The length of the screw and the injection stroke of the injection molding machine seem to be two different things. In fact, there is a subtle “quality and quantity” relationship between the two, and the ratio is a qualitative scale.
The length of the screw, generally do not use the contraband word length, and use the length relative to the diameter to measure. In this way, different diameter screw can be compared in length. This length is called the aspect ratio, represented by L/D. The length of the screw is, of course, only the part of the thread. A more accurate algorithm is to calculate the center line of the hopper, called the effective length or effective aspect ratio.
An injection molding machine usually has three screw options, called A, B, C screw, diameter respectively for small, medium (standard), large. Their aspect ratio is around 22, 20, 18.
Uneven temperature
The plasticized plastic, called melt, is stored at the top of the screw, ready for the next injection. The ideal melting is temperature uniform. But this is generally not the case. Because the heating tile does not surround the cylinder 360°, but there is a gap, so the annular temperature is not uniform. Heating tile heat transfer from the outside, coupled with the melting heat transfer is poor, so the radial temperature is not uniform.
When plasticizing, the screw moves backward. The effective length therefore gradually decreases. The larger the feeding stroke (injection stroke), the greater the change of effective length, the more uneven the axial temperature. Familiar with the extruder friends know that the extruder screw is not backward. As a result, the extrusion melt has no axial temperature difference. If the melting temperature difference is 15°C, the appearance and mechanical properties of the finished product will not be equal. The mold with multiple cavities will produce the difference of the finished product between the cavity and the cavity, and even one cavity is dissatisfied and one cavity flies. Moreover, this situation is not regular.
To improve the 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 case, the injection trip is 4.4 times the diameter of screw A and 3.7 times the diameter of screw C. The radial temperature difference of screw A is the largest and that of screw C is the smallest.
Increase the aspect ratio
Increasing the aspect ratio will reduce the axial temperature difference because the plastic has to make more turns to reach the end of the screw as the screw is longer. The more you stir, the more even the temperature. Under the condition that the injection stroke remains unchanged, the longer the screw, the “injection stroke ÷ screw length” decreases, so the axial temperature difference also decreases. If the B screw can have the aspect ratio of 22, it is better than the aspect ratio of 20.
In general, the injection stroke is large or the design of the screw length to diameter ratio is short, the injection weight will be large, but the melting axial temperature is not uniform, only suitable for the single cavity products with low requirements. The design that limits injection stroke and screw length-diameter ratio ensures the quality of multi-cavity products.
