The Pulse of Pultrusion: Pull Force Trending for Quality and Productivity Management
Authors: Sumerak Joseph E. , Martin Jeffrey D. 
Published: March 2023

A large number of studies in recent years have focused on the significance of the thermal environment within the pultrusion die and the effects of process parameters on the reaction of thermosetting resins within the die. Numerical models have been proposed which can simulate internal exothermic reaction dynamics for simple shapes. The rate and position of reaction can be predicted given thermal inputs and considering the heat transfer characteristics of the die, material, and profile geometry. On a practical plane, commercial instruments have been developed which allow the processor to easily conduct thermal analysis experiments which can measure the reaction characteristics for more complex profiles not within the current predictive realm of computer models.

Another area of investigation which has recently received attention is the influence of internal die pressures on processability and product quality. A variety of methods have been employed to quantify the internal pressures on both a continuous and intermittent basis. A number of investigators have studied the effects of different raw materials on frictional resistance within the die and have measured the resultant pulling force required to overcome the resistance. The results of such studies have established that internal pressures are substantial and desirable for good product integrity and dimensional reproduction. They have also established that there are numerous contributions to frictional resistance that are difficult to measure independently and even more difficult to model numerically. Such a model when perfected could be useful in helping to optimise materials and die design which would help to minimize internal porosity, improve surface quality, and reduce internal stress imbalances that result in dimensional tolerance problems. However, as is the case with pultrusion heat transfer models, simplifying assumptions often limit the applicability of a model's predictions.

In lieu of a technique to segregate the component causes of pulling resistance, the measurement of the pultrusion machine pulling force is an extremely useful tool for the processor. Factors which can cause changes in the pulling force include a loss or excess of reinforcement, poor fibre wetout, change in resin reactivity or viscosity, loss of temperature, or change in line speed. The stability of pull force, therefore, becomes a valuable operator diagnostic tool when displayed as a graphical historical trend line. Coupling such a display with a high-load alarm to signal the machine operator to process threatening conditions provides an important process monitor and control capability. Additional benefits of pull load and line speed trending can be realized by the production supervisor and the process engineer who are charged with optimising productivity and quality. This paper describes the use of pull force measurement techniques and commercial equipment which is now available to establish this necessary process insight. Practical examples of the use of this methodology are provided.