Plastic billets

[Sam StonesParticipant @samstones42903]

Something discussed quite recently in Model Engineer about the alignment of piston rods in early steam engines prompted the following (early 60's and slightly O/T) memory:-

In the plastics laboratory way back then, there came a need to produce some solid billets of plastics materials, e.g. polyethylene and polypropylene. They needed to be about 6" (150mm) in diameter and about 20" (500mm) in length. After solidification, the billet had to be machined into some sort of conveying screw. Of that, I’m not clear.

The most logical starting point was to use one of the available laboratory extruders to produce a continuous supply of quality melt (i.e. molten plastic). Upon reflection, the polymer melt could simply have been dribbled into a suitable ‘bucket’ and allowed to cool. The choice however, was to feed the melt into a horizontal chamber equipped with a piston and piston rod. See my sectioned schematics of the arrangement which, in reality, was designed to be stripped after cooling. Well of course!

My idea behind the piston was to apply enough backpressure to it to restrain the advancing polymer melt and hold it in a convenient cylindrical shape while it cooled. I figured that it wouldn’t take much pressure to hold the melt in place.

Imagine my surprise when I was ‘invited’ to see why the idea failed.

Can you spot the Achilles’ heel?

Sam (aka Dennis)

Edited By Sam Stones on 03/12/2017 01:50:34

[Sam StonesParticipant @samstones42903]

Filling a cylinder with hot plastic (melt)

[Jeff DaymanParticipant @jeffdayman43397]

Looks like a one use tool to me. If you can't get it out of the mould what use is it?

Zero draft is never good in moulds either, or zero venting.

Plastics pellet manufacturers probably would have given you some good hints on how to make large dia rod stock with decent properties by multi gate extrusion through a deep die if they had been consulted.

[Sam StonesParticipant @samstones42903]

Hi Jeff,

Some good points, but not the Achilles' heel.

In my spiel at the start of this thread I wrote –

… See my sectioned schematics of the arrangement which, in reality, was designed to be stripped after cooling. Well of course! …

I'd like to add – with the amount of shrinkage, the billets didn't need any draw taper. When cooled to a safe level, they would have fallen out.

As for venting, the actual assembly was riddled with them. The speed in which the melt entered the cylinder was very slow coming from the extruder screw. Nothing like the injection rate in an injection moulding machine.

As the raw material suppliers ourselves, there wasn't much in the way of 6 inch diameter polypropylene 'rod' in the early 60's.

More to come.

Sam

Edited By Sam Stones on 03/12/2017 08:54:18

[SillyOldDufferModerator @sillyoldduffer]

The idea looks sound to me. Was the problem to do with pressure changes in hot or cooling plastic, perhaps something like the way freezing bursts a water pipe?

Dave

[V8EngParticipant @v8eng]

Far too early on a Sunday for all this really, but as the Wife says we are going shopping!

Did the melt cool and solidify before it could fill the chamber? I Know nothing about plastics so if that is silly ignore it!

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Sam Stones on 03/12/2017 01:49:04:

… The choice however, was to feed the melt into a horizontal chamber equipped with a piston and piston rod. See my sectioned schematics of the arrangement which, in reality, was designed to be stripped after cooling. Well of course!

[ … ]

My idea behind the piston was to apply enough backpressure to it to restrain the advancing polymer melt and hold it in a convenient cylindrical shape while it cooled. I figured that it wouldn’t take much pressure to hold the melt in place.

Imagine my surprise when I was ‘invited’ to see why the idea failed.

Can you spot the Achilles’ heel?

Two thoughts, Sam

1. Things would probably be in better equilibrium if the cylinder was vertical, not horizontal
2. I suspect that the required Air-pressure would be much higher than you anticipated.

MichaelG.

[Neil WyattModerator @neilwyatt]

Once you had a relatively small amount of cylinder, it wouldn't be able to push the piston down the tube?

[not done it yetParticipant @notdoneityet]

I reckon Gillie is on the right track, differential cooling if done in air at ambient temperature (why not immerse the cylinder in a suitable heated medium?) and why the aid pressure – just not needed as that ram could better be controlled mechanically.

Done horizontally would require a perfect seal on the piston, I suspect. Any air leakage would mess up the casting.

[Ian S CParticipant @iansc]

I would think that you would need to heat the cylinder, and once the piston is right down increase the air pressure, or exert pressure on the piston rod to compress the billet in the cylinder until it is cool, then remove the input end and force the billet out.

Biggest I'v made from a number of different scrap plastics is a disc 2.5" x 5/8". The mold a piece of steel plate with a 2.5" hole, two 2.5" dia steel plugs, one plug in the bottom of the hole, plastic chips, the the other plug, sit the mold on the workshop electric hot plate(here's where it gets scientific), it starts to get a bit sniffy, so take the mold and put it sideways in the bench vise and squeeze the two plugs until they won't go any further, allow to cool and knock the plugs out. One casting became the hub of the fan on a wind turbine, I think it was made of PET from soft drink bottles.

Ian S C

[J HancockParticipant @jhancock95746]

It solidified round the piston immediately and seized in the bore.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by not done it yet on 03/12/2017 10:41:15:

I reckon Gillie is on the right track

.

I sign-off as MichaelG.

If we are going to start using other names … are you happy being called virgo intacta ?

[Sam StonesParticipant @samstones42903]

The Achilles’ heel

The idea of holding the piston against the advancing plastic melt using air pressure failed because…

1. I hadn’t recognised that the piston areas were dissimilar.
2. The cross-sectional area of the piston rod was about 0.8 in²
3. At 80 psi line pressure, the resultant (force) differential was more than 60 lbf.
4. Air leaked past the edges of the piston resulted in the air pushing the piston along.

After the laboratory technologists had carried out their initial tests they invited me to come downstairs from the DO to see my mistake

I seem to recall that they did try again. This time leaving the air turned off and the piston free to be shoved along by the melt. However, at some stage the person operating the extruder was not quick enough to stop the extruder screw before the piston reached the end of its travel. It was about then that the steel cylinder whose wall thickness was about 5/16" began to take on a permanent bulge.

[Normal operating pressures in a plastics extruder, range between 1,000 and 5,000 psi.]

I should point out that the piston rod had marks (rings of braze in grooves) on its diameter to indicate when the piston was approaching the end of its stroke.

Eventually, they had a much larger cylinder designed and built. The new one was about 20 inches in diameter and some 7 or 8 feet long. However, unlike my horizontal design which had been conveniently bolted onto the end of the extruder, the new cylinder was positioned upright, and stood on the floor. A stainless steel pipe suitably profiled to shape fed the melt up and over the top edge of the cylinder. During the extrusion stage, they had heaters wrapped around the feed pipe and the cylinder. As I recall, the billet took several days to cool before it was taken to the engineering workshop.

There they machined it into the shape of a very large socket head screw and a matching nut.

It became an exhibition piece.

Somewhere.

Sam

 

Edited By Sam Stones on 04/12/2017 08:19:53

[David ColwillParticipant @davidcolwill19261]

Posted by Michael Gilligan on 03/12/2017 12:38:59:

Posted by not done it yet on 03/12/2017 10:41:15:

I reckon Gillie is on the right track

.

I sign-off as MichaelG.

If we are going to start using other names … are you happy being called virgo intacta ?

Don't mess with Da Geesta.

(sorry)

David.

[David Standing 1Participant @davidstanding1]

Posted by Michael Gilligan on 03/12/2017 12:38:59:

Posted by not done it yet on 03/12/2017 10:41:15:

I reckon Gillie is on the right track

.

I sign-off as MichaelG.

If we are going to start using other names … are you happy being called virgo intacta ?

Superb!

Upright Dave

[David Standing 1Participant @davidstanding1]

Posted by David Colwill on 04/12/2017 08:26:10:

Posted by Michael Gilligan on 03/12/2017 12:38:59:

Posted by not done it yet on 03/12/2017 10:41:15:

I reckon Gillie is on the right track

.

I sign-off as MichaelG.

If we are going to start using other names … are you happy being called virgo intacta ?

Don't mess with Da Geesta.

(sorry)

David.

He is, indeed, Da Man

[David Standing 1Participant @davidstanding1]

Posted by Sam Stones on 04/12/2017 08:16:34:

It became an exhibition piece.

Somewhere.

Sam

Aka 'It went in the skip', probably

[MuzzerParticipant @muzzer]

A home-made 20 inch diameter / 7-8 feet long pressure vessel, pressurised to 80psi, loaded with molten polythene at 200C or more could have led to a truly horrific industrial accident. If you'd had a failure of any of the various couplings, there could have been very little to stop the entire contents exiting at great speed – and temperature. I wouldn't like to be standing anywhere nearby at the time. "Cleaning up" after such an accident would have been character forming. In contrast, injection moulding machines use pistons to force the plastic into the dies. There is no stored energy (compressed air) as such. A failure in the hydraulic or the heating / cooling circuits can still be hazardous but on a whole different level.

Murray

[Sam StonesParticipant @samstones42903]

A good one Murray,

Thanks for your observations and details of what might have happened had it been a pressure vessel.

In reality, this SOB (me) forgot to mention that…

Installed in the main plastics laboratory, the 20 inch diameter / 7-8 feet long vessel stood on end, was an open-topped stainless steel chamber into which the polymer melt ‘dribbled’. It settled at the bottom of the vessel under its own weight.

With one end bolted to the end of the extruder, the open end of the one-piece stainless steel (feed) pipe was shaped to carry the melt over the top edge of the vessel, and down past the top of the tank a short distance. There were no couplings other than the pipe’s very secure attachment to the extruder.

For further protection, guardrails and warning signs indicating ‘HIGH TEMPERATURE’ surrounded the vessel.

As for homemade, I suspect (because I was no longer a part of the project), that the device was designed and manufactured under the control of the refinery engineering group. Had it been a pressure vessel, I reckon they would have understood the requirements

Other than that, I can’t be more specific.

I beg to differ about injection moulding machines. For years, a large number have utilised nitrogen cylinders to store hydraulic fluid under high pressure. With this arrangement, smaller pumps and faster injection rates are possible.

Knowing about this while standing between the (installed) mould halves of a 3000 tonne IM machine can be a bit creepy and rather intimidating, even when you know that all the pumps are off and the power is isolated. At the time, we were standing on the spring loaded, steel-mesh floor, equipped with multiple micro-switches linked to the controls.

Even then it still affected the hair on the back of my neck, and maybe other places too

Regards,

Sam

A one-time Lancashire lad.

PS I almost forgot why I created this thread in the first place.

Something to do with Achilles? 

Edited By Sam Stones on 04/12/2017 22:06:05

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