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What is the L/D Ratio of the Injection Molding Machine

Aug 02.2022


As we all know, the injection molding machine, as an efficient production tool, is very expensive to invest in it. However, it will become very efficient and profitable for manufacturers with the expertise.

The aspect ratio of the injection molding machine is one of them. This article will reveal what an aspect ratio is and walk you through the associated logic and possible problems


What is the injection molding L/D ratio?

The L/D ratio is the ratio of the length of the screw to its outer diameter.

Ratio calculations are calculated by dividing the length of the screw by its nominal diameter. Although several injection molding machine manufacturers now offer options for injection units, such as PP and PE with length-to-diameter ratios in the 20-30:1 range. But most injection molding screws use a 20:1 L/D ratio.


Why Knowing the L/D Ratio Is Important

Different screws in different machines have different aspect ratios. Manufacturers need to know the aspect ratio of the screw they choose, as this affects the amount of heat and pressure applied to the plastic during the injection molding process.

Molding plastic with two different screws (and with two different aspect ratios) will cause the material to react in two different ways, possibly causing some plastics to fail at certain aspect ratios.


Try to understand the logic of L/D ratio


It should be clear that the channel depth and screw diameter of the screw in the feed zone will affect the output of the screw.

The increased L/D ratio increases the residence time of the material in the screw.

Suppose we obtain optimal melt conditions at a certain screw speed, such as N1 on a lower L/D ratio screw. N1 corresponds to a certain residence time of the material in the screw.

Therefore, it should be possible to obtain the same melt conditions by rotating the larger L/D dosing screw at a higher speed (greater than N1) to maintain the desired residence time. By doing so, its output increases. The increase in output here is not due to increased L/D ratio, but due to the possibility of increasing screw speed, which is only possible for thermally stable materials. Increased screw speed increases the shear rate of the melt in the compression zone. This is acceptable for thermally very stable commodity polymers such as HDPE, LDPE, PP, PS, etc.

But for engineering polymers, the increased shear is unacceptable. Therefore, for a given polymer and a given screw diameter, the screw speed is limited by its sensitivity to shear. You will observe this in the polymer manufacturer's recommendations. They specify limiting screw speeds for typical screw diameters, or limit speeds (0.05m/sec to 0.3m/sec) around the screw.


What's the problem with a longer aspect ratio?

For the overall machining situation, excessively long screws can actually limit throughput. This limitation is often manifested in excessively high melt temperatures, which can lead to problems such as polymer decomposition, color shift, loss of additive effectiveness, and plating. For polymers that melt easily, the ideal melt length should be shorter, as excessively long transitions will actually reduce the melt rate.


What should you ask your manufacturer about aspect ratio?

Knowing the aspect ratio is a good sign of a manufacturer's process knowledge and expertise. This is also the key to ensuring the final quality of the components. To learn more about the manufacturer's aspect ratio, the following questions should be asked:

  • What aspect ratio do you use in your injection molding machine?

  • How do you tune other parameters to efficiently mold different plastics at the available aspect ratios?

  • How do you ensure that the aspect ratio does not negatively affect the final quality of the product?

Answers to these questions will help clarify the manufacturer's level of process knowledge and ensure that they can meet the required component quality standards.

Log Machine has decades of experience in the plastics industry and understands how to make more suitable, high-quality injection molding machines. Check out our injection molding machine offerings and see some of the injection molding solutions we offer.

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