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How to select the right injection speed for a hydraulic injection molding machine?

2024/09/02 By Topstar

hydraulic injection molding machine (1)

The correct injection speed in the process of injection molding can ensure the quality and efficiency of the production process. The injection speed determines the speed at which the molten plastic is injected into the mold, and plays a key role in the final characteristics of the molded part. Injection speed is also a parameter we must consider when selecting injection molding machines. In this article, we will provide you with the process of choosing the appropriate injection speed for a hydraulic injection molding machine and let you know more about the injection part of the hydraulic injection molding machine.

The importance of injection speed in hydraulic injection molding machine

The injection speed in hydraulic injection molding machines is not just a parameter. It is also a key factor affecting the quality of the final product. The injection speed refers to the speed at which the screw or plunger moves during injection. The faster the speed, the shorter the time required to complete an injection stroke, and the greater the output. By setting the filling speed near the gate, in the body, and at the end of the flow and adjusting the corresponding injection position, you can produce products with a good appearance and less internal stress.

hydraulic injection molding machine （2）

It affects several key aspects of the molding process, including the flow of molten plastic, the filling of the mold cavity, the cooling time, and the total cycle time. The correct injection speed helps to achieve uniform part quality and reduce defects such as dents or welds. In a hydraulic injection molding machine, the injection speed is controlled by adjusting the flow rate of the hydraulic oil, thereby controlling the movement of the injection screw. Injection speed depends on several factors, including the type of plastic, the geometry of the mold, and the product requirements.

Effect of Mold Geometry on Injection Speed ​​Setting

Mold geometry includes the shape, size, and complexity of the mold cavity and features such as thin walls, ribs, and intricate details. These geometric features directly affect how molten material flows into the mold. Thin-walled mold sections require higher injection speeds, and thick-walled parts require a slow-fast-slow speed profile to avoid defects. Setting the injection speed balances the need for fast filling and prevents defects. This balance is achieved by considering the specific geometry of the mold. A multi-step injection speed profile is required for complex molds with varying wall thicknesses, and the speed varies throughout the injection process.

Injection molding machine mold geometry(1)

Multi-stage injection speed

Multi-stage injection speed refers to the ability of the hydraulic injection molding machine to adjust the speed of the injection piston or screw in multiple stages throughout the injection process. Unlike a single-speed injection system, the molten plastic is injected into the mold cavity at a constant speed, and multi-stage speed allows different speeds to be used at various points in the process.

Multi-stage injection speed(1)

The injection process can be divided into several stages (three stages, as an example). Each stage can have different speed settings, allowing precise control over how the molten plastic is injected:

Initial injection stage: During this stage, higher speeds can quickly fill most of the mold cavity, reducing cycle time and ensuring that the plastic reaches all areas of the mold before it starts to cool.

Press holding stage: After the initial fill, the injection speed can be reduced by applying pressure gradually. Slower speeds help fill the molten plastic into all the intricate details of the mold, reducing the risk of defects such as sink marks.

Holding stage: Once the mold cavity is filled and compacted, the speed is further reduced to maintain pressure. At the same time, the plastic cools and solidifies, allowing the final product to keep its intended shape and size.

What are the characteristics of high and low injection speeds?

High injection speed

  1. Faster filling time:
    It can fill the mold cavity faster, especially for large parts or molds with complex geometries. Faster filling time helps to reduce cycle time and improve the overall productivity of the molding process.
  2. Reduce viscosity:
    When you inject plastic at high speed, the shear rate increases, temporarily decreasing viscosity. This reduction makes it easier for the molten material to flow into the mold and fill intricate details and thin walls more effectively.
  3. Potential for defects:
    While high injection speeds can improve filling and cycle times, they also increase the risk of certain defects. For example, high speeds can cause jetting. Plastic enters the mold too quickly and forms a serpentine pattern. In addition, high speeds can cause increased internal stress in the material, resulting in warping, sink marks, or internal voids.
  4. Enhanced surface finish:
    In some cases, high injection speeds can help the material reach and fill the fine details of the mold before it cools, thereby improving the surface finish of the molded part.

Low Injection Speed

  1. Controlled Filling Process:
    Lower injection speeds allow for a more controlled and progressive filling of the mold cavity, ensuring that the material flows evenly into all areas of the mold, reducing the possibility of air pockets and voids.
  2. Reduced Shear Stress:
    Lower speeds reduce the shear rate, which minimizes the stresses on the material during the injection process. Reduced stress helps maintain the material’s properties and can result in better dimensional stability in the final part.
  3. Improved Part Quality:
    Infection defects such as burn marks, flow lines, and welds can be minimized by reducing the injection speed. These defects can occur when material enters the die too quickly, causing turbulence and improper fusion.
  4. Increased Cycle Time:
    The downside of low injection speeds is that they can increase cycle time. Since the mold fills more slowly, the total time required for each molding cycle increases.

How to choose the injection speed of hydraulic injection molding machine reasonably?

The type of plastic material used will affect the ideal injection speed. Materials such as polypropylene and polyethene generally benefit from higher injection speeds because they have lower viscosity and flow into the mold more easily. In contrast, more dense materials such as polycarbonate or nylon may require lower injection speeds. Always consider the material’s melt flow index (MFI); you should use slower speeds for materials with lower MFI and those with higher MFI.

As mentioned earlier, mold design must also be considered. Molds with intricate details, thin walls, or multiple cavities require slower and more controlled injection speeds. The surface finish required for the final product can also determine the injection speed. In a production environment that requires high-volume manufacturing, there is often pressure to use higher injection speeds to reduce cycle times. However, we must balance this with the need for consistent product quality.

Modern hydraulic injection molding machines (including Topstar’s TMll series) offer multi-level injection speed settings. This feature lets you use different speeds at various stages of the injection process.
By utilizing this feature, you can optimize the injection speed to suit the different stages of the process.

Master the Right Choice

By understanding the importance of injection speed and considering the key factors influencing its selection, you can achieve high-quality results while maintaining efficient production operations during injection molding. Ultimately, the goal is to find the injection speed that achieves the best balance between quality, efficiency, and cost. By optimizing the process and monitoring it in real time, you can ensure higher-quality injection molding work.

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