3 Ways to Reduce Cycle Time in Injection Molding

Reducing melt overheating, back pressure, and poor mold cooling can shorten cooling time and improve injection molding efficiency.

When quoting a new injection-molded component, processors often set the cycle time based on previous experience or similar parts. Once the process is parameterized to meet that cycle time, however, it is often no longer challenged.

You can also read: Plastics Injection Molding: Definition, Benefits and Applications

During my consulting work, I frequently find opportunities to improve cycle time. My first piece of advice is always the same: reduce the amount of energy entering the system. The more heat you carry inside the component, the longer it will take to cool.

Therefore, when looking for opportunities to reduce cycle time, examine the process stages where you may be inadvertently supplying more heat to the melt than necessary. Heat obviously comes from the heater bands around the plasticizing unit. However, the largest amount of heat often comes from friction.

Take Care of the Temperature Profile

The first step to avoid overheating the melt is to reduce the heat supplied by the plasticizing unit’s temperature profile. In a general-purpose screw under regular operating conditions, processors normally use a temperature profile that starts lower at the feed throat and increases toward the nozzle.

This low-to-high temperature profile is related to the design of the plasticizing screw and the function of each section. The first section, close to the feed throat, is the feeding section. Heat in this zone improves material intake and flow. The transition section, in the middle of the screw, manages the phase change from solid pellets to molten material. Finally, near the nozzle, the screw pumps the molten material forward. This is where the temperature should be higher.

Avoid setting a constant temperature along the entire profile. This only increases the amount of heat entering the melt without adding value. It can also reduce the allowable residence time and contribute to material degradation. Figure 1 shows how the mean temperature of a molded component decreases over time. The lower the initial melt temperature, the faster the part reaches ejection conditions.

Avoid Over-Plasticizing

To homogenize the melt during plasticizing, processors may increase back pressure. This parameter prevents the screw from returning freely during backward rotation and substantially increases friction.

Many times, when I analyze an injection molding setup, I ask why a specific back pressure value was selected. Very often, nobody knows the answer. Be critical and challenge the value. Back pressure should be set as low as possible while still achieving proper melt quality.

You know the back pressure is sufficient when the melt appears smooth and glossy. Increasing back pressure beyond that point only increases energy consumption and overheats the melt. By reducing the heat generated through over-plasticizing, processors can directly reduce cooling time.

Take Care of Mold Cooling

After avoiding melt overheating in the plasticizing unit, make sure the mold removes heat as efficiently as possible. An appropriate cooling layout is essential, but processors should also confirm that the cooling channels are clean and unclogged.

In addition, avoid poor hose connections entering the mold. I have encountered several molds where nobody knows for sure which hose is the inlet and which one is the outlet. When connections are incorrect, it becomes impossible to ensure proper water circulation. Simple practices, such as using blue hoses for the inlet and red hoses for the outlet, can reduce the margin for error. Another useful practice is to use a thermal camera to confirm that all hoses are active and operating at similar temperatures. Thermal images make it easy to identify clogged or inactive hoses. Today, several thermal camera models can be adapted to a cellphone. Finally, make sure the mold has enough water supply. Water temperature is important, but it means little without enough flow rate. Processors must ensure enough gallons per minute through the cooling channels to achieve turbulent flow inside the mold.