Step 21: Propeller Pt 4/5 - Hub
HDPE (or High Density Polypropylene) is a great material to work with as it is hard, easy to machine and has a low coefficient of friction. This site has a good breakdown of how it compares to other engineering plastics. When bought in sheet form it is fairly expensive but it is laying around your house in a variety of forms, milk jugs, other food grade containers, cutting boards etc.
The process I use has been refined after many trials and represents the best method I have tried so far. I started by searching instructables and the web to find what others had done and found this one. I tried this method several times changing different parameters each time but melting the plastic in oil didn't give the final consistency I was after. My next idea was to melt the platic in a tin can that was submerged in oil. I neglected the effect of bouyancy and had problems keeping the can submerged. My first thought was to place a weight on top of the can but this made it too top heavy and prone to tipping over, not something you want when it is in hot oil. I thought I found the perfect solution by attaching the can to the bottom of the pan using magnets but when the oil gets up to temperature the heat is enough to break down the magnetism of the rare earth magnets I used. The next step of the evolution of this process was to melt the plastic in the oven and is the method I used here.
The first step is to clean out some milk jugs, take off the labels and reduce them into flakes. I do this with a pair of scissors while watching tv. I can only do about four jugs at a time which takes around an hour before I start to get blisters from using the scissors. The flakes should be as small as you have the patience for, as reducing the size in half requires four times the cutting time. The smaller the flakes are the easier it will be to eliminate voids in the plastic. Cutting the milk jugs up by hand is very tedious and I have tried to come up with better methods. I can tell you one that doesn't work is using a paper shredder. HPDE is used in tick tac lids as it is very flexible and won't break due to fatigue when flexed. This property means that it will not shred in the paper shredder but will deform enough to go in half way and get stuck. I cut up four milk jugs aiming for flakes 3/4" x 1/8" in size (Photos #1,2).
I cleaned up three tuna tins for use as moulds. I have made billets from all sizes of cans from tomato paste for the shaft collar and larger tomato cans depending on what I want to use the plastic for. (Photo #3)
Next I filled the tins up with flakes. When melted these will compress and the tins will have to be topped up. (Photo #4)
With the oven preheated to 350 degrees Fahrenheit I put the tins in for half an hour. HPDE has a melting temperature of 266 F and an extrusion temperature of 350-500F. With a household oven the temperature can be expected to swing +/- 25 F as the heating element cycles on and off. Trying different temperature settings I fould that at 375F and over the plastic would burn and give off an unpleasant smell. I've settled on 350F as the best temperature to use taking longer to melt the plastic but avoiding overheating issues.
After the half hour I removed the tins from the oven and press down the partially melted flakes with an old spoon to compress the material and try to remove any voids (Photo #5). If the tins are out of the oven for over a minute or so the extremities will begin to cool and transition from opaque to white.
I kept putting in the tins in for half hour stints and after two hours they were fully melted. I gave them one last press with a spoon to try and get the surface as even as possible to minimize machining afterwards. Photo #6. When fully melted they are fully opaque. The material is not runny but has a sticky gooey consistency.
I have tried many different methods of heat cycling the plastic to minimize internal stresses. If the pieces are thin enough and taken out of the oven at this point and left to cool, they will warp. The easiest way to minimize the heat stess is to just turn off the oven and walk away. The plastic will retain heat remarkably well and will be too hot to handle even after several hours. I leave them overnight to be sure everything has cooled down slowly. There is enough shrinkage when cooled that the parts come out easily from the tins even when there is a slight lip on the inside. Photo #7.
After cooling the recycled plastic is ready to be machined into propeller hubs. Starting with the top hub the top surface is machined flat (Photo #8). The excess plastic is cut away using a hack saw (Photo #9). The piece is then flipped over and the bottom surface is machined flat (Photo #10). Next the five holes are machined with an end mill (Photo #11), and then the recess for the bolt head (Photo #12). A 1/8" groove is cut for the propeller shafts (Photo #13). The last step for this side is to cut out the circular profile. The part is now flipped over and recesses cut for the nuts (Photo #14).
The bottom hub is similar with the top surface of the piece machined flat (Photo #16,17) and then flipped over and flattened again (Photo #18,19). Holes and a 1/8" groove are machined next (Photo #20) and finally the profile cut out (Photo #21,22). Not shown, but the final step is to flip the piece over one more time and machine the recesses for the bolt heads.
As I progressed through the steps I tried different cutting speeds and depths. I discovered by the end that I was not taking deep enough cuts which resulted in the bit chattering on the surface. The facing cuts would have been much cleaner such the recess in Photo #8 had I taken off twice as much material per pass.
Drawings for the hubs are attached.
(Edit 29/4/2012, propeller hub drawing now attached)