Need Help with One-Off Part Building

[LMB]

I've had a customer provide me with a mold built out of plywood and bondo. The finished part will be a 4' x 4' cradle used to support patients during specialized medical proceedures. It will only have support around the edges and can not have bracing or reinforcments as it will interfere with the function of some of the medical equipment. So it will span 4' x 4' and need to support up to 300lbs. We really do not want it any thicker than necessary. Any suggestions on thickness? Laminate schedule? I have 1.5oz mat and 1808 bi-axle fabrics on hand. Not sure they will work given the compound curvatures of the piece. Also, suggestions for mold release? The mold surface is not sealed and is somewhat rough. Finish does not have to be smooth or pretty though. Trying to keep it simple and low cost-so keep that in mind before you start suggesting infusion and vacuum bagging. I do appreciate any input however.

[tinhorn]

I'm not going to suggest a laminate schedule, but having built many parts from quick-and-dirty molds I will suggest that you seal the surface of the mold. Either resin or gelcoat, but add some wax solution so the surface will cure. Then you can sand it smooth (or not).

Wax the heck out of it--at least four coats, using a different pattern each time. (I also alternate right and left hands for different waxings.) PVA it if you can. THEN after you've gelcoated the mold for the first part, blow all the gelcoat off with an air nozzle. If the first part is going to stick, it's easier to sand off a bit of gelcoat, than gelcoat AND a bunch of fiberglass. A stuck part could damage the plywood and Bondo mold.

[afrhydro]

it takes 2 tons of pressure to break 2 layers of 1808 with (1) 1.5 on both sides but it will also defelct quite a bit before it breaks sounds like your going to have to upgrade products to crabon fiber in the laminit schedule some where
i would have to see what the application is from all angles to help

[Herman]

The requirements your customer has are very different (and much lower) then I am used to in the medical industry.

I do not care too much for strength, but I suggest making the surface of the part as smooth as possible, for hygienic reasons.

Before being able to give decent advise, I have the following questions:

-how many parts will the customer take?
-what is the expectation of the customer, gelcoated on the outside? Perhaps he has a picture of a part from a competitor?
-is the part shaped as such, that it can be produced in a single mold, or does it need a split mold?
-what is your experience and possibilities? Polyester, epoxy, spray equipment, etc.

Do you have a photo, by any chance?

[Submarine Tom]

Is this for humans or other mammals?

[LMB]

I finished the lay-up last week and pulled the part today. I used packing tape as a surface sealer and release agent, along with a few coats of wax. Eleven layers of 1.5oz mat worked out to 5/16" thick. Took about twice as many layers as I had estimated based on some reference charts. I gel-coated both sides as a sealer and sacrificial sanding surface so if they decide to refine the finish at a later time. I was concerned about hygeine and appearance as well, but was assured the top side would be covered. It was a split mold which eased the removal. I stuck with the mat because I could make cuts/tears to help it conform to curvatures. The part seemed rigid enough to me, and I believe will be adequete with the proper support around the perimeter. They will also have some safety strapping as a back-up. There is a possibility we will make serveral more parts from this mold. If so we will consider sealing the surface to refine the mold.

[Submarine Tom]

Why??