Composites 101

This is a brief article describing a few different ways to manufacture composites, and where Torpedo Bags products fit into the mix.

There are a number of processes that fall under the “composites” umbrella, and they vary greatly in the resulting strength and weight characteristics, as well as appearance. Generally, for our purposes, any of the methods and materials described here would make a very durable case. But some are much lighter and look much better than others.

One of the lower-end processes is called “Chopped Strand Mat,” where a specialized gun chops and then sprays short strands of fiberglass soaked with usually polyester resin onto a mold. This process is smelly — that’s the polyester resin — and has a rough B side. It results in a very durable, heavy, and rather thick part. Many lawn mower manufacturers use this method to make parts of the deck because of its durability. The resin to strand ratio is high, which is related to its heavier weights. Torpedo Bags are not made this way.

The next best process is known as “Wet Layup,” where fiberglass and sometimes carbon fiber are soaked with an activated resin, and laid over or into a mold. Sometimes, the fabric is “wetted out” on or in the mold directly. Usually, this is also done with a cheaper polyester or vinylester resin, but sometimes the better performing epoxies are used. The ratio of resin to glass here is usually a little higher than optimal, but the result is a very strong shell that can have a decent finish on one side.

Our legacy Coyotes are made this way, with a marine grade epoxy resin. When we farmed production out previous to 2016, they were made with polyester resin. In nearly 10 years, we found one edge that was damaged after shipment, and that was because there was only one layer of glass in that spot, and we didn’t notice it — there should have been 3 layers. None of our rivets pulled out in all of that time. Wet layup is a great way to make a case, and is a relatively easy process to learn.

However, using one of 2 different vacuum techniques, a maker can achieve significant weight reductions, as well as achieve finish and strength improvements.

The first method is called “Light Resin Transfer Molding,” where a female mold is made and then matched to a slightly smaller male plug. These are usually heated and have vacuum and resin ports installed. The operator lays in usually a cheaper fiberglass matt, and polyester resin is pumped through it as air is evacuated by the vacuum pump. The result is an extremely uniform part, which can be considered nearly uniform from one to the next. Both the A side and the B side can have glossy “Class A” finishes, and the cure time can be rather quick in a heated environment. However, the weight is generally high compared to the methods described below.

One variation is called “Resin Infusion,” which is how the new Torpedo Bag fiberglass composites will be made. A mold is lined with dry fabric. A bag is then secured over the top, and sealed off with specialized tape. Infusion ports are installed, and catalyzed resin is drawn through with a vacuum. The A side can be a glossy, “Class A” finish, but the B side will usually reflect variations from pleats in the bag. The resin-to-glass ratio is nearly optimal, meaning, the best performance of strength and weight can be achieved — almost! Many different cores can be added to increase the flexural strength. Resin Vacuum Infusion can be considered a low-end aerospace technique.

Finally, for true aerospace parts, there are pre-preg fabrics that require heat and/or pressure to cure. “Pre-preg” stands for”pre-impregnated,” or, already wetted fabric, almost always with a high grade epoxy resin. A pre-preg fabric has the optimal ratio between resin and fabric, and this ratio is entirely uniform across the fabric. This is what Torpedo Bags uses to make our carbon fiber / honeycomb core cases, and using this particular core requires three cook cycles. This fabric must be kept frozen, thawed in a very careful way that doesn’t allow moisture to enter the bag. It is then cut, the backing is peeled off, and the fabric is carefully laid into the mold. A bag is then placed over the part to be cooked, and various ramp-up, heat, and dwell cycles are employed to achieve a properly cured part. The honeycomb must be glued onto the first layer after the first cure cycle, using an adhesive film for proper bonding to the edge of the honeycomb, which means the mold has to cool. The bag is removed, and the film and a new bag is placed into position. A third cycle glues another layer or two of carbon fiber to the inner edge of the honeycomb. Each layer of carbon fiber is shifted 45 degrees in orientation for flexural strength optimization. It is a labor-intensive, time consuming process.

The advantage here is significant: Since the core is 95% air or better, but because the honeycomb separates the inner and outer layer by some distance, the wall of the part is extremely stiff as well as extremely light. The resin used in pre-pregs is usually a very high quality epoxy resin, since most of these types of parts are being made for highly demanding aerospace applications. The adhesion properties of epoxy is about 4X better performing than polyester, and this is the best of the epoxy class of resins.

There are a number of fiberglass and carbon fiber cases made in the industry. Many are made using wet layup techniques, and some are made with a resin infusion process, often without a core, or with a soric core.

The only music instrument cases made on earth with pre-preg carbon fiber and a honeycomb core, as far as we know, is a Torpedo Bag case.

Please enjoy your super-strong, super-light, super-enduring Torpedo Bag case.