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A Production Methods Mystery (Involving a Time-Traveling Hollywood Actor)

By Rain Noe - March 28, 2013

in Materials

Here's a production methods mystery, albeit one we think will soon be solved by one of you.

On the Discussion Boards, a Core77 reader asked how a hot water bottle is made. A couple of votes came in for slush molding, which is like rotational molding without the spinning; the mold heats up, vulcanizing whatever part of the liquid rubber inside comes into contact with it, and leaving the stuff in the middle, well, slushy. You then pour the slush out and you've got your hollow bottle.

However: How the heck is this threaded insert added?

Then, like the rubber, the plot thickens: Our trusty Board Moderator LMO submitted this photo of that very bottle being produced by B.F. Goodrich in 1939:

Looks like it isn't slush molding at all. And if we zoom in on the photo, we can see the frying-pan-shaped mandrel that forms the negative space of the bottle inside the mold:

Which beg the questions: How the heck does the worker get the mandrel out of there?

Is it actually possible the bottle has that much flex? What about the threaded insert? And most importantly, how did Sean Penn travel back in time to work in a B.F. Goodrich factory?

Due to our deep readership, we know it's just a matter of time before someone with direct experience sounds off on how this is all accomplished (except for the Sean Penn time machine part). In the meantime, you may be wondering—where did this awesome and high-quality image of a 1939 manufacturing facility come from, and are there more like it? Stay tuned.

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SolidDesign Consultancy

11 years ago

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You can see from the ones hanging on the rack that they are moulded in halves. They will then be heat sealed and the neck piece cemented in place.

I would think now they would form it as one and use air pressure to remove it from the mandrel or they would be made by blow moulding, having a final process which involves cementing the neck piece in place.
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Rich Beck

11 years ago

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They are done in halves. Unvulcanized rubber is melted over the hot mold to make each half. The halves and insert in the spout are joined via a vulcanization process which makes the rubber cross linked and stable. It is how tires are made and how we patched bicycle tubes in the old days.
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Antti Eskeli

11 years ago

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Easy. The black insert is fitted in after the mold comes out (the elastic material allost the mold to squeeze out). The ribs around the neck comes from reheating the surface of the bottle, then squishing the insert and the bottle together with some tools that has the ribbing and that keeps the insert in place. Sorry for my horrible english.
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Manu T

11 years ago

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I think this fact might help:

Natural Rubber after vulcanization will have an elongation at yield (ie, stretch without any physical damage) of 400-1000%, depending on the compounding ingredients.
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Carter

11 years ago

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I am voting for the flexibility of the warm rubber. Also, if you look carefully at the one that Sean Penn is working on in the photo, you can see the under side of the mouth, absolutely disproving the made in separate halves theory.
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Julie

11 years ago

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I don't know guys, I think they're cast in halves....check out the castings that are hung on the rack above Sean's head. In the HiRes version, look at the second and third ones in, you can see that they're hollow.

And check out the castings lying on the bench to his right - they look too thin and flat to be the entire bottle. I have one of these bottles (or the modern equivalent). When lying down, it is never as uniformly flat as these due to the wall thickness. Instead, the center is depressed....

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brutherford

11 years ago

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i've spent the last 10 years working on hollow rubber and tpe parts. the process shown is compression molding, the bottle is molded in 1 piece around the mandrel, there is plenty of stretch in natural rubber particularly when it's freshly out of the mold. The process for removal is either brute force with a couple levers or in a modern environment you might use a pneumatic cylinder. Sometimes you can just blow air in to the cavity and the part will pop off. The threaded portion is held in place with a rubber cement.
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Blair Miller

11 years ago

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Here's a link to a much higher-resolution version of the photo: http://25.media.tumblr.com/tumblr_m7jr8svAJz1qchs1zo1_1280.jpg

Looking at the image, it's clear that they're formed with the mandrel inside the bottle. Look underneath and above-left the bottle he's lifting out of the mold--it's akin to a waffle-maker, with both sides being formed at once/together. It would seem silly to go through all that only to cut the two halves apart.

My guess is that they are, in fact, that flexible. The mandrel is simply pulled out through the neck, which returns to it's original shape, and then threaded insert is either glued or otherwise fused into place (maybe through a process similar to spot welding).
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WileECoyote

11 years ago

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The part of the process you're looking at is before the insert goes in, and it turns out the answer is less complicated than we hope: rubber is quite, you know, rubbery, especially so when it's hot. So just like pulling rubber rain boots off a mold, they do just slip off of the center mandrel. You'll notice the black rubber surround for the insert, which is intentionally smaller not simply so that they can glue the threaded part in, but also to give them a little more play on how much throat they can pull the mandrel through.

The threaded part is cast in to a black rubber insert, which is then either solvent "glued" in to place or heated to form a "weld" or sorts.

I don't think this is how it's done any more. Far more likely 3-4 steps in this process have been removed by doing blow molds similar to how plastic soda bottles are manufactured. This also cuts down unit time and quality issues that result from gas bubbles trapped in the mold. This is also much quicker than slush molding, where the process is available. A tube of plastic is extruded, dangled between two external mandrels, then clamped in and pressurized to fill the cavity. This is also how petrol canisters and Gatorade coolers are manufactured.

As for Sean Penn, I think we can assume everything was explained as clearly as it can be in Primer...
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Nathan

11 years ago

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MY best guess: He stumbled upon the designs for a flux capacitor from a 3rd party who although incredibly intelligent was also extremely prone to losing things on the morning commute.
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supafluous

11 years ago

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looks to me like that black part is an inner lining that was made first, and that the red compression molded part is just laminating around it.

Maybe for production? Mold the black part that can stretch off the form, weld in the threaded insert, seal it to a fixture to trap air inside and hold the form, compression mold a higher durometer around it?

Maybe the inner, black bag has superior insulation, and the outer red material is more durable?
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thomas

11 years ago

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The threaded insert is just cemented in, isn't it? And yes, I can see a hot water bottle having that much stretch at the neck.
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DJ

11 years ago

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My best guess: The bottles are moulded in halves. What he's lifting is not an entire bottle - it's just one half? The neck would be glued onto one side, then the remaining side is carefully glued on to finish the assembly, and the part is trimmed? Would be interesting to see the jig that holds two rubber parts together for lamination.
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