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Injection molding: what am I doing wrong?

  • This topic has 15 replies, 3 voices, and was last updated 2 years ago by Stan.
Karen Danenhauer karenrpcv

Injection molding: what am I doing wrong?

08/12/2019 at 23:43

I’m a novice at injecting; help!

I’m so stoked to have gotten an injection machine and mold.
I have been trialing using shredded polypropylene, temp set at 290 upper panel, 295 lower panel (though the thermometer reads 450 and 300).  I let the plastic sit in the machine x 12 minutes.  Creations still all look quite lumpy.  Does anyone else have this problem?  What’s the solution?

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09/12/2019 at 00:43

If you are following the guides in the download kit https://preciousplastic.com/en/videos/download.html  , the recommended starting temperatures for PP are in the 190-210 C range. Your temperatures (if they are in C) are way off to the point of potentially generating some nasty fumes. Be careful. I don’t understand some of your temperature terminology. If you have a typical setup with two temperature controllers (barrel and nozzle) you should have a set value and an actual value displayed for each location. If your numbers are in C, I would not do any more until you get the temperature control sorted out.
Post some pictures of how your thermocouples are mounted and the PID display.

13/12/2019 at 22:34

Thank you so much for your helpful response!  I had not seen those instructions on the download, and will follow them to a T next time I give it a go.  I think the “lumpiness” I described is probably the “sinking” from plastic being too hot.   Please see the attached photos of my PID display and mounted thermocouples.

I’m still concerned though that the set temperature on the barrel differs so much from the temperature reading (as seen in the photo).  And quite often as I’ve injected into the mould, the last bits of plastic that come out are not even melted.

13/12/2019 at 22:53

I can’t see how the thermocouples are mounted in those pictures but I would not run more plastic until the temperature control is working well. Can you take a picture of the two thermocouples as they are mounted on the tube?

13/12/2019 at 23:07

Sorry, I’m not exactly sure what part the thermocouple is…Do you see what you need in these photos?

13/12/2019 at 23:48

The thermocouples are the two thin wires with the stainless braid on the outside. Looks like they are mounted by being clamped under the heater bands. I think that’s the way the Precious Plastic design recommends but there can be issues. Without breaking the thermocouple wires, do they feel like they are clamped snugly (gentle wiggle does not move them)?

When you first turn on the PID controllers, do they show the right temperature (room temperature)? Does the upper one then shoot up quickly past the set point?

Looks like in your system the top three heaters are wired to the PID/SSR and the bottom two (near the nozzle) to the bottom PID/SSR, is that right?

Sounds like you may not have built the system, Is the builder local to you?

14/12/2019 at 14:50

The thermocouples don’t look very well fitted just clamped under the bandheaters like that. They are being heated well past the set point probably well before the barrel gets up to temperature leading to very erratic feedback and control.

14/12/2019 at 19:09

I don’t know why PP chose to clamp the thermocouple under the heaters. You really don’t want to measure the heater temperature, you want to measure the pipe. Adding a separate hose clamp to mount the TCs would have been an easy alternative. This is especially true of the typical low PID/SSR/TC kit that comes with a threaded mount for the TC tip. Instead of removing the threaded tip and chancing breaking the TC bead, just use a hose clamp to secure a threaded block or even just a nut to the tube and then screw the TC in until the tip is pressed against the tube. Easy and robust.

Getting back to @karenrpcv , If the builder is not around to fix this, and you are on your own, here are some simple things you can try.

First a question: From your photos (especially the second one) it looks like on some of your heaters the two ends of the cylindrical portion meet where the clamping bolt is. There should be a gap, otherwise the heaters are oversized and don’t clamp the pipe fully/tightly. Is that true? If it is, that would help explain why the thermocouple is not clamped properly. One way to fix this could be to cut up some aluminum cans into strips the same width as the heater and a little shorter than the perimeter of the tube. Add one or two of these to the tube as shims until you can tighten the heater without the two ends of the split cylinder touching when you tighten the bolt.

To solve the thermocouple issue, I would use a metal hose clamp to clamp the thermocouple bead to the tube between the heaters. I would use a couple of small strips of aluminum can between the hose clamp and the TC bead so the bead does not get pushed into the typical gaps in the hose clamp. Tighten the hose clamp but you don’t need to crush it. Also I would use a bare wire (otherwise the insulation may smoke) to secure the stainless wire near the TC so accidental tugs on the wire don’t pull on the TC bead. I would do this for both of the thermocouples.

With the thermal control working properly, you should not need to have the set temperatures above the 190-210C range suggested in the manual.

Also, if you can, during use, move the injection machine outside. You can not see the plastic fumes but they are there, even at 200C.

Hopefully with these minor issues out of the way, you can enjoy your new machine and make may of those round …whatever they are.. parts

Good luck and let us know how it worked out.

14/12/2019 at 22:06

Thank you for your clear advice!  I got the machine from out of the country and I’m going at this alone for the moment, so I appreciate your use of layman’s terms!

You are right that the thermocouples are loose; I will get a couple of hose clamps and secure those as you describe.

There are no gaps by the clamping bolts in the heaters, but they do feel tight, with only a tiny bit give – do you still think I need to add the shims?

I have one final question – the bottom heating ring has a tendency to turn along with the bolt or mold when I screw /unscrew it.  There has also been some plastic that has come out from somewhere I cannot see and is now stuck under that heater (see photo).  I’ve still been able to use it (before you advised to pause and get this fixed), but it doesn’t seem like a good thing.  Do you have an easy fix for that?

And the round things I’m making are soap dishes; a couple have turned out alright 🙂

14/12/2019 at 23:48

As you tighten the bolt, the heaters should be compressed against the tube before the you lose the gap in the cylinder portion. It should be tight enough that you can’t twist the heater on the tube with your hand.

I can’t see what holds the bottom heater on. Is it just the spring of the band? I’m guessing that what is turning is the bottom pipe fitting, it looks like you have some plastic oozing from that joint. If the heater just slips off, take it off and you can take a couple of pipe wrenches and tighten the fitting to the pipe. Hopefully the outside of that pipe fitting is a nice round cylinder for the heater to sit on without lumps or ledges to form gaps. There should not be any plastic between the pipe fitting and the heater. If that bottom heater is a loose fit without a clamping bolt you may need to add something to compress it. I would mount the bottom thermocouple on the tube just above that pipe fitting/bottom heater.

Yes, I figured that round thing had a better looking side. Note, depending on the plastic, it can take longer than 12 min for the plastic to fully melt in the tube. I would give it more time instead of raising the temperature. It also helps to cap or block off the nozzle while the plastic is melting and compress/compact the plastic a few times. It also helps to maintain pressure for a while while the plastic is starting to solidify in the mold.

20/12/2019 at 05:58

Thanks for all the tips!  I was just going to add the shims under the heating bands, and notice there is a wire mesh between the bands and the barrel (see photo).  Does it matter if the shims (of aluminum cans) are on the inside or outside of the mesh?


20/12/2019 at 09:53

Wow, I didn’t expect to see that. That’s acting like some insulation between the heater and the tube. I assume it was put there just to mechanically fill the gap that still existed with the heater closed as far as it will go. Do you have a way (caliper) to measure the diameter of the tube? Those layers of mesh look like they add up to close to a mm thick.

I would take the mesh out of there, I assume it is not connected to the heater. You will need something thicker than soda can to fill that gap. Ideally you would fill it with one layer but it will depend on what you have on hand and what you can shape around the pipe.  Some options might be aluminum from pie tins, hardware stores have sheets of aluminum (roof flashing?), brass, or copper in various thickness. Another option may be if you find a piece of thin walled copper pipe that is close to the diameter of the tube, you can cut a slit so it is C-shaped and compress it under the heater.

If this mesh was where the thermocouple was located, I am not surprised by the bad readings.

Good luck, sorry that you have to do all this rework.

02/02/2020 at 22:36

I made the necessary re-work!  And before I left for a holiday last month, I made a couple of soap dishes, more or less successfully!
Now, first day back, the fuse keeps tripping.  I only have the injection machine plugged into that circuit, and no other fuses in the house are tripping.  This has not happened before.  Does this signify the something has changed/gone wrong with the wiring while the machine has sat for a month?  Do I need to call in an electrician for this?
Frustrated but still determined and grateful for your advice!

03/02/2020 at 04:18

Sorry to hear you are having problems. There are some things you can check and do before you call an electrician. Every time the system goes hot and cold, there is stress and a chance for something to fail. Are there any sounds? (sparks?) when the circuit breaker trips?
Here is what I would do, stop if you are uncomfortable with any of these.
First, I would open up the electronics enclosure and do a careful inspection of all the components and wires/connections.  Is anything burned, melted, smells burnt, loose wire, etc.
I would disconnect the heater wires from the SSR modules and from the line connection. If you have a multimeter, measuring the resistance of each of the two heater circuits and also resistance to ground should tell you if the heaters and wiring are OK.
If the heaters are OK, I would work my way backwards, isolate each of the controllers and see if they power up without tripping the breaker.
It is frustrating but there are only a few components and they are relatively cheap to replace if they happen to be bad.

Good luck

03/02/2020 at 06:04

I looked more closely at the electronics in the box, and there was a connection that was very tenuous.  But I fixed it, and have had some success, finally.
Thank you for all your help; I am very grateful!  I will attach a photo of my best soap dish of the day.

03/02/2020 at 19:02

Happy to hear you are back in production.
Great colors in that soapdish. It is funny, when we are buying stuff like liquid laundry detergent, I am conscious of the color of the container and how I could use it.

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