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Basic knowledge of the pultrusion process
 May 6, 2022
1. What is pultrusion
Pultrusion is a continuous process of manufacturing scrap with constant cross-section. Combining "pull" and "squeeze", pultrusion stretches the material as opposed to the extrusion that pushes the material.
2. Processing of pultrusion
In a standard pultrusion process, reinforcements, such as fiber roving or woven roving cloth or chopped strand, are dipped into resin, followed by a separate preform system, which is then pulled through a heated stationary die to polymerize the resin.  

Impregnation can be accomplished by pulling the reinforcement material through a bath or injecting resin into an injection cavity usually connected to a mold. Pultrusion can use a variety of resin types, including polyester, polyurethane, vinyl ester, and epoxy.  

Resin provides resistance to the environment (ie, corrosion resistance, UV resistance, impact resistance, etc.), while glass provides strength in addition to fire safety.
Fiberglass mats can also be added to prevent erosion or "fiber bloom" and to provide corrosion and UV resistance. 

The technology is not limited to thermoset polymers. More recently, pultrusion has been successfully used with thermoplastic matrices such as polybutylene terephthalate (PBT), polyethylene terephthalate (PET) chopped strands by impregnating or wrapping the fiberglass powder in a sheet of thermoplastic matrix, which is then heated. 

Compared to composites based on thermoset resins, the ecological cleanliness of the finished product and the almost unlimited recycling (processing) possibilities after resource depletion seem to be compelling arguments in favor of reinforced thermoplastics. 

For these reasons, industrial production and the use of a given material have grown by 8-10% per year in highly industrialized countries in recent decades. New developments not only give a practical boost to the manufacture of both straight and curved profiles, but especially in the automotive sector, the demand for this technology is growing. 

The pultrusion technique for making fiber glass composites from a polymer matrix appears to be energy and resource efficient. 
Both economic and environmental factors favor the use of thermoplastic matrices, but due to the high viscosity of the melt, it is difficult to achieve high productivity and high quality fibrous filler impregnation with this type of matrices.