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Today, you will be granted a new superpower, very simple but very cool.
By the end of this video, when you will see anything made of plastic,
you will say: “I know. I understand”.
You will be able to read the lines of the plastic, and I mean that very literally, to
the point where you will know its history, what led it to become what it is today.
Because plastic, a bit like us, is born from the fusion of… plastic.
And its birth left it with a navel or even birthmarks.
I found the photos of the day that I acquired this superpower myself.
I was working on a practical work project during my last year of engineering school.
When I tell you about "the story of plastic", that means that we are looking at
how it was made.
It's a lot less sexy when you put it like that.
And looking back at the photos, I remember I really wasn't excited to go to this practical work session.
Because going to see a production line, on the list of things that excited my
brain... it was quite low.
And yet today, it is among my best memories from engineering school.
So I hope I get to bring little stars in your eyes too
just like I had when I left from this experience.
If you want to make this video interactive, try to find yourself a plastic bottle, if
possible a pot of yogurt too (no need for it to be empty yet), a medication
tablet and ideally also a container of some household product like this one.
The birthmarks on each of these objects will be different.
I'll let you gather everything, you can press pause if you need to,
and I'll just continue.
And I'll start my birthmark research with the bottle.
Normally, if you look at almost any bottle, you're going to have one
small plastic circle which is a little harder, more opaque.
Remember this information: there is a circle of excess plastic.
Then if we look at the shape of the bottle generally, we have something very symmetrical.
And if we look at one of the axes of symmetry, we will see a very subtle line in the plastic.
Remember this second piece of information: there is a line in the plastic that goes
all around the product.
Continuing the investigation.
Last clue that interests me is the thickness of the material.
In my case (and it should be the same for you at home), the bottleneck is significantly
thicker than the rest of the plastic.
You can feel it.
Remember this too: there is a variation in thickness.
Now there are also all kinds of sexy shapes on the bottle, with curves,
nooks and crannies, bumps, hollows... we'll put all of that aside for today,
these are shapes that are there either for aesthetic reasons, marketing, or in order to
add rigidity to the material, or for some other reason.
With our three clues (the little thingy, the line and the thickness variation),
I know exactly how this piece was made.
But since I don't really trust my memory nor my old report
to validate what I'm going to tell you — because as I read it again, I recognized my style...
But you could say it's a bit content-light... So instead, I went back to see the mothers
of the bottles: the machines themselves.
The process that makes it possible to manufacture this bottle is called injection
and the principle behind it is to… drumroll... inject plastic to
get to the final shape.
We have polypropylene in grain form like this, which we are going to introduce
into the funnel here to fill our injection press.
There is a screw here that you can't see which turns and will melt the polymer.
And then we will inject it directly into the mold.
Then we will squeeze the molds together with a certain pressure that can be adjusted
and then the polymer will fill the mold.
We release and the part simply falls, ready for use.
In the lab that day, they were printing parts that are used for pratical work
by students (these are test tubes for tensile testing if you really want to know).
With the explanation of the process, you may have gathered what is that
little plastic circle at the bottom of plastic bottles... it's simply
where the plastic is injected.
This is called the injection site.
The plastic that's melted here, goes down into that area.
It is pushed by a worm drive at a certain pressure, and it comes out through the small
hole here to fill the inside of the mold.
And once the room is full, we separate the two molds.
We say that the piece is born, we cut off its umbilical cord, and it falls.
So this circle, in a way, is the bottle's navel.
This is an analogy that has many limits, no one is fooled.
What I find pretty crazy with this injection mechanism is to imagine
just how much pressure the machine is under.
Because plastic, even when melted, is a very viscous thing.
I've put some honey in my syringe.
In order to manage injecting some honey, which is viscous too, at a high speed (because the injection
is done very quickly), I have to apply tremendous pressure to reach
my desired rate.
This is the role of the screw, to push melted plastic.
But the mold in which plastic is injected must also be extremely tight so that it may
not explode and to avoid the plastic (or honey) ending up between the two molds
rather than in the path that was laid out for it.
And when I say it must be tight, I'm talking several hundreds or even thousands of kN.
This probably means nothing to you as a unit and to be fair, it is so high
that it's hard to imagine.
I saw a machine that went up to 8500kN.
If we consider a 5 ton elephant, that gives us the force of 170 superimposed elephants.
All this to prevent the plastic from coming out of its mold… And, speaking of the mold.
We also understand where the injection line comes from.
This is where the two molds meet.
The junction is never completely perfect and that's what allows us to see it.
Moreover, on pieces that have been imperfectly injected, we can see
that little piece of plastic sticking out.
We call it the flash.
When you see this, you can tell yourself that the injection parameters were not optimal.
So, for the bottle, that means that we had a mold on one side, a mold of
the other side, and we injected the plastic here.
But there is still a problem.
If we had just that, the injection would allow us to make a full piece;
we would have a bottle completely filled with plastic.
How do we make the inside of a bottle hollow?
Well, we don't do it like that.
There's one more step.
We start by injecting the bottle, yes.
Except that at the end of the injection, it doesn't look like this at all,
it really doesn't.
It looks something like this.
So this is a pretty basic tube; it would look more like this, but
I don't have one so we'll make do with this model.
The way that stuff is injected is with a mold inside
and a mold on the outside.
Which implies that we won't see a line of injection.
We would see an injection line if we had a mold on either side of the piece.
But here, with a positive mold and a negative mold, there is no line.
This thing is kind of the bottle's fetus.
To turn it into a real bottle, all you have to do is blow into it.
Literally.
When the tube is still hot, we blow into it.
Except that when you breathe, obviously it's when it's constrained inside
of the mold that looks like the final bottle.
And boom, we have a bottle with the shape of the mold that we wanted.
Basically, the parison (this is the name of the fetus of the bottle) was of uniform thickness.
But by blowing it, we stretched the material, making it thinner.
Except at the bottleneck, since that is where it is held and not stretched.
Hence the fact that I listed the thickness change as a clue.
If there is a change in thickness, in general (because I'm sure we
can find an exception), it means that the material was stretched.
So the bottleneck is probably as thick as the parison, and everything else was stretched.
Those are all the plastic birthmarks that were created with
injection or injection blow molding.
You can take other plastic objects and try to see if you can spot these marks.
I will give you another example.
If I take my comb, I can see the injection line here too.
So I know my two molds were placed that way, one here and one there.
And I see here also a change in the plastic, and I'm guessing that's probably
my injection point.
Now, if you start observing plastic objects (which I hope
you will after this video), do not confuse this injection site
with these points, which are actually ejection points.
These are the marks of the rods that allowed to push the piece out of the mold.
But honestly, those points aren't always visible.
Now if you have this kind of bottle, you'll see the mold line that
we talked about, except that at the navel, you'll find something super weird.
This time it's not a dot at all, it's a long line.
That’s because it’s not the same process.
This was made with extrusion blow molding.
It is a molten plastic tube that is continuously produced and which we blow
into a mold.
It gives us that kind of bottle.
But we won't get into it because I want to look at my yogurt pot now, or
my medication tablet, since both follow the same process.
What is interesting is that we can hardly see any trace of its production.
We have no dots, no lines, in any case not any that would be related to the manufacturing method.
We do have a small rim there, but it's not at all related to the manufacturing method.
The only thing we still have is this change in thickness between the rim
here or there on the medication pack, and the rest of the body where it's much more flexible.
And a change in thickness, you now know what that means.
We started with a thicker material and stretched it.
As in the case of blowing.
Except here it's not blowing but thermoforming.
Do you do thermoforming too?
Yes, right behind you.
So here we have the heating element that heats the sheet.
So this time, instead of working with grains, we are working with polymer sheets.
We want it to be at a temperature which allows us to warp it without melting it.
So in comparison, we're going to be at a lower temperature than its melting temperature.
It will be placed here, in the middle of this frame.
We see a mold below here, which is actually a coffee cup lid.
We close the heating element, we let it reach the required temperature, and
then we mount the mold, we create a vacuum to draw the sheet around the mold.
All the little tiny holes that are in the piece all the way to the top, all over
the edges, they make the suction happen.
And bam, it's thermoformed.
They have a manual machine mostly used by students but of course
this process gets automated very well.
And since I find it very satisfying to watch, I'll show you a video that
I still have from the first time I saw this technique.
This is the thermoforming plastic sheet, we have the heating element coming on top,
which will therefore heat the sheet.
We see that it gets warped because it heats up.
The heating element goes away, the mold goes up, and here we create a vacuum, we suck all the air out
so that the plastic sticks to the mold.
And then there is a cooling off period.
Then we see that there are small imperfections here in the corners.
These are things that can happen in thermoforming and which must be optimized with
the mold, with the different parameters of this technique.
One thing about thermoforming is that you can always see thickness changes?
Not necessarily, because if we had cut the piece
at the edge of the mold here, we wouldn't have seen this change in thickness.
One of the signs is that we have no injection point.
It’s a shape that looks simple enough to be thermoformed.
So thermoforming only works with simpler shapes?
Yes, in principle we wouldn't be able to render lots of details, small grooves, etc.
The sheet must be able to fully hug the shape of the mold.
So if there are details that are too complicated, the sheet will have a harder time hugging all
those little details.
For example, this clothespin.
It’s a two piece assembly but we see that there is a cavity in one direction,
one cavity in the other; inside, we have a very complex shape... I can't
see how we could reach this level of complexity just by stretching a plastic sheet.
On the other hand, with injection, that seems more likely to me.
You now have a new superpower.
You are able to determine how a piece of plastic was made
based on 3 main clues.
First, the presence of an injection site, like a navel, if you will.
Then, the presence of an injection line... both clues indicating the use of the
injection technique.
There's also the navel in the shape of a line, in this case it points to extrusion blow molding.
And you also have the thickness changes, which are often associated with thermoforming
if the shapes are simple.
But it can also be due to injection blowing for example, for more complicated shapes.
Having said that, it's nice to have general guidelines, but sometimes you find a
plastic object that you have absolutely no idea how it was made.
It may be because all information is hidden.
For example, an injection site that would be inside of the piece
(this is probably the case with this one), and that we cannot see because there is
something around it.
Or an injection site that has been filed, where the plastic was polished
for example.
The same goes for the injection line.
Sometimes we can't see it at all because the mold has been designed so that the line
won't be visible.
As was the case with our parison from a moment ago, where there was a negative mold
and positive one rather than a right-left mold.
Or, as on this model that we have made in the laboratory.
Look here, the line is present but only on the part that is removed from the piece.
The piece we keep has no markings.
Same goes for this classic IKEA faux plant pot, I would have guessed that it was thermoformed,
because it is a classic thermoforming shape.
But there is an injection site... so it was probably made by injection.
Except I have no injection line that would have come to separate, that would have
indicated the presence of right and left molds.
And I think actually both parts of the mold came together like this:
one inside, one outside.
As was the case of our parison.
So there is no visible injection line.
The line was designed to be at the limit of the product in a way, which
means that we cannot see it.
Often, if we draw a general rule, the more upscale the product, the less
plastic birthmarks are visible.
All these processes (injection, injection blow molding and thermoforming),
are not the only ones out there.
And above all, there are plenty of subtleties within each of these methods.
I only gave you some general tidbits that I use all the time.
I really like looking at plastic objects and ask myself “mmh, how was this made?",
I check for injection sites, whether the
piece is assembled with Robertson screws, that kind of thing.
And now I hope that you will be like me and that you will start to
look at all your little plastic things to see how they were made
if you get caught up in the game.
If you want to know more, I have found a French-speaking YouTube channel that
goes into even more detail and is really well done.
So if you are interested in the subject, I will put some links in the description.
Check it out, there are really a lot of details that I found really, really interesting.
I watched every video even though I already knew pretty much how it worked.
And I would like to leave you with a little guessing game, see if it tickles your brain.
This ball.
It seems hollow, there is no sign of an injection site, at least I can't see any.
However, there is a line here.
How do you think it was created? Don't answer right away, I will
give you some time with a little ad break to think about it, and then
I will let Mélanie give you her hypothesis.
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I wonder if it wouldn't be… rotational molding.
A part that has been in a mold that has turned to be able to do it in its entirety,
and something empty in the middle.
Could it be blowing? Blowing injection? Because we can see a line.
Yes, it's true.
The problem is, it's not transparent, so I don't see how thick it is.
The little spikes seem to be full.
Then if it was blown in a mold, that would be hollow.
Whereas if it's been filled all over, it would be easier to make something full.
Again, this is really guesswork.
But you would lean more towards rotational molding.
Since I don't want to cut open my ball to be sure, we will not have the answer.
Unless… Decathlon… will you tell us how you made this ball?
We would like to know.
Bye.