.
But, to keep things simple, an adjustable timer outside the PC would work just as well. And limit switches that detect a mold in place could trigger that.
It could be hardwired, but the other member that recommended a small PLC was right on track: think of them as small programmable logic blocks. They run almost everything in industry. There is a small learning curve to learn how to program them, but nothing major. Ladder logic (the most prevalant way to program PLCs) is fairly simple.

[aadams]

Great ideas! I will surely spend hours sifting through PLC related material!

Before I do though (actually while I do!), my plan is to be able to line the table with many molds for making 30 or so small objects in a single run. Same shape/size of molds for a single run but will need to change out with different types each run (even changing from open faced molds to two part molds). Also each run will require different amounts of each color plastic. Would this be doable with PLC without major reprogramming? I was also hoping to be able to store on disk all the code for the runs I am doing to easily setup for a new run, perhaps this is all up the PLC alley?

I do agree simpler is better. I will re-approach this project with the z axis stationary holding the three pots and solenoids to dispense as well as the possibility of using PLC.

You don't need it to turn a 12v or 24 v motor on or off-a single bit can handle that. Same for solenoid control: a single bit driving a transistor driving a relay and you can turn hundreds of amps on and off.

Please excuse my ignorance, but how do you send the on/off bit to the selenoids? Would this be controllable by means of G-Code?

Thank you everyone for being so helpful (and patient!) I am sure with your help I'll be up and running in no time!

[Evodyne]

Hi! O.K, as far as the bit thingy: it was somewhat of a generic statement. Depending on what software you would be using you might be able to control a few of the lines on the parallel port as on/off outputs. Specifically I was thinking of the enable outputs for drive control or perhaps a spindle on/off line. You could "steal" the spindle on/off line and use the G-code that controls it to run something else. I can't comment on what software, exactly, gives you the most options: I just don't know.
You stated you wanted to do a mill to make molds. I don't know if you have the room for two machines, but think about a dedicated mill/router set up just for moldmaking. Use the CNC stuff to run it and be happy.
A dedicated "casting" machine doesn't really need fancy axis control. Let me explain-at lunch time. I'm at work and this "goofing off" is going to me in trouble! More in a while...