Principle of Operation of the Injection Unit

Principle of Operation of the Injection Unit (3)

The core of the injection unit is a screw shaft, also called a worm, which is located in a cylinder. The inner diameter of the cylinder is equal to the outer diameter of the screw. The barrel is called a screw barrel or plasticizing barrel. At the rear of the screw cylinder, there is either a funnel into which the granulate is filled and trickles through an opening (the filling block) into the cylinder or a feed eye through which cord blanks (no longer in use today) are drawn in. Turned by a drive, the screw rotates in the barrel and transports the raw material toward the tip of the screw.

In thermoplastic injection molding, the screw barrel is heated from the outside by means of electric heating bands. Due to this heat and the special geometry of the screw, the granulate is not only conveyed but also sheared, thereby melting the plastic and plasticizing and homogenizing it. At the tip of the screw barrel, there is a nozzle that forms the transition to the mold.

In thermoset injection moulding and elastomer injection moulding, on the other hand, the barrel is tempered in order to prevent an excessively high melt temperature caused by internal friction, as otherwise, the moulding compound would already react in the barrel.


Injection molding is a highly versatile process used in the mass production of plastic components with great precision and consistency. It involves the use of an injection molding machine that comprises several essential units, with the injection unit being one of the most critical components.

In the course of the metering process, the moulding compound is now usually transported through a non-return valve to the nozzle and accumulated in front of it. In order to provide sufficient storage space for the moulding compound, the screw is axially pressurized only slightly (dynamic pressure) so that it can move in the direction of the feed hopper, thus forming the so-called screw antechamber between the non-return valve and the nozzle, in which the volume of the compound is located.
The dynamic pressure acts against the melt so that the melt is compressed. The pressure exerted by the melt moves the screw back.

During the injection process, the screw is pushed axially towards the nozzle, whereby the non-return valve closes and thus the mass volume is injected through the nozzle into the mould.

After the volumetric filling of the mould, the system switches over to holding pressure. Mass must remain in the cylinder (residual mass cushion), otherwise, the pressure cannot act on the mass. The holding pressure is needed to compensate for the volume loss.

Principle of Operation of the Injection Unit (2)

Understanding Injection Molding

Injection molding is a revolutionary manufacturing process that has transformed the production of plastic products across various industries. In this comprehensive guide, we unravel the intricacies of injection molding, exploring the principles that drive this versatile technique.

From the basics to advanced concepts, this article serves as a go-to resource for anyone curious about the magic behind injection molding machines. We’ll explore the crucial role played by the injection unit—the heart of the molding process—examining its components and how they work together harmoniously to produce high-quality plastic products with remarkable precision and consistency.

Unraveling the Injection Unit’s Function:

At the heart of every injection molding machine lies the injection unit, responsible for the crucial stages of plastic material transformation. We delve into the purpose of the injection unit, unveiling the secrets of how it melts and injects the plastic material into the mold cavity, laying the foundation for the final product.

Key Components That Make It Happen:

The injection unit’s seamless operation depends on the intricate interplay of various components. In this guide, we highlight the key players—the hopper, screw, barrel, nozzle, and heating bands—and their vital roles in this dynamic process. You’ll gain insights into the initial feeding of plastic pellets, the melting of the plastic within the barrel, and the careful homogenization of the molten plastic to achieve uniformity.

From Molten Plastic to Mold Cavity- The Injection Process:

With the molten plastic ready, we embark on the fascinating injection process. We examine how the screw exerts high pressure to propel the molten plastic through the nozzle and into the mold cavity, where it assumes the desired shape of the final product. Understanding the precise control of pressure and speed during this phase highlights the importance of achieving flawless molded parts.

Cooling and Solidification- Transforming the Molded Part:

As the molten plastic fills the mold cavity, the cooling process commences. In this section, we elaborate on the cooling mechanisms that promote solidification, ensuring the molded part takes on its final form. Once the plastic has cooled sufficiently, the mold opens, and the molded product is gently ejected, ready for its intended purpose.

Ensuring Excellence- Quality, Efficiency, and Sustainability:

Throughout the injection molding process, maintaining quality and efficiency is paramount. We explore the significance of monitoring and adjustments to optimize the molding process, delivering superior results. Furthermore, we spotlight the injection molding industry’s role in promoting sustainability by enabling the use of recycled plastics and minimizing waste.

Celebrating the Art and Science of Injection Molding:

As we conclude our exploration of the principle of operation of the injection unit, we celebrate the art and science of injection molding. The precision and versatility of this transformative process have made it an indispensable cornerstone of modern manufacturing.


Plasticizing the Material

Plasticizing the material is a crucial step in the injection molding process, where raw plastic pellets are transformed into a molten state, ready for injection into the mold cavity. This stage sets the foundation for producing high-quality plastic products with precision and consistency.

1. Hopper and Screw Feeding

The plasticizing journey commences with the hopper and screw feeding mechanism. The hopper serves as a reservoir for the plastic pellets, which are gravity-fed into the injection molding machine. From here, the rotating screw takes center stage. As it turns, it pushes the plastic pellets forward through the barrel.

2. Melting the Plastic

Inside the heated barrel, the plastic pellets encounter the first taste of heat. The temperature is carefully controlled using heating bands, raising it to a point where the plastic undergoes a remarkable transformation. Slowly but steadily, the pellets begin to soften and fuse together, transitioning into a molten state.

3. Homogenization of Molten Plastic

To ensure uniformity and consistency in molten plastic, homogenization is essential. As the plastic particles melt and mix thoroughly, any variations in temperature or composition are smoothed out, guaranteeing a consistent material to be shaped into the final product.

The success of the plasticizing process hinges on precision, as different types of plastic require specific temperature profiles and melt times. The proficiency of the injection molding machine operator plays a crucial role in achieving the desired plasticized state.

Once the plastic material is perfectly plasticized, it is now ready for the next stage of the injection molding process—the injection itself. The molten magic, now transformed from simple pellets, will be injected into the mold cavity, where it will take on the shape and intricacies of the final product.

Principle of Operation of the Injection Unit (1)

Injection and Pressure

As the plastic material transforms into a molten state, the injection molding process enters its critical stage—Injection and Pressure. This dynamic duo takes center stage, working in tandem to shape the molten plastic into the precise form required for the final product. The art of injection and the science of pressure control are the key factors that determine the quality and integrity of the molded parts.

The Injection Process:

With the molten plastic in its optimal state, the injection process commences. The injection molding machine‘s powerful hydraulic system exerts force on the screw, pushing the molten plastic forward with controlled precision. The plastic material flows through the nozzle, entering the mold cavity under pressure.

The injection speed and pressure are carefully calibrated to ensure the plastic fills the mold cavity uniformly, avoiding any air pockets or defects. The entire process is a dance of finesse and accuracy, where the timing of each step is meticulously orchestrated.

Pressure and Speed Control:

In injection molding, the control of pressure and speed is paramount to achieving impeccable results. The pressure applied during injection directly impacts the final product’s integrity and surface finish. A delicate balance is struck, ensuring that the plastic material flows smoothly and uniformly through the mold cavity.

High pressure and rapid injection speed are preferred for certain applications, while lower pressure and slower injection are more suitable for others. Manufacturers must adapt the pressure and speed profiles based on the complexity of the product, the type of plastic being used, and the intricacies of the mold design.

Too much pressure may lead to excess stress on the mold, causing deformation or wear, while too little pressure can result in incomplete filling and undesirable cosmetic flaws. Striking the perfect equilibrium is a testament to the injection molding machine’s sophistication and the expertise of the operator.

The Complexity of Thin-Walled Parts:

The injection and pressure dance is especially intricate when dealing with thin-walled parts. These delicate components demand finesse, as the high pressure needed to fill thin walls must be balanced with the risk of potential defects like warpage, sink marks, or flow lines. Proper injection and pressure control ensure these thin-walled parts retain their structural integrity while meeting the required dimensional tolerances.

From Prototype to Perfection:

Injection and pressure are not fixed concepts, but rather variables that can be tailored to each specific project. Manufacturers often run prototypes to fine-tune the injection parameters, analyzing how different pressure and speed profiles affect the final product. Iterative testing and adjustments are performed to achieve perfection in the molded parts.

The Symphony of Injection Molding:

The combination of injection and pressure in the injection molding process creates a symphony of engineering and design, yielding a wide array of products that enrich our lives daily. Whether it’s intricate automotive components, durable household items, or complex medical devices, the dance of injection and pressure brings these creations to life.

As technology advances, the art of injection and the science of pressure control continue to evolve, pushing the boundaries of what’s possible in the world of injection molding. Precision, innovation, and a touch of creativity converge in this transformative process, shaping the future of manufacturing one molded part at a time.


Cooling and Solidification

As the molten plastic material fills the mold cavity during the injection molding process, it embarks on a transformative journey—cooling and solidification. This crucial stage ensures that the plastic takes on its final form, converting from a pliable, molten state to a solid, durable product ready to serve its intended purpose.

Cooling the Molded Part:

As the molten plastic enters the mold cavity, it starts to lose heat to the surrounding mold walls. The cooling process begins, gradually reducing the temperature of the plastic. The cooling rate is carefully controlled to strike a balance between rapid production cycles and achieving the desired structural properties.

Achieving Optimal Cooling Rates:

Achieving the optimal cooling rate is essential to the quality of the final product. Too rapid cooling can lead to internal stresses and surface defects, while slow cooling can cause warping and distortion. Injection molders carefully design the cooling channels within the mold to ensure even cooling throughout the part, minimizing potential flaws and ensuring uniformity.

Ejection of the Molded Product:

Once the plastic has sufficiently cooled and solidified, it’s time for the grand finale—the ejection of the molded product. The mold opens, and with precision and finesse, the newly formed product is gently ejected from the mold cavity. Ejection is a critical step that requires attention to detail, ensuring that the product is released without damage or distortion.

Post-Cooling Considerations:

After ejection, some products may require additional post-cooling to fully stabilize their shape and properties. Depending on the complexity of the part and the specific requirements, secondary cooling processes may be employed to achieve the desired characteristics.

Quality Assurance: Inspecting the Molded Part:

Once the product is released from the mold, it undergoes rigorous inspection to ensure it meets the required quality standards. This inspection includes checking for surface imperfections, dimensional accuracy, and overall structural integrity. Any defects or variations are meticulously analyzed, allowing manufacturers to make necessary adjustments for subsequent production runs.

From Melt to Masterpiece:

Cooling and solidification mark the completion of the injection molding process. What began as a molten concoction of plastic pellets has now been skillfully shaped into a high-quality, durable product. The cooling and solidification stages are the final touches that transform the molten material into a masterpiece, ready to serve its intended purpose in homes, industries, and various other applications.

Innovations in Cooling Technology:

As injection molding technology advances, innovations in cooling techniques continue to emerge. Manufacturers constantly explore new cooling methods and materials to optimize production cycles, improve product quality, and enhance overall efficiency.

Ensuring Quality and Efficiency

In the intricate world of injection molding, achieving both exceptional quality and optimal efficiency is the ultimate pursuit. The success of this manufacturing process hinges on meticulously balancing various factors to produce high-quality products with maximum productivity. In this section, we explore the critical aspects that ensure quality and efficiency in injection molding.

1. Monitoring and Adjustments

Constant monitoring is the backbone of ensuring quality and efficiency in injection molding. Advanced sensors and monitoring systems are employed to track various parameters throughout the process. Key variables such as temperature, pressure, injection speed, and cooling rates are closely observed to detect any deviations or inconsistencies.

The injection molding machine operators, armed with real-time data, are empowered to make swift adjustments when required. Fine-tuning the parameters during production runs ensures that each molded part meets the specified quality standards. Monitoring and making timely adjustments prevent defects, reduce material waste, and ultimately lead to higher production efficiency.

2. Process Optimization

Process optimization is a continuous endeavor that seeks to improve the overall efficiency and performance of the injection molding process. This involves a holistic approach to analyzing every stage—from plasticizing the material to ejection—and identifying opportunities for enhancement.

Through data analysis and simulation tools, manufacturers can explore different process scenarios, selecting the most efficient settings to maximize output while maintaining product quality. Optimized processes reduce cycle times, leading to higher production volumes and reduced manufacturing costs.

3. Automation and Robotics

The integration of automation and robotics has revolutionized injection molding, elevating both quality and efficiency to new heights. Automated systems can handle tasks such as part removal, sorting, and quality inspection, reducing human error and enhancing consistency.

Robotics play a significant role in handling complex tasks that require precision and repeatability. The use of robotics for repetitive actions not only enhances production efficiency but also contributes to improved safety and reduced labor costs.

4. Training and Skill Development

Skilled operators are the backbone of any successful injection molding operation. Investing in training and skill development programs is essential to ensure that operators possess the expertise needed to run injection molding machines optimally.

A well-trained workforce can troubleshoot issues efficiently, perform preventive maintenance, and make accurate adjustments during production. Their expertise directly impacts the quality of the molded products and the overall efficiency of the manufacturing process.

5. Recycling and Sustainability

Embracing sustainability initiatives is not only a moral imperative but also a way to enhance efficiency and reduce costs. Injection molding enables the use of recycled plastics, minimizing material waste and promoting a more eco-friendly approach to manufacturing.

Recycling initiatives contribute to cost savings by reducing the need for virgin plastic materials. Emphasizing sustainability resonates with consumers and businesses alike, further enhancing the reputation of the manufacturer.



The injection unit is the heart of an injection molding machine, and its principle of operation is essential to understand for producing high-quality plastic products efficiently. From plasticizing the material to injecting it into the mold cavity, each step requires precision and control. The versatility and reliability of injection molding make it a cornerstone of modern manufacturing processes.



Q: What is the injection unit’s role in injection molding?

A: The injection unit is responsible for melting and injecting the plastic material into the mold cavity.

Q: What are the key components of the injection unit?

A: The major components include the hopper, screw, barrel, nozzle, and heating bands.

Q: How is the plastic material melted in the injection unit?

A: The plastic pellets are heated and melted in the barrel using heating bands and a rotating screw.

Q: Why is precise pressure and speed control during injection important?

A: Precise control ensures high-quality molded parts with minimal defects.

Q: How does injection molding support sustainability?

A: Injection molding allows for the use of recycled plastics, contributing to sustainability efforts.

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Frequently Asked Questions

Daoben has the SOP (Standard Operation Procedure) and all the production steps must follow up this SOP. Every machine needs at least automatic running over 72 hours and must be inspected carefully before shipment.

Actually, the delivery time of the machines is depended on the machine options. Normally, the delivery time of a standard machine could be within 1-2 months.

12 months warranty after shipment for mechanical parts, hydraulic parts, and electric components excluding the screw and barrel 6 months (not including use for recycled material or reclaimed material).

Yes, Daoben will provide one set of standard spare parts to the customer including a heater, filter, screw head, screw washer, checking ring, sealings, etc.

Surely, we have experienced technical engineers for oversea service, they would help you install machines and support training to workers also. Currently, we provide visits online by video call for COVID-19.

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