Injection molding machine injection screw length to diameter ratio and injection stroke

Update:14-12-2018
Summary:

The plastic injection molding machine injection stroke […]

The plastic injection molding machine injection stroke expresses the size of the injection volume and is an important technical parameter. The amount of material filled by the screw during the injection process is basically a very uncertain factor. At the time of injection, the screw moves axially forward, and the material flows into the groove, but cannot fill the groove because the injection time is insufficient to meet the time required to completely fill the screw groove.

Since the material is sparsely filled, the air is easily inhaled, and if the air cannot be discharged in time, the plasticizing quality is lowered. Usually, the size of the metering stroke is the main factor determining whether air can enter the storage tank. Studies of various materials have shown that if the metering stroke is greater than 3D, there will be a trapping gas behind the check ring. At this time, if the aspect ratio of the screw is less than 18 (the aspect ratio of the screw that satisfies the 3D metering stroke), that is, the length at which the solid in the feed section starts to melt is too short, and the solid remains in the melt when the solid is converted to the melt in the compression section. A large increase in solids can even cause delivery to stop under severe conditions. Therefore, in order to obtain a metering stroke of more than 3D, it is necessary to increase the effective length of the screw so that the solid material can have a sufficient melting path for melting during the feeding process to reduce the solid content in the melt, so that the composition of the melt volume and the compression section The volumetric flow compositions are matched such that the required melt volume for metering is greater than the melt volume in the screw channels. Under normal circumstances, the screw length to diameter ratio reaches 20~25, which can meet the requirement that the metering stroke is greater than 3D. In addition, in order to solve the problem of feeding entrainment caused by the increase of the aspect ratio of the screw, the screw design must satisfy the speed of the solid plug at the time of plasticization is greater than the speed of the solid bed.

Now, with the advancement of screw design and processing, the injection stroke of general plastic injection molding machines has increased from 3D to 4.5D~5D, and some even reached 6.5D. The length-to-diameter ratio of the screw has also increased from 18 to 20~25, and even more. Large, thus increasing the economics of plasticizing institutions. For example, in the Ergotech series of DEMAG, the injection stroke of the standard screw is about 4.5D, the effective length of the plasticizing of the screw is 20D~24D, the length-to-diameter ratio of the special screw is 25, and the ratio of the pitch to the outer diameter of the screw is 1:1.