Propeller Molds
"the making of"
A personal interpretation of Bill Lee’s article – Part two

Now, try and figure out the quantity (by weight) of resin you will need by calculating the volume of the box, subtracting the volume of packing blocks, then subtracting the volume of propeller, mandrel, plasticine & the whole lot… quite messy, isn’t it? Anyway, try and come up with some kind of reasonable figure, then multiply by your favorite K factor (bigger than 1) and start mixing up.

Once you are REALLY convinced that all is looking right, that the release agent has been applied properly, that you have mixed enough resin, and that one day, you WILL positively have your propeller back, then you can start pouring:

 Pour just enough resin as to obtain this nice "bulge" over 
the edge of the box.


Of course you’ll need to set the mold exactly level, otherwise resin will spill all over the place.

img13-resin bulge

What? After all your calculations you ended up with a lot of resin left in the can?
Don’t worry: this always happens to me too! Next time decrease that K factor.

Now it is quite possible that you really had enough, so, set the mold aside for a couple of days, depending on the type of casting resin you used, and forget everything about it. More than anything else, don’t spend all your time wondering if you’ll ever see your precious master propeller again! 

When the resin is properly set, the top of the mold –that nice bulge- can be finished flat by filing, grinding, planing, or whatever you like: I use a Stanley Surform rasp for this job. Remember that this will be the side the mold will lay on while you’ll be laminating the propellers, and this also will be one of the sides that will take the big squeeze once you close the mold; so it’d better be nice and flat.

Finally, unscrew the base plate from the box and remove the packing from the mandrel, then a light tap should be enough to separate the half mold from the plate….

and you will be staring at this mess:

img14-plate removed

If you like chinese food, choose a restaurant that still gives you bamboo sticks, and not ugly plastic ones.
Ask then for a fork and a knife and pocket some sets of sticks.
img15-scraping away
Use those sticks with properly shaped points, to remove all the plasticine from the cavity of the mold: use only wooden tools so as not to damage the propeller.

Don’t worry: all this will come out one day!

Take your time, and work with patience, DO NOT for any reason, move the propeller from its place: if this happens you’ll have to restart again from the beginning.
The propeller is held there (strangely enough) only by the release agent,
so avoid using solvents to remove the plasticine and go gently with those chinese sticks.

Cotton buds will also help in the final cleanup, until you obtain this result:

img16-clean female

When you are satisfied of this first half of the mold, and you're quite sure that the propeller is stil solidly stuck inside the cavity, apply again your release agent, one or two coats.
Caution: the bottom spacer does NOT need the release agent: you will instead degrease it properly,
and slide it back onto the stem of the spindle, just before pouring the resin.

The top box can now be securely screwed onto the female mold, after a coat of release agent on the mating surface only. You didn't forget to predrill the holes and mark the position, right?
Img17-male box screwed on      Here again, if you wish to save resin, 
     you can add some blocks 
     of scrap wood inside the box

Just in case, it is better to seal the outside of the mating surfaces with plasticine or/and a layer of masking tape.
By now you might get the feeling that this will probably end up a nice piece of work, therefore you will not feel happy if you have to bash it with a hammer in order to take it apart. 
This is why, at this point, I place some simple extractors on the inside of the male mold.
Img18-extractor on male half
Extractors are made with  8mm i.d. threaded bushes of the type used in making furniture, holding a flat point grub screw just long enough to protrude past the edge of the box. 
Do not use conical pointed grub screws, they will damage the male half of the mold; grind them nice and flat.

The bushing is held in place with cyano, while the screw must be well waxed, otherwise you'll leave it there forever! 

Two of these units, placed at diagonally opposite corners of the male box, will generally suffice for a 6-7" prop.

Once again, try and calculate how much resin you need to fill the box, remember that K factor?
And when ready, pour!
Img19-pouring male half

Obviously, you are dead sure that the release agent is all where it has to be, 
that the mating surfaces are nice and flat, that everything is sealed, 
and what had to be waxed 
had really been waxed.....

Pour 'till you get again that bulge over the edge of the box:
Img20-bulge with extractor grub

Well, it's time to relax: set that sticky thing aside and forget it for at least 48 hours.
Oh, yes, your beloved master prop is definitely buried in that epoxy!


End of Part 2

Proceed to Part 3
Back to Part 1