Nylon moulding.

Hi,

I recently bought Vincent Gingery's book on making a small injection moulding machine, I can't post it here for copyright reasons.

But it's basically a small metal frame with a machined mild steel cylinder and a cartridge heater and thermostat attached to it and all activated by a piston and lever handle. It also sports an adjustable base to raise and lower the height of a mould, closer or further away from the nozzle head.

Here is a picture of a modified one.

Not really the point i'm trying to make but I hope it gives you a little bit of context to the equipment.

I was thinking of making a similar machine, but instead out of a small arbour press and using that as the basis for my design.

In Vincent's book he frequently talks about all the different kinds of plastics you can use in the machine, but is quite light on the specifics of that. All of his examples were cast into two-part aluminium molds using HDPE.

HDPE is an easy one because it can be melted down, and reheated with no lasting consequences for doing so. Whereas other plastics are a one-shot affair, they can be heated up once, set and then they must remain like that. They simply burn or become brittle if you try to do it twice.

Now my question is whether you can do with Nylon what you can do with HDPE, can you say, heat up recycled nylon 66. swarf and get it to set again? Or do I need to buy virgin pellets for the trick to work? Is there any thing you need to be mindful of with this plastic?

Michael W

It's vicious stuff to inject at home... pops bangs and spits , unless you put it in an oven to drive all the water out of it ..and it will have water in it ,even virgin pellets .

The distinction you need to be aware of is that HDPE is a thermoplastic material, and you are worried about thermosetting materials. In the first, there is a chemical change as the plastic sets, and these include Phenol-formaldehyde, Urea-formaldehyde, Aniline-formaldehyde, Glycerol-phthalic-anhydride, Epoxies, Silicones, and the bases of Bakelite, Tufnol, Melamine, etc. They can sometimes be moulded as pastes (not yet reacted) or as mixtures of powders, but one set, that is it.

The stuff you need includes a selection of thermoplastics - stand by for alphabet soup - CA, CAB, HDPE, PA (nylons), PBTB, PC, PE (polythene), PETP, PMMA (Perspex), POM (acetal), PPO, PP, PS, PVC, PUR (polyurethane), SAN, SB, etc - and some synthetic rubbers. With a few exceptions, the setting plastics are hard and opaque, the remeltable ones are bendy and translucent, or even clear. And most of them are hopeless above boiling point.

So now you know what to ask for - which of the above list is available to hobby users, or suitable for your application, is something I can't help with.

Regards, Tim

Thank you tim, I didn't know the specific term by which to describe it.

I would imagine that naturally a pellet nylon would melt in a much more controllable fashion and be far less prone to defects and inclusions that might come from oily or unclean swarf. (dictionary doesn't seem to like the word swarf ?).

This said, I wondered more specifically if there were certain chemicals that are combined with these pellets, like an activator of sorts to help the process along?

And yes hacksaw, I've heard horror stories about needing to keep an eye on moisture content and discoloration in the castings.

Clearly the only real test will come from making the machine and seeing how it performs under the combination of heating and pressure, but that's only once I've invested the time and effort into making it, that's always the price of discovery I guess. 

Michael W

Edited By Michael-w on 07/01/2018 17:14:15

Please note - my message 16:35:43 says 'In the first, there is a chemical change ...' but it should read 'In the second, there is a chemical change ...'

Tim

My main concern about the machine in the pic is the flimsy mould support, it looks like a small square of plate on the end of a bit of threaded rod. Unless the mould cavity is really small, like 6 mm dia or less, there will be serious risk of something bending or breaking during injection and allowing the mould to slip sideways and damaging it and the machine and maybe squirting hot resin out sideways.

If you need a lot of moulded parts and have moulds already, I would not recommend the use of such a machine, I would find a small injection moulding firm and ask them to run the parts in a commercial press. Small commercial presses do come up for sale, some quite cheaply, if you want one for home. Power requirements are usually high and many commercial machines are three phase though, and some are for use with 415 or 660 VAC. Worth checking before buying. Small Arburg Allrounder and MTT presses are two examples that are small but can produce excellent consistent parts. A colleague of mine recently bought a very old but still serviceable Arburg press and a couple of novelty game piece moulds for basically scrap value, under 100 UK pounds.

If you have no moulds yet I would recommend studying up on the basics of injection moulding and mould design before building anything. There's lots of free info about it on the web. Also lots of info on the various available thermoplastic resins and thermosetting resins.

If you want to do only a few parts with very small moulds the Gingery type machines may work OK but again I would recommend better support for the mould than your picture shows.

Whatever resin is chosen you will have almost no control of moulding variables (pressure, temperature zones, time of injection, cooling etc) in such a Gingery type machine as you would in a commercial press so you will not have any consistency of properties or dimensions in your produced parts.

This machine is really a toy just for having a dabble in injection moulding, in my opinion. My opinion is based on over 30 years direct involvement with tool and part design for injection and compression moulding, and injection moulding press selection, in industry.

As usual I'm sure there will be LOTS of contrary opinions from all manner of "experts" on the subject.

Why wouldn't you use ABS? It's thermoplastic, sets hard and is readily available in the form of filament for 3D printers which can be easily chopped up.

Thanks jeff, I think when you use it, you hold the mold cope/drag together with either bolts or g clamps or a drill press vice. The picture doesn't show that. And yes, it's only intended for small parts, I think he made no bones about the fact that it couldn't produce either serious quantity or large components.

The power quoted for the cartridge heater is 250W (some have doubled this figure with either more cartridges or a larger one) 3/8" diameter by 3" long. Running on an American voltage of 110VAC but i'm pretty sure there wouldn't be any problems substituting and adjusting the rated components for 240VAC U.K equivalent. 5AMP max toggle switch and 250-600 degree adjustable thermostat.

Michael W

I'd certainly consider it if nylons a P.I.T.A!

Michael W

The plastic will degrade if you hold it in the molten state for any period of time. Getting it up to temperature and molten just before it's needed, then shifting it quickly into the cooler mould is critical and requires endless buggering about with pressures and temperatures. As suggested, the quality of an experimental thing like that will be very variable. It's tricky enough to get the real ones working smoothly. I'm sure some materials will be more forgiving than others for this kind of environment.

Murray

Just FYI injection moulds do not have copes and drags as used in casting, they have core and cavity halves. I encourage you to read up on moulds and moulding to understand the technology and its' terminology. There's a lot to know to get reasonable results.

You will need injection moulding grades of the materials suggested they are a lot more runny than the extrusion grades. We had a small commercial injection moulder on the same principle as the one in the picture it was pretty useless. The success rate for getting a good moulding was low and it was a simple test piece. It ended up in the metal skip, if it was any use I would have brought it home.

Anything you can 3D print will be suitable.

Nylon MUST be dry for good results.

PLA should work well.

Temperatures need to be higher than for 3D printing to give you the fluidity you need.

Commercially, Nylon needs to go through a dryer before it can be pre heated and then melted in injection moulding machines. If it requires to be ultrasonic welded afterwards, then it requires that there be about 20 to 25% reground material also feed into the system. It will in high temps and high pressures go through very small gaps. 0.01mm is considered a big gap in Nylon injection moulding tools. Typically venting is in the 0.003mm range .

Neil

Commercially, Nylon needs to go through a dryer before it can be pre heated and then melted in injection moulding machines. If it requires to be ultrasonic welded afterwards, then it requires that there be about 20 to 25% reground material also feed into the system. It will in high temps and high pressures go through very small gaps. 0.01mm is considered a big gap in Nylon injection moulding tools. Typically venting is in the 0.003mm range .

Neil

Interesting that you reckon regrind is actually required. Generally I've had to battle to minimise or eliminate regrind. Regrind can only be degraded and contaminated compared to the virgin material and the motivation for moulder to use it is normally to reduce costs and minimise scrap. I'm not familiar with ultrasonic welding, only laser welding - what's the explanation for the benefit of adding regrind?

Murray

Muzzer, you can't get it to ultra sonic weld properly with virgin material. Like the Styer gun buts, they were nylon with a glass fill. There was a run that they could not weld. The fix was either to use a solvent with the ultrasonic welder , or to use a percentage of reground material in the blend. It was discovered by mistake and then used from that point on. So the 1st lot of mouldings went straight into the grinder, specific for that machine and material. The solvent and ultra sonic welded ones were not was strong as the ultrasonic only welded ones that used the regrind blend. Needless to say, the mould were cleaned of any mould saving product before the run, and a ban on any silicon spray or release spray that contained silicon in the are and vicinity of the machines. There are loads of different types of nylons these days and nylon filler blends. But all nylons are hygroscopic.

Neil

Thinking about the stated example of the gun butts being hard to ultrasonically weld. This sounds like it may have been related to shorter fibre length in the regrind maybe allowing more vibration of the parts, which might improve welding. The first moulding and re-grinding process for regrind shortens the fibres by 25% or more, and fibre length is further reduced with every subsequent regrind. Long fibres as found in new resin tend to strengthen the part and reduce flexing. When ultrasonic welding there must be high frequency flexing and relative vibration between parts which generates heat for the partial local melting and fusion of the parts.

I have ultrasonically welded many types of unfilled nylon including the high temp high strength HTN grades from DuPont, supposedly very hard to weld, without any regrind just FYI, without problems, as well as many other thermoplastic resins both hard and soft , filled and unfilled.

Success in ultrasonic welding is also very geometry sensitive. The easiest joints to weld are ones with round joint path like the bottom of a round salt shaker. The hardest to weld are long skinny rectangle shape weld paths with small radii in the corners like a TicTac package lid. Larger rads in the corners of such welds almost always help weldability. A long thin ellipse shape like a gun butt might be a difficult shape to begin with.

Unfortunately regrind is one sure way to increase profits by moulders, by making waste runners and parts into saleable product. You can insist on zero regrind on your part dwgs and most moulders will stand there and swear there is none but there almost always is. Some compounds degrade progressively with every re-heat for moulding and these compounds can be detected by gas chromatograph analysis to prove that there is or is not regrind, and what percentage, but this testing is very expensive and takes time, so unless parts are critical to function for things like medical machines, space suits or aircraft navigation or other life dependent functions, it is not done, so regrind goes undetected and usually causes no big issues. When defects start occurring in parts that had no such defects in previous batches that is often a hint that more regrind is being used (sometimes regrind from other resin families) or that someone sped up the process somewhere.

Neil and Jeff - that's interesting to hear. As I said, I have no experience of ultrasonic welding (nor of Nylon) but a fair bit recently of laser welding of PPA, where regrind adversely affects the laser transparency of the "transparent" half of the assembly. Fundamentally that's because the material progressively degrades, the longer it is held in the molten state. That problem is worsened when you start out making early samples in a multi-cavity mould that has only one of the cavities initially, since the material spends even longer in the screw.

We ended up buying a transparency tester to test the bare mouldings to ensure the final assembled product would be suitable for welding, whether the result of regrind or thermal degradation. And of course we prohibited any regrind.

For laser welding, you pass the laser through one of the mating halves. When it hits the other part, the beam is absorbed and the plastic at the join melts and fuses. You can join dissimilar materials in this way. If the transparent half needs to appear to be black it typically contains a very dark red dye. The other half can be truly black and certainly needs to be opaque, eg loaded with carbon black etc. These were loaded with 40% glass fibre for strength, which didn't impede the laser welding process. The process is widely used in automotive components, the most obvious of which are external lighting clusters etc.

Murray