[Hallicrafters] Casting Old Knobs

Thu Jan 12 21:07:01 EST 2006

I don't know if this will help but I thought I'd at least offer to throw it 
into the pot.

I've spent a lot of time working with Vinyl Ester Resin and a fair amount 
with West System epoxy the past few years. Other than *usually* being of a 
much lower viscosity than epoxies, working with the resin is much like 
working with epoxy.

What I've found as to mix ratios is a couple % one side or the other from 
optimum makes little difference, or at least it hasn't for me and I'm 
counting on structural strength and high stress conditions.

Cabosil (Silicon Dioxide powder), microspheres, and even mill fiber make 
good filler (wear a mask), depending on end use and availability.  I prefer 
Cabosil for filler when making small parts.  It adds strength to the epoxy, 
or resin and can be added in any proportion desired to get the wanted 
characteristics.  Mircospheres do not add any strength, but they make the 
mix much lighter.  You can add over 50% before the strength is materially 
affected.  Mill fiber is for strength, but the surface finish may not be as 
good as either straight resin, or with the other fillers.  Mill fiber adds a 
lot of strength to the straight resin and I usually add about 10% Cabosil to 
the mix for an improved finish and more mechanically stable mix. (It keeps 
the resin from running out).

Use what ever the manufacturer recommends for a means of measuring be it 
weight or volume.
The West system just uses a pump to get the proper amounts of resin and 
catalyst.

When using Vinyl Ester Resin (Dow Derakane) in up to 200 gram batches Grams 
and CC are interchangeable.
Most epoxies and resin mixes are "exothermic", meaning they give off heat 
when they cure.  The West System gives off a *lot* of heat and you do not 
want to get the stuff on you when it's *that* hot. The stuff is as bad as 
getting dosed by a hot glue gun.  It won't come off and it will 
burn/blister.

Vinyl Ester Resin *usually* doesn't get that hot, but its cure time is 
highly dependent on temperature. Hence with both processes the idea is to 
mix no more resin that is absolutely necessary at a time (in a batch). 
Vinyl Ester is nicer to work with, but it has several drawbacks.  It's more 
expensive than most epoxies and it has a relatively short shelf life which 
is even shorter after being promoted. Which reminds me the stuff has to be 
promoted which is a simple procedure. It also uses Methyl Ethyl Keytone 
Peroxide as a catalyst.  You need to keep the MEKP and  promoter well apart 
as they can develop a nasty disposition when mixed directly. If you are 
going to be working in cool temps there is also an accelerator which is an 
Amine.  I've forgotten which one, but being an Amine it STINKS!  The resin 
has a rather sweet odor with a touch of Styrene while the Catalyst is a bit 
pungent.  Once mixed the batch takes on a rather strong odor that'll make 
your eyes water and at this point a mask with an activated charcoal filter 
is recommended.  OTOH I know of several guys who have worked with the stuff 
every day for the last 6 or 8 years without a mask.

Even with the extra chemicals and equipment I much prefer working with the 
resin compared to the epoxy.

So with either:
1.    Make certain that the environment is clean, the proper temperature, 
and with plenty of fresh air.
2.    Measure carefully even if there is some wiggle room.
3.    Mix thouroughly, scraping the sides of the container to make sure the 
material is well mixed.
        Do not mix so energetically that it adds air.
4.    After mixing, let the pot set a couple of minutes to "de air" before 
using.

One thing we avoid in making structural parts is pressure. Pressure will not 
get rid of air.  Normally it will force the airbubbles to get smaller, join 
together, or even push them into cracks. When the pressure is releases the 
bubbles then want to get larger creating voids.

At this point, if possible instead of pressure we apply a vacuum.  It does 
not have to be a great vacuum, nor does it have to last more than a minute. 
The vacuum will cause any trapped air to expand and climb to the top of the 
mix where it can escape.  When the vacuum is released it puts the mix under 
pressure without the trapped air.  Let the mix de-air normally for at least 
one of not two minutes before applying a vacuum or it may become "instant 
foam".

Even a vacuum cleaner can provide enough vacuum to do the de-air procedure.
De-airl, pour, and then deair again if possible.  Following this procedure 
there is normally no problem with trying to get the mix into corners and 
tight places as the atmosphere forces it in.

Some epoxies and resins list an elevated cure temperature with will speed 
the cure and *may* increase the strength. Either way it *usually* eliminates 
the partial cure problems.

Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
www.rogerhalstead.com

> Hi,
>
> Having done this a few times I forget what I do
> automatically now.
>
> Also try using a digital scale with the measurements.
> The two mixtures might require a weight measurement
> rather than a liquid quantity measurement, don't
> forget to zero the scale to the measureing cup and to
> get the bubbles out swish the mix around inside the
> mold while filling and gently tap the mold on the
> sides and bottom to dislodge the air. Shwish, tap,
> fill, shwish, tap, fill then get it in a pressure pot.
> Be aware of undercuts where air can get trapped and
> work the mix into the areas.
>
> You'll get it, it just takes time to get the technique
> down.
>
> 73,
> Douglas
>
> --- Roger K8RI <hallicraftersgroup at rogerhalstead.com>
> wrote:
>
>> Cook 'em?
>>
>> With epoxy and vinyl ester resins we just elevate
>> the temps a bit. (not a
>> lot though)
>>
>>
>> Roger Halstead (K8RI and ARRL 40 year Life Member)
>> N833R - World's oldest Debonair CD-2
>> www.rogerhalstead.com
>>
>> > In a message dated 1/11/2006 2:42:48 PM Central
>> Standard Time,
>> > iluznst at yahoo.com writes:
>> > Any little bit, short or extra can  throw your
>> > product off. The knobs will either come out sticky
>> and
>> > never  dry or can be rubbery and never harden.
>> > __________
>> >
>> > I paid attention to the A and B amounts using the
>> small measuring cups
>> > supplied, but the knobs ARE slightly sticky.  And,
>> the one that  feels the
>> > best is
>> > bleeding liquid at one small spot in the center of
>> the  knob.  The mix is
>> > old,
>> > but the bottles appeared to have been well
>> sealed.  I do have some
>> > additional casting material, and will see if it
>> makes a difference.
>> >
>> > Also, one very small air pocket appears on each
>> knob.  I thought I was
>> > pouring slow enough to let the air escape, but
>> maybe not.  It looks like
>> > the one
>> > air bubble appears where the mix is first poured.
>> >
>> > Joe
>> >
>>
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