In production, we often encounter this phenomenon: Previous injection molding run went well, and we recorded process parameters on a process card. However, next time we run machine and apply those same parameters, we still end up with a poorly executed product.
Some colleagues blame this on injection molding machine's instability; others on unstable mold temperature; and some even completely deny usefulness of process cards, believing them to be of little value.
So, what is real reason? Do process cards offer any guidance?
This is a representative issue, so let's analyze it.
First, stability of injection molding machine, including stroke control, temperature control accuracy, and repeatability, is a factor to consider. This is why injection molding machines require daily inspection and calibration.
However, even the most precise injection molding machines can experience this phenomenon, demonstrating that machine accuracy isn't the only factor; other factors can also play a role.
This problem stems from a common misconception: assuming set mold temperature equals actual mold temperature.
During injection molding process, mold acts as a heat exchanger. Plastic melt introduces heat, raising mold temperature; cooling water removes heat, lowering mold temperature (of course, there are also heat conduction and radiation). Therefore, mold temperature fluctuates within a certain range.
Figure below shows temperature curve of mold cavity surface during injection molding process. It can be divided into two parts: average temperature field and fluctuating temperature field.

 

As shown in figure, during initial stages of injection molding, mold cavity surface temperature gradually increases with time (or number of injections). After a certain number of production cycles, cavity surface temperature reaches a relatively stable value on a macroscopic level, while exhibiting stable cyclical fluctuations on a microscopic level. This is because once injection molding production enters a stable phase, heat transferred from melt and heat removed by mold cooling system are essentially equal, and mold temperature tends to stabilize.
Because heat introduced by plastic melt during injection molding process is intermittent, cavity surface temperature fluctuates with cyclical action of injection molding cycle, resulting in microscopic sawtooth-shaped curve.
Fluctuation curve in figure also shows that mold temperature fluctuates significantly at the beginning of production. As production progresses, fluctuation gradually decreases, eventually stabilizing. Smaller fluctuations indicate a more stable mold temperature field and improved injection molding repeatability.
Injection molding process parameters are generally set after injection molding process reaches a stable state. However, using these parameters in the early stages of injection molding often results in suboptimal products. Primary reason is that mold temperature at the beginning of injection molding is not as high as temperature at stable state. As a result, defects such as insufficient mold fill, shrinkage, and noticeable water inclusions often occur.
If process parameters are adjusted, such as increasing injection pressure, speed, or material temperature, good products may be initially achieved. However, as production progresses, after a period of time, when mold temperature reaches a stable state, overfilling phenomena such as flashing and white tipping may reappear. At this point, returning to stable state process parameters is appropriate.
Therefore, it is appropriate to make appropriate adjustments to parameters on original process card during initial stages of injection molding. Returning to original parameters after injection molding process reaches a stable state is also correct. This is due to different mold temperature conditions during injection molding process. Please note that this is a revision, not a complete overturn. Guiding significance of process card adjustment machine cannot be mechanically denied just because of existence of differences.