Making plastic servo parts (not injection modling)

Typically this sort of thing is done with a polyurethane casting resin in a
rubber mould. I'm not sure that the result would be as strong as you need,
but resin development is a hotbed of activity.

Injection moulding need not be out of the question. Simple aluminium moulds
and hand operated (prototype) machines would probably be adequate for
smallish parts with no requirement for rapid production. [see for example
"The secrets of building a Plastic Injection Molding Machine" by Vincent
Gingery.]

If strength is important I'd go down the route of CNC machining from solid
plastic. You will have finished a batch of 60 units before you have even
completed the moulds for casting or injection moulding. Additionally, you
can modify the design without significant penalty. Finally, when you do want
to make an injection mould, you already have the cnc program to make the
mould !

have fun !
Dave

Yes, I heard that they are going nuts with this right now. There goes
injection molding... .


I thought about that and read some books, but it seems extremely time
consuming. The prototyping machines seem relatively expensive. And since
there is a CNC machine not too far...


... I'll probably go with this.


I'll do my best ;-)


  Thanks!

Matt

Does anyone by any chace know of what plastic exactly servo cases and
servo horms are usually made? Browsing the catalog (or catalogoue ;-) of
my local dealer brings up about 20 different basic plastics with many
subtypes each. Maybe they will know if I sent them samples?

    --IIRC horns are usually made of nylon; cases could be anything;
maybe styrene? If you've got a pal with a CNC, unless you've got a problem
with weight in the application, I'd have him make the horns out of
aluminum. It's no more work than nylon; just gotta lubricate it better
while machining..

--
        "Steamboat Ed" Haas         :  My shop is open to  
        Hacking the Trailing Edge!  :  visiting dog-nitaries...
                   http://www.nmpproducts.com/intro.htm
                   ---Decks a-wash in a sea of words---

But you probably want to use it with stainless steel control rod or
clevis. Nylon / stainless steel is self lubricating. Aluminium on steel
will wear out.

Correct. I should have mentioned that.

   If you already have the originals then you just need to look up different
types of hand mould-making practice (as opposed to machine moulding)
to give you an insight into what`s required from a mould.
   Cheapest and quickest method would be to use some two-part
silicone putty which you simply press around the original parts;
this is then filled with low-viscosity polyester resin for the peices.
Note that polyester resin on its own is extremely brittle but an additive
(5%) can be mixed in with it called a plasticizer: the ratio of which will
determine tensile strength, which can range from an indestructable
rubber type object to a hard and glass-like one.
Electronic circuitboards and sockets are made with this usually
containing fine glass-fibre hairs (as well as colour paste, for any colour).
   You can also add wire reinforcement set inside the piece.
-----------------------------------------------------------------------
Ashley Clarke
-------------------------------------------------------

How about something like Polymorph?

It's mouldable at 62°C, so can be heated with hot water/hairdryer. Cools to
be strong as nylon & can be drilled etc or re-used if desired.

Is available at www.maplin.co.uk  & elsewhere (www.kelvin.com & probably
eBay!).

More info at: http://www.mutr.co.uk/pdf_files/LIT0048.pdf

Dave.

Awesome stuff! I will use that one day, no question! Not on this project
though because 60 deg are easily reached in a servo.

I found this site, but have not used them.

http://www.protomold.com/

Yes, thank you. It's too expensive for this project. However, the
combination of rapid prototyping and injection molding seems to become a
new industry. I am glad that they finally admit that an injection mold
does not need to cost 20k and up.

Try this site. They have how-to videos and a whole bunch of good stuff:
http://www.freemansupply.com/

This is one awesome link. I started to download all the knowledge just
now... .

On Sat, 13 Aug 2005 20:30:35 +0200, Matthias Melcher


As no one else has mentioned them, I'll suggest you look at ALUMILITE
@ http://www.alumilite.com  . Very useful line of molding and casting
products.  Some time back I took a gear with a 1/8 inch hub and
drilled it out so that the outboard protrusion of a round servo horn
would be centered in the gear, then glued the horn to the gear with CA
and filled the remaining voids in  the sides of the gear / servo horn
with modeling clay and pored thin mold making compound (HSII /HSIII)
around and over this "master". The results being a mold which I could
use to cast servo spline hub gears which fit the splined output of the
servo perfectly. If you wanted to reinforce the arms of a servo horn
you might consider using strands of fiberglass or carbon fiber
"roving" placed in the mold before the casting material is poured. If
I were doing something like this, I might also consider using short
sections of stainless steel tubing as a bushing to reinforce pushrod
connection points.

ALUMILITE has some good info in the Applications, FAQ, and How To
sections. Lots of helpful people in the Forums also.

No, I don't work for them, I just like the products.

M.L.

Interesting stuff for sure. Thanks for the link!

    --One thing to know about Alumilite: keep cross-sections from
getting too thick, as the stuff gets HOT when it cures. Neat stuff;
I used it quite a bit years ago..

--
        "Steamboat Ed" Haas         :  My shop is open to  
        Hacking the Trailing Edge!  :  visiting dog-nitaries...
                   http://www.nmpproducts.com/intro.htm
                   ---Decks a-wash in a sea of words---

2 sources I have used who have real nice "how to" sections on creating
a mold and then casting.  Once you figure out the hardness of the
plastic you are looking for you narrow down your products to cast.

http://www.polytek.com/
http://www.smooth-on.com/

my local shops had nothing so I mail ordered direct to the above folks
check out the trial purchase / intro packs to get your start.

The stuff is easy to work with.  With the items I chose weight and
time were critical so planning is important before you mix / pour.

regardless of where you buy, I recommend the how-to's that are on the
sites.
        

Good links. Thank you! I was able to mill a few part out of a block of
Delrin, but when I need more, I relly would like to pour them instead.
So this was very helpful.

Matt