Low Shrinkage Additives (LSA) for molding resins are functional auxiliaries specifically designed for molding processes (such as SMC, BMC, and general molding resins). They typically consist of thermoplastic polymers or composite systems. Their primary function is to compensate for the polymerization shrinkage and thermal shrinkage of the resin during the curing process through a mechanism of microscopic phase separation. This effectively reduces the molding shrinkage rate of the finished product, minimizing defects such as sink marks, deformation, surface depressions, cracks, and flow lines. Simultaneously, LSAs enhance the product's dimensional stability, surface finish, and flatness. They are well-suited for thick-walled and complex-shaped molded parts, and are particularly ideal for products—such as decorative molded items and industrial components—that demand high molding precision and superior aesthetic quality.

Common types fall into four categories, allowing for precise selection based on the specific requirements of the finished product: ① Non-polar type (e.g., Polystyrene/PS): Offers good internal colorability but provides only moderate low-shrinkage performance at a relatively low cost; suitable for general-purpose molded parts. Note that due to its low polarity (approx. 0.3), it exhibits poor compatibility with unsaturated resins (which have a polarity of approx. 20) and therefore requires the use of a compatibilizer. ② Weakly polar type (e.g., Polymethyl methacrylate/PMMA): Exhibits moderate compatibility with the resin and offers better low-shrinkage performance than PS, though its colorability is somewhat inferior; suitable for mid-range molded parts with moderate aesthetic requirements. ③ Polar type (e.g., Polyvinyl acetate/PVAc, Saturated polyesters): Delivers the most effective low-shrinkage performance and demonstrates excellent compatibility with unsaturated polyester resins (with a polarity of approx. 1.3, closely matching that of the resin); suitable for products requiring high precision and superior aesthetic quality. ④ Combination type (e.g., Core-shell polymers): Offers excellent comprehensive performance, balancing low shrinkage properties, colorability, and mechanical strength; effectively prevents phase separation and is suitable for high-end decorative molded items and precision industrial components. Selection Focus: Prioritize matching the specific resin type and the precision requirements of the final product, while also taking into account cost-effectiveness and processing convenience. It is recommended to request samples for testing to verify compatibility before making a final selection.

The standard addition ratio is 5%–10% of the total resin weight. However, this specific ratio should be adjusted based on the resin type, product wall thickness, and precision requirements: for thick-walled or high-precision products, the ratio may be appropriately increased to 8%–12%; for standard molded parts, it can be maintained within the 5%–8% range. If inorganic fillers (such as calcium carbonate or talc) are also being used, the ratio may be fine-tuned to strike a balance between shrinkage control and processing flowability. Impact of Addition: ① Adding too little: This fails to effectively compensate for resin shrinkage, making the product prone to defects such as sink marks, deformation, surface depressions, and cracks, resulting in poor dimensional stability. ② Adding too much: This leads to reduced product toughness and increased brittleness. It may also exacerbate phase separation, causing surface mottling, additive migration (blooming), and delamination from the resin matrix. Furthermore, it increases production costs and can negatively impact the flowability of the molding compound, potentially resulting in incomplete mold filling.