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Pultrusion Preforming and Curing

The main function of preforming is to guide the impregnated, flattened fibrous materials to gradually take the shape closest to that of the final pultruded product. It also involves squeezing out excess resin from the reinforcement material and eliminating bubbles introduced into the material, to obtain a structurally dense pultruded product. The preforming process is accomplished with the help of a preforming mold, which transitions from simple to complex, occupying a length of about 0.6-1.2 meters. The reinforcement material gradually takes on the designed shape in the preform and ensures the fiber distribution in the product meets design requirements. Usually, tubular preforming molds are used for pultruding rod materials. The simplest design involves creating a certain number of radiating yarn holes on a board. For producing tubes, a mandrel preforming mold is required. When manufacturing profiles, about 2 to 6 preforming molds are often needed to ensure a smooth and reasonable transition of fibers and felt materials into the appropriate shape, close to the profile’s cross-sectional shape. The design of the preforming mold is a very worthwhile area of study in pultrusion technology. It requires flexibility, a broad mindset, rich experience, and strong practical skills from the designer. The normal pultrusion of a complex product depends on a well-designed, innovative preforming system. Understanding the critical role of preforming allows for not being confined to any specific preforming pattern, fostering innovation and developing a unique system.

Here is one design approach for preforming channel steel in pultrusion: After the material is pulled out from the preforming mold, it enters the heating mold, where it is cured and shaped before being pulled out of the mold. This process is the most important and primary in the pultrusion technique. The length of the curing mold is generally between 0.5m-1.55m, depending on the product thickness, pultrusion speed, and the chemical reaction characteristics of the resin system. Molds are typically made of tool steel and then chromed or nitrided to improve hardness, reduce wear, decrease traction force, and extend mold life. Heating methods for the mold include steam heating, heat conduction oil heating, and electrical heating, with the latter being more common due to its ease of controlling temperatures in different areas along the length of the mold. Pultrusion machine molds typically contain one to four heating zones, determined by factors such as the resin system, pultrusion speed, and mold length.

In the design of the curing mold, besides considering any dimensions of the cross-section, two main factors should be primarily considered: one is the chemical and physical characteristics of the resin system’s curing reaction; the second is the frictional performance between the pultrusion material and the mold walls. In many cases, based on the resin reaction characteristics and related material properties, the mold is designed with three different heating zones: the preheating zone, gel zone, and curing zone, with temperatures in these zones being coordinated with each other. The resin-fiber composite material first enters the preheating zone to reduce the viscosity of the resin, improve its flowability, and further impregnate the reinforcement material; then the material enters the gel zone, where the resin begins to react and changes from a viscous liquid to a gel state; finally, the material enters the curing zone for complete curing; the resin’s reaction mainly occurs in the gel zone. The point where the resin reacts at a higher temperature to reach the gel state is known as the “gel point,” and the process of curing reaction within the gel zone is an exothermic reaction, with the point of fastest exothermic reaction rate known as the “exothermic peak”; when the resin cures into a solid, due to curing shrinkage, the pressure drops, and the product detaches from the mold surface, this point is called the “detachment point.” Successful pultrusion technology ensures that the gel point, exothermic peak, and detachment point are close and concentrated in the gel zone; otherwise, poor mechanical properties of the product and phenomena like sticky films may occur.

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