Nov 9th 2021
Our friend Chris Rogers from ExploreComposites.com joins us today with this first installment of the various ways you can build with composite materials. Make sure to check-out more of Chris' posts and great information at ExploreComposites.com .
There are lots of different ways to build with composite materials! The variety can make it really confusing to choose what path to go down with your project. Adding to the complexity is the fact that two very different techniques can produce what looks like the same outcome. This article is an overview of the most commonly used methods of manufacturing composite parts – with a focus on thermosets.
Because the processes are so different and suited to such a wide variety of part sizes and types, here is a quick chart to give an idea of methods by part size and required cycle time:
Hand layup (or wet layup) is any process where the laminator manually aplies the resin and the reinforcement. From there it can be rolled out and left to cure, or vacuum bagged. Hand layup requires that all the details and laminating features be accomplished before the resin starts to cure. This can really limit the amount of tailoring and detail that is possible. In general wet-layup can range from among the cheapest methods using a chopper gun and a roller to a very expensive relatively high performance option with vacuum bags and epoxy.
There are machines that mix resin and spray it out of a gun along with little short strands of fiberglass roving. Generally called “chopper guns” or “spray-up” equipment, these machines combined with a skilled operator can laminate really fast – and depending on the attention paid to rolling the laminate – potentially neatly. The resulting laminate is not high performance. It is resin rich: easily 60% resin by weight – and thickness is variable and often hard to control.
For parts where weight isn’t an issue, but gelcoated surfaces and complex geometry are – it can be ideal. Machinery housings, vehicle panels, shower enclosures, tanks… there are many applications where spray-up laminating is a great option. It can be miserable work though and requires good personal protection equipment and a good air circulation system. The output is highly worker-dependent and poor attention to detail can result in voids, porosity and surface distortion.
Open molding is the process of “wetting-out” dry reinforcements with resin in a mold without using any additional consolidation step. The reinforcements can be wet-out in place or on a table or even run through a fabric impregnator – a machine used to wet out large volumes of reinforcement at a time. The results are highly dependent on the skill of the team doing the work, and like spray-up, rolling the laminate to remove air is a critical step. Because reinforcements are cut off rolls and placed in the mold, the thickness is much more consistent. Laminators must be aware of slip joints in corners and be careful to orient plies correctly. All this happens under the constant ticking clock of the resin’s gel time. Once the resin starts to cure, times up!
Open molding is suitable for projects that need the strength of continuous fibers in addition to complex geometry and gelcoat surface finish. Parts can have variable thickness and additional reinforcement as needed and can make use of cores like Coremat, foam and balsa. Resins are usually polyester or vinyl-ester though epoxy can be a good option for certain types of work. Generally, because of viscosity and cost, it will make sense to vacuum bag epoxy laminates to take advantage of higher performance laminates that don’t make extensive use of chopped strand mat.
Next week Chris will take us through the pros & cons of Vacuum Bagged Wet Layup.