Scratchbuilding the Dove part 14

BUILDING A HOLLOW GRP COCKPIT MASTER

THE INGREDIENTS

OK, I've taken you through the why and how of building a two-element hybrid tool. Now to use the thing:

First, let me point out that the same fiberglass cloth, resin, and thickening agent I used to create the before mentioned mother mold are used here to make the actual GRP cockpit part. The only ingredients added to the process is a gray pigment - used to give me a solid opaque look. I prefer gray.

I refer you to the last photo of part 13: there we saw four small containers. The container on the left contained a fibrous 'thickening agent', used to thicken the epoxy 'gel-coat' to a point where it would not readily sag from vertical surfaces.

That same thickening agent, when mixed with the epoxy resin, epoxy hardener, and coloring pigment, produces the gel-coat, which is the first layer of material brushed onto the surface of the tool (mold). This gel-coat, of course, becomes the surface of the finished piece.

And note in that same photo that I have already 'painted' the un-reinforced (glass, carbon, burlap, horse hair, or any other none soluble substance added to the resin to afford strength is termed an 'reinforcement') gel-coat within the cavity of the glove mold.

The first step in producing a hollow GRP (glass reinforced plastic) part: brushing in the 'gel-coat'. Consisting of catalyzed resin mixed with a gray pigment and thickening agent to prevent sagging, the gel-coat forms the surface (substrate) of the finished part.

A fortunate situation exists here: the RTV rubber used, even when not treated with a spray mold release agent, is little effected by epoxy resin (unlike how the RTV rubber reacts to polyurethane, which eventually degrades the rubber to the point of destruction). I take advantage of this nearly inert interaction between epoxy and silicon rubber by omitting the application of a mold release and brushing the gel-coat straight onto the raw rubbers surface. Since there is a great deal of adhesive attraction between the wet gel-coat and unprotected rubber I don't suffer much sagging of the gel-coat off of vertical surfaces. A soft brush is needed for application of the gel-coat to limit 'brush marks'.

It's important to let the gel-coat get to a hard tack, but not a complete cure, before proceeding with the first lamination of fiberglass reinforcement.

THE PROCESS

The size and shape of the fiberglass cloth should be as close as you can get it to the actual geometry of the cavity, with no more than a quarter-inch of glass projecting past the tools flange plane. It is very useful to make a marking/cutting template to assist you as you cut out the many layers of glass cloth needed to build up the laminates within the tool.

After the gel-coat had cured to a hard tack, I laminated two pieces of four-ounce fiberglass cloth into the cavity. Care was taken to wick out the excess resin with the brush. To the extreme left is the first GRP piece created in this tool. It will be made into a detailed interior. The piece seen here in work will be mounted directly to the upper hull master.

Making the glass cloth marking template is done by taking an oversized square of the glass you're going to use, pushing it into the virgin tool cavity, and marking onto the glass the outline of the flange plane with a dark colored 'clothes marking pen'. Carefully pull the glass cloth piece out of the tool and place it on an oversized piece of cardboard paper already sitting atop the cutting table. Cut through both the glass cloth and cardboard, making the cut one-quarter inch outside of the pen marked outline. I prefer the disc type cutting wheel described in the previous installment for this kind of work.

Save the glass piece and use the cardboard as a marking template to outline the other pieces of glass cloth needed to produce the remaining GRP lamination reinforcements.

After the gel-coat had cured to a hard tack, I laminated two pieces of four-ounce fiberglass cloth onto the gel-coat layer, saturating the work with catalyzed epoxy resin. Care was taken to wick out the excess resin with the brush. Note that extra piece of pre-cut fiberglass cloth to the right with its marking template just over it, in the first photo? To the extreme left is an initial part produced from this tool - kept on hand as a spare.

After twelve-hours of cure, a hardened cockpit part is pulled from the tool. The slight undercut present at the base of the part (near the flange line of the tool) defines the flash cut line. The rubber element of the tool permits the incorporation of deep drafts and even undercuts such as this - the rubbers flexibility tolerates a great deal of flexing, permitting easy extraction of even the most complicated of shaped GRP parts without damage to tool or part.

EXTRACTION OF GRP COCKPIT PART FROM THE TOOL

After twelve-hours of cure, the hardened GRP hollow cockpit part was pulled from the tool. The slight undercut present at the base of the part, near the flange line of the tool, defines the cut point of the part. The rubber element of the tool permits the incorporation of deep drafts, and even undercuts such as this. The rubbers flexibility permits distortion (after removal of the mother mold) of the glove mold to permit extraction of the GRP part without damage, no matter the degree and depth of undercut.

(The careful reader of this article will note that I have made a change in the sequence of operations employed to achieve the final two cockpit masters: I will not work the second GRP piece produced from this first tool. It will be discarded. I now plan to finish one hollow GRP cockpit/access hatch master by outfitting it with the two outboard air-scoops and window recesses. That reworded GRP piece will then be used to form a new glove mold insert to fit the existing mother mold).

Take a look at the finished GRP part, to the left, and the tool that formed it, on the right in the third photo. Note that the outline of the access hatch has been captured perfectly on the part. I will take that GRP part and cut out its windows. I'll install two vacuformed air-scoops on either side, and use this piece to make another glove mold. From that took I will produce the two masters used for production. One will be glued atop the upper hull master and will represent the access hatch closed. The other GRP cockpit will be built as two pieces: the access hatch, and the cockpit without the access hatch. On these pieces their interiors will be detailed as seen in the movie. This 'beauty' cockpit/access hatch set will stand as an alternative to the closed cockpit piece that will become an integral part of the hull master. This will give me (and those who purchase a kit based on this tooling - more on this later) the option of building an 'in flight' version, or one with the hatch open revealing the details within.

Another look at the finished GRP part and the tool that formed it. Note that the outline of the access hatch has been captured perfectly on the part. Two GRP parts are made. One will be permanently mounted to the upper hull master and will represent the access hatch closed. The other GRP cockpit will feature a detailed interior.

TRIMMING THE GRP PART TO FIT THE HULL

Examine the fourth photo. Cutting off the excess 'flash' material from the base of the cockpit parts is the next chore. This flash is seen as that lower portion of the GRP part formed over the clay used to support the master during tool fabrication. And the excess fiberglass cloth that projected past the tool flange plane. The pieces base was trimmed back to outline with the aid of a moto-tool outfitted with a glass reinforced carbide cut-off wheel.

(Take care when you do this kind of work, even though the cut-off wheel I use is reinforced, it can still fly apart sending sharp pieces shooting everywhere at high velocity. Use of eye protection is a must. Also, lots of fine fiberglass/resin dust is liberated during this rather messy operation, so ware a mask as well).

Both GRP cockpit pieces were cut loose from the flash at their bases and were then sanded to exact fit atop the upper hull master. Note that I have employed a masking tape 'handle' to give me something to grab onto as I slide the cockpit piece over the hull mounted sandpaper.

The hollow GRP cockpit piece was cut loose from the flash and then sanded to an exact fit atop the upper hull master - employing the same sand-in-place technique used when I was matching the original solid cockpit master base to the upper hull. Photo four shows how I taped #240 grit sandpaper atop the hull to accomplish the final fit between it and cockpit. Note here that I have employed a masking tape 'handle' to the hollow GRP cockpit piece giving me something to grab onto as I slid it over the hull mounted sandpaper. In the same shot you can see the discarded flash that was removed with the moto-tool.

In Part 15 we open the cockpit windows...