This process is similar to profile extrusion, but it does not provide flexibility and uniformity of product control, and automation. Used for continuous production of simple shapes (rods, tubes, and angles) principally incorporating fiberglass or other reinforcement. High output possible.
Similar to extrusion but with much higher Strengths- even used to make road bridges. Glass or other fibres are incorporated into the extrusion and so loadings of up to 60% glass can be achieved with very good fibre alignment. Materials are generally thermosetting type materials such as epoxy.
The process starts when reinforcing fibres are pulled from the creel: the reinforcements, are guided towards the impregnation station passing through a preforming station where rovings and mats are carefully guided in successive steps; the impregnation of all filaments of each roving and mat occurs as they pass through the bath containing the resin formulation.
The composite material then enters a heated steel die that has been precision-machined to the final shape of the profile to be produced. While the material is being pulled through the die, the heat initiates an exothermic reaction in the thermosetting resin matrix.
Finally, a GRP profile, fully cured and with its final cross-sectional dimensions, exits the pultrusion die; the pulling mechanism can either be a caterpillar-type puller or two reciprocating clamps to ensure continuous movement.
Finally, the profile is cut to the desired length by an automating flying cut-off saw.
Due to its nature of continuous process, the pultrusion is arranged on three shifts and is particularly suitable for a mass production.
The key elements of the manufacturing process are:
- Reinforcement Handling
This consists of roving creels, mat and veil dispensers, and winders or braiders where other than axial reinforcement is required.
- Resin Impregnation
This can consist of a simple resin bath or pressure/vacuum impregnation device
- Preforming Dies
These serve to guide the impregnated reinforcement into the correct position, remove excess resin, provide pre-compaction to the approximate profile in order to assist air removal, wetting and to reduce the pressure in the main die.
- Pultrusion Dies
This is a machined steel or ceramic die which is heated and produced the final profile. Dies can be more than 1m in length. As the material passes though the die, heat transfer initiates the cure reaction and the pulling speed is such that the resin has fully cured by the time it leaves the die.
- Pulling Device
By allowing a suitable gap between the die exit and the pulling device, the product cools to a stage where the resin is sufficiently hard to be gripped by the pulling device. This can be a pinch roller or a caterpillar type haul off mechanism. Pulling speeds depend upon the resin system and the size and shape of the product. Typical values from industry are in the range 0.5m/min – 1.5m/min.
- Cut-off Device
This is a flying cut-off saw which is programmed to cut the product to the desired length
Sample Resin formulation: Product: 2.5 x 1/8 inch rectangular section
Sub Processes in pultrusion
- Resin Impregnation
The reinforcement is passed through a resin bath and it gets adequately impregnated with resin.
The resin impregnated reinforcement enters a heated die, where it gets cured into a solid composite. The cured composite exits from the die.
- Pulling and Cut Off
The solid cured composite is continuously being pulled to give continous production. An automatic saw at the downstream end cuts the product to required length.
Reinforcements and resins for pultrusion
Advantages of Pultrusion
- Structural properties of laminates are greatly improved
- Resin impregnation area can be enclosed limiting volatile emissions
- High glass content can be achieved
Disadvantages of pultrusion
- Profile with complex cross section difficult to pultrude
- High initial investment
- Uniform cross section profiles only feasible