The relationship between the length of injection molding machine screws and the injection range

At first glance, the screw length and injection stroke of an injection molding machine are two different things, but there is actually a subtle relationship between quality and quantity, and the ratio is a measure of quality.
The length of a screw is generally measured relative to its diameter rather than absolute length. In this way, screws with different diameters can also be compared in length. This length is called the aspect ratio, represented by L/D. The length of the screw is of course only considered as the threaded part. A more accurate algorithm is to calculate the centerline of the hopper, which is called the effective length or effective aspect ratio.
An injection molding machine usually has three optional screws, called A, B, and C screws, with diameters of small, medium (standard), and large, respectively. Their aspect ratios are around 22, 20, and 18. Jinyang screw
Injection molding machine screw
Uneven temperature
Plasticized plastic is called melt and stored at the top of the screw, ready for use during the next injection. The ideal melting is at a uniform temperature. But in general, this is not the case. Due to the fact that the heating tile is not 360 ° surrounding the material cylinder, but has a gap, the circumferential temperature is uneven. The heat of the heating tile is transmitted from the outside to the inside, coupled with poor melting heat transfer, resulting in uneven radial temperature.
When plasticizing, the screw moves backwards. The effective length gradually decreases as a result. The larger the feeding stroke (injection stroke), the greater the change in effective length, and the more uneven the axial temperature. Readers familiar with extruders know that the extrusion screw does not move backwards. Therefore, there is no axial temperature difference in the melting process of extrusion. If the melting temperature differs by 15 ° C, the appearance and mechanical properties of the finished product will not be average. A mold with multiple cavities can lead to differences in the finished product between cavities, even if one cavity is not satisfactory and the other cavity has burrs, and this situation is irregular. Jinyang screw
To improve this situation, the injection stroke should be designed to be 4 times the diameter of the B screw. The change in effective aspect ratio is therefore 4. In this way, the injection stroke is 4.4 times the diameter of screw A and 3.7 times the diameter of screw C. The radial temperature difference is maximum for screw A and minimum for screw C. Jinyang screw
Increase aspect ratio
Increasing the aspect ratio will reduce the axial temperature difference because the screw is longer and the plastic needs to rotate several more turns to reach the end of the screw. Stir more, and the temperature becomes more uniform. When the injection stroke remains unchanged, the longer the screw, the lower the "injection stroke ÷ screw length", resulting in a decrease in axial temperature difference. If the B screw can have a aspect ratio of 22, it is certainly better than a aspect ratio of 20. Overall, designs with large injection stroke or short screw aspect ratio result in larger injection weight, but uneven melting axial temperature, making them only suitable for single chamber products with low requirements. The design that limits the injection stroke and the large aspect ratio of the screw ensures the quality of multi chamber products. Jinyang screw