Propeller Molds
"the making of"
A personal interpretation of Bill Lee’s articlePart one
by Mario A. Ferrero
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Start off with all the necessary hardware:
  • a mandrel of the appropriate diameter  (7 & 8 mm shown)
  • a bottom spacer (same dia. as the mandrel)
  • an upper plate
  • a back spacer (the brass article)
  • a well polished master propeller

Then build the outer box for the first casting: the female mold,
and obtain a suitable base plate for mounting all the hardware on.
The base plate can be reused for more molds, 
provided it is strong, flat, and well waxed.

This one shows that it was, several times.

Slide the mandrel into the propeller, front side up, then the bottom spacer onto the mandrel,
and the mandrel through the base plate in this fashion:
img03-hub assembly
Yes, the prop is not the same as the one in the whole story, because I screwed up the original photo –sorry.
Anyway, this is also to show that a master prop can be readjusted if necessary before copying.
This one in particular, had the hub reshaped and an aluminium front plate fitted to make things really nice and square, and to cancel usage marks.

Propeller courtesy of Parent & Kelly (circa 1974)

Securely fasten the whole lot to the base plate by suitably packing the protruding stem of the mandrel (with the bottom spacer, or anything else, as in this case) and tightening 
the 4 mm socket screw 
(you did tap the hole, in the mandrel, didn’t you?)

Aligning the propeller with the marks you will have traced on the base plate will help afterwards, 
in positioning the upper box.

img04-back bolt

Once satisfied with all this, start packing up the back of your prop with plasticine,
taking care not to warp the blades upwards by packing too tight:

img05-plasticine packing

img06-template scraping

Then scrape away the excess plasticine by running a 20 degree template all around the leading and trailing edges of both blades (unless you are making a single blade...)

The tricky story will begin around the hub, when the template switches from a trailing edge, which is on the bottom, to a leading edge, which is on the top. 

The plasticine will be reduced to an extremely thin blade, which will then be the same thin resin blade on the male mold and I always thought it would break, therefore I prefer to make the transition by packing up more plasticine and trying to guess the correct separation line.

In the end, you should come up with something like this:

img07-finished hub

Once you are satisfied with the finish of the plasticine, and convinced that there are no undersquares, it is time to give a good coat of your favorite release agent, including all of the base plate.  And maybe another, after a while.
Be extra careful at this point: the release agent will bring your precious master prop 
back to you, when all this mess is over!

img08-aluminium top plate
Next, when the release agent is nice and dry, 
and polished if necessary, 
mount the upper aluminium plate on top 
of the mandrel cone with another 4mm 
socket head screw 
(you tapped that hole too, hopefully…).
To give access to the screw once everything is buried solid in resin, you’ll have to install a piece of tubing over the screw head, long enough to protrude over the edge of the box. 

Fasten the tube from the inside with a well packed blob of plasticine.

img09-screw funnel

After all this, you might as well fasten the box to the base plate with 4 wood screws driven through from the bottom of the base. A good idea is to pre-drill both pieces before starting with all the plasticine stuff. 
Don’t forget to mark the relative positions!

To save resin I usually cyano some blocks of scrap wood inside the box, about 5 mm below the edge. Personally I don’t believe in making the box much shallower than about 50mm, as this will be the part of the mold in which most of the work will be done while making the props, therefore I like it to be rather solid and stable.

Once ready for the first casting, the inside of your job will look like this:


img10-inside the box      
The scalpel marks on the walls of the box are there to give a better grip to the resin.

Actually they are just useless: that damn’ stuff will grip to about anything!

While on the outside, it will be better to seal the joint 
between box and base with some nice fillets 
of finger-smoothed plasticine.

It is very unlikely that the resin will seep through,
unless you went really dirty on the mating surfaces.

img11-lateral sealing

End of Part 1