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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.
What wattage is each of the 3 bandheaters? Are you using a SSR or relay output? You may be trying to draw too much current and have burnt something out. Note that the cheapo SSRs can’t cope with the current they are supposedly rated for, so play it safe. If you swap the connections does the PID for the 3x heaters work on the single one?
Thanks @s2019 Stan,
Ah, yes! That was actually a different machine. I probably didn’t explain it clearly, here’s a picture. There isn’t a sleeve inside the barrel and the bore of the cylinder doesn’t change. There is a plug/bushing in the end of the barrel that can have different sized ID’s for different diameter pistons. The piston doesn’t have to match the bore of the cylinder at all, it just changes the volume inside the cylinder. This was a two stage machine and the plastic was introduced (by a screw) into this cylinder through a valve, and then injected by the piston into the mould. I used this method to vary the pressure/volume as I was using a pneumatic cylinder to push the piston and had a limited range of force available.
Good advice from @friedrich but there’s also a third way you can reduce the sinking and it’s quite easy. You need what’s called a ‘chill’
Basically you need to make your bottom plate much thicker, probably around 50mm+, and thicker than any of the other walls of the mould. This thick plate will cool the face of the part and solidify it first while the rest of the part is still molten. The shrinkage will still occur but most of it will happen in other areas such as the curved sides where it will not be noticeable. Also make sure to apply pressure for some time after the mould has filled until the part has had time to form a solid skin.
There are two commonly used ways to do this much more easily. Compressed air and extractor pins.
Works better with injection moulds than compression. Drill a small hole through the core (part which forms the inside of the pot) from the bottom all the way to the top. If you can taper the hole or at least the start of it (taper reamer), wide end at the bottom, this will help. Blow compressed air into the top of the hole and the part should pop off easily. Ideally you want to prevent the plastic going up the hole as much as possible which is why it should be small and works better with injection as the plastic cools and solidifies before it goes to far.
Again you have holes drilled through the core and tightly fitting pins which are flush with the bottom surface. Push the pins out to push the part off. An easy low-tech way to do this is to use bolts with the ends turned down and thread the top part of the hole. Just screw in the bolt to push the part off. Works great with compression or injection, I use this method a lot. In this video you can see the heads of the extractor bolts (cap screws) at the top of the core, I found usually I didn’t need to use them as the mould is quite smooth and the part comes off easily, so I didn’t show this in the video.
To avoid the uneven top edge/flash on the parting line, what I do is weigh the amount of plastic going into the mould so I can get the volume exactly right every time.
It’s a UCFL204 bearing. Should be available just about anywhere and they are cheap. As for steel if using stainless you probably want AISI 304 also known as A2, other grades are more expensive and no better for this application.
I suspect the broken hinge is preventing an interlock switch from closing properly, even if the switch is nowhere near the hinge (check the opposite side of the panel).
Oldham couplers are great for high speed low torque applications where there is quite a lot of misalignment. For the shredder look for the jaw/spider type, these are available in bigger sizes and can handle more torque. The duplex chain couplings are also good for this sort of thing.
That’s an interesting idea. I particularly like the reuse of the bottle’s threads for securing the blades (of course there would have to be some way of adjusting it for the correct pitch). Though I’m not sure PET is the best material for this, from the limited experiments I’ve done with it, it can become very brittle and I think good results might need more sophisticated equipment and precise control of temperature, humidity etc. Though I’ve only tired injection moulding, not blow-moulding.
Another way might be to make an injection or compression mould which would enable use of easier to work with plastic, such as HDPE or PP.
I think the weakest point of the Hubs design is the screws in the end grain of the wood. This is generally regarded as bad practice by woodworking types, screws do not hold well in end grain and are more likely to split the wood.
I like the slot with the cross drilled bolt design much better, you can see the plastic failed before the wood, it also aligns the beams nicely which the Hubs do not. I suppose you could adapt the ball part of the Hub and put a square socket or a flat tongue on the end of it instead of using a screw.
I think what thegreenengineers means is that once the point of the tooth has penetrated the plastic (assuming it has enough positive rake) then the cutting action only takes pace along the two edges of the blade, so its width is irrelevant.
I’m not convinced this is a good way to calculate the power requirements though, it’s not like a punch that would be perpendicular to the material, the cross section varies throughout the cut and also according to the shape of the tooth and thickness of the plastic.
First off, the green with yellow stripe wire is a safety earth connection, it should be earthed.
In a split phase motor you have two coils, these are between connections 1-4 and 2-5. It looks like 1-4 is the main winding and 2-5 the start winding (there should be a capacitor in series with this winding, may be hidden inside the case).
Note that I’m not familiar with US wiring, but it looks like you should connect what I believe is called the ‘hot’ wire to pin 1 and the neutral to pin 4. Then if you also connect 2 to 1 and 5 to 4 the motor will spin in one direction, 5 to 1 and 2 to 4 it will spin in the other direction. (The connections are left permanently in place).
BE VERY CAREFUL WITH LINE POWER! Note the warning on the motor and insulate/cover the terminals so you can’t touch them.
Thanks for this, never heard of ‘Fordite’ before.
“6. I also teach them that bottle caps wth hinges are always PP because it is good at repetitive bending.”
This is called a ‘living hinge’ and while PP is usually regarded as the best material for this, it’s no guarantee the item is PP. HDPE is often used for living hinges, (particularly in larger items like blow molded tool cases), even PET can be used.
Where does it actually say 5HP?
As S2019 said, 15 Amps (“Full Load”) at 120v is not 5HP, actually less than 2.5 and that will be electrical input power, the mechanical output is probably around 2HP, but at a 50:1 ratio looks good enough (the calculator does take into account the mechanical inefficiency of the motor/gearing).
Arena at Glastonbury to be made entirely from recycled plastic
No Problem, thanks for bumping this thread, the calculator is quite buried in the forum and a lot of people might not have seen it.
The tapered square crank spindle on a bicycle is hardened steel (just try cutting it with a hacksaw), and it needs to be to withstand the torque. The shredder shaft (mild or stainless steel) won’t last long if reduced to these dimensions as you have found out. The crank arm attached to the large sprocket is soft steel or even aluminium, better to bore/drill this out to match the shredder shaft, easy if you have access to a lathe, but could be done with hand tools.
Probably HDPE, as it’s most common and easy to use.
HDPE (2) and LDPE (4) are both types of polyethylene. There is no symbol for just PE.
Wow! That looks incredibly thorough! Must have taken a long time. Did you grind them with a kitchen blender?
It looks like coffee grounds and pine cones/needles worked well, but what products would these be suitable for actually making? Do they have any residual smell of the original material?
Looking forward to hearing more about the mould and how you were able to apply pressure in a controlled way.
PET (Polyethylene terephthalate) is not the same as PE and it’s properties are quite different (harder to work with).
HDPE is type 2 and is common in household bottles, containers and their lids.
LDPE is type 4 and is often used for thin plastic sheets such as carrier bags and wrapping.
Both LDPE and HDPE are Polyethylene, just different size/shape molecular chains. (there is also LLDPE, MDPE and UHMWPE, all are different types of PE).
Below are some pictures of the clamping unit from my 2nd machine, you can see a video of it in operation here
I prefer the over-centre toggle method to a screw, it’s what nearly all industrial machines use, it’s faster and applies all the force at the end of the stroke where you need it, though a linear screw clamp is simpler to construct.
This clamp applies around 5 tonnes I think, though I don’t have a good way to measure this accurately. It’s powered by a cordless drill motor with a large gear reduction, all spur and planetary gears for maximum efficiency. There was quite a lot a machining involved in making it, I had to make all of the gears plus some more you can’t even see. The two arms are supported in pairs of heavy duty bearings inside the plates.
There’s something called the 80-20 rule in injection moulding, which is that the part should be between 20-80% of the machines maximum shot capacity for a successful shot. The max. shot of the PP injector is probably a little less than the theoretical max. volume. I’d guess the absolute maximum part volume would be around 150cc including runners and sprues etc. but you’d have to go slowly, pack it well and allow time to fully melt.
150cc is actually very large for a hand operated machine.
I tried PET just to see what would happen, though I’d heard it was awkward to work with so always avoided it in the past. It needed a temperature of around 270°C the moulded part had a nice shiny surface but quite a lot of sinking and was quite brittle. Also some of the runners broke off in the mould and had to be picked out piece by piece, they were quite well stuck, this never happened before with HDPE or PP. I don’t think I’ll be using PET again.
s2019 is right, the pressure you get in the barrel is the pressure you will get at the nozzle, the change in diameter makes no difference. An easy way to calculate it from your air pressure driving the pneumatic cylinder is:
P= line pressure x (dia. cylinder)² / (dia. barrel)²
So if you have a 100mm cylinder at 8 bar (0.8 Mpa) you will get approx. 50 bar (5Mpa) Which is acceptable and probably a little higher than the PP lever operated machine. Less than around 30 bar and you will start to have problems and will need to heat the mould to get good results.
I have built 2 pneumatic semi-automatic injection machines (now building a 3rd) and I try and aim for 1000-2000psi (65-130 bar). I’ve found the biggest problem is the clamping system which has to be able to cope with the pressure in the mould, which multiplied over it’s area can result in much higher forces.
200mm is a huge cylinder! I look forward to seeing that!