Tricks of the Trade: Manufacturing Pultrusions
One of the major attractions of the pultrusion process is the simplicity of tooling and low labor requirements. At first look, pultrusion seems like a straight forward process: fiber reinforcement is saturated with a thermosetting resin matrix, and pulled through a heated die. Sounds simple enough…… right?
However, there are many “tricks of the trade” in manufacturing pultrusions where knowledge can only be found through experience. Listed below are some needed tips one must learn in order to become a successful pultruder.
Die Shape, Length, Mounting
Before one plans to pultrude, a pultrusion die must be manufactured to precise dimensions. Harden steel is ground down to the correct tolerance and is chrome plated to reduce friction. In designing the die, one must decide on the length. This will effect the time the pultrusion will be exposed to the heat, it will also effect the inside surface area, friction, and production line speed.
Pultrusion Die
Once the die is manufactured to specific length, mounting to the heating plate can sometimes require special shoes, depending on the die shape. As well, if the pultrusion is hollow, a floating mandrel must be correctly installed.
Machining of dies and tooling set-up are generally the largest fixed cost in an individual pultrusion run.
Fiber Reinforcement/Density/Organization
Depending on the end application and through use of finite element analysis (FEA), the pultruder must decide on the appropriate “ply schedule.” What type of fiber is to be used, unidirectional, woven or stitched fabric, what weight fabric, and how many layers to use are all questions which must be addressed. The decision to use continuous strand matting and surfacing veils must be taken into account as well.
Once the given reinforcement is determined, organizing and preforming tools must be fabricated and installed. This ensures the fiber stays in the proper location without folding, creasing, or jamming in the die.
Resin Mixture
This discussion is limited to thermoset resins, by far the most utilized resin in the pultrusion process. The resin chosen for a particular pultrusion is not as simple as mixing two simple parts. Fillers, pigments, and other additives such as fire retardants all need to be mixed with the desired ratio. Pot life of the resin needs to be taken into consideration, as well as the resin viscosity. Viscosity is important to ensure proper wet-out of the reinforcement, and will also affect line speed.
Resin Mixture
Resins in thermoset have the draw-back of containing styrene, if polyester or vinylester are used, whereas epoxy-based and urethane resins do not. Pot life is less with epoxy and urethanes and these are used almost exclusively with die-injection techniques. Polyester or vinyl ester resins are sometimes “open bath” systems, but be sure to check local emissions regulations before committing to this approach.
Pultrusion Die Temperature
Die temperature is extremely important. Too low of temperature and the composite will not fully cure. Too high a temperature and the composite could blister, crack, or worse, the composite could get stuck in the die. There is an ideal temperature for every profile, ply schedule, and resin matrix. Line speed, die length, and the exothermic properties of the resin need to be taken into account as well. It is no surprise pultruders take detailed notes during every run.
Pultrusion Line speed
Finally there is line speed. Every manufacturer wants its pultrusion machines to run as fast as possible. Line speed is the rate at which the pultruded profile is produced, and is most often measured in feet-per-minute or meters-per-minute.
When calculating the optimum line speed, one must take into account the profile thickness, die length, die temperature, and resin formulation. Once again, experience is the key to line speed optimization with quality pultrusions.
In conclusion, the process of pultrusion is not nearly as basic as it may appear. Experience and trial and error is the only true method of mastering the manufacturing of pultrusions.
Stay tuned is we will be placing new “tricks of the trade” periodically on www.pultrusions.org.
Photos courtesy of Ebert Composites Corp.