The molding process of halogenated butyl rubber is the core step in transforming rubber compounds with specific molecular structures and performance characteristics into semi-finished or finished products that meet different application requirements. Its process design must consider the material's inherent low permeability, high elasticity, and vulcanization characteristics, while also taking into account the product's shape, dimensional accuracy, and performance requirements. This is achieved through the organic integration of mixing, molding, vulcanization, and post-treatment processes to unify structure and function.
Mixing is the first step in molding, aiming to uniformly disperse halogenated butyl rubber with various compounding agents (such as vulcanizing agents, reinforcing agents, plasticizers, antioxidants, etc.) to form a processable rubber compound. Because halogenated butyl rubber molecules are saturated and more polar than butyl rubber, its roll wrapping properties and powder absorption rate differ from ordinary rubber. The mixing temperature is typically controlled between 40 and 80°C to avoid scorching or molecular chain degradation caused by high temperatures. The order of feeding is crucial: generally, raw rubber is added first to the thin-walled roll, followed by activators, antioxidants, and reinforcing fillers (such as carbon black and silica), and finally vulcanizing agents and plasticizers. This ensures uniform dispersion of all components and avoids premature vulcanization. During mixing, the roll gap and number of re-rolling cycles must be controlled to prevent overheating or air bubbles. The Mooney viscosity of the mixed rubber should meet the processing requirements of subsequent molding equipment.
Molding processes vary depending on the product shape and batch size, and can include calendering, extrusion, compression molding, or injection molding. Calendering is suitable for preparing thin sheet-like semi-finished products (such as airtight sheets and waterproof membrane substrates). Roller temperature and speed ratio must be controlled to ensure uniform spreading of the rubber compound between the rolls, resulting in a smooth surface and thickness tolerances meeting standards. Extrusion molding is mostly used for continuous cross-section products such as pipes, profiles, and tire tread base rubber. The screw length-to-diameter ratio and compression ratio must match the viscoelasticity of halogenated butyl rubber. Die head temperature and pressure should ensure stable melt flow, avoiding rough surfaces or spongy defects in the extrudate. Compression molding is suitable for complex shapes and high dimensional accuracy, such as seals and shock absorbers. The clamping pressure and holding time must be sufficient to fill the mold cavity with the rubber compound. The frequency and timing of venting should be carefully controlled to reduce internal porosity. Injection molding, on the other hand, uses screw pre-plasticization and high-pressure injection to quickly fill the closed mold cavity with the rubber compound. It is suitable for mass production of small, precision products, offering advantages such as short cycle time and good dimensional repeatability. However, it requires higher standards for mold runner design and clamping force.
Vulcanization is the decisive process for imparting the final properties of halogenated butyl rubber. Vulcanization systems can include sulfur vulcanization, resin vulcanization, or peroxide vulcanization. Sulfur vulcanization is the most widely used due to its ease of operation and lower cost. The amount of vulcanizing agent and the ratio of accelerator need to be adjusted according to the halogen content and product performance requirements. The vulcanization temperature is usually set between 150 and 180°C, and the time depends on the product thickness and heat transfer efficiency. Over-vulcanization should be avoided, as it leads to increased hardness and decreased elasticity, while under-vulcanization causes permanent deformation and insufficient strength. For thick-walled products, segmented heating vulcanization can be used to ensure uniform heat transfer from the surface inwards, guaranteeing consistent vulcanization levels between the inner and outer layers. Pressure control during vulcanization is also crucial; appropriate molding or vulcanizing tank pressure can prevent foaming and deformation, improving dimensional stability.
Post-processing includes trimming, inspection, and surface treatment. Trimming removes burrs and flash to ensure a clear product outline. Inspection items cover dimensional accuracy, hardness, tensile strength, elongation, airtightness, and aging resistance to ensure compliance with relevant standards. Some products require surface coating, flocking, or lamination with other materials to improve adhesion or specific functions. In this stage, process compatibility should be carefully considered to avoid damaging the surface properties of the halogenated butyl rubber.
In general, the molding process for halogenated butyl rubber is based on uniform mixing, adapting the molding method to the product shape, locking in performance through vulcanization conditions, and ensuring the integrity of appearance and function through post-processing. Precise control of parameters and process coordination at each stage are key to ensuring the reliable application of products in fields such as tire airtightness, pharmaceutical sealing, and chemical corrosion protection, and also provide process assurance for maximizing material performance.