Thin-wall injection molded parts have been paid much attention to the lightweight saving from viewpoints of natural resources saving. In the injection molding, skin-core structure can be found in the parts. This skin-core structure affects the property of completed injection molding parts (bulk property) even if in thin-wall injection molding. However, there is a few research about the relationship between bulk property and internal property distribution in the injection molding specimen. In this study, thin-wall injection molded parts of polypropylene (PP) were prepared by 4 different molecular weight and molecular weight distribution to reveal the relationship between bulk property and property distribution. These characteristics were investigated by using tensile test, fracture toughness characterized by Essential Work of Fracture (EWF) method for bulk property and film tensile test by sliced sample for tensile property distribution. The property distribution test results revealed that the highly bulk property sample had thicker highly mechanical property layer on its surface.
Injection molding is one of the most popular methods to make plastic products. The parts made by this technique were used in various situations such as transportation equipment, electric appliance, and supplies. Recently, light-weighting is very important theme because their weight-saving causes low emission of carbon dioxide, convenience and economy. The one of the simple ways to light-weighting is thin-wall injection molding. However, the resin was rapidly cooled by mold, so that the skin and core layer of molding article formed different structure and this structure influenced bulk property strongly [
In this study, 4 different kinds of homo-PP resins were used. The molecular characteristics were summarized in
To measure the static tensile property behavior, tensile properties were measured using anuniversal testing machine (Instron Corp., Type; 55R4206). The 1 mm thick dumbbell specimen (JIS K-7162-1BA) was used and the longitudinal direction was set along the FD direction. Tensile tests were performed under room temperature (RT) condition at a crosshead speed of 10 mm/min.
Mw (×104) | Mn (×104) | MWD (Mw/Mn) (−) | |
---|---|---|---|
LN | 19.0 | 6.4 | 3.0 |
LM | 19.0 | 3.5 | 5.4 |
HM | 40.0 | 7.4 | 5.4 |
HW | 41.0 | 4.2 | 9.8 |
Fracture toughness of thin-wall specimens was evaluated by EWF tests using the universal testing machine with double edge notched specimens (see
where, l is ligament length, t is the thickness of the specimen, and φ is shape factor of plastic zone.
To measure the difference of mechanical property distribution, tensile property distribution was measured using universal testing machine. The samples were sliced into every 50 μm thick films from the sample surface to 500 µm in depth with a rotary microtome (Leica Biosystems Ltd., RM2235) at room temperature. After this thin- sliced process, dumbbell specimen (JIS K-6251-7) for tensile property distribution was stamped out. The selected sample layers were 0 - 50, 50 - 100, 100 - 150, 150 - 200, 200 - 250 and 450 - 500 μm depth from the surface on each sample.
Modulus (GPa) | Yield Stress (MPa) | Stress at Break (MPa) | Strain at Break (%) | |
---|---|---|---|---|
LN | 2.4 ± 0.2 | 44 ± 0.0 | - | - |
LM | 1.9 ± 0.1 | 40 ± 0.0 | - | - |
HM | 2.5 ± 0.2 | 48 ± 0.0 | 43.2 ± 1.1 | 331 ± 17 |
HW | 2.2 ± 0.1 | 44 ± 0.1 | 52.7 ± 3.1 | 679 ± 15 |
From these results, the higher molecular weight and narrower molecular weight distribution sample showed higher modulus, yield stress and fracture toughness as compared with the other samples and these highly property samples had the highly property layer near the surface. Especially, this relationship can be observed at bulk yield stress and yield stress distribution. The high molecular weight and middle molecular weight distribution sample (HM) showed highest yield stress and modulus. In the modulus and yield stress distribution, it was found
that the HM sample had a thicker layer with high values near the surface as compared with the high molecular weight and wide molecular weight distribution sample (HW). In addition, it was found that the difference of distribution between HM and HW were not observed in the low molecular weight samples (LN and LM). In near future, the relationship between the molecular orientation, crystallinity, and mechanical properties of thin-walled PP molding parts will be investigated using X-ray deflection analysis and mechanical test.
In this research, the relationship between bulk mechanical property and mechanical property distribution of thin-wall injection molded polypropylene with different molecular weight and molecular weight distribution was investigated. The bulk mechanical properties were characterized in terms of static tensile property and fracture toughness by essential work of fracture method. The mechanical property distribution was measured by tensile test with thin-sliced dumbbell film obtained from bulk specimen. As results, it was found that the higher molecular weight and narrower molecular weight distribution sample indicated the higher modulus, yield stress, and fracture toughness as compared with the other samples. In addition, these samples had the highly property layer near the surface, it was considered that there is a correlation between these mechanical properties and the skin- core structure. The property distribution test results revealed that the highly bulk property sample had thicker highly mechanical property layer on its surface. Therefore, the value of mechanical property layer in the surface rather than the thickness of highly mechanical property layer in the surface affect the increasing bulk mechanical property.
KeisukeMaeda,KojiYamada,KazushiYamada,MasayaKotaki,HiroyukiNishimura,11, (2016) The Relationship between Bulk Property and Property Distribution in Thin-Wall Injection Molded PP at Different Molecular Weight and Molecular Weight Distribution. Advances in Materials Physics and Chemistry,06,1-8. doi: 10.4236/ampc.2016.61001