Among of the most valuable of “shop accessories” that I made and one that I ended up using almost everywhere were my “shot bags”. I used them in ways similar to what Vern describes as well as to hold curved parts in place until they cured. The uses are numerous as you can imagine.
Here is how they are easily made. Go to your local ammunition “reload” shop and buy a 50 pound bag of bird shot. Buy some standard “Zip-lock” sandwich bags. Pour 2 pounds of shot into each sandwich bag, press the air out while positioning the shot in the bottom half of the bag and seal the zip-lock, then fold the empty top half over on the lower half. Now wrap the bag in duct tape, making sure to reinforce the ends and corners with overlapping tape.
These bags will last many years (a couple decades in my case). If they start to wear, you just add more duct tape. The 2 pound size is almost perfect size for the individual bags and can be used singly or stacked on top of one another or laid edge to edge. You can toss them to the floor and they will not break or leak shot. You can use masking tape to hold them in place, and they will easily pull loose from any cured epoxy that you may have inadvertently set them on top of.
Here is a picture of some in use when I was laminating sheets of plywood used to build my overhead C-beam.
This is the idea with caul sheets, which are made of dense high temp resistant rubber and then the parts are made in the autoclave where pressure (can easily exceed theoretical 15 psi permitted by vacuum) is equally applied during the cure. The pre-preg cure is accelerated and the temps taken right up to the heat saturation point so the structure will withstand higher ramp temps in Arizona for instance.
No post cure required as is done on tool-less composite with wet resin layup. The speed of the transition within the autoclave from a cold sticky pre-preg to a runny resin mix is just as critical as the stipple and squeegee process. Higher pressures also bring into the picture "core crush" problems which solid foams are not prone to. There are effective methods to prevent this problem (which for some reason Nordam had no idea about and they refused to listen to Contractor engineering folks that already solved this at other factory settings like at for instance, Raytheon!!)
The short of it is.. yes.. the sandbag method works and with rubber sheeting on top of the bagging beautiful contour parts within reason can be made. Your getting into a tooling condition tho... but it is very acceptable. Advantage of shot bagging or sand is there are no vacuum leaks to be concerned about. Bleeder cloth are still involved and that is usual because the layup cures to an excellent correct ratio. By careful preparation you'll end up with near perfect parts with zero risk of a leaky tape seal or pump failure at the worst possible time.
Just a thought. Would it help to vacuum bag parts and then put sand or lead on top of the outside bagging material to use weight to press the resin and glass together tighter into the weave? It would be messy and you would have to keep the mold from creeping but what does the group think and does any one have any experience to share about that? Just thinking outside the box.
---------- Original Message ----------
From: "Stuart Grant" <smgrant@...>
Subject: Re: [Q-List] Resin volume placed on foam
Date: Sat, 27 Mar 2021 07:18:37 -0700
Here is a link to a YouTube video where Cozy Girrl Randi explains about how Cozy Girrls make composite parts, including mixing epoxy, mixing micro, using gloves, low pressure vacuum bagging, peel ply, sanding etc. The video was recorded at Sun-N-Fun 2019 and the beginning has a lot of noise from the air show but Randi has great tips. https://youtu.be/fmuDOWTr_3c
If the link doesn't work search YouTube for
Cozy Girrrl's LoVac Composite Tools & Tricks