The Role of Temperature and Pressure in Injection Moulding

Jun 13, 2024

Plastic injection moulding is one of the most widely used manufacturing processes due to its efficiency, precision, versatility, and cost-effectiveness. It’s used by many industries to make high-quality parts and components, but not many know what goes on behind the scenes. In this blog, we’ll delve into the role temperature and pressure play in injection moulding.

Temperature

Temperature is a crucial variable in plastic injection moulding as it can impact many different areas of the process from the flow of the molten plastic to the properties of the part. Understanding how temperature affects aspects of the process is essential for achieving consistent results.

Temperature Control

In plastic injection moulding, thermocouples regulate and sense temperatures. The temperature required is set on the control instrument of the plastic injection moulding machine, acting as the set point. Once the point is reached, the power cycles on and off to maintain the temperature so it doesn’t fall below the set point.

Like any production process, some factors and conditions can influence the condition of the product, especially melt temperature and mould temperature.

Melt Temperature

The melt temperature is the temperature of the plastic material as it exits the nozzle and enters the mould. Measuring the melt temperature is carried out through an air shot technique or the nozzle. The melt temperature plays a role in the viscosity and final molecular weight of the final product.

Mould Temperature

Mould temperature has a bigger impact on the final product than the melt temperature as using a higher mould temperature reduces moulded-in stress. It also affects crystallinity in polymers which influences properties like creep resistance and dimensional stability. The mould temperature however mustn’t be too high as it can introduce de-moulding and dimensional issues in finished parts. Longer cycle times may then be required to overcome these issues.

Temperature Settings

Injection moulding machines allow for the precise adjustment of temperature settings for both the barrel and the mould. For easier control, the injection cylinders are divided into several zones. These zones have different temperature values but can also be set to the same value if necessary.

When operating with higher temperatures, or with longer cycle times, the first zone temperature can be lowered to prevent premature bridging and melting. It’s crucial to check that the barrel, mould, hopper block, and hydraulic oil are all appropriately heated before starting the process.

Temperature Recording

Temperature recording is essential for ensuring the consistency and quality of your plastic injection moulded parts. The machine setting record indicates the temperature to be recorded, including melt temperature, mould temperature, water temperature, and oil temperature. These recorded temperatures offer valuable insights into the performance of the injection moulding process so operators can adjust where needed.

For a more convenient checking of temperature, tools like electronic portable temperature probes can be used as a thermometer to quickly check temperatures at various points. By using these temperature recordings, plastic injection moulding manufacturers can maintain tight control over parameters.

Pressure

Pressure management is crucial in plastic injection moulding as it ensures efficient material flow and mould filling.

Injection Pressure

Injection pressure is a critical parameter as it guides the flow of materials throughout the moulding process. The pressure is usually measured by a transducer located either at the hydraulic line or nozzle. While its value remains constant, it increases proportionally as the mould becomes more complex. By carefully controlling injection pressure, manufacturers can achieve optimal mould filling for high-quality parts.

First and Second Stage Pressure

First stage pressure is the pressure applied to the molten plastic during the mould-filling stage. It’s often needed to ensure the quick and complete filling of the mould. However, once the mould is full, maintaining the high pressure may become unnecessary, or even counterproductive.

Second stage pressure is applied after the mould cavity is filled and is typically a lower pressure than the first stage. Second stage pressure is applied to compress the molten plastic to fill any voids and remove any air bubbles.

By controlling both stages, manufacturers can optimise the process to produce consistently high-quality parts.

Clamping Pressure

Clamping pressure refers to the force applied to the mould halves by the machine’s clamping unit to keep them securely closed during the injection process. The level of clamping needed depends on various factors like the size and complexity of the moulded part, the material being used, and the mould design. Without sufficient clamping pressure, excess material can escape from the mould resulting in defects. The amount of pressure needed for clamping pressure must be estimated from the projected area.

Back Pressure

Back pressure in injection moulding refers to the resistance applied to the screw of the machine during the plasticising phase. Essentially, it’s the pressure exerted against the screw as it rotates and moves forward to push the molten plastic through the barrel towards the mould.

Applying back pressure ensures product quality, and the optimisation of colour dispersion and material melting. However, this type of pressure can strain the injection moulding machine, leading to reduced fibre length within the system and potentially making the screw retraction time longer.

Cavity Pressure

Cavity pressure refers to the pressure applied by the molten plastic material inside the mould cavity during the injection process. It’s an important aspect that provides insights into the filling and packing phases of the moulding cycle.

To monitor this pressure, a transducer is often used to measure the pressure within the mould cavity. The signal produced can then be used to trigger the switch from first stage pressure to second stage pressure. This process is known as cavity pressure control (CPC). By implementing CPC, manufacturers can optimise the injection moulding process.

Nozzle Pressure

Nozzle pressure is the pressure exerted on the molten plastic at the entrance of the mould cavity through the nozzle of the injection moulding machine. It affects the material flow and distribution in the mould. This pressure doesn’t have a consistent value, but increases based on the growing complexity of the mould filling.

How Material Selection Affects Temperature and Pressure

Material selection plays a huge role in determining the temperature and pressure parameters during plastic injection moulding. Different materials have unique characteristics, behaviours, and requirements, so it’s natural you must treat them differently when it comes to injection moulding.

Factors such as melting point, viscosity, and thermal conductivity can all influence temperature settings with different materials needing different temperature settings for the proper melt flow and to prevent degradation. Pressure sensitivity also varies among different materials. Plastic injection moulding operators can make careful adjustments to achieve optimal mould filling without defects.

Temperature and pressure are important variables in the injection moulding process and affect the quality, production efficiency, and overall manufacturing costs. However, by trusting an experienced plastic injection moulding company, you can be sure the temperature and pressure parameters are optimal for the material used.

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