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What effect does injection molding pressure holding have on the shrinkage of injection molded parts?

Table of Contents

Introduction

Injection molding process is a commonly used method in modern manufacturing industry, and injection molded parts are widely used in electronics, home appliances, automobiles, daily necessities and other industries. Pressure holding is an indispensable part of the injection molding process.

Usually, the process of injection molding and pressure holding requires adjusting processing parameters according to different types and shapes of materials, such as working pressure, injection speed, mold temperature, etc., to ensure that the material fully melts and flows during the injection molding process and maintains temperature and pressure. Conditions for solidification and shaping.

This technology is widely used and suitable for various types of plastic materials, such as polyamide, polystyrene, etc., and can produce plastic products of various specifications and shapes. In this article we will delve into what pressure holding is and the role of pressure holding.

What is pressure holding

Definition of injection holding pressure

Injection molding pressure holding refers to the operation of increasing the injection pressure from the stage when the mold is about 90% full to before the liquid material solidifies, so that the liquid material in the mold does not shrinkage occurs.

Holding pressure is a common technique in the injection molding process. It means that after the plastic material is injected into the mold, a certain cooling time is maintained under given pressure holding conditions so that the plastic material can fully melt and flow, thereby obtaining a high-quality finished product.

This technology plays a vital role in the injection molding process, which can effectively avoid product defects caused by poor injection molding processes and ensure the stability and quality of the finished product.

The purpose of injection molding pressure holding

The purpose of holding pressure is to compensate for the defects caused by the cooling mold shrinkage rate of the melt and the drop in melt temperature, while also ensuring the longitudinal and transverse consistency of the product. The process of holding pressure is to maintain the pressure of the injection molding machine for a period of time immediately after injection molding. This allows the plastic material to be partially melted through the interaction of pressure and temperature to ensure the quality and performance of injection molding.

The role of injection molding pressure holding

Solve the problem of shrinkage holes and burrs

Injection molding pressure-holding can effectively solve the problem of differential shrinkage caused by melt cooling shrinkage and drop in melt temperature. When the pressure is released after injection molding, the melt will shrink, easily leaving shrinkage holes or burrs at the corners of the molded product. By maintaining pressure, the injection molding machine can be maintained in a good pressure state and prevent product defects.

Improve the firmness of the product

The pressure holding of the injection molding machine can ensure that the plastic material is well filled into every corner and blind hole of the injection mold. When the melt is compacted, the molecules of the plastic are arranged more closely, which makes the product stronger and more durable, and also increases the density of the product.

Improve the molding accuracy of the product

During the pressure maintaining process of the injection molding machine, the plasticity of the melt is increased. This allows the plastic material to fill the injection mold more completely, thus effectively improving the molding accuracy of the product.

Pressure switching method

Any plastic raw material has its own no-flow temperature, and the value of the no-flow temperature is slightly greater than the glass transition temperature of the raw material.

When the high melt temperature vertical flow direction drops to its corresponding no-flow temperature, the melt will no longer flow. The no-flow temperature before melting can be easily obtained through the dynamic temperature display of the filling process, thereby determining the exact starting moment of delayed holding.

When the cavity is almost full during the injection molding filling process, the movement of the screw switches from flow rate control to pressure control. This conversion point is called the pressure-holding switching control point, that is, the V/P conversion point.

Pressure switching

Pressure switching means that the priority of the machine is pressure. When the machine detects that the pressure at the moment of injection reaches the V-P switching pressure you set, the injection action will switch to a pressure-holding action until the pressure-holding is completed.

Pressure switching. The switch is triggered once the mold cavity pressure reaches a predetermined pressure. Since this method is based on a stable and reliable pressure absolute value signal, this switch is the most effective.

The use of pressure monitoring eliminates the screw stroke and backstop. Due to the influence of the valve, this method cannot compensate for changes in hydraulic oil, melt and mold temperature, that is, changes in injection rate.

The faster the pressure build-up during the compression phase, the more effective this method is, since in this case precise and timely switching avoids pressure peaks. When switching pressure, since the minimum detection data of pressure can reach 1KGF, it is sensitive to pressure accuracy and is suitable for making small precision plastic parts only. It is mostly used in Japanese machines.

Position switching

The position switching is different. When the screw injection action reaches the V-P switching position you set, the injection action will be directly converted to the pressure maintaining action.

Time switching method This method requires that a switching signal is released after a predetermined time after the injection starts, and the holding pressure is switched when the time is up.

Time switching

Time switching method, this method does not consider changes in melt compression, melt viscosity, feeding accuracy or hydraulic pressure at the front end of the screw, nor does it consider changes in screw advancement rate, and changes in hydraulic pressure will cause changes in screw position. (The end of the feeding stage and the beginning of the pressure holding stage) and the corresponding stroke changes (feeding stroke, injection stroke), as a result, the quality of the parts, especially the weight and size of the parts, fluctuates greatly.

It is suitable for making some ordinary products with low requirements. Therefore, it is generally not recommended to use the method of switching based on time.

Itinerary switching

The method of trip switching is the most commonly used method and has been proven to be effective in practice.

The switching signal is sent by the limit switch in flow direction of the holding position. Since the injection stroke is basically constant, it can be considered that this switching occurs once when filling the same volume.

The holding stroke is very short. This method is unreliable because it is very small. The change will cause the switch to not start accurately every time. In this case, it is best not to perform the switch.

Conclusion

Injection mold holding pressure is a key link in the injection molding process. Different holding pressure settings will directly affect the dimensional stability and physical performance stability of the product. Therefore, in actual production, it is necessary to comprehensively analyze the specific materials, mold structure and other factors to reasonably set the holding pressure.

In short, the holding pressure of the injection molding machine plays a decisive role in injection molding. Reasonable use of holding pressure can achieve better injection molding product quality and process efficiency improvement.

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