Injection molding is a widely used manufacturing process for plastic product manufacturing. In this process, cooling time often occupies a large part of the production cycle, even up to 60-70%. Therefore, it becomes very important to minimize cooling time, improve production efficiency, and minimize production costs. This article aims to discuss different technologies and methods for reducing cooling time in injection molding.

Understanding the Importance of Cooling Time

During the injection molding process, the plastic material is melted and then injected into the mold. After injection, it must go through a cooling process so that the material hardens in the mold and forms the final shape and size. The cooling time not only has an impact on the quality of the manufactured product, but also has a direct impact on the production process and the efficiency part of the cycle. Therefore, reducing the cooling time is an important goal to improve the quality of the injection molding process.

The cooling process primarily includes two stages: cooling process of the material through the liquid state, to the glass transition state and from the glass transition state to the completely crystallized state. The first stage typically takes longer because of plastic’s high specific heat capacity at high temperature meaning that it will take longer for the material to shed the heat. Although the second stage is shorter, it significantly affects the product’s dimensional stability and internal stress distribution.

Common Cooling Methods in Injection Molding

Water Cooling

Water cooling is the most common cooling method used in injection molding machines. It usually involves circulating water through the cooling channels of the mold or injection molding machine.

Advantages:

1. Effective Cooling: This is because water is widely known as being a very good heat transfer medium and it provides steady cooling.

2. Controllable Process: The flow and the temperature of the water can be controlled and therefore it strictly regulates the cooling process.

3. Low Cost: Water is relatively cheap hence choosing water cooling is pocket friendly.

Disadvantages:

1. Bacterial Growth: Hot water results in bacterial activity and therefore water must be disinfected before use.

2. Requires a Circulating System: Introducing a water circulation system is also an additional cost in the system investment.

3. Water Stains: Water could be utilized in the process but stains the items during the process and hence, the need for constant washing.

Air Cooling

Air cooling uses fans to blow cooling air directly into the mold area of ​​the injection molding machine and then exhaust the hot air.

Advantages:

1. No Need for Circulation Pumps: Saves on water circulation pumps which are costly equipment investment for heating system.

2. No Bacterial Growth: Without using water, there is no risk of bacterial growth.

3. Long-Distance Cooling: The air cooling distance can be longer, which is suitable for some equipment that is difficult to cool with water.

Disadvantages:

1. Less Effective than Water Cooling: Air is not as efficient as water as a heat transfer medium because it cannot transfer as much heat as quickly as water.

2. Uncontrollable Temperature: The air temperature of the fan cannot be accurately adjusted.

Oil Cooling

Oil cooling is a specialized cooling method using an oil cooling system to remove heat.

Advantages:

1. Effective Cooling: Oil has high thermal conductivity, providing stable cooling effects.

2. Space-Saving: Oil cooling systems occupy less space compared to water circulation systems.

3. Controllable Temperature: The oil temperature can be controlled, ensuring a well-regulated cooling process.

Disadvantages:

1. High Cost: Oil is a valuable chemical, making it a more expensive cooling option.

2. Maintenance Challenges: Oil requires periodic replacement and maintenance, which can be cumbersome.

Factors Affecting Cooling Time

Material Selection

1. Thermal Conductivity: There are great distinctions in the thermal conductivity of various types of materials. It means that materials with higher thermal conductivity (for instance metal filled plastics) cool faster than the ones with lower thermal conductivity (some of the engineering plastics for instance).

2. Crystallinity: Semi-crystalline polymers (e. g. , polyethylene, polypropylene) take more time to cool than amorphous polymers (e. g. , polystyrene, poly carbonate) because of the formation time of crystalline structures.

3. Specific Heat Capacity: Materials with higher specific heat capacities require more energy to cool to the same temperature, resulting in longer cooling times.

Mold Aspect

1. Cooling System Design: It is important to have a well-designed cooling system for purposes of minimizing on the time that is taken for the component to cool. Among them, the layout, diameter, and length of cooling channels have a direct impact on the cooling ability of the liquid.

2. Mold Material: The thermal conductivity of the mold material influences cooling time. Copper alloy molds also are good conductors of heat, and, therefore, requires less time to cool, as compared to steel molds.

3. Mold Surface Treatment: The cooling time is also related to the surface characteristics of the mold. A rough surface and a thicker coating will slow down the cooling speed, while a smooth surface and a thermally conductive coating can improve the cooling efficiency.

4. Mold Water Channel Layout: In the process of injection mold water channel design, it is necessary to analyze all factors that may cause problems in the product structure. To this end, different circulating water channel designs may need to be proposed. Mold water channels should choose straight-through water channels as much as possible, minimize designs containing many cooling wells and corners, and minimize dead water. In the mold design stage, the cooling water channel layout can be reasonably arranged to ensure the cooling effect.

5. Mold Temperature: It is established by the calculation of the real temperature of the points of water inlet and outlet on the mold cavity. If thermal variation between the several regions and thermal variation with the set temperature can be regulated in the range of ±5℃, it still means that the fundamental cooling is satisfactory.

6. Daily Maintenance of Injection Molds: If there is oil or dirt on the mold surface, the cooling efficiency will generally be reduced. It is necessary to clean the cavity surface regularly and use a cleaning machine to clean the mold water channel. During normal operation work, especially during the daily inspections of local startups, it is necessary to pay more attention to the monitoring of cooling water flow, and various abnormalities require timely handling.

Processing Parameters

1. Mold Temperature: The higher the mold temperature, the longer the cooling time. Lowering the mold temperature can shorten the cooling time.

2. Injection Temperature and Pressure: The heat inside the mold will rise as the injection temperature and pressure increase, which will increase the cooling time. Adjusting these parameters can reduce heat buildup.

3. Injection Speed: The higher the injection speed, the more shear heat will be generated, so the cooling time will be longer. One of the measures that operators can take to reduce cooling time is to reduce the injection speed.

Environmental Factors

1. Ambient Temperature and Humidity: The cooling process is influenced by the temperature and the humidity levels within the production area. High temperatures and humidity prolong cooling time, while low temperatures and humidity shorten it.

2. Cooling Medium Temperature and Flow Rate: The rate of temperature and flow also affects the cooling in the system. It is also realizable to improve the cooling speed through decreasing the cooling medium’s temperature and raising the flow rate.

Methods to Reduce Cooling Time

Optimize Material Selection

Choose materials with higher coefficient of thermal conductivity, it can be plastics with additional additives. The following measures are recommended to enhance the resultant formula’s thermal conductivity: Including heat-conductive additives in the composition, for example, additives based on aluminum powder or copper powder of powdered plasticizers.

Improve Mold Design

1. Optimize Cooling Channel Design: Correctly design cooling channels to ensure uniform cooling of the mold surface. Another method is to use spiral cooling channels or multi-circuit cooling systems.

2. Use High-Efficiency Cooling Media: Cooling can be done with other media that has a higher capacity to cause cooling, such as oil and refrigated gases other than water.

3. Select Mold Materials: Utilize materials that have higher thermal conductivity than steel, these materials may be copper and alloys including aluminum.

4. Mold Surface Treatment: One should apply heat-conductive coating or plating on the mold surface to improve the thermal conductivity.

5. Balanced Cooling Design: Optimize mold design to ensure uniform cooling of all parts of the plastic product, preventing internal stress and deformation.

Adjust Processing Parameters

1. Lower Mold Temperature: You can also use cooling devices to cool the mold temperature like a cooling water system, a cold air system, or cooling sprays.

2. Optimize Injection Temperature and Pressure: Optimize the temperature and the pressure of the injection to control the build up of heat without affecting the quality of the product.

3. Control Injection Speed: Adjust injection speed to prevent excessive shear heat, thus reducing cooling time.

4. Extend Holding Time: Higher holding time under pressure can enhance the cooling effect.

Use Auxiliary Cooling Technologies

1. Pulse Cooling Technology: Since the cooling rate of the cooling medium may also have an effect on the efficiency of the process, it is recommended that cooling flow rate and medium temperature be changed periodically.

2. Variable Frequency Cooling Technology: Analyze for necessary adjustments the flow rate and temperature of the cooling medium according the changes in mold temperature.

3. Mold Surface Treatment: One could use thermal conductive coating, thermal conductive plating layer to help optimize the heat conduction of the mold.

Leverage Computer Simulation Technology

1. Computational Fluid Dynamics (CFD) Simulation: By using simulation software, different cooling water layouts are tried and the convection of fluid in the mold cooling channels is studied to achieve the optimal design.

2. Mold Temperature Field Analysis: Analyze temperature changes that occur during molding to check for areas that take longer to cool, and make adjustments accordingly.

3. Injection Process Simulation: In order to find other parameter that will shorten the injection molding cooling time, use the simulation software to develop the injection process.

Conclusion

Cooling time is one of the most important parameters of injection molding as it decides the quality of the final product, the cycle time needed, and productivity of the process. To reduce cooling time in injection molding, it is crucial to consider factors such as wall thickness, maintaining a proper cooling time, proper cooling temperature, ensuring adequate cooling time, and implementing efficient mold cooling techniques, as insufficient cooling time can negatively impact the overall injection molding cycle time. Therefore, to decrease the cooling time, there are several approaches that can be applied, such as material selection, mold design, process parameters, and applying auxiliary cooling technologies. New technologies and materials that will be developed over time will create better chances for the cutting time to be reduced further in the future.