Schematic diagram of the formation processes of the unfoamed skin layer in microcellular injection molded part: (a) during the filling stage; (b) after the filing stage. 

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... of the flow front, the cells on both sides of the cross section are deformed and stretched further, the structure of the skin region is becoming more and more compact. In the tail position of 20 mm away from the flow front, as shown in Figure 6(6), the deformed cells in the skin region almost disappear, a compact solid-like skin layer is formed. The formation mechanism of this kind of unfoamed skin layer will be analyzed in the following section. Through analyzing the morphologies of the unfoamed skin layers in microcellular injection-molded parts with different shot sizes, we can find that the structural morphology is different at different locations of a same microcellular injection- molded part. Generally speaking, the unfoamed skin layer in microcellular injection molded part has two kinds of morphologies: one is that the unfoamed skin layer and foamed core region have a transition region and without obvious boundaries between them, the other is that the unfoamed skin layer and foamed core region have no transition region and with obvious boundaries between them. Obviously, the formation process and formation mechanism of these two kinds of unfoamed skin layers are different and also complex, it cannot be easily explained by just the cooling solidification behavior of the polymer-gas solution. In our previous studies, we have found that there are two cell forming processes, ‘‘foam during filling’’ process and ‘‘foam after filling’’ process, in microcellular injection-molding process, and the melt pressure in the filling stage is the dominant factor affecting the cell forming processes. In filling stage, the melt near the flow front with a pressure lower than a critical pressure value will foam during filling. But the melt near the gate with a pressure higher than the critical pressure value will not foam during filling, and it foams in cooling stage after filling stage. 27 These two kinds of cell forming processes in microcellular injection molding are shown in Figure 7. Also, we know that the molten polymer melt is a typical non-Newtonian fluid and renders typical viscoelastic behavior. In the filling stage, the polymer melt filling flow is a type of laminar shear flow and exhibits fountain flow at the flow front. Figure 8 schematically gives the melt filling flow behaviors in the filling stage. From this figure, we can find the melt flow velocity is smaller at the mold wall but higher at the center of the mold cavity, which results in a large shear rate gradient. Figure 8 also shows the fountain flow effect at the flow front. This effect forces the melt at the flow front lay down a skin layer towards the two mold cavity surfaces and the melt at the center of flow front flows forward and fills the mold cavity. Meanwhile, for most of the injection molding processes, the temperature of mold is much lower than that of the polymer melt, so when the skin melt contact with the mold, it will cool down quickly and form a thin frozen layer. From the above analyses, it is known that in microcellular injection molding process, the melt with pressure lower than the critical pressure foams during the filling stage. And the region of this foamed melt during filling stage is from the flow front to the position where the critical pressure lies. Opportunely, this region just corresponds to the place where the laminar shear flow behavior of the polymer melt and fountain flow behavior of the flow front are all relatively severe. So the melt filling flow behaviors will have a significant effect on the structure of cells formed in the foaming process during filling. The bubbles or cells formed in ‘‘foam during filling’’ process will be deformed by the shear stress of the laminar flow field. As the shear rate gradient shown in Figure 8, the bubbles in the center layer will have a near-spherical shape due to a zero shear rate at this region. But since the shear rate becomes large gradually from the center to the cavity wall, the bubbles’ shape will be deformed more and more severely from a near sphere to a spindly ellipsoid. 28 At the same time, the filling flow behavior of the melt at the flow front is not laminar flow but a fountain flow behavior. The deformed bubbles will be continu- ously pushed from the center layer out to the skin layer and finally to the surface. As the flow front moves forward, the bubbles’ shape will be deformed more and more severely, changed into long and thin strips or eventually broken down into a series of smaller bubbles under the large shear rate and extrusion forces. For the polymer melt with a pressure higher than the critical pressure value, it will not foam during filling, so the melt filling flow behaviors have a small effect on it. Because the melt filling flow behaviors have different effects on the two cell-forming processes, here we also divide the formation of the unfoamed skin layer in microcellular injection molding into two processes: ‘‘during filling’’ and ‘‘after filling’’. Figure 9 gives the two forming processes of the unfoamed skin layer ‘‘during filling’’ and ‘‘after filling’’. In ‘‘during filling’’ process, as shown in Figure 9(a), the melt near the flow front generates ‘‘foam during filling’’ process, the laminar shear flow of the melt causes the formed cells deforming and the fountain flow at the flow front push the deformed cells out to the cavity walls. Once the deformed cells in frozen layer region contact the cold mold walls, they will be frozen quickly and retained in the frozen layer. While for the deformed cells outside the frozen layer region, as the flow front move forward, the cells are deformed further into thinner and longer cells by the shear flow of the melt. Meanwhile, the pressure of the melt at this position increases gradually and the contact surfaces between the cells and polymer melt increase too due to further deformation of the cells. Under a certain pressure, these severely deformed cells will re-dissolve into the polymer melt to form a solid-like compact skin layer. This is the formation process and formation mechanism of the unfoamed skin layer in ‘‘during filling’’ process. In ‘‘after filling’’ process, as shown in Figure 9(b), the melt near the gate begins to foam. At this moment, the melt is beginning to cool and solidify, and the temperature of the melt near the mold walls decreases quickly. Therefore, only the melt in the core region has a relatively high temperature and can foam. The melt near the mold cavity walls has a relatively low temperature and will not foam, and finally form an unfoamed skin layer. This is the formation process and formation mechanism of the unfoamed skin layer in ‘‘after filling’’ process. Once the unfoamed skin layer is formed in ‘‘during filling’’ process, its viscosity is much different from that of the cell and polymer mixture which continue to move through the centre. This difference in viscosity makes their flow velocities have obvious difference and form a clear boundary between them. For the unfoamed skin layer formed in ‘‘after filling’’ process, it is formed in cooling stage. Since the transition of temperature field on the cross section is relatively smooth, the boundary between the formed skin layer and the foamed core region is not observed obviously. The closer to the core region the melt is, the higher the temperature of the melt is, the smaller the foaming resistance is and finally the bigger the cell size and the cell density are. On the contrary, the closer to the skin layer the melt is, the lower the temperature of the melt is, the bigger the foaming resistance is and finally the smaller the cell size and the cell density are. Furthermore, if the temperature of the unfoamed skin layer formed in ‘‘during filling’’ process is relatively high enough, it may generate the second foaming process, and a small number of cells are formed in the skin layer near the foamed core region. But these newly formed cells are small both in cell size and cell density due to the overall low temperature in this moment. According to the above analysis on the formation process and formation mechanism of the unfoamed skin layer, it is known that the unfoamed skin layer in microcellular injection-molded part consists of two regions, the outer region is a thin frozen layer that has deformed and broken cells inside, and the inner region is a relatively compact solid-like layer that has no visible cells inside. Figure 10 shows the SEM pictures of the unfoamed skin layer in microcellular injection-molded part and the amplified views of the frozen layer. It can be seen that the structural characteristics of the unfoamed skin layer is not identical, the outer frozen layer region is relatively loose. From the amplified views of this thin loose layer, it can be seen that there are a number of deformed cells. When the melt contacts the cold mold walls in filling stage, it is frozen quickly, the deformed cells are solidified and their morphologies remained directly. Therefore, this loose layer is the frozen layer formed very quickly in melt filling process. Besides, from the amplified views of the frozen layer, it can also be seen that there are rough and clutter cells on the surface of the microcellular injection-molded part. This is because in the filling stage, the cells on the flow front were broken and pushed out by the fountain flow behavior. When they contact with the cold mold wall, they freeze quickly, the gas has no time to escape and retains on the part surface. This is also the primary reason that the flow marks usually exist on the surface of microcellular injection-molded parts, as shown in Figure 11. Since the inner region of the unfoamed skin layer in microcellular injection- molded part has no visible cells, it looks like the solid part molded by the conventional injection molding. However, they are different in nature. The essential difference is that there is dissolved gas in the unfoamed skin layer. Figure ...