Casting HDPE - Introduction

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rmac
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Re: Casting HDPE - Introduction

Post by rmac » Sat May 08, 2021 11:20 am

Mr Ron wrote:
Fri May 07, 2021 10:32 pm
Isn't this the same as "machinable wax" used for prototypes?
I don't think so. Machinable wax is a mixture of wax and plastic. There are recipes online that use paraffin for the wax and either HDPE or LDPE for the plastic. I don't know what the commercial versions are made of.

-- Russell Mac

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rmac
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Casting HDPE - Melting

Post by rmac » Sun May 09, 2021 7:47 pm

When it's time to melt a batch of HDPE, about all you have to do to get it ready is to make sure it's clean, then cut it up into pieces that will fit in your melting container. If you have a bunch of really odd-shaped items, it might be useful to shred them so they pack efficiently into your container. But otherwise I haven't seen that shredding has any benefit.

You'll want to melt your plastic in an oven at about 350 degrees F. HDPE has very poor thermal conductivity, so if you're dealing with thick blocks of material, it will take a long time for them to heat throughout. Avoid the temptation to speed the process by raising the oven temperature. If the oven gets too hot, the plastic will burn and smoke and you'll get an icky brown layer of oxidation on the surface of the material.

If you're using plastic with a high melt flow index, it will be easy to see when the material has become semi-liquid and started to flow into the pan. But it's not so easy to tell when the bigger pieces have melted completely. It's also not easy to tell when all the bubbles have risen to the top and popped. For both of these reasons, it a good idea to let the batch cook for a good long while before you let it cool and solidify. Again, just be patient.

More to follow.

-- Russell Mac

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Casting HDPE - Mold Options

Post by rmac » Tue May 11, 2021 5:27 pm

As shown in Randy's video, a non-stick brownie pan works well if you plan to cool the material in the same container you melt it in. The non-stick surface and the sloped sides make it easy to remove the cooled plastic from the pan.

However if you want to use low-MFI material, you will need a mold that somehow allows you to clamp down on a lid to force the plastic into the mold under pressure.

I prefer the high-MFI, cool-in-the-melting-pan approach myself, but sometimes don't want to make a whole brownie pan full. So I sort of went overboard and made this mold where you can position the sides in various ways to make blocks of various sizes:

mold.jpg

This thing just barely fits in my thrift shop toaster oven. It doesn't have a non-stick coating or any draft to aid in removing the cast plastic, but since it comes apart, this isn't a problem. Naturally, you have to plug up the unused holes to prevent the plastic from leaking out. When I do find myself needing to compress some low-MFI material under pressure, I just cut a piece of plywood to match the current positions of the moveable partitions, and squish it down with C-clamps.

More to follow.

-- Russell Mac

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Casting HDPE - Working the Material

Post by rmac » Wed May 12, 2021 11:33 pm

Once you have a nice block of HDPE, you'll be happy to discover that you can work it with most any of your woodworking or metalworking tools. In particular, it's a lot of fun to machine on a mill or a metal lathe because you can remove material much faster than you would ever dream of with metal.

About the only thing that doesn't work is sandpaper. HDPE is pretty resistant to abrasion, and when you try to sand it--especially by hand--nothing much happens. It just kind of sits there.

Here are some of the things I've made from cast blocks of HDPE:

wobbler2.jpg
HDPE tends to be slippery (sort of like Teflon), so it worked well for this model engine's simple bearings, piston action, and the seal between the wobbling cylinder and the stationary upright.

trunnion.jpg
These trunnions for the tilting table on a disk sander are another place where HDPE's slipperiness made it possible to get a tight, low-friction fit between two sliding parts.

wheels.jpg
HTPE is waterproof, so it worked well for these replacement axles, bushings, and wheels for a pool sweep.

ski.jpg
This replacement walker ski takes advantage of HDPE's abrasion resistance. It and its mate were in use for a couple of years and showed almost no sign of wear.

More to follow.

-- Russell Mac

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rmac
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Casting HDPE - Limitations

Post by rmac » Thu May 13, 2021 7:06 am

Obviously, you'll be limited in how big a piece of HDPE you can cast by the size of your oven. What's less obvious is that it's difficult to make very thick pieces. In my experiments, blocks more than about 1-1/2" thick almost always have internal voids in them. These flaws are not round like bubbles would be, but appear more like tears.

This is pure speculation on my part, but here's what I think causes these voids. HDPE shrinks a lot when it cools. (That part is a fact.) Before it has a chance to solidify, this shrinkage is relatively uniform throughout, and the block just gets smaller. But since the block naturally cools from the outside in, eventually the top, sides, and bottom solidify to make a rigid shell that surrounds the still-molten material inside. Then as that material tries to shrink within the rigid shell, something has to give and the inner material basically rips itself apart.

Even if these internal voids don't materialize, I believe that the cooling process creates internal stresses in the material that may cause the material to deform as it's machined, or even slowly afterward. As it turns out, you can anneal the material to help mitigate this problem. This website has more details on that process.

One last problem is that most adhesives won't stick to it at all. I've never had any luck trying to weld it, either. But bolts and nuts and screws all work, so it is possible to fabricate items with multiple parts if you don't mind using mechanical fasteners.

The End

-- Russell Mac

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Re: Casting HDPE - Introduction

Post by BadDog » Thu May 13, 2021 12:01 pm

Thanks for the great information!

I wonder if cooling it slowly would help. It does for similar issues in some metals, glass, etc. Maybe wrap it in a heavy blanket when it comes out of the oven? Or if a small toaster oven type affair, just turn it off and wrap the whole thing.
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rmac
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Re: Casting HDPE - Introduction

Post by rmac » Thu May 13, 2021 12:47 pm

BadDog wrote:
Thu May 13, 2021 12:01 pm
Thanks for the great information!

I wonder if cooling it slowly would help. It does for similar issues in some metals, glass, etc. Maybe wrap it in a heavy blanket when it comes out of the oven? Or if a small toaster oven type affair, just turn it off and wrap the whole thing.
You're welcome!

Slow cooling might very well help. I generally just turn off the oven and then let the plastic, mold, and oven cool together, but I've never tried wrapping it all up like you suggest.

The website describing the annealing process mentions cooling rates in the range of 10-50 degrees F per hour. I suppose you could approximate something like that by slowly lowering the oven temp over a period of several/many hours instead of just turning it off. I seem to remember trying something similar once a long time ago. But, I don't remember the outcome, and it would have been before I had figured out that not all HDPE is the same. Might be worth revisiting the next time I fire up the plastic foundry.

-- Russell Mac
Last edited by rmac on Thu May 13, 2021 2:54 pm, edited 1 time in total.

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Re: Casting HDPE - Introduction

Post by pete » Thu May 13, 2021 1:26 pm

My thanks as well Rmac. Very informative and interesting.

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Re: Casting HDPE - Introduction

Post by Harold_V » Thu May 13, 2021 4:00 pm

The voids mentioned on thick materials is a common problem with casting metals. That's the reason risers are employed, so the casting can be fed molten metal to prevent voids as it solidifies. I don't see that as a solution for this process. Just agreeing that your thinking is on the right track.

Quart oil containers are offered in colors, as well as large buckets. In your experience, have you tried using them, and, if so, have you mixed types?

What types of containers have you used with success? What combination have you tried and not achieved success?

Thanks for your very welcome information on this subject. It is something that has always held my interest.

H
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rmac
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Re: Casting HDPE - Introduction

Post by rmac » Thu May 13, 2021 11:24 pm

Harold_V wrote: Quart oil containers are offered in colors, as well as large buckets. In your experience, have you tried using them, and, if so, have you mixed types? What types of containers have you used with success?
I think I mentioned earlier that I prefer to cool my material in the melting container rather than transfer it to a different mold and force it to cool under pressure. So for me, material with a high melt flow index (MFI) works best. That rules out containers like motor oil bottles, milk jugs, detergent bottles, etc. since they are all blow molded using a low-MFI HDPE variant. It's also really hard to get oil bottles in particular perfectly clean.

Five-gallon buckets are injection molded and work well for me. You'll see some orange bits in a couple of pictures above. Those were Home Depot buckets in a former life. The plastic in them is good, but it's a pain to scrape off the labels. Also remember that the guy in the video mentioned in my first post was using what looked like brand new bucket material for his slingshots, with extremely good results. If I was going to buy new material to work with, I'd probably follow his lead.

Around here (Phoenix), it's not uncommon to find lids from pool chemical buckets that have blown onto the street from the backs of the pool servicemen's pickup trucks. They're also injection molded, and so work well for me. These lids are mostly from the same (local) manufacturer, so there's an excellent chance they're all made from identical material. Anything that's blue or yellow in my pictures came from these. The pool chemical buckets themselves are also good, except that 1) they don't often blow out onto the street, and 2) like the Home Depot buckets, it's a pain to remove the labels.
What combination have you tried and not achieved success?
Before I figured out the difference between high-MFI injection molded items and low-MFI blow molded items, I would just throw a bit of everything into the pan and hope for the best. And depending on what "everything" was on a particular day, the results would be sometimes good and sometimes not so good. Items in my pictures with lots of different colors came from the times I simply got lucky with a mix of unknown materials.

These days I don't mess with blow molded containers at all. I will mix injection molded items with different colors if I think they are the same material (like the blue and yellow bucket lids), but otherwise I try to stick to material from identical containers within any given batch.

-- Russell Mac

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Re: Casting HDPE - Introduction

Post by Harold_V » Fri May 14, 2021 1:59 am

Thanks. I hadn't figured out the blow mold versus the injection mold thing.

Don't know when I'll give this a go, as I'm up to my eyebrows in solving the cooling problem with my induction furnace, but I'll get to it some day!

H
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Re: Casting HDPE - Introduction

Post by RSG » Fri May 14, 2021 6:51 am

Very educating Russell thanks, some day I'll try it! Should work well for fishing reel handles.
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