The relationship between injection stroke and screw length of injection molding machine

The length of the screw and the injection stroke of the injection molding machine seem to be two different things at first glance, but in fact, there is a subtle "quality and quantity" relationship between the two, and the ratio is qualitative scale.
 

The length of the screw is generally not measured in absolute length, but in length relative to the diameter. In this way, the lengths of screws of different diameters can be compared. This length is called the length-to-diameter ratio and is represented by L/D. The length of the screw is, of course, only the part with threads. It is more accurate to count to the midline of the hopper, which is called the effective length or effective L/D ratio.
 

An injection molding machine usually has three screws to choose from, called A, B, and C screws, with small, medium (standard), and large diameters respectively. Their length-to-diameter ratios are about 22, 20, and 18.
 

Injection molding machine screw length and injection stroke, suddenly look like two different things, but in fact, the two exist in a subtle "quality and quantity" of the relationship, the ratio is a qualitative scale.
 

The length of the screw is generally not measured in absolute length, but in length relative to the diameter. In this way, the lengths of screws of different diameters can be compared. This length is called the length-to-diameter ratio and is represented by L/D. The length of the screw is, of course, only the part with threads. It is more accurate to count to the midline of the hopper, which is called the effective length or effective L/D ratio.
 

An injection molding machine usually has three screws to choose from, called A, B, and C screws, with small, medium (standard), and large diameters respectively. Their length-to-diameter ratios are about 22, 20, and 18.

 

Uneven temperature

The plasticized plastic is called melt and is stored at the tip of the screw, ready for the next injection. The ideal melt is uniform in temperature. However, this is generally not the case. Since the heating tile is not 360° around the barrel but has a gap, the temperature is not uniform in the circumferential direction. The heat transfer from the heating tile is from the outside to the inside, and because of the poor heat transfer from the melt, the radial temperature is not uniform.

 

When plasticizing, the screw recedes. The effective length is thus gradually reduced. The larger the charging stroke (injection stroke), the greater the change in effective length, and the more uneven the temperature in the axial direction. Those who are familiar with extruders know that the extrusion screw does not go backward. Therefore, there is no axial temperature difference in the extruded melt. If the melt temperature differs by 15°C, the appearance and mechanical properties of the finished product will not be even. The multi-cavity die will produce cavity to cavity differences between the finished product, or even a cavity dissatisfaction, a cavity flying edge, and this situation is not regular.

 

To improve this situation, the injection stroke should be designed to be 4 times the diameter of the B-screw. In this case, 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 length-to-diameter ratio

Increasing the L/D ratio reduces the axial temperature difference because the longer the screw, the more turns the plastic has to make before it reaches the end of the screw. With more mixing, the temperature is more uniform. The longer the screw is, the longer the "injection stroke ÷ screw length" decreases, so the axial temperature difference also decreases. If the B-screw can have a length-to-diameter ratio of 22, it is certainly better than a length-to-diameter ratio of 20.

 

In general, a design with a large injection stroke or a short screw L/D ratio will result in a large injection weight, but the melt axial temperature is not uniform and is only suitable for single-cavity products with low requirements. The design with limited injection stroke and large screw L/D ratio can ensure the quality of multi-cavity products.