The process technology of foamed PVC baseball involves multiple aspects such as raw material selection, formula design, molding methods and process control:
Raw material selection: In the production of foamed PVC baseball, PVC resin is the main raw material. PVC varieties with a K value of 58 to 65 can be used to make hard foamed bodies. Generally speaking, suspension polymerization, emulsion polymerization and PVC bulk polymerization are all suitable, and suspension polymerization and bulk polymerization PVC are the best.
Formula design: Foaming agents need to be added to the formula of foamed PVC baseball. Foaming agents can be divided into two major categories: physical foaming agents and chemical foaming agents. Physical foaming agents can be liquids or substances that have vaporized under normal conditions, including aliphatic hydrocarbons (pentane, hexane, decane, etc.), chlorinated hydrocarbons (chloroform, trichloroethane, etc.), chlorine-fluorocarbons [trichloromonofluoromethane (R11), dichlorodifluoromethane (R12), etc.], CO₂, N₂, rare gases and air, etc. Chemical foaming agents are substances that release gases such as nitrogen and carbon dioxide when heated. Widely used ones include inorganic foaming agents like sodium bicarbonate and ammonium carbonate, as well as organic foaming agents like azoformamide and azodiisonitrile. The ideal chemical foaming agent should have a relatively narrow and stable decomposition temperature range, a fast and controllable decomposition speed, a large gas generation per unit mass, and the decomposition gas should be non-flammable, non-corrosive and non-toxic. The decomposition residue should be colorless, odorless and mold-free, and have no impact on the performance of raw materials. It should be low in price and have storage stability. The heat generated during the decomposition of the foaming agent should not be large. The foaming agent and its residue should be related to P Plastic melts such as VC should have good compatibility and other properties.
Molding methods: Foamed PVC baseball can be formed by extrusion molding, compression molding and other methods. Extrusion molding is a process in which physical or chemical foaming agents, PVC resin, additives and other components are heated in the extruder barrel during the extrusion process and melted and plasticized above the decomposition temperature of the foaming agent. Due to the back pressure of the screw and the sealing effect of the die head, the gas produced by the decomposition of the material is strongly suppressed and cannot be foamed in the barrel. Therefore, this material has a large expansion force. This plasticized material is squeezed into the mold at a relatively fast speed, and the molten material foamed and shaped in the mold by the release of the expansion force. Form a material with a skinning surface layer and a honeycomb-like core layer. Compression molding is a method in which expandable materials are placed in a mold and formed and foamed through pressure and heating.
Process control: Taking extrusion molding as an example, the control of extrusion temperature can enable the decomposition rate of AC foaming agent to be controlled. In the feeding section, to prevent the prematurely decomposed foaming agent gas from escaping from the feeding hopper, a lower temperature should be used. In the compression section and metering section, to increase the gas evolution of the foaming agent during the extrusion process and evenly dissolve it in the PVC melt, a higher temperature should be adopted. The temperature control at the die should be appropriately lower to prevent excessive foaming and also maintain the pressure distribution inside the die head. When producing PVC skin foamed rods, it is necessary to maintain sufficient pressure inside the die head to facilitate foaming after the material leaves the die. As the extrusion pressure increases, the cell size and foaming density decrease significantly, the number of cells increases, and the performance of the product improves accordingly. The screw speed is directly proportional to the extrusion pressure. An increase in the screw speed is beneficial for extrusion foaming, but an increase in extrusion pressure will cause an increase in the load on the extruder, making it difficult for the motor to bear. Meanwhile, the strength of the die head mold and the connection parts must also be enhanced. The extrusion pressure has a significant impact on the performance of the product. The key to controlling the extrusion pressure lies in controlling the relationship among the extrusion temperature, the screw speed and the amount of lubricant in the formula. The extrusion speed for producing PVC skin foamed rods is controlled at 15 to 20r/min, and the traction speed of the profiles is controlled at 1 to 2m/min. In addition, the melt viscosity is affected not only by the specifications, types and addition amounts of PVC resin and additives, but also by process conditions such as screw speed and extrusion temperature. During the foaming process, PVC needs to simultaneously form a large number of evenly distributed bubble nuclei and supersaturated gases in the melt, thereby obtaining PVC foaming materials with uniform cell distribution, fine structure and light weight. Appropriate viscosity is indispensable. Generally, when the melt viscosity is high, the expansion speed of the bubbles decreases, resulting in a higher density of the foamed body. This makes it easy to continue foaming during the secondary processing of heat treatment, affecting the surface quality. Therefore, during processing, it can be considered to appropriately increase the amount of lubricant used.