Re: [CAD_CAM_EDM_DRO] Re: fibre reinforced part casting

Graham,

You talk about the high price of books that contain information on plastic injection molding, you are right, they are very expensive and sometimes hard to follow. I recommend the book called "Cutting Cost in Short-Run Plastics Injection Molding" by Morgan Industries, http://www.morganindustriesinc.com/plastic-molding-guide.htm

The book is very good and informative for the Do It Yourselfer and anyone just learning about the process, and mold making. And it's very economical, $25.00 US, the last time I bought one. You will not be disappointed, and it does talk about fillers and modifed materials, i.e fiberglass fibers.

I am not connected with this company in any other way other than as an owner of one of their plastic injection molding machines and their book.

Victor A. Estes
Mold Designer and Short Run Injection Molder

----- Original Message -----

From: Graham Stabler
To: CAD_CAM_EDM_DRO@yahoogroups.com
Sent: Thursday, December 21, 2006 4:32 AM
Subject: [CAD_CAM_EDM_DRO] Re: fibre reinforced part casting



Qualtity = enough for me. This is not commercial. For example a
triaglide requires 12 rod ends.

I actually have a bench injection moulder, it's pneumatic and will do
a reasonable shot size. I've not used it yet (I was given it) but
after moving house in the new year I should have space for it (and the
compressor).

That said I like the idea of using longer fibres for the
reinforcement, if something had round apertures (such as for bearings)
it makes sense to have fibres wrapping around this part to add
strength as they are put in tension. Most parts I want could have an
open side to the mould, I was wondering about just over filling and
then compressing. I'd CNC my mould from ali, I can't see RTV holding
the tolerances for the bearing housings etc.

I've seen some stuff on resin transfer moulding and was wondering if I
could layup some long reinforcement strands in the mould and then
vacuum some resing with short fibres in to fill the mould completely.

Its seems hard to get decent practical info on many of these subjects,
the info is locked up in the manuals of machines and in books costing
100's of dollars (which is fine if they turn out to be good).

Graham

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Marcus <implmex@...> wrote:
>
> Hi Graham:
> How many?
> How complicated?
> These kinds of parts are best injection molded if you need best
possible
> physical properties.
> A moderately glass filled polycarbonate or similar engineering resin is
> tough as hell when molded.
> Knocking together a simple mold is often not a big investment...I've
built
> tooling like this all my working life, and a prototype or low volume
tool
> can often be knocked out for a couple of thousand bucks.
> Make a thousand parts and the tool amortization becomes a couple of
bucks a
> part.
> Casting them in RTV silicone is a very poor second best if you need
> dimensional stability and good physical properties, but it does work.
> Problem is, you'll spend a good bit of time or money or both on that
process
> too, and the cycle time per part is so much longer that your
productivity is
> severely constrained.
> If I build a quick and dirty in-house tool for molding, I'm usually
> competitive at around 250 parts, compared to RTV silicone, and if I
build a
> REAL rude and dirty mold, I can compete at 50 parts or less.
> If you're comfortable with machining and have a good eye for detail, it
> ain't rocket science to roll your own.
> There are some things you definitely need to know so you build a
safe and
> effective tool...the main one being that the forces involved are
> huge...20,000PSI injection pressure at times, and tens or hundreds
of tons
> of clamping pressure.
> Best path forward if you're interested in doing it this way, is to
go to a
> local molder and see if they run MUD sets.
> This saves you the effort and cost of building a mold base.
> They'll give you a set of inserts so all you need to do is cut the
cores and
> cavities and do the fitting and finishing.
> Not bad at all if you know what you want and are handy with a mill.
> Cheers
>
> Marcus
>





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