In injection molding production, we often find such a phenomenon: the last time the injection molding production was done well, the process parameters were recorded and made into a process card, and the next time the production was started, the process parameters of the last time were pressed, but the process could not be done well. The product?
For this phenomenon, some colleagues said that the injection molding machine was unstable;
So, what is the real reason? Does the craft card have any guiding significance?
This question is representative, let's do some analysis here
First of all, the stability of the injection molding machine, including stroke control, temperature control accuracy and repeatability are factors to be considered. Therefore, this is the reason why the injection molding machine needs to be inspected and calibrated daily.
But even with the most sophisticated injection molding machines, this happens, indicating that the accuracy of the injection molding machine is not the only factor, there are other reasons.
The reason is that there is a misunderstanding in understanding: the set mold temperature is considered to be the actual mold temperature.
In the injection molding process, the mold is a heat exchanger. The plastic melt brings heat to increase the temperature of the mold; the cooling water takes away the heat to reduce the temperature of the mold (of course, heat conduction and heat radiation). Therefore, the mold temperature will also fluctuate within a certain range.
The figure below is the temperature change curve of the mold cavity surface during the injection molding process. It can be divided into two parts, one is the average temperature field and the other is the fluctuating temperature field.
picture
It can be seen from the figure that at the initial stage of injection molding, the temperature on the surface of the mold cavity gradually increases with time (or the number of injections). After a certain number of production cycles, the temperature on the surface of the cavity will reach a relative macroscopically. Stable value, microscopically showing stable periodic changes. Because when the injection molding production enters the stable stage, the heat transferred to the melt is basically the same as the heat taken away by the mold cooling system, and the mold temperature tends to be stable.
Since the heat brought by the plastic melt during the injection molding process is intermittent, the temperature on the surface of the cavity changes with the periodic action of the injection molding cycle, so the curve is microscopically zigzag.
The fluctuation curve in the figure also shows that the fluctuation range of the mold temperature is relatively large at the initial stage of production, and as the production progresses, the fluctuation value gradually decreases, and finally the fluctuation value tends to be stable. The smaller the fluctuation value, the more stable the mold temperature field and the better the repeatability of injection molding.
Our injection molding process parameters are generally made after the injection molding enters a stable state, but even if this parameter is used at the initial stage of injection molding, good products are often not obtained. The most important thing is that the mold temperature at the initial stage of injection molding is not as high as that in the stable state. . Therefore, there are often defects such as insufficient glue, shrinkage, and obvious water lines.
If the process parameters are corrected, such as increasing the injection pressure, speed or raising the material temperature, good products can be obtained at first, but as the production progresses, after a period of time the mold temperature reaches a stable state, and the front will appear again. , Top white and other overfilling phenomena. At this time, the process parameters in the steady state are used again.
Therefore, at the initial stage of injection molding, it is correct to correct the parameters on the original process card appropriately, and it is also correct to return to the original injection molding parameters when the injection molding enters a stable state. This is due to the different states of the mold temperature during the injection molding process. due to.
Please note that this is a correction, not a complete overthrow, and the guiding significance of craft card swapping machines cannot be mechanically negated because of the existence of differences.
Why Can't The Injection Molding Process Parameters That Were Done Well Last Time Not Be Used in The Next Production?
Saved process parameters record static machine inputs (temperatures, pressures, speeds), but plastic quality relies on real-time melt behavior inside the mold cavity. Between production runs, subtle shifts in materials, machine mechanics, and plant environment alter this behavior-meaning identical machine setpoints produce entirely different physical results.
Key Variables That Shift Between Production Runs
Material Lot-to-Lot Variations:
Raw resin properties change slightly between batches. Differences in Melt Flow Index (MFI), moisture content, or regrind percentages alter melt viscosity. A stiffer lot requires higher pressure to fill the mold under identical velocity settings.
Environmental Humidity & Dryer Performance:
Hygroscopic plastics (such as Nylon or ABS) absorb ambient moisture rapidly. Seasonal humidity swings or minor changes in desiccant dryer performance alter moisture levels, causing splay, voids, or sudden drops in viscosity.
Thermal Realities of the Mold:
Setting a mold temperature controller (MTC) to 50°C doesn't mean the steel reached that exact state instantly. Scale buildup inside water lines, ambient shop air flow, or insufficient pre-heating times change actual heat dissipation, leading to warpage and dimensional shifts.
Machine Hydraulic & Mechanical Drift:
As hydraulic oil heats up during a shift, its viscosity changes, altering injection response times. Furthermore, wear on the barrel's check ring (non-return valve) allows backflow during injection, causing unstable cushion positions and inconsistent shot weights.
Machine Inputs vs. Melt Realities
| Saved Setting (Machine Input) | Physical Reality (What Shifts) | Part Defect Result |
| Injection Speed | Resin viscosity fluctuates between batches | Short shots or flash |
| Barrel Heat Zones | Moisture/MFI changes alter actual melt enthalpy | Splay, burning, or sink marks |
| Hold Pressure | Check ring wear causes variable backflow | Dimension variations / weight loss |
| Cooling Timer | Water line scaling reduces heat transfer | Warpage and structural stress |
The Solution: Scientific (Decoupled) Molding
To eliminate parameter drift, modern manufacturing relies on Scientific Molding. Instead of trusting saved setpoints, process engineers monitor the plastic's rheology via cavity pressure sensors and dynamic machine data:
Match Fill Time, Not Set Velocity: Set the machine to achieve a fixed actual fill time (e.g., 1.15 seconds) so the controller automatically adjusts pressure to overcome viscosity shifts.
Measure Melt Temperature Directly: Use an optical pyrometer or purge thermometer to measure true polymer temperature rather than relying solely on barrel thermocouples.
Monitor Cushion Trends: Track the final screw position at the end of hold time to detect check ring leakage or material inconsistencies before defects appear.
