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Pultrusion Preforming

The primary purpose of preforming is to guide the flat-band-shaped fibers, post impregnation, to gradually evolve into a shape that closely resembles the final pultruded product. This process also involves squeezing out excess resin from the reinforcing materials and eliminating air bubbles that might have entered the materials, ensuring a structurally dense pultruded product. Preforming is accomplished using preforming molds, which transition from simple to complex designs. These molds typically span a length of about 0.6 to 1.2 meters. The reinforcing materials are progressively shaped into the designed form during preforming, aligning the fiber distribution within the product as per design requirements. Typically, tubular preforming molds are used for pultruded rods, with the simplest design featuring a set number of radially distributed yarn holes on a plate. For producing tubing, a mandrel preforming mold is necessary. The creation of specialized profiles often requires 2 to 6 preforming molds to ensure a smooth and reasonable transition of fibers and felt materials into the appropriate shape, closely matching the profile’s cross-sectional shape. The design of preforming molds is a subject worthy of study in the pultrusion process, demanding flexibility, broad-minded thinking, and rich experience from the designer, along with strong practical skills. The successful pultrusion of a complex product relies on a well-designed, innovative preforming system. Understanding the importance of preforming allows for flexibility and innovation beyond any single preforming pattern, leading to the development of unique systems.

One design approach for the preforming of pultruded channel steel is as follows: After being pulled from the preforming mold, the material enters a heating mold, where it is cured and shaped before being pulled out. This process is the most important and primary in the pultrusion technique. The length of the curing mold typically ranges from 0.5 to 1.55 meters, depending on factors like product thickness, pultrusion speed, and the chemical reaction characteristics of the resin system. Molds are generally made of tool steel and then undergo chrome plating or nitriding to enhance hardness, reduce wear, lower traction, and extend mold life. Common heating methods for molds include steam, thermal oil, and electrical heating, with the latter being more common for controlling temperature in different zones along the length of the mold. Pultrusion machine molds usually contain one to four heating zones, determined by the resin system, pultrusion speed, and mold length.

In the design of the molding mold, besides considering every dimension of the cross-section, two main factors must be taken into account: the chemical and physical properties of the resin system’s curing reaction, and the frictional properties 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 distinct heating zones: the preheat zone, gel zone, and curing zone, with coordinated temperatures. The resin-fiber composite material first enters the preheat zone, reducing resin viscosity, enhancing flowability, and further impregnating the reinforcing materials. Then, the material enters the gel zone, where the resin starts reacting, transforming from a viscous liquid to a gel state. Finally, the material enters the curing zone for complete solidification; the resin’s reaction primarily occurs in the gel zone. The point at which the resin reacts at a high temperature to reach a gel state is known as the “gel point,” and the exothermic reaction during curing in the gel zone is known as the “exothermic peak.” The “release point” is when the resin solidifies into a solid, and due to curing shrinkage, the pressure decreases, allowing the product to detach from the mold surface. Successful pultrusion process aims to concentrate and align the gel point, exothermic peak, and release point in the gel zone. Otherwise, issues such as poor mechanical properties and sticky films in the products may arise.

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