Injection molding is used in a variety of industries as a process to efficiently and accurately produce plastic products. Among these, ‘internal cavity pressure’ is an important indicator of the quality of the molded product. In this article, the role of this internal cavity pressure and how to read the waveforms are explained in detail.
What is internal cavity pressure?
Internal cavity pressure is the pressure generated in the cavity (the space in the mold that forms the molded product) during injection molding. This pressure varies throughout the entire molding process, from the injection process through the
Relationship between the injection molding process and internal cavity pressure
In injection molding, the cavity inner pressure changes in three main processes. Let's take a look at the characteristics of the internal cavity pressure in each of these processes.
1. injection process
In the injection process, the resin is heated to a molten state and then injected at high speed into the cavity by the movement of the screw.
Characteristics of cavity internal pressure:
When resin starts to flow into the cavity, the internal pressure increases rapidly. The rate of this pressure increase is highly dependent on the injection speed and the viscosity of the resin.
Key points of this process:
If the pressure increase is not smooth, the air in the cavity may not be released (insufficient venting).
It is important to ensure uniform filling through appropriate injection speed and mould design.
2. the compaction process
As the injection process proceeds and the cavity is almost filled, the resin is gradually compressed until the internal pressure reaches its peak. This compression ensures that the resin is spread to every corner of the cavity.
Characteristics of cavity internal pressure:
The internal pressure peaks and the entire cavity is filled with resin.
The apex of the pressure waveform indicates when filling is complete.
Key points of this process:
If the peak pressure is too high, there is a risk of burring in the molded part. On the other hand, if the peak is too low, there is a risk of underfilling (short shots) and voids inside the product.
Appropriate injection conditions need to be set.
3. pressure holding process
After the compression process, the pressure holding process maintains a constant pressure until the gate solidifies. This pressure serves to compensate for shrinkage during cooling of the molded product.
Characteristics of cavity internal pressure:
The internal pressure is stable at the start of pressure holding, followed by a gradual decrease in pressure as the gate solidifies.
The waveform remains relatively flat after the peak and then falls gently.
Key points of this process:
If the pressure holding is insufficient, voids (cavities) and shrinkage marks will occur in the molded product.
If the pressure holding time is too long, the cycle time will increase, so an optimum balance is required.
▸ Analysis and utilisation of cavity internal pressure waveforms
How to read and interpret waveforms
The waveform of the cavity internal pressure visualises the health of the molding process. Focusing on the following key points enables early detection of problems and optimisation of the process.
Rise of the waveform
The initial stage when the resin is filled into the cavity. If the rise is slow, the injection speed may be insufficient or there may be a resin flow problem.
Peak pressure.
The timing at which the cavity is completely filled. If the peak is too high, it indicates excessive injection pressure; if it is too low, it indicates underfilling.
Flatness of the holding pressure phase
If the holding pressure is stable, the waveform will be smooth and flat. If it is unstable, the pressure holding settings and material properties should be reviewed.
Troubleshooting examples
Short shots: if the waveform has a low peak, the injection speed or material temperature needs to be increased.
Burring: if the peak pressure is too high, the injection pressure or holding pressure should be optimised.
Shrinkage marks and voids: if pressure holding is insufficient, adjust the pressure holding time and pressure.
Conclusion.
Cavity internal pressure is like a ‘health certificate’ in injection molding quality control. Precise analysis of the waveform and setting the right conditions during injection, compression and pressure holding can help stabilise the quality of the molded product.
The use of data is essential for optimising injection molding. The cavity internal pressure waveform is not just data, but a powerful tool to improve your molding site. Use it to improve quality and reduce defects!
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