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Why Does Injection Molding Cycle Time Gradually Increase During Production?

2026/09/16 By le zhan

Injection Molding Cycle Time

When the injection molding cycle time gradually increases during production, most operators tend to blame the injection molding machine. However, in this case, we need to analyze and identify issues with auxiliary equipment other than the injection molding machine.

The auxiliary equipment surrounding the injection molding machine continuously determines the various conditions entering the molding process. The feeder must supply resin continuously; the dehumidifying dryer must maintain stable resin temperature and humidity, and so on. If these auxiliary devices gradually decline in performance, the injection molding machine will be forced to extend its waiting time or compensate by prolonging the cooling phase. Therefore, we derive another diagnostic principle:

If the molding cycle is normal at startup but lengthens after several hours of operation, investigate whether deviations occurred in material supply, mold water circulation, chilled water temperature, or resin drying conditions during production.

Failure of auxiliary equipment to maintain process conditions extends the Injection Molding cycle time

The injection molding cycle consists of a series of actions, including injection, holding pressure, cooling, plasticizing, mold opening, part removal, and mold closing. Although the injection molding machine controls these actions, some of them rely on auxiliary equipment.

The most obvious example is cooling.

At the start of production, the mold, cooling water, and peripheral equipment may be at relatively low temperatures. As the machine runs through hundreds of cycles, heat generated by the molten resin continuously transfers into the mold. Mold temperature controllers and chillers must remove this accumulated heat at a rate roughly equivalent to the process’s heat generation rate. If the equipment lacks sufficient capacity, the mold temperature will gradually rise.

On the surface, it may appear that the injection molding machine is running more slowly, but the actual cause is a decline in thermal control capability.

The same principle applies to the material feeding process. If the filter in the feeder becomes partially clogged, the equipment can keep running. Still, the amount of material delivered per suction cycle may gradually decrease, eventually causing the injection molding machine to stall while waiting for material replenishment. Similarly, instability in the resin drying process may force operators to slow down production or increase process allowances to ensure product quality.

Therefore, simply adjusting the injection molding machine parameters can only address the surface-level issues and fails to resolve the problem at its root.

Failure of auxiliary equipment to maintain process conditions extends the Injection Molding cycle time

Four Common Auxiliary Equipment Issues That Cause Injection Molding Cycle Time to Increase

Changes in peripheral auxiliary equipment operating conditions can also lengthen injection molding cycle times. In most cases, we attribute the primary issues to the following four factors:

  1. Insufficient mold cooling water flow;
  2. Declining performance of the chiller;
  3. Slower automatic feeding speed;
  4. Unstable resin drying.

These issues affect cooling efficiency, material supply, and resin condition, causing the molding cycle to gradually exceed the values set at the start of production. Next, we will analyze these four causes and explain how to troubleshoot and adjust the injection molding auxiliary equipment by comparing data from normal production with that from periods of extended cycle times—before adjusting the injection molding machine parameters.

Four Common Auxiliary Equipment Issues That Cause Injection Molding Cycle Time to Increase

A Gradual Decrease in Mold Water Flow Rate May Lead to Longer Injection Molding Cycle Times

The cooling phase typically accounts for a significant portion of the entire injection molding cycle. The mold temperature controller must ensure adequate water circulation within the mold to remove heat continuously. If the flow rate decreases due to scale buildup, blockages in the cooling circuit, insufficient pump performance, or partially clogged filters, the temperature difference between the incoming and returning water may increase.

This slows the heat dissipation rate.

For large molds, this issue becomes particularly pronounced, as longer cooling channels and higher thermal loads require a greater effective water flow rate. Therefore, if the injection molding cycle gradually lengthens—accompanied by rising mold return water temperature, uneven cavity temperatures, or longer part solidification times—operators should inspect the cooling water system before adjusting injection parameters.

Key Checkpoints

Compare the mold inlet and outlet temperatures at startup with those after several hours of operation. At the same time, check the flow rate, pressure, filters, hoses, cooling channels, and valves. If the temperature difference increases and the flow rate decreases, the extended injection cycle is likely due to the temperature control system.

A Gradual Decrease in Mold Water Flow Rate May Lead to Longer Injection Molding Cycle Times

Increased Thermal Load Leads to Reduced Cooling Capacity of the Chiller

The chiller is located upstream in the process flow.

Even if the mold temperature controller has sufficient water circulation, the system cannot maintain the expected cooling effect if the factory’s chilled water supply temperature gradually rises.

As the number of injection molding machines increases or daytime temperatures rise, total cooling demand may exceed the chilled water system’s effective cooling capacity. Additionally, a dirty or clogged condenser, insufficient airflow, poor chilled water quality, or scale buildup in the heat exchanger can further reduce the chiller’s performance.

Therefore, the key to our inspection is not whether the chiller is running, but whether it can maintain the chilled water supply temperature at the setpoint during full-load production. If the setpoint is 15°C but the supply temperature gradually rises as the system’s heat load increases, the mold cooling rate will slow down even if the injection molding machine and mold temperature settings remain unchanged.

Increased Thermal Load Leads to Reduced Cooling Capacity of the Chiller

Slower Automatic Feeding Speed Leads to Longer Molding Cycles

Even minor and recurring feeding delays in the automatic feeder can extend the actual molding cycle.

When dust accumulates in the filter, or partial blockages occur in the conveying lines, the automatic feeder may require a longer suction time to convey the same amount of resin. If the material consumption rate approaches or exceeds the replenishment capacity, the hopper level will eventually drop.

At this point, because the feed port cannot receive a continuous supply of material, the injection molding machine may shut down due to material shortage or experience uneven plasticization. If the feeder previously required only a short suction cycle but now needs to run repeatedly or for extended periods, inspect the material conveying path before a material shortage halts production.

Slower Automatic Feeding Speed Leads to Longer Molding Cycles

Situations Arising from Resin Drying Conditions

Although drying does not have as significant a direct impact on the molding cycle as mold cooling, it still affects production stability.

Dehumidifying dryers must strictly maintain the set resin temperature, drying time, air flow rate, and dew point. If the airflow rate decreases, thermal efficiency changes, or material throughput exceeds drying capacity, the resin may be unstable when it enters the injection molding machine.

Insufficient drying affects melt characteristics and product quality. Operators often have to take compensatory measures to ensure material stability. Conversely, if the material stays at the set drying temperature too long, it may over-dry and waste energy. Therefore, when molding cycles fluctuate and are accompanied by changes in melt characteristics or material condition, check the resin drying process first.

Situations Arising from Resin Drying Conditions

Diagnosing Injection Molding Cycle Time by Comparing Data from Different Production Stages

The most effective troubleshooting method is to compare operating conditions at two different points in time: during normal production startup and when cycle times are prolonged.

Auxiliary EquipmentCheck at Startup vs. LaterWarning Sign
Mold temperature controllerFlow, pressure, supply/return temperatureLower flow or larger ﾎ乃
ChillerActual chilled-water supply temperatureWater temperature gradually rises
Material loaderSuction time and loading frequencyLonger or more frequent cycles
Dehumidifying dryerMaterial temperature, airflow, dew pointDrying condition drifts

This method allows for faster troubleshooting than repeatedly adjusting the injection molding machine’s process parameters.

Controlling Auxiliary Equipment Can Shorten Injection Molding Cycle Times

Before changing injection molding machine settings, first ensure the surrounding auxiliary equipment is operating stably.

We recommend following a practical optimization sequence to stabilize each stage:

Material supply → Resin condition → Mold water flow → Chilled water temperature → Optimize injection molding machine settings.

This approach helps shorten the injection molding cycle while avoiding issues such as warpage, mold sticking, inconsistent shrinkage, or increased scrap rates. It enables the shortest, most stable, and reliable cycle times during continuous production.

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