Recycling HDPE and PP Plastic

I've been playing around with my old lathe a lot lately (its a pre-WW2 Myford ML1 museum piece, slop everywhere, but its surprisingly useful for working in soft metals and plastic), and browsing Youtube to that end.

But an article about recycling HDPE (high density polythene) into blocks for machining purposes really piqued my interest, as I am always on the lookout for plastic scraps to use for machining stuff.

The great thing about HPDE and PP (Polypropylene) is that they are both quite durable plastics; not too brittle like melamine and perspex (acrylic) can be . They both also melt at low temperatures and don't give off any toxic fumes while doing so.

A bit of experimentation in the oven found that HDPE melts at round 180 C, PP at 200; though these can vary a bit according to the exact sample. The HDPE (recycle code 2) comes from plastic milk bottles and the PP (code 5) from ice-cream containers! When I say melt, it is doughy rather than fluid, but quite workable.

Once out of the oven, it hardens quickly so you have to work fast. I made up some 5mm spacers out of ally tube so I could squish the blob between two boards, and the spacers would maintain the resulting sheet at a consistent thickness.

 As you can see due to insufficient care I ended up with a crease or two where the material was under that thickness, but apart from that it was bang on, so I decided to press (!) on; It was for a boomerang, so I could carve the thickness down a little to remove the variations. Incidentally PP remelts fine, so if you stuff-up, just re-do the process. HDPE seems a little more difficult to re-melt; I haven't yet established how much higher in temperature I need to go for success.

I had laid out the material to be melted on the oven tray in the rough shape I needed, to avoid waste, and it was easily cut out from that.

I'm really excited about all this; at last I don't have to make do with materials and thicknesses that aren't suitable, as I can control that; and all I have to do to get them is eat a lot of ice-cream!  win-win, as they say.

Presenting - the Variorang!

The photos explain it all, really - a boomerang made out of strips of material (in this case polypropylene, a really good material for boomerangs)
joined with screws and pins.

I'd wondered when making my first booms the traditional way (cutting a flat shape from sheet ) whether joining strips with a simple overlap joint would be feasible, since it creates a boom with a layered 3D shape; would it fly?

Then i saw a number of "pocket knife" type booms on Pierre Boillon's site, and that answered my question.




It was obvious once I considered this method of construction that it lent itself to the creation of a design of boomerang the geometry of which is very easily modified;  a central screw allows elements to rotate to any desired angle, and pins pressed thru the elements lock them in position; to change the shape again, just re-pin.

I added the "winglets" not just for more variability,  but also because I needed something for balance at the end of the main arms to compensate for the weight of the added screw joining the two arms.

Boomerang making part 2; a few mis-steps.

As mentioned in my previous post, difficulties in finding suitable materials has played a large part of my self-education in the practice of making boomerangs.  A secondary aspect has been the fabrication side.

My first foray into the sphere of lap-jointed boomerangs, for example. Wood would (!) seem an ideal material, light but rigid.  Since NZ is home to some of the world's largest Pinus radiata plantations I rather unsurprisingly chose to make mine out of that (radiata pine).

The lap-jointing, courtesy of some careful sawing,  went surprisingly well, and i ended up with quite a nice looking, relatively light boomerang. However in the process of planing what was a 10mm thickness down to 5-7mm (= a lot of shavings!) and subsequent shaping exposed a difficulty in maintaining a sufficient joint strength. On an early test flight, a mild crash broke the rang on a part of the joint. dang!   I'll try that again some time.

Meanwhile, the idea of moulding my own cloth-reinforced resin rang had taken hold.

I had some polyester potting resin on the shelf;  add some strips of polyester/cotton tee-shirt, a plasticine mould built on a foil-covered board in the outline of my selected plan, and presto!  ...oops, whats this?  my composite rang is a rubbery flop! not enough hardener?

More on boomerangs - the making of them.

Once I got into the subject of boomerangs I was soon fascinated by the scope of the topic. Combining the skills of technology, craftsmanship and creativity,  the making of boomerangs is more art than science, but no less alluring for that.

The first art to boomerangs is that of actually throwing them, and in the absence of any expertise in my neighbourhood I had to resort to the internet for some help.

Of course the easy path is to go buy one; even boomerangs have their mass-produced commercially available models.

The Aussie Magic, a typical A-pattern sport 2-blade boomerang

But since when did I choose the easy way? Attempting to achieve a good returning boomerang by building them at the same time as I learn to throw them has definitely complicated the whole exercise, but hey, I have the time, so why not.

MATERIALS

One issue I have struck early on is that of the suitability of materials. Ideally the material should be light enough to fly well (weight can always be strategically added), durable, adequately rigid in narrow thicknesses, and easily worked.

Another self-imposed criterion is that the material should be readily available and easily affordable, to avoid my research becoming too expensive.

Plastics of various sorts are an obvious option. My first problem is understanding what material I am using; for example one of my first boomerangs was made from a 4mm kitchen chopping board. It is a waxy translucent material, fairly dense and therefore a little heavy, but easily worked; I think it is polypropylene. PVC (polyvinyl chloride) is another possibility. I have also used ABS (acrylonitrile butadiene Styrene). Polystyrene is a little brittle, melamine even more so.

I also intend to experiment with creating my own reinforced resin laminates, so I can achieve the required attributes.

I also intend to explore the use of aluminium extrusion, as aerofoil-shaped sections are available quite cheaply. I believe that metal boomerangs are not used for safety reasons in competitive boomerang events, but that is not an issue for me at the moment.

DESIGN

There are a myriad of designs to draw inspiration from. the early design decision is the number of effective blades the boomerang should have; four and three-bladers are general suitable for short fast accurate flights; I personally find the two-bladers,  traditional or otherwise,  for long range slower flights, more intriguing.

FIRST ATTEMPTS

The first design I attempted to build was an A-shaped 2-blader similar to those pictured above, called Aussie_Round, as the plan showed appropriate airfoil cross-sections.

I built two, one in 3mm pvc, the other in 4mm pp; The latter I got flying fairly well, until a recent careless throw landed it firmly in a treetop, where it hooked over a branch and will need a decent storm to dislodge.

CONSTRUCTION

I found construction to be a quite straightforward:

• download pattern and scale to required size
•  print on A4  paper (make sure print size is a full 100% ),  and cut out paper pattern  (paste together several sections if larger than A4)
• tack pattern to material and transfer outline
• cut out outline with jigsaw
• carve aero profile with surform file, and scrape smooth with craft knife

The scraping is used because trying to smooth these plastics by sanding produces a furry mess, whereas scraping works well.

THE LEARNING PROCESS

 

Insolite - 3mm abs version

  Insolite (a small weighted 2-bladed design by P Boillot - see my index of his plans) in 3mm abs - this version of abs was a bit brittle, and the rang broke on a crack introduced while fabricating.

Insolite mkII - 1.5mm polypropylene

A version in 1.5mm polypropylene proved too flexible, and unstable.

It's a bird, it's a plane, no , it's a ....boomerang?

George Dimantiev's "Trigger" mta boomerang.

Yup, it's nothing like the Australian aboriginals bent sticks we all know as boomerangs, but that's what it is.
Did you know that experiments have proved that boomerangs still work in outer space? i.e. the basic mechanics are more to do with physics than aerodynamics, although here on earth the latter does come into play.
 
My interest in boomerangs started a few weeks ago as a lightbulb moment during thoughts about how to keep Pukekos out of our vegetable patch; Pukekos are a sort of avian combine harvester, and our plastic bb gun wasn't fierce enough to deter them, on the rare occasions of a direct hit.

Actually it started long before that. When I was a kid, at one stage a cereal manufacturer actually gave away small plastic boomerangs in their cereal boxes, and I recall hours of fun flying my Weetbix Boomerang.

I haven't got anywhere near accurate enough with them, but is been fascinating research.  like almost anything, boomerangs have evolved a competitive sports element, where distance , accuracy, and mta (maximum time aloft) are competed for.

Interest in boomerangs seems to have waned in recent years, which is a pity, for their creation combines elements of art, science, and craft.

A guy called Pierre Kutek has kindly created a big collection of plans, and another Pierre, Mr  Boillon, has an even larger gallery of thousands of different designs - he's even reviewed their performance. I've take the liberty of loading his index into a database and created a report so I can sort by name or his boomerang performance score.