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Injection Stroke vs. Screw Length: Understanding the Thermal Trade-Off in Injection Molding
Views: 487416 Author: Site Editor Publish Time: 2026-11-02 Origin: Site
Relationship between the injection stroke of an injection molding machine and the length of the screw
The length of the screw in an injection molding machine and the injection stroke may appear to be two separate issues; in fact, they share a subtle relationship between "quality" and "quantity," and their ratio serves as a qualitative measure.
The length of a screw is generally not measured in absolute units; instead, it is expressed as a length relative to the diameter. This allows screws of different diameters to be compared in terms of length. This length is referred to as the length-to-diameter ratio, denoted as L/D. Naturally, the screw length is calculated only for the threaded portion. A more precise calculation extends the measurement to the midpoint of the hopper; this is referred to as the effective length or effective length-to-diameter ratio.
A single injection molding machine typically offers three screw options, designated as screws A, B, and C, with diameters corresponding to small, medium (standard), and large sizes, respectively. Their length-to-diameter ratios are approximately 22,20, and 18.
The length of the screw in an injection molding machine and the injection stroke may appear to be two separate issues; in fact, they share a subtle relationship between "quality" and "quantity," and their ratio serves as a qualitative measure.
The length of a screw is generally not measured in absolute units; instead, it is expressed as a length relative to the diameter. This allows screws of different diameters to be compared in terms of length. This length is referred to as the length-to-diameter ratio, denoted as L/D. Naturally, the screw length is calculated only for the threaded portion. A more precise calculation extends the measurement to the midpoint of the hopper; this is referred to as the effective length or effective length-to-diameter ratio.
A single injection molding machine typically offers three screw options, designated as screws A, B, and C, with diameters corresponding to small, medium (standard), and large sizes, respectively. Their length-to-diameter ratios are approximately 22,20, and 18.
Temperature unevenness
Plastic that has been plasticized is referred to as molten; it is stored at the top of the screw and is prepared for use during the next injection cycle. Ideal melting results in uniform temperature throughout the material. In practice, however, this is generally not the case. Because the heating element does not surround the barrel completely—instead, it has a gap—the circumferential temperature is uneven. The heat from the heating element is transferred from the outside toward the inside; combined with poor heat transfer during melting, this results in radial temperature unevenness.
During plasticization, the screw retreats, causing the effective length to gradually decrease. The greater the feeding stroke (injection stroke), the greater the variation in effective length, and the more uneven the axial temperature distribution becomes. Those familiar with extruders know that an extrusion screw does not retreat; consequently, the extruded melt exhibits no axial temperature gradient. If the melt temperatures differ by 15°C, the appearance and mechanical properties of the finished product will not be uniform. In multi-cavity dies, differences between cavities become even more pronounced: one cavity may remain underfilled while another develops flash, and moreover, these variations occur without any discernible pattern.
To improve this condition, the injection stroke should be designed as four times the diameter of screw B; consequently, the effective length-to-diameter ratio will also be four. Under these conditions, the injection stroke will be 4.4 times the diameter of screw A and 3.7 times the diameter of screw C. The radial temperature difference will be greatest at screw A and smallest at screw C.
Increase the length-to-diameter ratio
Increasing the length-to-diameter ratio reduces the axial temperature gradient because, when the screw is longer, the plastic must rotate several additional revolutions before reaching the end of the screw; increased mixing results in a more uniform temperature distribution. With the injection stroke remaining unchanged, a longer screw causes the ratio "injection stroke ÷ screw length" to decrease, thereby reducing the axial temperature gradient. Furthermore, if screw B has a length-to-diameter ratio of 22, it will naturally perform better than screw B with a ratio of 20.
Overall, designs with a long injection stroke or a low screw length-to-diameter ratio result in a higher injection load, but cause uneven axial melt temperature; therefore, they are suitable only for single-cavity products with modest quality requirements. In contrast, designs that limit both the injection stroke and the screw length-to-diameter ratio ensure the quality of multi-cavity products.
Temperature unevenness
Plastic that has been plasticized is referred to as molten; it is stored at the top of the screw and is prepared for use during the next injection cycle. Ideal melting results in uniform temperature throughout the material. In practice, however, this is generally not the case. Because the heating element does not surround the barrel completely—instead, it has a gap—the circumferential temperature is uneven. The heat from the heating element is transferred from the outside toward the inside; combined with poor heat transfer during melting, this results in radial temperature unevenness.
During plasticization, the screw retreats, causing the effective length to gradually decrease. The greater the feeding stroke (injection stroke), the greater the variation in effective length, and the more uneven the axial temperature distribution becomes. Those familiar with extruders know that an extrusion screw does not retreat; consequently, the extruded melt exhibits no axial temperature gradient. If the melt temperatures differ by 15°C, the appearance and mechanical properties of the finished product will not be uniform. In multi-cavity dies, differences between cavities become even more pronounced: one cavity may remain underfilled while another develops flash, and moreover, these variations occur without any discernible pattern.
To improve this condition, the injection stroke should be designed as four times the diameter of screw B; consequently, the effective length-to-diameter ratio will also be four. Under these conditions, the injection stroke will be 4.4 times the diameter of screw A and 3.7 times the diameter of screw C. The radial temperature difference will be greatest at screw A and smallest at screw C.
Increase the length-to-diameter ratio
Increasing the length-to-diameter ratio reduces the axial temperature gradient because, when the screw is longer, the plastic must rotate several additional revolutions before reaching the end of the screw; increased mixing results in a more uniform temperature distribution. With the injection stroke remaining unchanged, a longer screw causes the ratio "injection stroke ÷ screw length" to decrease, thereby reducing the axial temperature gradient. Furthermore, if screw B has a length-to-diameter ratio of 22, it will naturally perform better than screw B with a ratio of 20.
Overall, designs with a long injection stroke or a low screw length-to-diameter ratio result in a higher injection load, but cause uneven axial melt temperature; therefore, they are suitable only for single-cavity products with modest quality requirements. In contrast, designs that limit both the injection stroke and the screw length-to-diameter ratio ensure the quality of multi-cavity products.
Achieving this balance in practice depends not only on correct machine sizing but also on the precision and consistency of the screw itself.
About SUPERIOR MACHINERY
Established in 1992, SUPERIOR MACHINERY has been engaged in the rubber and plastic molding machinery industry for more than 30 years. With an increasing market share, the company has established itself in the field of high-quality fully hardened screws and bimetallic barrels that are highly resistant to wear, corrosion, high pressure, and high speed.
This article examines the relationship between injection stroke and screw length-to-diameter (L/D) ratio in injection molding machines. Because the screw retracts during plasticization, a longer injection stroke reduces the effective L/D ratio, creating axial temperature gradients that cause inconsistent melt temperature and uneven part quality—especially in multi-cavity molds. Increasing the L/D ratio improves mixing and reduces axial temperature variation. The article recommends limiting injection stroke to approximately four times the diameter of the B screw and concludes that long-stroke, low-L/D designs suit single-cavity products with modest quality requirements, while constrained stroke and higher L/D ensure multi-cavity product consistency.
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On this National Day, Ningbo Superior Machinery extends our warmest wishes to clients, partners, and colleagues around the world.We grow with our nation, and we move forward because of you.May our country prosper, and may our partnerships stay green and lasting.Happy National Day!
