Casting HDPE - Introduction

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rmac
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Casting HDPE - Introduction

Post by rmac » Thu May 06, 2021 3:42 pm

Five or six years ago I ran across an article where some guy had melted down some empty plastic milk jugs and used the material to make the head for a mallet. While I'm not exactly an environmentalist, the cheapskate in me was attracted to the idea of making something useful out of apparent trash, so I decided to try it myself.

On the surface it's all pretty simple. You melt a bunch of plastic scraps together, maybe transfer the resulting blob into a mold, and let it cool.

For background, there's a pretty good series of three YouTube videos that start here where a gentleman named Randy Knapp shows how he does it, with what I consider to be exceptional results. (Actually, you may only care to watch the first video. The other two are mostly about making slingshots from the resulting material.)

I plan to add to this thread as time permits over the next several days with some detail about what I've learned by experimenting with this process in my own shop. If you're interested, stay tuned!

-- Russell Mac

John Hasler
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Re: Casting HDPE - Introduction

Post by John Hasler » Thu May 06, 2021 3:58 pm

> If you're interested, stay tuned!

I will.

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Bill Shields
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Re: Casting HDPE - Introduction

Post by Bill Shields » Thu May 06, 2021 7:31 pm

I hate to point out the obvious..but were it me -> I would do that outdoors and be sure I was standing upwind of the oven.

all kinds of nasties come from melted plastic (of every type)
Too many things going on to bother listing them.

pete
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Re: Casting HDPE - Introduction

Post by pete » Thu May 06, 2021 8:19 pm

Bills point was the exact thought I had after watching the first video. Even with the little I'd like to think I know about plastics there's also some that are highly poisonous when heated due to the generated gases they produce. Knowing exactly the type of plastic your using beforehand would be high on my list. It's still an interesting concept and there's a lot of helpful information in that video.

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Bill Shields
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Re: Casting HDPE - Introduction

Post by Bill Shields » Thu May 06, 2021 9:18 pm

Heck. I will not even put food in a plastic anything to warm it in my microwave.

Maybe I am paranoid but two cancer strikes have firmly put me in that corner..
Too many things going on to bother listing them.

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Rich_Carlstedt
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Re: Casting HDPE - Introduction

Post by Rich_Carlstedt » Thu May 06, 2021 9:44 pm

Milk jugs are Low Density Polyethylene (LDPE) or Linear low Density Polyethylene (LLDPE) a pretty safe plastic..I mean it has milk in it !
What is dangerous - heated - is PVC . When heated , PVC gives some nasty gases that qualify you for the Intensive Care Ward
Doing it outside is always a good idea.
I made dies for the plastic industry for many years and worked at a Film plant where they produced recycled plastic pellets so heating polyethylene was a daily fairly safe nonoccurence .High density poly (HDPE) is stiffer and usually white .
Melting it requires more heat than low density. You will find the that LLDPE melts about 300 and keep it under 370 to reduce oxidation
Rich
Not seen the video yet

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Rich_Carlstedt
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Re: Casting HDPE - Introduction

Post by Rich_Carlstedt » Thu May 06, 2021 9:49 pm

Bill Shields wrote:
Thu May 06, 2021 9:18 pm
Heck. I will not even put food in a plastic anything to warm it in my microwave.....
Excellent advice Bill !. Using plastic is a No-No in a Microwave unless it is specifically for that ( restricted additives)
Even though i worked in the industry, I use pottery , and not plastic in the microwave
Rich

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Lew Hartswick
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Re: Casting HDPE - Introduction

Post by Lew Hartswick » Fri May 07, 2021 8:45 am

Interesting, not so much the carving and routing but I'm surprised he didn't use a bit of vacuum to pull the bubbles out. It would be fairly simple to build a suitable container and pull a vacuum on the pan while it's still hot. The clamping of the board on the top could be implemented a lot better so it maintained a good parallel to the bottom and even in the ultimate eliminate the thickness planner op.
I am disappointed there as no coverage of attaching the rubber bands, or what ever is used "now-a-days". The strips of old innertube
wrapped around the stick and tied with string from my slingshot days is all I know. Oh what sort of "projectile" pocket is used now and how is it attached??
...lew...

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rmac
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Re: Casting HDPE - Introduction

Post by rmac » Fri May 07, 2021 11:12 am

Bill Shields wrote:
Thu May 06, 2021 7:31 pm
all kinds of nasties come from melted plastic (of every type)
There's nothing wrong in being careful, of course, and lots of ventilation certainly can't hurt. Having said that, I think this statement is a little broad. It looks to me like the hard data tends to agree with Rich's opinion that melting HDPE is relatively safe. Take a peek at this graph:

hdpe.png

This shows how HDPE loses mass as its temperature is increased. Note that this loss doesn't start until the material reaches about 300 C (572 F).

That means it's not giving off any toxic material (or any material at all for that matter) below that point. Since we're only talking about melting HDPE at around 180C (350 F), there's plenty of margin for error (from a poorly calibrated oven thermostat, e.g.) before you get into trouble.

Besides this graph (and others like it), I also looked at a whole bunch of material safety data sheets (MSDS) for HDPE from various manufacturers. I was surprised to see that each company has its own MSDS, with varying levels of detail.

Most describe the inhalation hazard during processing as irritating, rather than toxic or dangerous, and it appears that the concern is related more to airborne HDPE dust than to any kind of fumes.

The few data sheets that do actually mention high temperatures use language like "Avoid temperatures > 300 C" or "Decomposition temperature: > 300 C". They consistently give 300 C (572 F) as the cutoff temperature, which supports the conclusion based on the earlier graph that above about 300 C (572 F) is where you need to start worrying.

-- Russell Mac
Last edited by rmac on Fri May 07, 2021 2:24 pm, edited 2 times in total.

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Bill Shields
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Re: Casting HDPE - Introduction

Post by Bill Shields » Fri May 07, 2021 2:19 pm

good to know...thx
Too many things going on to bother listing them.

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Casting HDPE - Material Selection

Post by rmac » Fri May 07, 2021 10:17 pm

So let's stick with High Density Polyethylene (HDPE) here because it's plentiful, it melts at a reasonably low temperature, and it won't give off a bunch of toxic fumes unless you get it way too hot. You can easily identify HDPE by looking for a triangular recycling symbol with the number 2 in it.

Unfortunately, not all HDPE is the same. When heated, some variants form into a rubbery sticky blob that has to be more or less forced into a particular shape. Others become kind of semi-liquid so that they will settle by gravity alone to completely fill whatever container they're in. The measure of this "runniness when heated" quality is called the melt flow index, or MFI. Wikipedia has lots more to say about it here. If you poke around on Dow Chemical's website, you'll find a huge variation in the MFI (and other parameters, too) among their HDPE products.

This brings up the question: Do you want material with a high or low MFI? The answer depends on how you plan to cast the material into the shape you want. One way, as shown in Randy's video, is to let it cool in the same container you melted it in. In this case, a high MFI is needed so that the plastic will flow nicely into the container. Any bubbles will (eventually) rise to the top, and with luck the result will be a solid block of uniform material. Although Randy clamps a board over the top of the material after it has started to cool, I haven't seen where this has much benefit when cooling in the same container that was used for melting. Maybe it helps squeeze out bubbles. Maybe not. I'm not sure.

As mentioned, HDPE with a lower MFI doesn't really melt. Instead, it sort of softens into a rubbery, sticky blob. You can deal with it, however, by manually kneading it and twisting it while it's hot (You'll need gloves for sure!) to force out the bubbles, perhaps several times as more and more material is added to the mix. Then when enough material is ready, you can let it solidify in a mold with a lid that allows you to apply clamping pressure to force the blob into the mold. The first 2-3 minutes of this video show how this works.

You'll notice (and be disappointed) that the HDPE items in your trash can don't have the MFI printed on them along with the recycling symbol. So, short of heating it up and seeing how it behaves, how do you tell what you've got? Here are a couple of hints based on what I've observed:

1. The two common ways to make HDPE parts are blow molding and injection molding. There's a good description of the differences between them here. In general, the MFI of blow molded items like milk jugs, bottles, and similar hollow, thin-walled containers will be lower than that of injection molded items. You can expect, then, that milk jugs and bottles will soften into the "sticky rubbery blob" state, and that material that was originally injection molded will tend to settle by gravity into the container it's melted in.

2. Among different samples of injection molded material, those with the highest MFI will be the most flexible for a given thickness. I say this based on my own observations, as well as the statement from the Wikipedia article mentioned earlier that "The plastics engineer should choose a material with a melt index high enough that the molten polymer can be easily formed into the article intended, but low enough that the mechanical strength of the final article will be sufficient for its use."

One last hint on material selection is to try to use as close to the same material as you can in any given batch. It's tempting to just throw together bits and pieces of whatever's lying around, but if you try to mix material with different characteristics, you're much more likely to have problems with voids, cracks, and places where two different kinds of plastic don't fuse together properly.

More to follow.

-- Russell Mac

Mr Ron
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Re: Casting HDPE - Introduction

Post by Mr Ron » Fri May 07, 2021 10:32 pm

Isn't this the same as "machinable wax" used for prototypes?
Mr.Ron from South Mississippi

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