0

Making the mold

What a week! We started to make the mold and had a couple of very long and late ending sessions. In the fist session we spread a thin coat of tooling gel coat (Formenharz P, R&G Faserverbundwerkstoffe GmbH) and strew a mixture of glas shred and cotton fluffs (coupling layer) on top of the fresh gel coat:

P1060261_s

 

Properly spreading the gel coat is the most important step to obtain a high gloss surface (along with the pattern’s finish). The coupling layer is needed to have a surface on which glas fabrics can be laminated in a later step. Without a coupling layer one has to keep on laminating before the tooling gel coat has completely cured, i.e. after it started to gel. Going on wouldn’t be problem for a small mold. However, when you have several m², such as here, mixing and spreading the gel coat is already enough hard work for one session:

 

One day later, after the gel coat cured, we went on laminating several layers of glas fabrics to stiffen the mold. We started with filleting the corners and filling the surface with thickened epoxy resin (glass shred and cotton fluffs). This reinforces the corners substantially and prevents air bubbles. Then we laminated four layers of glas fabric (two 160 g/m² and two 280 g/m²). Each layer of a given weight was laminated with two different orientations (0°/90° and ±45°) to obtain a quasi-isotropic laminate. We would have wanted to keep on with the next two layers of 390 g/m², but it was late enough (about 3 o’clock in the morning!), and we, thus, decided to stop and close the session with a layer of peel ply:

 

Two days later, we laminated the two remaining layers of 390 g/m² (quasi-isotropic), glued the core material and cured everything in vacuum. This time we wanted to get sooner to bed, but to be honest, we had a terrible time to build enough vacuum. Somehow it was again 3 o’clock in the morning and we were still looking for a large leakage. As in the last session, we had to get up at 7 o’clock the next morning and get to work… A consequence of this, was that I decided to buy a stronger vacuum pump with much more volume flow!

Anyway, we started to get everything ready for the next and final layers of the sandwich. I separated the external ribs and trimmed the surpluous border. We hope to end this side of the mold this week.

0

AERO 2016

Yesterday, we made a short trip to Friedrichshafen and visited the AERO 2016 fair. We saw many very interesting airplanes, including some very light constructions, which provided us with new ideas for Schneewittchen.

Take for example this extremely light Czech airplane:

P1060233_s

P1060232_s

 

Of course we had to make also a short visit to the good old vintage gliders (Ka 3). Even nowadays an infite source of inspiration:

P1060241_s

 

Despite all the nice airplanes, our principal aim was to get information about following things:  rescue devices, electic motors, wheels, towing releases and canopies. Chatting with Plexiweiss GmbH, Tost GmbH Flugzeuggerätebau, Junkers Profly  GmbH and Geiger Engineering GmbH was, thus, a must:

P1060247_s

 

0

From you have I been absent in the spring

 

When proud pied April, dressed in all his trim,
Hath put a spirit of youth in every thing,
That heavy Saturn laughed and leapt with him.

P1060154_sP1060158_s

Yet nor the lays of birds, nor the sweet smell
Of different flowers in odour and in hue,
Could make me any summer’s story tell,
Or from their proud lap pluck them where they grew:

Nor did I wonder at the lily’s white,
Nor praise the deep vermilion in the rose;
They were but sweet, but figures of delight,
Drawn after you, you pattern of all those.

P1060169_s

Yet seemed it winter still, and you away,
As with your shadow I with these did play.

Sonet XCVIII, Shakespear

2

Pattern is almost ready

The last weeks I have been intensively working on the paint job of the pattern. I began with spraying three times epoxy primer-filler (EP-Grundierfiller, MIPA SE):

P1050964_s

Between the spray sessions I had to grind with the orbital sander (400 and 600 grit) to get a smooth surface for clear coating.

I then sprayed one and a half layers of acrylic clear coat (CC8, MIPA SE). I’ve never sprayed before and had to learn straight off. For obviuos reasons, the result was faur enough but not perfect. So I grinded the sufrace with 2000, 2500 and 3000 grit and polished to high gloss with a grinding compound (3M green cap):

P1050998_s

That was some hard work, but the result is worth the effort:

P1050996_s

I have to grind and polish the other side. After that I’ll be able to prepare evething for making the mold.

0

Tempering of composites

Composite materials need to be heat treated to achieve best results. Both their final strength and heat-resistance increase. The reason is that epoxy resins do not polymerize completely at room temperature. Though a higher final strength is without question relevant, heat resitance is from a practical point of view much more important. Who wants to have a nicely shaped airplane deform in a hot summer day? Not me!

Heat resistance of a cold-hardening epoxy resin is usually about 30 °C over the curing temperature. This means that if the composite material is cured at 20 °C, the piece will not soften below 50 °C. The latter temperature is called the glass-transition temperature Tg, which depends on the degree of polymerization. Temperatures of over 50 °C are easily reached in the sun and some pieces can get very hot in motorized airplanes, such as the cowling. Even temperatures below but near to Tg should be avoided. Essentially, composites should be heat treated at the highest temperature they will be subjected to loads. The German Federal Aviation Authority (LBA) requires 54 °C for sailplanes and 72 °C for motorized airplanes, which means that pieces should be tempered at—or over—these temperatures.

Ok, so let’s heat  the piece up to 55 °C and everything is fine! Well, it is not that easy, as there are some pitfalls:

  1. During tempering the composite is heated over the original glass-transition temperature and deformation can take place.
  2. Composites can deform heterogeneously when heated.
  3. Tighly regulated heating of cubic meters of space is challenging.

The solution to the first problem is to heat up slowly and if possible, to temper inside the mold. Consequently, this means that the mold itself has to resist the temperature. Slow heat production provides the remaining resin components enough time to react resulting in a continuously rising glass-transition temperature. In other words: if you do it right, you never reach Tg. Consensus is that the heating rate should stay below 10 °C per hour. The second point is solved by distributing heat as good as possible and by employing quasi-isotropic laminates in the mold. Quasi-isotropic laminates tend to deform homogeneously (at least in the plane of the laminate). And the solution to the third point is at hand: power. However, you need to account for points one and two! Power, thus, has to be regulated tighly, or else, the composite might be spoiled.

 

Today, I’d like to present my solution to these problems. There are industrial solutions, which are too expensive to be used by private persons. RC-Model builders use boxes of isolating material. They heat by turning light bulbs on, and regulate the heating rate by the amount of lighted bulbs. Other people heat using old electric ovens or with gas heaters. They usually regulate the rate and final temperature by allowing heat to escape up to a certain degree. An approach that is energy-intensive and needs a careful hand to setup correctly.

I decided to use a microcontroller which drives an electric heating source through a solid state relay :

P1050995_s

The board is an Adafruit Adalogger based on a Cortex M0+ microcontroller. It has a OLED display and a SD card reader/writer. The temperature profile is written on the SD card and the current state is easily read from the display.

As a heating source I decided to use heating foil, which is usually installed in room ceilings:

P1050986_s

This foil provides about 200 W per square meter. I started by building a small prototype to desgin the software:

P1050980_s

 

Basically, the heating foil is driven by a pulse-width modulated signal. The microcontroller decides with the current temperature the duty cycle and, thus, the effective heating power.

Regulating a constant temperature and a heating rate is more challenging than I thought at the beginning. It is not difficult when variation of several degrees is allowed, such as in a fridge. Here, however, I wanted to have tighter control. The temperature sensor I use (DS18B20) provides a discrete temperature, something that controllers do not like at all. So I had to come up with a solution to that…

A test run of the prototype showed that my ideas work quite well. Depending on the heating rate, the control error is small below 0.3 °C. This is certainly better than regulating heat escape by hand!

tempern

I built a larger box to test if everything works well on a larger scale. It is large enough to fit the center section:

P1050991_s

The test is running since a couple of hours and I am eagerly waiting for the results. The room is cold (about 6 °C) and even so it achieved yesterday night a temperature of over 40 °C. The power should be enough to reach 55 °C when the room has 20°C or more!

 

0

How should we finish the pattern?

I have to admit that finishing has never been my favorite job when building an rc-model: everything is ready to maiden and extra time has to be invested „just“ for protecting the surface. This is one reason why I have done many of my maidens without a finish, besides that repairing a finished model is much more elaborate. Now, I have no other options than learning about two component paints and using a good spray gun.

The pattern of the center section is almost ready since a couple of weeks. „Almost“ because it has to be finished before creating a mold. The surface of patterns has to be in general at least as good as the one of the final parts, as these are usually painted directly in the mold before lamination to avoid excessive weight. Thus, any defect of the pattern is transferred almost directly to the final finish. Though the molds can be sanded and polished to a certain degree, it’s best to have a perfect pattern.

Finishing the pattern turned out to be more subtle than we thought before. We have been testing some techniques and decided to use a HVLP spray gun with clearcoat used normally in finishing cars. This should provide a clean and even surface suited to create a mold. However, not only the coat is important for the final result, but also the layers below.

The pattern has a wooden surface with many smal pores and cracks. Priming this surface would inevitably result either in a bad surface for the clearcoat or in having to spray and sand repeatedly. We, thus, decided to use Ahrweitex Schnellspachtel to fill the pores. This is a sort of one component filler, which is soft and gives you plenty of time to spread over a large surface with a spatula.

Here’s an example of two similar surfaces after priming. On the left without and on the right with Ahrweitex:

abporen

I think this picture speaks for itself.

We filled this way the surface of the pattern:

P1050896_s

Soon, we will spray gun a two component epoxide-primefiller and a clearcoat from Mipa SE. I hope to get a great surface and good separation when producing the mold…

1

Baking a sligthly different Christmas turkey

We have been preparing to vacuum bag the pattern of the center section for a couple of days. Time runs once the resin is mixed and a good preparation is half the work.

The pattern is covered by thin abachi sheeting (1.2 mm, 1/20 inch). This creates a good surface, which is filled and sanded easily—an approach used often in rc-models. Abachi veneer has to be primed to prevent unnecessary resin consumption. As neither the foam core nor the primed veneer are well wettable, a coupling layer is recommendable. We used 86 g/m² glass fabric and an epoxy resin with a processing time of 110 minutes (HP-E110L). The HP-E110L provides enough time to prepare everything before vacuum bagging.

We proceeded as follows:

  1. Cutting and priming the abachi sheets:
    P1050839_s
    P1050849_s
  2. Making a vaccum bag that is large enough (not as trivial as it sounds!)
  3. Laminating the glass fabric, setting the pre-assembled abachi on top of the foam and vacuum bagging the „turkey“:
    https://vimeo.com/150053703

Right now, everything is curing at -0.3 bar (-43.5 psi) and a temperature of about 20 °C (68 °F). We will see the result tonight!

1

Joining the center section parts

One word that fits perfectly to what happend today in one of my workshops is:

wedding

  1. an act, process, or instance of joining in close association

Defintion taken from Merriam-Webster’s Dictionary, December 19th, 2015.

The last couple of weeks, we have been cutting core by core until we had all six segments ready that compose the pattern of the center section. One issue was that the foam panels were too small and had to be glued together to obtain the needed length of 2.84 m (9 ft 4 in).

We tried silan modified polymer from the hardware store. It worked quite well, but this adhesive stays very elastic after curing. This is troublesome, when you have long pieces. Combining a highly elastic adhesive on a small contact surface with long lever-like pieces is a bad idea.

Thus, we tried the good old polyutherane foam spray, which worked better but not perfectly. Foam spray is suboptimal when you need small gaps between pieces. Honestly, this foam is too foamy and gross in handling for our kind of application. It was good enough to glue the panels together, but I really needed a better solution for joining the six sections!

I found finally the solution: one component, foam building, fluid polyutherane adhesive. I know this adhesive already for some years and used it a couple of times before. However, the European Union (EU) started to regulate its distribution a couple of years ago. Some important componet is believed to be carcinogenic. It was sold in every hardware store before regulation. Now, it is hard to get for private persons… I said „hard“ but not impossible! Either you buy it in Switzerland—a neighboring country outside the EU were it is not regulated—or you do like I did and buy it from Bacuplast Faserverbundtechnik GmbH (Germany). They sell it provided you read a somewhat lengthy technical information sheet and sign a disclaimer. Believe me, this U-600 is great and worth the extra trouble.

After having the the right adhesive, we started to join section by section until we had the left and right halves. Today, these two halves had their wedding. A moment I have been eagerly waiting for the last weeks. Questions had been plaguing my mind: Will the parts fit together? Did we make a mistake while hot-wire-cutting? How will the shape look like?

Not a single of these fears was truly justifiable. It was a perfect day for a perfect wedding:

P1050835_s

0

First foam core

Yesteday, after a somewhat longer preparation time, everything was ready for cutting the first foam core of the center section’s master model. I chose styrofoam for the core, because it can be cut with the hot wire and it is easily sanded further in shape afterwars. I had to exepriment a little bit with the glue to get the sheets together. Though UHU-Por is ideal for styrofoam, aligment cannot be corrected after joining. This is a potential problem when you have large pieces. I ended up in using a silan modified polymer.

Here’s a time-lapse video of the procedure: