When looking at new parts or attempting to optimize current production it is often helpful to ask “What determines cycle time?”. A typical answer is usually the Cooling Time. This is often the largest part of time taken up by an injection molding cycle but there are a lot of factors that go into determining a cycle time. It may be helpful to look at the part to be molded and steps of an injection molding cycle to better understand what determines Cycle Time.
The wall thickness of a part is usually a deciding factor on how fast you can push a molding cycle. If the wall of a part is thick, it will take longer to cool and solidify. Until this solidification is far enough along the part will not be able to be ejected without potential issues such as pin push and warpage. This solidification hasn’t fully started until after the part has been injected and the gate is sealed. The cooling time therefore is directly related to the wall thickness.
A material supplier will often include a chart that shows wall thickness vs estimated cooling time. Mold flow simulations can also be used to estimate cooling. If neither of these are an option, trial and error can be used to determine the required cooling time.
Directly related to the wall thickness is the gate diameter. The larger the gate, the longer the time for the gate to freeze. Until the gate of a part is frozen the part can not be fully solidified. Once a gate is frozen the hold time can end and the cooling time can begin. It is also important to note that sometime the thickest area may not be in the part but instead in the runner system.
The material is also a big factor in the cycle time. As expected different materials run at different temperatures. The higher the temperature, the longer it can take to reach ejection temperature. There is also differences between Semi-Crystalline and Amorphous materials. A Semi-Crystalline material has chains that “set” in place which usually can be ejected quicker. Certain materials such as urethanes can take a long time to set which controls the cooling time.
In order to eject a part, a cavity has to be filled. This includes any delay time for valve gates, injection fill time, and hold time. These parameters combine determine how long it takes to fill a part before recovery for the next part can begin. Once the gate is frozen recovery for the next shot can begin. If a shot is large it will take longer for the screw to recover in preparation for the next shot. Shear sensitive materials may require lower screw recovery speeds and injection speeds to avoid burning.
Once a part has reached an appropriate temperature the mold can be opened to eject the part. The time it takes a mold to open and close is based on the capabilities of the machines. Toggle electric presses are often faster but more expensive up front than hydraulic presses. With the mold open the part can then be ejected or removed with automation. Robots can only enter once the mold area is safe. Many robots are pushing 100in/sec but can still add a few seconds to a cycle. When allowing parts to free fall enough time has to be left to ensure all cavities have cleared the bottom of the mold.
There are many more factors that can go into cycle time such as cooling line layout, mold construction material and mold actions/inserts. Hopefully this brief overview of what makes up an ejector molding cycle will help you better understand what is affecting your cycle time.