Scratchbuilding the Dove part 13

The finished mother mold next to the cockpit and access hatch master. Note the three matching 'indexing' holes in the mother mold flange and moldboard. These, and the securing fasteners that screw into them act to register and secure the mother mold down over the master as I pour the inner 'glove mold' element of this two piece tool.

MAKING THE RTV SILICON GLOVE MOLD

The first photo of this installment shows the finished mother mold next to the cockpit master mounted atop the moldboard. Note the three matching holes in the mother mold flange and moldboard. These were drilled before popping off the mother mold after its lay-up. The holes accept wood screws that index and secure the mother mold over the master with near perfect registration. This alignment would be needed late as I poured the rubber element of the tool.

Note the rough, uneven inner surface of the GRP mother mold element. When the rubber is cast between this cavity and the surface of the master, the hardened rubber will retain the exact shape of the masters surface (in negative, a cavity) and the rubber glove will also key exactly with all the little distortions of the mother mold. This assures exact alignment of the rubber glove mold to the GRP mother mold. See the oblong hole I cut into the top of the mother mold? That's the sprue hole through which I will introduce the catalyzed RTV rubber mold material. That event coming up.

The inner, glove-mold element of the hybrid tool would be made from my old stand-by, BJB brand TC-5050, RTV Silicon mold making rubber. The rubber, while still in its liquid state, would be introduced into the mother mold through the open sprue hole, heightened here with a dam of clay.

The inner, glove-mold element of the hybrid tool would be made from my old stand-by, BJB brand TC-5050 RTV Silicon mold making rubber.

BJB is in California, but they ship quickly to any part of the world. Their address is BJB Enterprises, 14791 Franklin Ave., Tustin CA 92780. Phone them at, 714-734-8450. The casting resin of choice from BJB is TC-810. And be sure and get their spray mold-release, another excellent product from this fine company.

The TC-5050 Silicon rubber is a very robust tooling material and can be used for resin and low temperature metal casting as well as the application here: as a surface upon which GRP pieces can be laid-up to give them form.

Note that in the second photo I have built up a sort of 'stand-pipe' over the mother mold sprue hole. This sprue extension eases the introduction of the liquid rubber into the annular space between master and the mother mold cavity. Even a half-inch of sprue height produces a 'pressure head' sufficient to quicken significantly the introduction of the very viscous RTV rubber.

So, how much RTV rubber to mix? Good question. As this stuff is expensive you should determine, by some means, how much you need for the job. No more. No less. Two methods work for me:

One, if you have used clay as a standoff material between surface of the master and inner cavity of the mother mold, as in this case, is to take the removed clay and weigh it. Clay has about the same density as RTV silicon rubber so a direct measurement of clay weight will be a close determination of the weight of rubber you'll need.

Sure, you can experiment and determine closely the density of the clay to the RTV and work out a conversion factor. But, since you'll find that number to be one-point-zero-something, why bother? Keep it simple!

Weigh the clay and that's the weight of the rubber and catalyst you have to mix. Done!

The other method of determining how much rubber to mix is to assemble the mother mold over the master, then pour rice (uncooked rice) through the sprue to completely fill the annular space. Overturn the tool and dump the rice into the same container you're going to mix the RTV, and mark with a pen the height of the rice within the container and then dump the rice.

Pour into the container your RTV rubber to the pen mark, add catalyst, and mix. Of course, with the addition of the catalyst, a bit more rubber will be made than established, but that slight extra rubber may be needed to account for any errors you made in measuring. The BJB rubber I use is mixed at a ten-to-one ratio with catalyst, so there is not that much of an increase in volume with a small job likes this. But, the quantity of catalyst is indeed something to take into account for pours over two pounds in weight. On a job this small I usually wind up with only a spoon-full of rubber left over - not bad.

After mixing the rubber and catalyst the liquid rubber is de-aired in a vacuum vessel. De-airing is the process of subjecting the still liquid rubber to a vacuum of at least twenty-eight inches of Mercury - which is pretty much a hard vacuum.

Why do this?

Bubbles of air are introduced into the RTV rubber during mixing and if encapsulated in the cured rubber those bubbles may cause distortion problems to parts cast (or, in our case, laid-up) within the tools.

De-airing works like this: as the mix is subjected to the vacuum, air bubbles within it expand greatly. The bigger bubbles posses greater buoyancy and rush to the surface of the mix, break, and the gas discharged to atmosphere. Subjected to a hard vacuum, the rubber is seen to froth a great deal and to then calm down, A typical de-airing cycle takes about two minutes after which the vacuum is broke, the mix poured into the tool, and left to cure at room temperature and pressure.

Here's a closer look at why you don't want air bubbles entrapped within the rubber tool: The part produced from an air-riddled tool that is subjected to differentials of heat (metal casting for example) or application of a vacuum (some resin casting techniques) evidence depressions or pits on the cast/laid-up parts surface. If the casting process involves pressurization of the tool (most of my resin casting is done this way) then the distortions to the piece will show up as dimples.

De-air your tools!

Though not a vital requirement for tools that will not be subject to high temperatures or differential pressures I always de-air my rubber before pouring it to form a tool. Just to make sure that no bubbles will hang up within the pour, spoiling the finish of any part produced within.
 

The de-aired catalyzed RTV rubber is poured into the mother mold and takes form around the internal master.

In the third photo I'm pouring the freshly mixed and de-aired RTV silicon rubber through the mother mold sprue hole. Not much science here: just keep pouring until no more rubber will fill the cavity. After sitting the tool aside for about twelve hours to cure hard, I cut the sprue material (clay and rubber) off flush with the top of the GRP mother mold. At that point the three retaining/registration screws were undone and, with the aid of a putty knife blade I pried the tool (containing the entrapped master) clear of the moldboard.

So, how to extract the cockpit master from the tools?  I had, it would seem foolishly, introduced to the clay forming the mounting material between cockpit master and mold board a deep undercut, defining the perimeter of the masters base. This undercut would have been disastrous had the tool been made exclusively of a ridged material like the GRP mother mold.

However, this hybrid tool, containing a flexible rubber element, can easily shrug out a part possessing moderate to extreme undercuts.

The rubber glove mold, while being held rigid in the mother mold will not easily give up the master, I simply punched the glove mold element out of the mother mold element. (This done by pressing, with a thumb through the sprue hole while holding the mother mold flange securely with the other hand) and out fell the glove mold with encased master. Unsupported, the flexible rubber was easily pulled back on itself, without damage, and the master easily extracted.

And the above describes exactly how future GRP thin walled cockpit pieces would be extracted from this tool. It's the ability of the rubber glove mold to be stretched easily off the work that points to its the main virtue of a hybrid glove mold-mother mold tool used to produce GRP cockpit parts.

Next installment I'll get to the details of how to produce a multi ply GRP hollow shell piece within the hybrid cockpit tool. 

Things To Come

In part 14 I'll get to the details of how to produce a multi ply GRP hollow shell piece within the hybrid cockpit tool.