The rheological behavior of thermosetting plastics in compression molding is much more complicated than that of thermoplastics. The entire molding process is always accompanied by chemical reactions. The materials in the initial stage of heating are in a low molecular viscous flow state, and the fluidity is still good. Partial cross-linking occurs, the fluidity of the material gradually becomes smaller, and a certain degree of elasticity is generated, so that the material is in a gel state, and then heating is continued, the molecular crosslinking reaction tends to be perfect, the degree of crosslinking increases, and the material is gelled. The state changes to a glassy state, and the resin body has a body structure, and the molding is finished.
In the plastic molding process, the "three-state" change (viscous flow state, gel state, glass state) of the material is often regarded as three stages: the flow stage, the gelation stage, and the solidification stage. In the flow phase, the resin molecules have an amorphous linear shape or a branched molecular structure, and the flow pattern of the resin belongs to the entire macromolecular displacement. The molecular weight and structural complexity of the resin determine its fluidity. It is generally considered that a linear structure having a small molecular weight or a resin having a small amount of branched molecular structure has good fluidity, and vice versa. The flow phase is the best stage for the material to fill the cavity and is the critical period for ensuring the molding of the product. This timing should be mastered to fill the cavity during the best fluidity.
In the gelling stage, the molecular structure of the material belongs to a linear structure with a large branch density, and may also be a network structure in which a large part has been crosslinked. Therefore, the fluidity is poor, and the flow is difficult, but it is still flowable. At this time, the viscosity of the material is significantly increased. The purpose of understanding the rheological behavior of the material at this stage is to allow the operator to further fill the cavity by taking advantage of the flowable properties of the material at this stage.
During the curing stage, the resin gradually becomes insoluble and infusible, completely loses fluidity, the product has been formed, and the molecular structure has been formed into a body structure, although a very small amount of low molecules exist at this time. However, the product can still be demolded.
Molded molding can also be molded into thermoplastic parts. After the thermoplastic is added to the mold cavity, it is gradually heated and pressurized to convert it into a viscous flow state, filling the entire cavity; then reducing the mold temperature, curing the part, and then pushing it out. Since the mold needs to be alternately heated and cooled, the production cycle is long and the efficiency is low.