Plastics Molding & Manufacturing/Time
Injection time
[edit | edit source]The total amount of injection time consists of initial injection and holding injection.
Initial Injection
[edit | edit source]When the mold closes completely, a limit switch will signals the injection screw to push forward and inject the molten plastic into the mold cavity. The screw does not turn at this point but acts only as a plunger to force the material into the mold. This initial injection is performed in the fastest practical amount of time which in most cases lasts less than 2 seconds and rarely more than 3 seconds.
Holding Injection Time
[edit | edit source]Visualizing the plastic being injected into the mold ater the initial injection, the molten plastic enters the mold cavity image through a gate. When all the required material goes through the gate and packs the cavity image, the plastic is allowed to cool, under hold pressure, down to the point at which it all solidifies. The internal timer will decides how long is the hold time based on the settings. The hold time is the amount of time the injection screw maintains pressure against the plastic after it has been injected into the mold. Once the gate solidifies does, there is no reason to keep pressure because the plastic in the cavity lies beyond the solidified gate and the pressure from the injection unit no longer has any effect on it. So the length of time required to hold pressure against the gate is only long enough for the gate to freeze (solidify). In most cases this is only a matter of a few seconds. A gate with a thickness of 2 mm would take approximately 6 seconds to solidify.
Cooling Time
[edit | edit source]Cooling time is the most important time phase in the entire injection process. Cooling time is the amount of time required for the plastic material to cool to the point at which it has solidified, and become rigid enough to withstand the ejection process. Ejection of the part is the process that pushes the finished molded product out of the mold after the entire cycle is completed. Even the plastic may cool enough to solidify, it may not be rigid enough to eject. The cooling process actually takes as long as 30 days to finalize. The initial cooling is rapid, and 95% of the total cooling takes place in the mold. But the other 5% takes place outside the mold. If the outer skin of the plastic product is solidified to a sufficient depth, the remaining cooling will not have an appreciable effect on the molded part.
In conclusions, If cooling time is ↑ , the part size will stabilize but will decrease output and higher costs If cooling time is ↓ , the output will increase but will causes instability in part sizes