How to Make (Almost) Anything | Anders Häggman

WEEK 8 // Composites

I have some previous experience with composites, mainly using glass fiber weaves and cloths with vacuum assisted resin transfer moulding (VARTM), although I haven't done any composites in several years.

For this assignment I was hoping to make something relatively simple, that would look nice, and be useful. In other words, try and limit the scope of the project and instead try to focus on execution. Therefore, I decided to make a crumpled burlap bag, that I could use to put my wallet, keys and other odds and ends in when I get home. Other ideas that I had (that I discarded due to being too broad were making a balance bike frame for my daughter, or a carry-on suitcase).

Although for this weeks assignment, it would have been overkill, when designing composite structures, there are several different programs that can be used to determine the strength of the resulting part, which way fibers should be laid in each layer, etc. One such program is ESA Comp (which is what we used when designing glass fiber structures).

This weeks assignment also brought to mind my grandfather who at one point in his life worked for Finnish luxury sailboat manufacturer Nautor laying composites into boat hulls Nautor is based in Jakobstad, Finland The actual factory he worked at is located in the middle of the Finnish countryside.

Level of previous experience:
Time taken:
Tools used:
Vacuum bagging material & equipment
Got help from:
Charles Guan
Main learnings:
Check equipment beforehand. Especially vacuum pump. (Pressure should read around 0 when nothing is connected.)
Be meticulous when making the vacuum bag. Making it totally airtight is not trivial, and although a good vacuum pump will manage small leaks in the bag, the pump will have to work harder (and in the case of the cheapo pump at the IDC, spew oil mist / smoke that will fill up the room).
Wet layup with burlap is somewhat annoying, as the burlap is pretty heavy and doesn't stick that well, so precise placement of the fabric (and making sure it stays there) is a bit of a challenge.
Make sure you are using the correct release fabric.
Check cure times, estimate realistic layup times and schedule enough time for your work (making sure the equipment you want to use is free). I found the best solution to this was coming in early in the morning.
Getting more wooden sticks, plastic cups, or [insert item name here] with rubber gloves covered in epoxy is difficult, so take some out before beginning, so you have an easily accessible stash of surplus materials.

This is the rough shape that the final product was supposed to be, covered in burlap, but unfortunately due to using the wrong release fabric, the end result is a big pile of release fabric, burlap and epoxy. It was quite disheartening and annoying to make such a mistake at the end of the process, after having been very careful during all the previous steps. This may stay to bug me enough that I may have to redo it after the class when I have a bit more time.

I started by making a mould for the bag. I considered modeling it in CAD, or trying to find an .stl file on-line, but after giving it a little bit of thought, both of those approaches seemed inappropriate for what I was trying to do. I figured the best would be to use an actual paper bag for the mold. The problem is though, that most paper bags are either the incorrect size or stiffness (large bags could be stiff enough, but are too large, whereas small paper bags are too thin to work as moulds). Therefore I took a couple of large grocery store paper bags, cut them up, and created my own custom paper bag to use as a mould.

I made my paper bag three plies thick, which seemed fine until I began to wrinkle it. It was surprisingly stiff, and much more like thin cardboard that paper.

After a bit of violence, I got the aesthetic I was looking for: a subtly wrinkled paper bag. At this point I was no longer regretting the three ply thick bag, as it was now a benefit that the bag was stiff and held its shape well.

I wanted to leave the paper bag inside the final composite structure, to give the interior of the bag a nice paper feel, and smooth surface. I was concerned that the epoxy resin would seep through the paper bag, and be visible on the inside. Therefore, I decided to paint the bag with some green oxide coloured acrylic paint in an effort to try and make the bag impermeable to the epoxy. We had black, green, and white paint in the lab, and I figured the green would be the less likely to shine through.

I painted the bag twice, letting the coats of paint dry completely in between.

Although the bag was stiff, it was not stiff enough that it would hold up under the pressure of vacuum bagging. Therefore I took some sand that I placed in a thin plastic bag, that I then placed inside the paper bag mould. The sand was still quite loose inside the bag, and did not hold it's shape well. I therefore decided to draw a vacuum on the sand bag which was meant to lock its shape in place, similar to vacuum packed ground coffee bags. Unfortunately, the plastic bag I was using was very thin, and kept getting sucked into the vacuum tube, thereby blocking it and preventing a vacuum from being drawn on the whole bag. After a few attempts I gave up, although I still think the basic idea is valid. (See for example this interesting video about a universal gripper that uses the same phenomena of particles jamming against each other inside and elastic bag during a vacuum.) Since the sand was loose in the bag, when I turned it upside down, most of the sand rushed to the bottom (the opening of the bag). This meant that the bottom of the bag would get crushed more than I had anticipated. Luckily, the design of the whole object was a somewhat random artistic process, so it did not matter too much. I would have hated to have tried to make a very precise object in this way.

I aimed to create a net shape composite, where minimal post-processing would be required. I therefore pre-cut the burlap fabric into different shapes that would fit the paper bag mould nicely. To make that easier, I made this cardboard template to help me cut the required pieces.

A trick I learned from buying fabric by the meter in stores, was that one can take a single yarn, and pull it out of the fabric. This creates an easily recognizable trace to cut along, and you will end up with threads that run the whole length of the fabric. If one were to use a ruler and cut along a straight line one would most likely end up cutting some of the threads along the edge of the cut leaving threads that do not run the full length of the fabric, since the fabric always lives a little and the threads would most probably not be perfectly aligned under the ruler. This is especially useful with fabrics that have small threads. With the burlap it would probably be possible to cut along a specific yarn just by being careful, but this method made it much easier.

The equipment was once again an issue of concern and wasted time. The vacuum pump had been on for a couple of hours when I arrived (a previous student had their work in a vacuum bag) and the whole room was filled with an oil mist haze. This was shown to staff, who determined that due to the quality of the pump there was nothing that could be done, and the pump would just create oil haze as it ran, but that it would not be a safety concern. (I would not have personally had the guts to keep running the pump, without someone else taking responsibility and saying we should push on with the pump. So, we opened a window...) When arriving, I also noticed an issue with the previous student's vacuum bag: it was loose and filled with air. First I thought it might be an issue with a hole in the bag, but since holes are somewhat hard to track down, I decided to make sure the pump was working as it should, so I disconnected the pump, and tested the suction. There was definitely some suction, but it seemed a bit low. Also, even if a finger was not placed on the hose, the pressure read almost 30 in Hg (whereas it should have been around 0 in Hg, since there should have been minimal resistance). I summoned the shop manager Charles, who took apart the filter, and we noticed that the nylon insert that connects the filter with the hose was so long that it mostly blocked the air inlet to the filter. This was then sanded down, and re-inserted into the filter. The suction was better, and the pressure dropped to almost 0 inches of mercury, although not quite. It was deemed 'good enough'. I got to work.

Before beginning to apply epoxy, I mocked-up all the different pieces onto the dry mould, just to make sure that everything seemed to fit, and that there were no surprises once I began with the epoxy. I also placed the pieces in a pile with a specific order, so that when I would begin the wet layup of the material, the piece I would need next would always be the top one in the pile. I was very happy I did this, because it made things a lot easier. Having gloves sticky with epoxy made life a lot less convenient, and having everything at the ready was totally worth the extra time spent on preparation. I didn't realize this initially, but I should have also taken out some extra wooden mixing sticks, plastic cups, and paper towels since I ended up needing them later one, and as mentioned, everything is more difficult with sticky messy epoxy hands.

Unfortunately, as often seems to be the case, there was a deficit of certain materials. This time around it was vacuum bagging materials. When I arrived on Saturday evening, there was only about half a roll of mastic tape left. This posed a bit of a problem, and I had to carefully measure up the mastic pieces first, and then make the vacuum bag based on how much tape I had available. Having had more mastic tape, I would have definitely made my vacuum bag larger.

Inserting the sand into the mould.

I did not want to fill the whole inside with sand, as it was heavy enough that it was pushing the bag outwards, removing most of the wrinkles. In order to make the contents a bit lighter, I inserted a foam block into the middle of the bag. As mentioned earlier, I was unable to pull a vacuum on the sand bag (thereby locking it in place), so I was forced to resort to simple closing the bag with a rubber band. Unfortunately this meant that when I turned the bag upside down, a lot of the sand rushed downwards and expanded what will be the bags opening, while leaving less sand at the bottom of the bag, potentially allowing it to wrinkle too much.

As the wetting of the fabrics was a bit messy, and required gloves that were surprisingly hard to take off, I don't have any pictures of wetting the fabric. I used a wooden tongue depressor, and spread the epoxy evenly over the fabric so that all sections were wet, but there were no 'pools' of epoxy on the fabric anywhere. I then placed the fabric sections on the bag mould, added the release layer, and the absorbent material. I had pre-made the bag so that three of the four sides were already taped together, and once I got my part inside, I removed the backing tape from the last mastic tape piece, and sealed the bag. I thought I did a pretty good job, but once I started the vacuum pump, I noticed there were several leaks in my bag (I could hear faint 'hissing'). Luckily, this was not a problem as the pump was strong enough to counter the small leaks. The only thing that this meant was that I could not turn off the pump, as the air leaked back into the bag rather quickly. With the pump on, I was initially able to get around 25 in Hg, but after re-working the seals, the gauge rose to almost 30 in Hg. I was able to locate the largest of my (possibly several) leaks to the area around the inlet tube. I had anticipated this might be a problem, and had saved extra mastic tape to place around the tube. Clearly, I had not done a good enough job with it. As mentioned, however, that did not matter as long as the vacuum pump was on. The tack free cure time for the epoxy was listed at 4h, but I was only able to run the vacuum for a bit over 2h as it was almost midnight and I needed to go. However, at this point the epoxy had already mostly cured, so it was not an issue.

This is a diagram of the fiber layup. Unfortunately the burlap fabric was not wide enough to make the parts in part (A), so I had to cut two separate parts. The workflow was as follows, (A) >> (B) >> (A) >> (B) >> (C) >> (D) >> DONE. When placing the (B) sections, I made sure that the seam would be on different sides each time, so that there wouldn't be a pile-up of seams all along the same edge. Also, I had not sufficiently taken into account the added thickness of the earlier layers, so the last (D) section did not quite fit all the way around. Oh well, having one seam which was a little uglier would not be the end of the world.

Anticipating that the tube area would be problematic, I did not want the inlet tube to move around by accident, possibly causing a leak, or exacerbating an existing one. I therefore taped down the tube to the table, to prevent it from being accidentally moved.

This is the bottom of my bag (as it is upside down in the vacuum bag). You can see how the epoxy seeped through the absorbent material. I was worried that this was Bad news. I was hoping this would not cause an issue. Not sure if this was the reason, or if the release fabric I used was solely at fault, but as mentioned earlier, the end product did not turn out well (I was unable to get the release fabric off).

Some excess epoxy even started to slowly make its way towards the vacuum pump filter. Luckily, the progress was slow, and the epoxy actually cured before it got anywhere close to the pump.

After curing for 2h+ under vacuum, and another day or two at ambient pressure, I came to check on my part, and try and remove the release cloth. It was immediately clear something was very wrong. The release fabric would not budge.

After about 45min of work, this was how far I had gotten. Based on the e-mails I have gotten from students and course staff, it seems that the release fabric I used was incorrect, even though the West Systems website describes it as 'peeling easily', and from what I understand, being designed to be a release fabric. So, it is still a bit unclear to me where things went so terribly wrong. I am not sure if it was in fact the process I used, the release fabric (perhaps it was placed in the wrong bag and it was not actually release fabric), or something else.

Whatever the cause, this was the result. I was very bummed. After all the time I spent being careful, and getting excited about the end product I was left with a mess of absorber, burlap and epoxy.

I was so disappointed, that I think I might have to redo this weeks assignment after the course is over and I have more time.