About three quarters of a year transcurred since we started to make the mold of the cabin. Since it’s ready, we’ve been eagerly waiting to make the canopy’s die. Some open questions regarding release agents and spray adhesives delayed the production. Yesterday and today have been the days: dry layup and vacuum resin infusion of the laminate.
Making a die is a huge enterprise, as material costs and effort are considerable. The die has to have a flawless surface, it should be a full laminate (no sandwiches allowed), have a thickness of more than 6 mm (1/4 inch), should be produced in one shot, and should have no air bubbles. Looks like vacuum resin infusion is the way to go to fulfill these requierements.
The resin for the die has to have a high temperature resistance (glass transition temperature over 140 °C or 280 °F). We decided to use RIM 935 from HEXION Inc., an epoxy resin designed for infusion and high temperatures. We were lucky that a local company (Lange+Ritter GmbH) distributes it. I took my bike and got back with roughly 7 kg (14 lb) of resin and a couple hundred € less in my pocket. Temperature resistent resin is not cheap!
Don’t let you fool by the nice blue color. That stuff is nasty and should not be handled without precautions!
The layout of the laminate is pure glass fabrics, starting with some lightweight stuff and going over to heavy fabrics to build thickness up. Originally, we thought of using a couple of layers of 105 g/m² Interglas 91111 and then 600 g/m² Interglas 04367 for the thickness (both from GRM Systems). However, MÜHLMEIER Composites GmbH was so kind to provide a thick 600 g/m² glass frabric for free. So we made a nice mixture. More on that below.
Before we took a shot on the die, we made a small test with a cheap infusion resin (HP-E120RI from HP-Textiles):
Besides a small leak, the test was very successfull. Usually the dies are hand laminates of milky/frosty appearance. This one looks almost transparent, though the laminate is about 7 mm thick. The test helped to know that infusion works good for such thick laminates and provided an estimate on the needed amount of resin (about 7 kg per m²). The surface was perfect, and we were ready to go for the big one. That’s what we thought—at least. In contrast to the small test, the fabrics need to be fixed for the full die…
Normally, the layup for resin infusion is fixed using spray adhesive. The adhesive should dissolve in the resin without effecting the quality. That may be right when you have the right adhesive and the surface does not have to be perfectly shiny. Before we spend hundres of Euros in material and the adhesive turns out to affect the surface’s quality, we made a small test with carbon frabrics. No comment on the outcome:
The test was actually part of another one we made to decide how to make the shell of the wings. More on that on a later blog entry. Anyway, the used adhesive was INFUTAC from Diatex. It’s very strong, but it affected the surface and should not be used on the surface. The problem is that the spray’s aerosol is too gross. Lucky that we tested it before using that expensive RIM 935 resin. We bought another adhesive form HP-Textiles, with a much finer spray. We also used the adhesive only at the edge of the die, so that it should not affect the surface at all.
Yesterday we started to make the dry layup and today we were ready to infuse the laminate. We used two layers of 105 g/m² 91111, four layers of 600 g/m² 04367, four layers of MÜHLMEIER’s 600 g/m² donation, and two final layers of 04367. This should end up in a thickness of 6 to 6.5 mm:
The layup is quasi-isotropic, which means that always two layers of orientation 0/90° and ±45° are used. A quasi-isotropic laminate tends to deform less when it’s heated up. That’s what we need for our die.
Having a solid vacuum buildup, we were ready to make the infusion. Again, that RIM 935 looks really nice, but take care when handlig it! We made three batches of about 2 kg (9 lb), which were degassed as usual. Degassing of RIM 935 worked pretty well. It needed slightly less time than, for example, the HP-E120RI. No matter how much resin is degassed, count that it will take about 30 min to be ready. That’s why I would not use a resin with a pot life of less than an hour.
Infusion worked as expected:
From mixing the first batch till the end, the whole process took about four hours. We’ll see how the die turned out 🙂