Low Profile Additives (LPA) and 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.

Primary Function: Their core 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, LPAs enhance the product's dimensional stability, surface finish, and flatness. They are well-suited for thick-walled and complex-shaped molded parts, particularly those demanding high molding precision and a superior Class A surface.

Common types fall into four categories, allowing for precise selection based on product requirements:

① Non-polar type (e.g., Polystyrene/PS): Offers good internal colorability but provides only moderate low-shrinkage performance at a relatively low cost. Note: Due to its low polarity (approx. 0.3), it exhibits poor compatibility with unsaturated resins (polarity approx. 20) and therefore requires a compatibilizer.
② Weakly polar type (e.g., Polymethyl methacrylate/PMMA): Exhibits moderate resin compatibility and offers better shrinkage control than PS, though colorability is somewhat inferior.
③ Polar type (e.g., Polyvinyl acetate/PVAc, Saturated polyesters): Delivers the most effective low-shrinkage performance. It demonstrates excellent compatibility with unsaturated polyester resins (PVAc polarity is approx. 1.3, closely matching the resin), making it ideal for high-precision, Class A surface additive for SMC products.
④ Combination type (e.g., Core-shell polymers): Offers excellent comprehensive performance, balancing shrinkage control, colorability, and mechanical strength while preventing macroscopic phase separation.

Selection Focus: Prioritize matching the specific resin type and the precision requirements of the final product. We highly recommend requesting samples for compatibility testing before mass production.

The standard addition ratio is 5%–10% of the total resin weight.

Ratio Adjustments: For thick-walled or high-precision products, the ratio may be increased to 8%–12%; for standard parts, 5%–8% is sufficient. If inorganic fillers (such as calcium carbonate or talc) are used in the formulation, the LPA ratio must be fine-tuned to balance shrinkage control and paste flowability.

Impact of Addition Errors:
• Adding too little: Fails to effectively compensate for resin shrinkage, leaving the product prone to sink marks, warping, surface depressions, and poor dimensional stability.
• Adding too much: Leads to reduced product toughness and increased brittleness. It can exacerbate phase separation, causing surface mottling, additive migration (blooming), and delamination from the resin matrix. Additionally, it negatively impacts paste flowability, potentially causing incomplete mold filling.