One of my all-time favorite movies, Randy Quaid did a fantastic job as Lenny, so much so, and as I was quite young when I saw the movie, I thought for years he truly was mentally disabled. And Berretta played his part very well too.
I have made the first rigid polyurethane ribs from my rib mold. The ribs have turned out to be very strong and the foam bonds very well to the aluminum as expected. However there are some aspects to the process I did not anticipate and it looks like I am going to abandon this process.
The foam is much stronger when expanding than first thought. In my research I had been warned that the foam had the ability to exert great pressures against any enclosing structure (probably a uni-directional force as opposed to a true pressure). So I expected the first rib to lift the top plate of the mold and create an overflow between plates. Therefore I put a 20lb weight on the top plate, but alas it wasn’t enough and added an additional 20lbs too late in the cycle to make any difference. Because I didn’t know exactly how much foam to put in the mold it was a big guess on the first pour. I mixed 150ml of A and B, 300ml total on the first pour. The foam lifted the top plate nearly a half an inch. But no big deal as this was just a test anyway and it would give me a good idea of how the foam would stick to the top plates. Once the foam was cured I removed the top plate quite easily. It gave a bit of resistance at first but then popped loose. But O MY! The static electricity it made! Several good zaps before I got it completely off. The overflow had to be chipped away until I reached the Inside Mold Line (IML). However the rib would not slide out of the profile plate even after chipping the overflow from both sides, I had to take a hacksaw blade and very carefully cut it out. After removing the rib the profile plate cleaned up rather easily with a putty knife. The foam doesn’t actually adhere to the HDPE, it appears to be a purely mechanical attachment to the tooling marks left in the material during machining.
I lined the profile plate with a 1” aluminum strip and used 100ml each of the foam components (200ml total) for the second pour. The blue tape is to hold the aluminum strip in place. After mixing the foam and pouring the mix into the profile plate you have to swish it around to cover all the bottom of the mold area, it doesn’t flow well once it is expanding. I quickly put the top plate on and added the 40lbs of weight. I was astonished to see the top plate had lifted about 3/16” when I went to remove the mold. So since this mold still had overflow I had to chip the overflow away up to the aluminum strip. The rib then slipped out of the mold with a bit of persuasion. This second pour could be a viable rib but it is too thick. It turned out exactly as expected, the capstrip is well adhered, the rib is extremely strong and is a third heavier than the original Lazair rib, but is a third thicker too. The new ribs will likely be nearly the same weight as the originals once the thickness is under control.
Several unforeseen difficulties have arisen that threaten this process. The primary one is the foams ability to lift a significant amount of weight while expanding; the second is the relatively small amount of foam necessary to fill the void. The lifting of the foam drives the need to have very accurate amount of foam mix poured in the cavities, coupled with the already small amounts of foam means that for this to be a repeatable manufacturing process the foam mix must be carefully controlled. With already small component volumes this becomes very difficult with equipment at hand. If the foam lifts the mold plate and overflows removing the rib from the mold becomes too labor intensive to be economical and the rib requires extra rework to make it a uniform thickness and meet quality controls. Therefore, unfortunately, I am abandoning this process and looking to a more traditional method for making ribs.
The new method will be to buy the foam pre-expanded to a uniform thickness and route it to profile with a template and flush cut router. The drawback to this method is the foam boards are rather expensive, somewhere on the order of $200.00 in materials for a complete shipset. It may turn out to be a wash when the reduced labor is taken into account, but this means the rib capstrips will have to be bonded in a separate operation. I wonder if JB Weld would work. I’ve already purchased a suitable router table and router. I need to make some templates and order some foam.
In other news I have pictures of the 2SI engines that are for sale. They are 2SI 215F’s with Prince P-Tip Props, you can find info on them on the 2SI website. If interested please contact me. I am also working on a wheel and brake replacement. I am having adaptor bushings machined now and will post pictures as that project progresses.
More Next Time,
Gene
Fltofancy@gmail.com
