Effect of film types on thermal response, cellular structure, forming defects and mechanical properties of combined in-mold decoration and microcellular injection molding parts

doi:10.1016/j.jmst.2021.03.026

Abstract

Abstract:

Different types of polymer films were used in the combined in-mold decoration and microcellular injection molding (IMD/MIM) process. The multiphase fluid-solid coupled heat transfer model was established to study the thermal response at the melt filling stage in the IMD/MIM process. It was found that the temperature distributed asymmetrically along the thickness direction due to the changed heat transfer coefficient of the melt on the film side. When polyethylene terephthalate (PET) films were applied, the temperature of the melt-film interface increased faster and to be higher at the end of melt filling stage in comparison with the application of polycarbonate (PC) and thermoplastic polyurethane (TPU) films. And the effects of film types on the cellular structure, forming defects and mechanical properties of IMD/MIM parts were also studied experimentally. The results showed that the film types had no obvious effect on the cells size in the transition layer and the mechanical properties of the parts. Under certain film thickness, the offset distance of core layer was the largest with PET film used, while the offset distance was the smallest with TPU film used. And similar results were found for the warpage of the parts. However, an exactly opposite change occurred for the thickness of film-side transition layer and the bubble marks on the surface of the parts.

Key words: Microcellular injection molding, In-mold decoration, Thermal response, Cellular structure, Forming defects, Mechanical properties

Wei Guo, Zhihui Yu, Wenting Wei, Zhenghua Meng, Huajie Mao, Lin Hua. Effect of film types on thermal response, cellular structure, forming defects and mechanical properties of combined in-mold decoration and microcellular injection molding parts[J]. J. Mater. Sci. Technol., 2021, 92: 98-108.

Figures/Tables 17

Fig. 1. Simplified coupled heat transfer model of IMD/MIM process: (a) the melt filling stage of IMD/MIM process; (b) the partially enlarged drawing of mold, film and melt; (c) the temperature of melt along the thickness direction; (d) the heat transfer path and thermal resistance of mold and film, film and melt, melt and mold.

Fig. 2. Schematic diagram of the geometric model and boundary condition type of the flexural spline and the mold in ANSYS/Fluent: (a) 3D geometric model of the flexural spline mold and cavity; (b) partial enlarged detail of flexural spline coated with different types of films; (c) symmetric section diagram of the flexural spline.

Fig. 3. Schematic diagram of the geometric model of the flexural spline and the mold in Moldflow.

Table 1 Physical parameters of the materials.

Material	Density (kg/m³)	Specific heat capacity (J kg^-1 K^-1)	Thermal conductivity (W m^-1 K^-1)
PP	806.1	2555	0.22
N₂	1.138	1040.67	0.0242
Air	1.225	1006.43	0.0242
PET	1054.1	2400	0.1985
P20	7800	460	29

Fig. 4. Sampling position of flexural spline used for SEM observation.

Fig. 5. Temperature history of monitoring point in the melt-film interface with different types of films: (a) the position of the monitoring point in the melt-film interface; (b-d) the temperature history of the monitoring point under different types of films with thickness of 0.1 mm, 0.2 mm and 0.3 mm.

Fig. 6. The cellular structure in the vertical direction of IMD/MIM parts with different types of films: (a) 0.1 mm PET; (b) 0.2 mm PET; (c) 0.3 mm PET; (d) 0.1 mm PC; (e) 0.2 mm PC; (f) 0.3 mm PC; (g) 0.1 mm TPU; (h) 0.2 mm TPU; (i) 0.3 mm TPU; (j) without film.

Fig. 7. IMD/MIM parts with different types of films in the vertical direction: (a) the cell radius; (b) the offset distance in the core layer; (c) the thickness of each transition layer..

Fig. 8. SEM on the film-side surface of IMD/MIM parts with different kinds of films: (a) 0.1 mm PET; (b) 0.2 mm PET; (c) 0.3 mm PET; (d) 0.1 mm PC; (e) 0.2 mm PC; (f) 0.3 mm PC; (g) 0.1 mm TPU; (h) 0.2 mm TPU; (i) 0.3 mm TPU.

Fig. 9. 2D surface contour on the film side of IMD/MIM parts with different types of films: (a) 0.1 mm PET; (b) 0.2 mm PET; (c) 0.3 mm PET; (d) 0.1 mm PC; (e) 0.2 mm PC; (f) 0.3 mm PC; (g) 0.1 mm TPU; (h) 0.2 mm TPU; (i) 0.3 mm TPU.

Fig. 10. 3D surface contour on the film side of IMD/MIM parts with different types of films: (a) 0.1 mm PET; (b) 0.2 mm PET; (c) 0.3 mm PET; (d) 0.1 mm PC; (e) 0.2 mm PC; (f) 0.3 mm PC; (g) 0.1 mm TPU; (h) 0.2 mm TPU; (i) 0.3 mm TPU.

Fig. 11. The polymer surface temperature and the surface roughness on the film side of the flexural spline coated with different types of films.

Fig. 12. The comparison of warpage difference of parts coated with different types of films.

Fig. 13. The temperature difference and warpage change on both sides of the part at the end of melt filling stage with different types of films.

Fig. 14. Tensile test results of IMD/MIM splines with different types of films: (a) tensile strength; (b) tensile stress-strain curve.

Fig. 15. The flexural experiment result of IMD/MIM spline with different types of films: (a) flexural strength; (b) flexural stress-strain curve.

Fig. 16. The impact strength of IMD/MIM splines with different types of films.

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