Mold Temperature for Injection Molding: Optimization and Control

I. Introduction

Injection molding, as an essential forming process in modern manufacturing, is widely applied to the production of plastic products. In the process of injection molding, mold temperature is one of the key factors that affect product quality and production efficiency. Mold temperature is not only related to the fluidity, cooling, and crystallization process of plastics, but also directly affects the dimensional stability, appearance quality, and mechanical properties of the product. Therefore, optimizing and controlling mold temperature is of great significance to improving the quality and production efficiency of injection molding products.

II. The Importance of Mold Temperature

Mold temperature is one of the critical parameters in the injection molding process, which directly affects the flow, cooling, and crystallization process of plastics in the mold. Firstly, mold temperature has a significant impact on the fluidity of plastics. During the injection process, plastics need to fill the mold cavity under certain temperature and pressure. If the mold temperature is too high, the plastic fluidity will be too good, which may lead to defects such as flash and burrs on the product. Conversely, if the mold temperature is too low, the plastic fluidity will be poor, possibly resulting in insufficient filling and material shortage in the product. Secondly, mold temperature also has an important impact on the cooling and crystallization process of plastics. The high or low mold temperature directly affects the cooling rate and crystallinity of plastics, further influencing the properties such as shrinkage rate, internal stress, and deformation of the product. Finally, mold temperature also affects the dimensional stability and appearance quality of the product. Both excessively high and low mold temperatures may lead to problems such as unstable product dimensions and increased surface roughness.

III. Factors Influencing Mold Temperature

1. Type and Properties of Plastic Materials

Different plastic materials possess varying degrees of thermal stability and melting points. Generally speaking, plastics with good thermal stability require higher mold temperatures to ensure excellent fluidity and filling performance; whereas, plastics with poor thermal stability require lower mold temperatures to avoid overheating and decomposition. Furthermore, the fluidity and viscosity of plastics are also influenced by temperature, thereby affecting the choice of mold temperature.

Here are the mold temperature requirements for common plastics:

PP (Polypropylene):

Mold temperature requirement: 40-80℃

Note: It is recommended to use 50℃ as the mold temperature for the injection molding machine to ensure uniform flow and cooling of the plastic.

PPS (Polyphenylene Sulfide):

Mold temperature requirement: 120-180℃

Note: PPS material requires a higher mold temperature to ensure its fluidity and molding effect.

POM (Polyoxymethylene):

Mold temperature requirement: 80~105℃

Note: For products with a large surface area, a higher mold temperature may be required to ensure sufficient cooling and prevent deformation.

PC (Polycarbonate):

Mold temperature requirement: 70~120℃

Note: Polycarbonate can achieve good dimensional stability and appearance quality within this temperature range.

PBT (Polybutylene Terephthalate):

Mold temperature requirement: For unreinforced materials, 40~60℃

Note: Ensure adequate fluidity during the molding process and rapid cooling.

PA6 (Polyamide 6 or Nylon 6):

Mold temperature requirement:

Thin-walled and long-cycle plastic parts: 80~90℃

Plastic parts with wall thickness greater than 3mm: It is recommended to use a low-temperature mold of 20~40℃

Note: Select an appropriate mold temperature based on the wall thickness and molding cycle of the product to optimize dimensional stability and mechanical properties.

ABS (Acrylonitrile Butadiene Styrene):

Mold temperature requirement: 25-70℃

Note: The mold temperature will affect the smoothness of the plastic part, with lower temperatures resulting in lower smoothness.

2. Injection Molding Process Parameters

Injection molding process parameters such as injection speed, injection pressure, holding time, and cooling time also affect mold temperature. The injection speed and injection pressure determine the flow speed and filling pressure of the plastic in the mold, thus influencing the choice of mold temperature. The holding time and cooling time determine the holding and cooling process of the plastic in the mold, which also need to be reasonably set according to the plastic material and product structure.

3. Mold Design and Material

The structure and material of the mold also affect the choice of mold temperature. The mold design should reasonably consider factors such as hot runner, cooling channel, etc., to ensure uniform flow and cooling of the plastic in the mold. The choice of mold material should consider factors such as thermal conductivity and thermal expansion coefficient to ensure good stability and durability of the mold under high temperature and high pressure.

4. Environmental Factors

Environmental factors such as workshop temperature, humidity, and cleanliness also affect mold temperature. Excessively high or low workshop temperatures may lead to unstable mold temperature, further affecting product quality and production efficiency. Therefore, it is necessary to maintain a stable workshop environment during the injection molding process.

IV. Optimization and Control of Mold Temperature

1. Principles for Setting Mold Temperature

When setting the mold temperature, initial settings should be made based on the plastic material and product structure. Generally speaking, for plastics with good thermal stability, a higher mold temperature can be chosen; for plastics with poor thermal stability, a lower mold temperature is required. Meanwhile, factors such as product size, shape, and wall thickness should also be considered to ensure uniform flow and cooling of the plastic in the mold. During the actual production process, adjustments may need to be made based on the specific conditions of the product.

2. Heating and Cooling Systems for Mold Temperature

The heating and cooling systems are crucial for optimizing and controlling mold temperature. Heating systems typically use electric heating tubes, hot oil circulation, and other methods to heat the mold to achieve the desired temperature. Cooling systems utilize water cooling, air cooling, heat pipe technology, and other methods to reduce mold temperature and control the cooling rate of the product. In the design of heating and cooling systems, full consideration should be given to factors such as mold structure and material to ensure uniform and stable heating and cooling.

3. Temperature Control Devices and Sensors

Temperature control devices and sensors are essential equipment for precise control of mold temperature. Temperature controllers can automatically adjust the heating and cooling systems based on the set temperature value to ensure stability and accuracy of the mold temperature. Temperature sensors are used to monitor changes in mold temperature in real-time and transmit the data to the temperature controller for processing. In selecting temperature control devices and sensors, factors such as precision, stability, and reliability should be considered to ensure precise control of mold temperature.

4. Methods for Optimizing Mold Temperature

Methods for optimizing mold temperature primarily include the experimental method and data analysis method. The experimental method involves observing the impact of different mold temperatures on product quality and production efficiency through actual production tests to find the optimal mold temperature. The data analysis method involves collecting and analyzing data from the production process to find the optimal mold temperature range. In practical applications, these two methods can be used comprehensively for optimizing mold temperature.

V. Analysis of the Impact of Mold Temperature on Product Quality

1. Impact of Mold Temperature on Product Shrinkage

Mold temperature is one of the key factors affecting product shrinkage. The level of mold temperature directly affects the cooling rate and crystallinity of the plastic in the mold, thereby influencing the shrinkage rate of the product.

High mold temperature: When the mold temperature is high, the cooling speed of the plastic melt in the mold is slow, resulting in the melt maintaining fluidity for a longer period. This favors the full filling of the mold cavity and reduces the shrinkage rate. However, excessively high mold temperatures may cause the plastic to stay in the mold for too long, leading to incomplete crystallization and ultimately increasing the shrinkage rate of the product.

Low mold temperature: Under low-temperature conditions, the plastic melt cools rapidly in the mold, resulting in fast crystallization but possibly insufficient filling of the mold cavity, thus causing a higher shrinkage rate. Additionally, low temperatures may cause the plastic to harden prematurely in the mold, forming significant internal stresses that further increase the shrinkage risk of the product.

2. Impact of Mold Temperature on Product Internal Stresses and Deformation

High mold temperature: Under high-temperature conditions, the cooling speed of the plastic melt in the mold is slow, which helps reduce the flow resistance and shear stress during the filling process, thereby reducing internal stresses within the product. However, excessively high mold temperatures may result in slow surface cooling of the product, leading to deformation during and after ejection from the mold.

Low mold temperature: At low temperatures, the rapid cooling of the plastic melt in the mold may cause significant internal stresses within the product. These internal stresses may gradually release during subsequent processing or use, leading to deformation or cracking of the product. Additionally, low temperatures may cause the plastic to harden prematurely in the mold, forming significant internal stresses that further increase the risk of product deformation.

3. Impact of Mold Temperature on Product Surface Quality and Glossiness

High mold temperature: Under high-temperature conditions, the plastic melt exhibits good fluidity in the mold, favoring the formation of a smooth and even surface. Additionally, high temperatures help reduce defects such as surface bubbles and shrinkage cavities, improving the surface quality of the product. Furthermore, high temperatures enhance the adhesion capacity of the material, making surface irregularities finer and improving the glossiness of the product.

Low mold temperature: Under low-temperature conditions, the poor fluidity of the plastic melt in the mold may lead to a rough and uneven surface of the product. Additionally, low temperatures may hinder the escape of gases from the melt, resulting in surface defects such as bubbles and shrinkage cavities, reducing the surface quality of the product. Furthermore, low temperatures may cause internal stresses on the product surface, leading to issues such as surface cracking or crazing.

VI. Case Study

Alpine Mold recently received a project to produce shower head covers. As everyone knows, shower head covers have numerous small holes on their surfaces, and during the injection molding process, a common issue encountered is the weld line.

How to solve this problem? Mold temperature plays a crucial role.

Alpine Mold has introduced a new technology, a high-low temperature steam machine. It can instantly raise the mold temperature to 210°C during the plastic injection process. Once the injection is completed, it quickly reduces the temperature to between 70°C and 50°C.

What are the benefits of this?

When the mold temperature is increased to 210°C, the weld line on the surface of the shower head cover is eliminated. When the mold temperature drops to 50-70°C, the plastic product can cool down normally without deformation.

VII. Conclusion

Mold temperature is one of the key parameters in the injection molding process, significantly affecting product quality and production efficiency. By optimizing and controlling mold temperature, we can reduce product shrinkage, minimize internal stresses and deformation, and enhance surface quality and glossiness. Alpine Mold is committed to meeting customers' high-quality requirements. If you have similar challenges, please feel free to contact us for the best solution.