Vibration analysis is an analysis performed to avoid the phenomenon of extremely large amplitude vibration (resonance) when a vibration equal to the natural frequency of a vibrating object is applied externally.
Vibration analysis can be broadly classified into the simplest "modal analysis (eigenvalue analysis)" and "frequency response analysis.
Vibration analysis includes Modal analysis and Frequency response analysis.
Fig. 1 Modal analysis (eigenvalue analysis)
Fig.2 Frequency response analysis
What Vibration Analysis Can Do
Obtaining vibration characteristics (eigenfrequency and magnitude of vibration) can lead to measures against resonance and noise.
Fig. 3 Vibration characteristics
When glass fiber reinforced material is used, the following measures can be taken to prevent resonance and noise.
① Set the natural frequency to a frequency outside the range of use.
② Suppress vibration amplification
By examining the glass fiber orientation and adding ribs or changing the rib thickness, it is possible to remove the natural frequency from the frequency range at which the product is used (Fig. 4 ①). In addition, when replacing metal, the viscoelastic properties of the resin will increase the vibration damping effect (Fig. 4 ②).
Fig. 4 Resonance and noise countermeasures
Here is an example showing that the natural frequency can be changed by fiber orientation. As shown in Fig. 5, after forming a flat plate of 120mmx80mmx2mm, we cut it out in each direction and conducted a damping characteristic evaluation test using the central excitation method.
Fig. 5 Test piece cutting image (left) and center excitation method image (right)
The experimental results are shown by the dashed line in Fig. 6. It can be seen that the natural frequency is higher in the 0° direction (= higher fiber orientation). Here, we can see that the primary natural frequency differs by several hundred Hz between the 0° direction and the 90° direction.
The solid line in Fig. 6 shows the results of a similar test reproduced by Vibration analysis. Here, Digimat was used to create material data considering anisotropy and reflected in the analysis. Similar to the experimental results, the results obtained from the analysis show that the higher the fiber orientation, the higher the natural frequency, and the difference between the natural frequencies obtained from the experiment and the analysis was only 5%.
Fig. 6 Glass-fiber orientation direction and natural frequency
As mentioned above, the eigenfrequency can be changed by changing the fiber orientation, but we will introduce how to actually change the fiber orientation using an application example of Box parts.
Changing the injection molding gate position is an effective way to change the fiber orientation in the actual product. As shown in Fig. 7, when gate A (left) and gate B (right) were compared and evaluated, changing to gate B changed the natural frequency to a higher position, as shown in Fig. 8. This approach is possible with glass-fiber reinforcements if the desired natural frequency cannot be obtained.
Fig. 7 Gate A (left) and Gate B (right) filling pattern
Fig. 8 Measurement point (left) and frequency response analysis results (right)
The natural frequency can be changed by adding ribs or changing the thickness of the ribs, in addition to glass-fiber orientation. As shown in Fig. 9, adding ribs increases the natural frequency, and reducing the thickness decreases the natural frequency (Fig. 10). In injection molding, it is relatively easy to change the shape like this, so it is effective to search for a better shape while analyzing.
Fig. 9 Change in natural frequency due to rib addition
Fig. 10 Change in natural frequency due to thickness change (2.5mm→1.5mm)