One injection molding machine usually has three screws, […]
One injection molding machine usually has three screws, called a, B and C screws, with the diameter of small, medium and large respectively. Their aspect ratio is about 22,20,18.
The screw length and injection stroke of injection molding machine are two different things. In fact, there is a delicate relationship between the two. The ratio is a quality scale.
The length of screw is generally measured by the length relative to diameter rather than absolute length. In this way, the length of screws with different diameters can also be compared. 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.
The plasticized plastic is called melting, which is stored at the top of the screw and is ready to be used for the next injection. The ideal melting temperature is uniform. 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. Readers familiar with extruder know that 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 finished product will not be average. The multi cavity mold will produce the difference between the cavity and the finished product, even a cavity is not satisfied, a 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.
Increasing the aspect ratio and increasing the aspect ratio will reduce the axial temperature difference. The reason is that the screw is long and the plastic needs to rotate several times before it reaches the end of the screw. Stir more, the temperature will be more uniform. When the injection stroke is constant, the longer the screw, "injection stroke ÷ screw length" decreases, so the axial temperature difference also decreases. If the length diameter ratio of screw B is 22, it is better than that of screw B 20.
Generally speaking, the design with large injection stroke or short screw length diameter ratio will result in large injection weight, but the melting axial temperature is not uniform, which is only suitable for single cavity products with low requirements. The design of the injection stroke and the large ratio of the length and diameter of the screw are limited, which guarantees the quality of the multi cavity products.