Key knowledge: What is injection molding?

Overview

Injection molding (injection molding) is a method in which a thermoplastic or thermosetting molding compound is uniformly plasticized in a heating cylinder and then pushed by a plunger or a moving screw into a cavity of a closed mold.

The granulated or powdered plastic is added to the heated barrel through the hopper of the injection molding machine, and the plastic is heated and melted. Under the pressure of the screw or the piston of the injection molding machine, the nozzle enters the mold cavity through the nozzle, and the plastic fills the cavity and is hardened by cooling. After shaping, after demoulding, a plastic part having a certain shape is obtained. Injection molding is suitable for almost all thermoplastics.

The molding method can be divided into the following six categories:

(1) Exhaust injection molding: The vented injection machine for vented injection molding has an exhaust port in the middle of the barrel and is also connected to the vacuum system. When the plastic is plasticized, the vacuum pump can be placed in the plastic. The combined water vapor, monomer, volatile substances and air are pumped away through the exhaust port. The raw materials do not need to be pre-dried, thereby improving production efficiency and improving product quality. It is especially suitable for the molding of moisture-absorbing materials such as polycarbonate, nylon, plexiglass and cellulose.

(2) Flow injection molding: Flow injection molding can be used with ordinary moving screw injection machines. That is, the plastic is continuously plasticized and squeezed into a mold cavity having a certain temperature. After the plastic fills the cavity, the screw stops rotating, and the thrust of the screw is used to maintain the material under the pressure for a suitable time, and then cooled and shaped. The plasticized article is not stored in the cartridge, but is continuously squeezed into the mold, so it is a combination of extrusion and injection.

(3) Co-injection molding: Co-injection molding is a method in which plastics of different varieties or different colors are simultaneously or sequentially injected into a mold by using an injection machine having two or more injection units. In this way, composite products of various colors and/or multiple plastics can be produced. Representative co-injection molding is two-color injection and multi-color injection.

(4) No-channel injection molding: a molding method in which a split runner is not provided in a mold, and a molten nozzle is directly dispensed into each cavity by an extension nozzle of the injection machine. During the injection process, the plastic in the flow path maintains a molten flow state and does not come out together with the product during demolding, so that the workpiece has no flow path residue.

(5) Reaction injection molding: The principle of reaction injection molding is that the reaction raw materials are metered by a metering device, pumped into a mixing head, collided and mixed in a mixing head, and then injected into a closed mold at a high speed, rapidly solidified, demolded, and taken out. . It is suitable for processing some thermosetting plastics and elastomers such as polyurethane, epoxy resin, unsaturated polyester resin, silicone resin, alkyd resin.

(6) Injection molding of thermosetting plastic: granular or agglomerated thermosetting plastic, which is plasticized into a viscoplastic state by the action of a screw in a cylinder with strict temperature control. Under a high injection pressure, the material enters a certain temperature range. Cross-linking curing in the mold.