Simulation in compression molding - can we accurately predict material flow?

By Hans Fredrik Sandberg MSc/Sealing Expert

1. Summary

The mode of operation at Seal Engineering is short production runs combined with strict specifications of finished parts. Our aim with this study was to investigate if simulation could provide new insight to improve our mould design activity. It was concluded from the studies that numerical simulations have a high probability of being a useful tool for predicting and optimizing the compression molding processes to manufacture high-quality rubber products.  

2. Introduction to Compression molding

Compression molding is widely used and a fairly simple manufacturing method where materials ready to be processed are placed in a two-plate cavity system and then pressed between preheated metal plates at controlled temperature and pressure [1].

2.1 Molding challenge

During first phase of compression molding, unvulcanized rubber is squeezed by the closing mould, ensuring its flow into the cavity. The required amount of rubber will fill the mould cavity completely, thereby displacing the air that was present in the tool. This step is critical to obtain a defect-free vulcanized product. Typically, voids as small as 0,1 mm in size make the rubber products unusable in critical-service applications in the energy or aerospace industries.

3. Methods

To obtain accurate and reliable results from numerical simulations, an appropriate material model is a necessity. Whereas a vast literature exists discussing material models for thermoset rubbers (spanning over 70 years of research in the field), rather scarce background information has been published on constitutive modeling of unvulcanized rubber. Extensive experimental testing has been undertaken in Seal Engineering’s metrology laboratory to establish a test protocol for material modeling of an unvulcanized fluoroelastomer (Seal Engineering RU23™).

Three different compression tests were run, with varying strain speed and levels as well as varying holding times and levels. The experimental data was loaded into “Data Fit” function of the FEA software (MSC Marc) using PRF (parallel rheological framework). This allowed for a material model that captures different aspects of material behaviour.

Simulations, using the PRF model, were subsequently validated through compression molding experiments. The compression press is equipped with high-precision sensors for accurate measurements of position and pressure. The molded test specimens fabricated with varying press settings were thoroughly inspected.

4. Results

Experimental data was collected for three different test programs, fast, medium and slow speed. Duration of these test program were 600, 800 and 1400 seconds. Result from medium speed test shown in Fig.2. Data fit calculation was committed with all three data set loaded. Results for medium speed data fit is shown in Fig. 3, displaying a good correlation between experimental data and computed constants for the material model set. The physical part used for simulation validation was O-RING 164,69 x 3,53. Material was RU23.

Three compression cycles were defined, all with five parallels. The three cycles were normal full cycle, abort at 30 bar bumping and abort at 10 bar bumping, called A, B and C respectively.

Examination of test specimen showed that the thickness, which should reflect different closing position of the press, did not match the position data recorded during testing. The difference in end position of cycle A and C was 0,58mm. This value represents the larger gap in cycle C. However, the difference in test specimen thickness from cycle A and C was in range 0,04mm. Root cause of this unsuspected result need to be investigated.

Position data from rubber press was extracted and used for position data in simulation. Simulations were executed and various results like flow pattern, deformation, strain and stress data was investigated

5. Conclusions

Our effort to do a validation of simulation versus real life rubber curing was obstructed by unforeseen results from rubber press data logging. However, we have gained a lot of detail knowledge about our molding process. Some findings have also raised need for further investigation.

6. References

[1] Shakeel Ahmed, Advanced Green Materials, 2021


Fig. 1. Compression molding process. Sketch by Seal Engineering AS

Fig. 2. Experimental data plot

Fig. 3. Curve fit evaluation

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