Friday, December 3, 2021

Manson Hot Air Engine - Free Plan


I got interested in hot air engines earlier this year and wound up building the popular Moriya model designed by Dr.  James Senft.  I got lots of help and encouragement from the guys on the Home Machinist forum.  There's a pretty lively discussion of the entire build here, including a link to a copy of Dr. Senft's original plans.

Sometime later I got interested in the Manson-style hot air engine, mostly due to its simplicity.  Surprisingly, I wasn't able to find a complete set of drawings for a Manson engine, so I tried to come up with an original design.  My first attempt was similar in a lot of ways to the Moriya engine, but it didn't run very well.  The next effort was kind of a cross between the partially complete plans found here and the build log for yet another example found here.  It ran much better than the first attempt, but still wasn't great.  But once again, the guys on the Home Machinist forum came through with some great hints for improvement, and with them, a third engine very similar to the second one ran like a champ.  There's even a video:

 
 
The Plans

This file contains a Fusion 360 model for the third version, 2D drawings based on that model, photos of most of the parts, and a few miscellaneous notes.  If you have any questions or comments about these plans, I'd love to hear them, either here or in a comment attached to the video mentioned above.

Thursday, May 6, 2021

Trash to Treasure -- How to Make Stuff from Scrap HDPE

 Overview

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.  I've been fooling with the process off and on since, and figure it's time to add what I've learned to the existing folklore.

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.  There's a pretty good series of three YouTube videos that starts here where a gentleman named Randy Knapp shows how he does it, with what I consider to be exceptional results.

Material Selection


The first thing to figure out is what kind of plastic to use.  From what I can tell, the best candidate for doing all this at home is High Density Polyethylene, or HDPE.  You'll find plenty of it in your trash can.  It melts at a reasonably low temperature, and it won't give off a bunch of toxic fumes when heated.  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 almost 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 better because the HDPE 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.

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 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.

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, along with 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.

Melting

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 don't think 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 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.

Mold Options


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 move one side to make blocks of various sizes:

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 position of the moveable partition, and squish it down with C-clamps.

Working the Plastic


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.

My Projects


Here are some of the things I've made from recycled HDPE.

I made this extremely crude oscillating cylinder engine a few years back as an exercise to see what I could do with the material.  HDPE tends to be slippery (sort of like Teflon), so it worked well for the engine's simple bearings, piston action, and the seal between the wobbling cylinder and the stationary upright.  Unfortunately, over time the plastic parts warped enough that the ending bound up too tight to run.  (More on that under "Limitations" below.)


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


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

 


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

Limitations

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's 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 tears itself apart.

Even if these internal voids don't materialize, I believe that the cooling process creates internal stresses in the material.  These are what probably caused the warping over time of the parts in my little wobbler engine.  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.