2. When the pressure holding stage is underway, the plastic material undergoes partial compression due to the high pressure involved. This means that certain areas of the plastic become more compact and dense, while other areas become looser and less dense. As time goes on, the distribution of density throughout the plastic changes, based on the location and pressure dynamics at play.
During the pressure holding stage of the two-color mold, the presence of plastic in the mold cavity leads to a high back pressure. As a result, the screw of the injection molding machine can only make slight forward movements, causing the plastic to flow at a relatively slow speed. This type of flow, known as pressure-holding flow, occurs when the plastic cools and solidifies against the mold wall. Consequently, the viscosity of the melt increases rapidly, creating significant resistance within the mold cavity.
During the pressure holding process, flow rate of the plastic is significantly reduced, and pressure becomes the primary determinant of the process. At this stage, the mold cavity has already been filled with plastic, and the partially solidified melt plays a crucial role in transmitting the pressure. The plastic in the cavity transmits the pressure to the mold wall, exerting a force that tends to expand the mold. To prevent this, an appropriate level of mold clamping force is needed. Ideally, the mold should expand slightly, promoting effective exhaust for the two-color injection mold. If the mold expansion force is too high, however, problems like burrs and overflow may occur, and the mold may even open prematurely.
During the later stage of the two-color mold holding pressure, there is a continuous increase in material density, allowing the plastic parts to gradually take shape. It is crucial to maintain the holding pressure stage until the gate solidifies and completely seals off. At this point, the cavity pressure reaches a peak value, ensuring the successful formation of the desired product.