Guest Post: Creating with Composite Materials – Part 4 – Pre-Pregs, Resin Transfer Molding & Pultrusion

In the penultimate post in this guest series from Chris Rogers from, Chris dives into using Pre-Pregs, Resin Transfer Molding & Pultrusion.


With pre-pregs, the fabricator gets the resin and the reinforcement pre-combined – suspended in a partially cured state. The manufacturer of the material has applied just the right amount of resin and then partially cured (usually) and frozen the “pre-impregnated” reinforcements, stopping the cure and allowing the pre-pregs to be stored in frozen state for months or years. Fabricators will thaw the pre-preg and laminate it into molds, adding cores and features with high precision and repeatability. To cure the pre-pregs into completed parts, they have to be “cooked” in an oven or autoclave. The elevated temperature starts up the curing reaction and the resin flows and then hardens – leaving a laminate with a very precise resin content.

Pre-preg is really only a good option if you are building relatively high performance stuff – and if you have the budget for it. Mandrel molding, press molding and layups consisting of heavy unidirectional “spars” are usually best done with pre-pregs. And really light stuff, like airplanes, race-cars and high end sporting equipment are usually built using pre-preg materials.

Here are a few Laminate Samples showing pre-preg in action:

Resin Transfer Molding (RTM)

Resin Transfer Molding is like infusion, but instead of using the difference between vacuum and atmospheric pressure to drive resin into and through a dry reinforcement stack – RTM uses a pump or a pressure pot to push it in at much higher pressure. RTM “injection pressures” are often around 10-20 bar (150-300psi) but can be as high as 100bar (1500psi) in certain high production methods (High-Pressure RTM, or HP-RTM) with very short cycle times. This is a lot of pressure! Typically RTM molds are either bolted together or clamped in a large hydraulic press with flat platens to keep the molds aligned. Because there is so much pressure, and it is often done at high temperature, resin injection can be very rapid and cycle time very fast. Many resin manufacturers have developed “snap-cure” resins that are meant for RTM and compression molding. Their goal is to reduce cycle time and make composites more competitive with metals and injection molding for automotive and other high-volume applications.

With vacuum infusion, you might have parts every few hours at best. With highly automated HP-RTM it can be every few minutes. Naturally with all this heavy tooling and presses and clamp frames, this is best for huge runs of smaller parts. You’ll pay up front with much higher tooling and equipment costs but for thousands of parts per month, RTM is one of the best options.


In the pultrusion process, wet fiber is pulled thorough a shaped die that is really hot. The fiber and resin goes in one end all goopy and comes out a fully cured part. Like extrusion (which is a push process), pultrusion (a pull process – can’t push on wet fiberglass!) produces long things with a consistent sectional shape. It produces them very accurately and economically, so pultrusion is a great option for long skinny things that might replace metal extrusions.

The down-side of pultrusion is that setting up can be a lot of work and specialized equipment is needed. Dies that form and cure the finished profile are expensive and must be carefully designed. You aren’t just going to head out to the backyard and do a little hobby-scale pultruding! But you can find a pultrusion specialist or buy already-manufactured stock profiles which are very handy. Railings, tubes, decking, structural profiles – there are lots of uses for composite pultruded parts.