Mechanical principle of injection molding machine Injection molding machine

Update:19-10-2019
Summary:

Mechanical principle of injection molding machine Injec […]

Mechanical principle of injection molding machine Injection molding machine is mainly divided into injection part and mold clamping part in mechanical structure. The function of the injection part is to melt the plastic and inject it into the mold cavity. The function of the mold clamping part is to control various actions such as opening and closing of the mold and ejection of the product. The action program of the molding machine: nozzle advance → injection → pressure holding → pre-plastic → retraction → nozzle retreat → cooling → mold opening → ejection → retraction → opening the door → closing the door → closing the mold → nozzle advancement.
There are two main forms of injection: piston and reciprocating screw. Piston type injection molding machines are rare now and will not be introduced here. The reciprocating screw type injection molding machine melts and mixes the solid plastic particles (or powder) by the rotation of the screw in the heating barrel, and squeezes into the cavity at the front end of the barrel, and then the screw moves forward in the axial direction, and the plastic in the cavity The melt is injected into the mold cavity. During plasticization, the plastic is compacted in the groove under the push of the screw rib, and receives the heat transfer from the barrel wall. The plastic and the plastic, the plastic and the barrel and the surface of the screw are heated and the temperature gradually rises. High to the melting temperature. The melted plastic is further mixed by the screw, and enters the front of the barrel along the screw groove and pushes the screw back. The components related to plasticization in the injection part mainly include: screw, barrel, split shuttle, check ring, nozzle, flange, hopper, and the like.

(1) The screw  is an important part of the injection molding machine. Its role is to transport, compact, melt, agitate and apply pressure to plastics. All of this is done by the rotation of the screw in the barrel. When the screw rotates, the plastic will rub against each other on the inner wall of the barrel, the bottom surface of the screw groove, the screw advance surface, and between the plastic and the plastic. The forward advancement of the plastic is the result of this combination of motion, and the heat generated by the friction is also absorbed to increase the temperature of the plastic and melt the plastic. The structure of the screw will directly affect the extent of these effects. Ordinary injection screw structure, also designed to improve the plasticizing quality into a separate type of screw, barrier type screw or split type screw.

(2) The structure of the barrel of the barrel is actually a round tube with a feed opening in the middle. In the plasticizing process of plastic, the power of advancement and mixing is derived from the relative rotation of the screw and the barrel. According to the different forms of plastic in the screw groove, the screw is generally divided into three sections: a solid conveying section (also called a feeding section), a melting section (also called a compression section), and a homogenization section (also called a metering section). In the textbook on plastic plasticization, the plastic in the solid conveying section of the screw is regarded as a solid bed without plastic movement between the plastic particles, and then through the solid bed and the barrel wall, the screw edge pushing surface and the screw groove The calculation of the ideal state of surface motion and friction to determine the speed at which the plastic is transported forward. This has a lot of gaps with the actual situation, and can not be used as a basis to analyze the feeding of plastic pellets of different shapes. If the particles of the plastic are not large, they will stratify and roll when they are pulled forward by the inner wall of the barrel, and are gradually compacted to form a solid plug. When the diameter of the particles is similar to the depth of the groove, their trajectory is basically a linear motion along the radial direction of the groove plus a linear motion at an angle.