Lakhasly

Process Description

In the pultrusion of thermosetting resin composites, the steady-state reaction process is initi-
ated by the application of heat to the chemically active material. The reaction progresses while
under pressure within the die. It is exothermic, and at some position within the die the relation-
ship of heat flux is inverted and the degree of cure progresses to the point where shrinkage
allows the part to release from the die wall. Reinforcing material is assembled and oriented to
enter the dic in the configuration necessary for the development of the desired mechanical prop- erties of the produced structural member. Heat is supplied to the process by electrically heated metal platens. Cooling water at the die entrance controls the transition from ambient to die temperature. Hydraulic pullers provide a continuous movement of the product.
Precise control of the thermal and chemical phenomena occurring within the die is of utmost importance. If the reaction proceeds too rapidly, the composite can bond to the die surface. This results in a loss of production, a low-quality product, and possible damage to the die. Once the process is in progress, the pulling force must be regulated to ensure that the line speed does not vary, since fluctuations thereof translate directly into variations in cure conditions.
Outwardly the process is deceptively simple in that the function is well understood. However, without an in-depth knowledge of the interaction of the various system variables, one cannot achieve efficient performance of the process. Essentially, only general qualitative information about what occurs inside the pultrusion die is presently available. In the liquid zone of the material, the temperature of the die exceeds that of the resin, with the temperature of both increasing. Within the gel zone, the peak exotherm of the resin is reached, usually being well above the temperature of the die. In effect, over the remainder of the process the die is drawing heat from the curing composite, thereby reducing thermal shock to the product upon its exit from the die. The material traveling through the process undergoes a number of dynamic changes as a result of the temperature environment within the die. The nature of these changes is manifested, to an extent, through the variation of the viscosity of the resin over the length of the die. Over the initial portion, the viscosity decreases as the temperature of the material in- creases through conduction. This reduction aids in the continuing wet-out of any unsaturated fibers. At the point at which the chemical reaction is initiated, the change in viscosity is re- versed, and the viscosity rapidly increases through the stages of gelation and final cure. It is desirable for this reaction to occur under sufficient pressure to ensure composite integrity and to minimize internal porosity that can occur from vapor pressures within the reacting material.
The pressure at the material-die interface is a measurement of normal surface forces which, combined with the appropriate coefficients of friction, yield a measurement of frictional force, or resistance to pull. Pressures can be associated with the viscosity of the resin, the volumetric ratios of fiber and resin, the coefficient of thermal expansion of the materials, the cross-sec- tional geometry of the cavity, the length of die over which there is material contact, the coeffi- cients of friction of the die with respect to the liquid, gei, and solid, and the degree of shrinkage of the solid. The efficiency of the process can be greatly enhanced through a better understand- ing of the pressure distribution.
The purpose of this description has been to define the pultrusion process with respect to
thermosetting resin systems, highlight the most important process variables, indicate the com-
plex interdependence among these variables which renders an intuitive grasp of the process
almost impossible to attain, and, thus, emphasize the need for analytical definition of process
variable interactions in order to gain insight into the process[3]