In the present study, a plastic product made of PP+EPDM+20%Talc was chosen as research object. The CAE software MoldFlow was employed to investigate the influence of the gas foam type and processing parameters on microcellular injection molding process. DOE was used to find out the significant processing parameters to warpage and sink mark, furthermore, the interactions between each processing parameters were also investigated.

2. Experimental setup

The aim of this section is to illuminate the geometry model and finite element model of the part and mold, and to explain the thermoplastic material of the part.

Fig. 1. 3D model and dimensioning of standard tensile. Fig. 2. FE model of product and mold.

2.1. Geometry model and thermoplastic material

In order to investigate the influence of processing parameters on product defect and product performance, a 3D geometry model is necessary. Hence, the standard tensile sample was selected as a research object, which was established in UG according to ISO 527-2, the dumbbell tensile test specimen was moulded as the following size: Gauge length (L0 =50.0), Length of narrow parallel-sided portion (L1 =80.0), Distance between broad parallel-sided portions  (L2 =110),  Overall  length  (L3 =150),  Width  of  narrow  portion  (b1 =10.0),  Width  at  ends  (b2 =20.0), Preferred thickness (h=4.0), which is shown in Fig. 1. A proper mold design is a necessity to assure accomplishment of the product injection. Hence, the runner system and cooling system were established in MoldFlow software according the standard mold design guidelines, which is shown in Fig. 2.

The thermoplastic material for conducting the finite element analysis is selected from the MoldFlow software.

The material structure is crystalline. Its properties were given in Table 1.

2.2. FE model of the part and mold

The 3D geometry model was imported into the CAE software MoldFlow through format conversion. A midplane finite element model was created by meshing the 3D model with 369 linear triangular elements. The average aspect ratio of the mesh was 1.51, and the maximum of aspect ratio was 3.0. The mesh was thoroughly checked to eliminate mesh-related errors, which is shown in Fig. 2. Meanwhile, the FE model of runner system and cooling system were also created by using meshing tools in MoldFlow, which is shown in Fig. 2.

Table 1. Properties of material.

Material Description Mechanical properties

Solid density (g/cm3) 0.90211 Elastic modulus (MPa) 918.603

Melt density (g/cm3) 0.77072 Shear modulus (MPa) 329.589

Ejection temperature (oC) 115 Poisson’s ratio 0.3936

Material characteristics PP+EPDM

Filler 20% Talc

3. Experimental method

The purpose of this study was to investigate the influence of processing parameters on microcellular injection molding. In the first section, fractional factorial design method was employed to conduction to identify the most influential variables to warpage. In the second section, the four most influential factors were selected as research object, which were identified on the basis of fractional factorial design.