The screw length and injection stroke of injection molding machine are two different things. In fact, they have a wonderful relationship between quality and quantity. The ratio is a quality scale.

The length of screw is generally measured by the length relative to the diameter rather than absolute length. In this way, the screw with different diameters can also be compared with the length. This length is called the ratio of length to diameter, represented by L / d. Of course, the screw length is only the part with thread. A more accurate algorithm is to calculate the center line of the hopper, which is called the effective length or effective length diameter ratio.

A injection molding machine usually has three screws, called a, B and C screws, with small, medium (standard) and large diameters. Their aspect ratio is about 22,20,18.

Uneven temperature

The plasticized plastic is called melting, which is stored at the top of the screw and is ready for use in the next injection. The ideal melting is temperature averaged. But this is not the case in general. Since the heating pad does not surround the barrel 360 ° but has a gap, the circumferential temperature is uneven. The heat of the heating pad is transferred from the outside to the inside, and the melting heat transfer is poor, so the radial temperature is uneven.

When plasticizing, the screw moves backward. The effective length is thus gradually reduced. The larger the feeding stroke (injection stroke), the greater the effective length change, and the more uneven the axial temperature. Familiar with the extruder readers know that the extruder screw is not backward. Therefore, there is no axial temperature difference in extrusion melting. If the melting temperature difference is 15 ° C, the appearance and mechanical properties of the waste products will not be uniform. Multi cavity mold will produce the difference between cavity and cavity, so that one cavity is not satisfied, one cavity is flying, and this situation is not regular.

To improve this situation, the injection stroke should be designed to be 4 times the diameter of B screw. The change of effective aspect ratio is also 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 the largest for screw a and the smallest for screw C.

Increase aspect ratio

Increasing the ratio of length to diameter will reduce the axial temperature difference, because the screw is long, and the plastic needs to rotate several times before it reaches the end of the screw. If you stir more, the temperature will be more even. Under the condition that the injection stroke is constant, the longer the screw is, "injection stroke ÷ screw length" drops, so the axial temperature difference also drops. If the length diameter ratio of screw B is 22, it is better than that of screw B 20.

In general, if the injection stroke is large or the length diameter ratio of screw is short, the injection weight will be large, but the melting axial temperature is uneven, so it is only suitable for single cavity products with low request. The design which limits the injection stroke and the large ratio of screw length to diameter ensures the quality of multi cavity products.