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Applications in Die Construction | |
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We would like to inform you about the latest innovations in unsaturated polyester resins for use in FRP (Fiberglass Reinforced Plastic); mainly, a new generation of low shrinkage resins called "Low Profile" resins. Low or zero shrinkage constitutes their main feature; in some cases, expansion occurs during resin cure, after acceleration and catalysis. Normally, linear shrinkage in polyester resin ranges between 1.9nd 2.9which results in serious inconveniences in most FRP products; this is due to tensile strain in parts, which leads to lack of dimensional stability, fiberglass print-through surfaces, parts that do not match the mold or die, etc. These resins are expected to cover most of the sector's needs, and are characterized by low or zero shrinkage. They may be classified as follows:
Low-shrinkage resin for die construction: POLIAL LP-531/A
Die costs and quality are significant means that should be considered in composite production. Usually, die and mold construction is the most expensive and longest stage in composite part production. Die quality is critical, since it defines whether a composites project will succeed or fail. Generalized use of unsaturated polyester dies started in the early '50s. From then on, shrinkage has been compensated by both manufacturers and designers. Mainly due to the characteristics of the materials used, no changes were introduced into die construction until the early '90s. Unfortunately, there were no fast methods for the production of high-quality, zero-distortion dies, guaranteeing dimensional stability throughout useful life. Die construction based on a model takes several weeks of intensive and methodic work, including hand lay-up, fiberglass mat and woven fabric impregnation. Slow processes were necessary in order to minimize the effects of lamination exothermic reaction and, most importantly, volumetric shrinkage. Complex techniques were used in an attempt to minimize print-through in reinforcements, warping, early demolding, internal strain and surface distortion, applying thin layers and low initiator levels. Dies were cured by aging before use, alternating lamination and post-cure by high temperatures, or a combination of the two. This process was followed by intensive hand sanding and glossing to minimize die distortion.
In order to simplify the die construction process, minimize time and costs and produce fully cured dies with a permanently smooth surface, some producers, including ourselves, have developed low shrinkage unsaturated polyester resins for die construction.
Construction sequence is as follows:
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A model is prepared with mold release, and Poligel MT, a die Gel Coat, is applied using conventional techniques. (Request Technical Report on Gel Coat Application.)
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Once Gel Coat is adequately cured, low shrinkage resin Polial LP-531/A is applied, and initiators, loads and sprayed up fibers are added.
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Air is eliminated and fibers pressed by using rollers and brushes. Layers of up to 3mm are applied one at a time.
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Consecutive layers may be applied immediately afterwards, even before reaching gel stage or cure of lower layers.
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Depending on die specific requirements, several layers of chopped fiber or special reinforcement may be applied.
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As in conventional construction, new structural materials, such as non-woven fabrics and foams, may be applied to achieve the adequate rigidity level.
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Shrinkage control has been the key element in die construction revolution. As regards low shrinkage, such systems have a linear shrinkage of 0.1 to 0.05as compared to 1.8or traditional polyester. | |
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