What Is Dosing in Injection Molding?

Injection molding is a complex manufacturing process that transforms raw plastic pellets into finished parts with precision and consistency. Among the many critical parameters that determine product quality, dosing plays a pivotal role in ensuring that the right amount of molten plastic is prepared for each molding cycle. Understanding dosing—what it is, how it works, and how it affects production efficiency—is essential for achieving consistent part quality and optimizing machine performance.

What Is Dosing in Injection Molding?

In injection molding, dosing refers to the process of metering and preparing a specific amount of molten plastic (known as the shot size) for the next molding cycle. During dosing, the screw inside the injection barrel rotates backward after an injection to draw in, melt, and accumulate a controlled quantity of plastic material in front of the screw tip.

Essentially, dosing is the preparation phase that occurs between two injection cycles. It determines how much material will be injected into the mold cavity during the next shot—directly influencing the accuracy, consistency, and quality of the molded parts.

The Dosing Process Explained

Dosing takes place during the plastication phase, where the screw performs two simultaneous actions:

1. Rotation: The screw rotates to convey plastic pellets forward from the hopper into the heated barrel. As the material advances, it melts under the combined effect of barrel heat and shear forces generated by screw rotation.

2. Back Pressure and Accumulation: As molten plastic accumulates in front of the screw tip, resistance from the material buildup forces the screw to retract backward. This backward movement determines the dose volume, also called the shot size.

Once the desired shot volume is reached, the screw stops rotating, and the dosing phase ends. The molten material in front of the screw is now ready for the next injection stage.

Key Parameters Affecting Dosing

Several parameters influence dosing accuracy and consistency:

1. Screw Speed

Screw speed controls how fast the screw rotates during dosing. Higher speeds increase throughput but can lead to excessive shear heating, while slower speeds improve melt uniformity but reduce cycle time. The optimal screw speed must balance thermal control with productivity.

2. Back Pressure

Back pressure is the resistance applied against the screw’s backward movement during dosing. It helps to homogenize the melt, remove trapped air, and improve color dispersion. However, excessive back pressure can cause overheating, increased energy consumption, and material degradation.

3. Melt Temperature

Proper barrel temperature settings ensure that the plastic reaches a consistent viscosity during dosing. Variations in temperature can lead to inconsistent dosing volumes, affecting shot weight and dimensional stability.

4. Screw Position and Shot Size

The screw’s retraction distance defines the amount of molten material accumulated. This distance must correspond to the part and runner volume plus a cushion to maintain consistent packing during injection.

5. Material Characteristics

Different polymers exhibit varying flow properties, viscosities, and melting behaviors. Semi-crystalline plastics (like PP or PA) and amorphous plastics (like ABS or PC) require different dosing conditions to achieve stable shot preparation.

Importance of Proper Dosing

Accurate dosing is fundamental to the overall stability of the injection molding process. Inconsistent dosing leads to variation in shot weight, which can result in defects such as short shots, flash, warping, or sink marks.

Proper dosing ensures:

• Consistent shot weight: Each cycle receives the same amount of molten material.

• Stable melt quality: The melt remains homogeneous and properly mixed.

• Improved dimensional accuracy: Uniform dosing results in parts that meet tight tolerances.

• Efficient cycle time: Optimized dosing minimizes downtime and maximizes productivity.

• Reduced material waste: Controlled dosing prevents overfilling and material loss.

Dosing vs. Metering vs. Plasticizing

These three terms are closely related but have distinct meanings in injection molding:

• Plasticizing: The melting of solid plastic pellets into a homogeneous molten form.

• Dosing (Metering): The act of accumulating a precise volume of molten plastic for the next injection.

• Metering (Alternate Definition):  Some sources use metering interchangeably with dosing, particularly in reference to the screw’s metering zone where the melt is homogenized.

In essence, plasticizing creates the melt, while dosing measures and stores it for injection.

Challenges in Dosing

Despite automation, several issues can arise during the dosing stage:

1. Inconsistent Screw Recovery: Caused by material inconsistencies, temperature fluctuations, or inadequate back pressure settings.

2. Overheating or Degradation: Excessive shear heat during fast screw rotation may degrade sensitive materials.

3. Color Streaks or Poor Mixing: Insufficient back pressure or low screw speed can cause uneven color distribution.

4. Shot Size Variation: Errors in screw position sensors or inconsistent melt density can lead to variable dosing.

Addressing these challenges requires a balance between mechanical settings, material behavior, and process control.

Optimizing Dosing for Better Results

To achieve optimal dosing performance, injection molders should focus on these best practices:

• Calibrate Screw Recovery Distance: Regularly verify that the screw’s retraction matches the designed shot volume.

• Fine-Tune Back Pressure: Use enough pressure for good mixing but avoid overheating.

• Monitor Melt Temperature Stability: Use multi-zone barrel temperature control and ensure even heat distribution.

• Adjust Screw Speed Dynamically: Set speeds according to polymer type and viscosity.

• Implement Closed-Loop Control: Advanced machines use sensors and software algorithms to adjust dosing parameters automatically for each cycle.

Modern Advancements in Dosing Technology

Modern injection molding machines are equipped with servo-driven screws and digital control systems that allow precise dosing with minimal human intervention. Smart systems monitor parameters like screw torque, melt temperature, and recovery time in real-time, enabling predictive adjustments for consistent shot preparation.

Additionally, material feeding systems are increasingly integrated with dosing controls to regulate the exact amount of raw material entering the barrel, reducing waste and improving process repeatability.

Conclusion

Dosing in injection molding is far more than a simple preparatory step—it is a cornerstone of process stability and product quality. By carefully controlling how much molten material is prepared for each shot, manufacturers can ensure that every part meets dimensional accuracy, mechanical performance, and aesthetic standards.

In modern injection molding operations, optimizing dosing through advanced control systems and parameter fine-tuning is essential to maintaining efficiency, reducing waste, and achieving consistent, high-quality production. Whether for automotive components, consumer goods, or precision electronics, mastering dosing is key to mastering injection molding itself.