Comparative study of crystallization and lamellae orientation of isotactic polypropylene by rapid heat cycle molding and conventional injection molding

2.1 RHCM part preparation

To study the morphology of CIM and RHCM parts, a semi-crystalline iPP (T30S, Zhenhai branch of Sinopec Corp., Ningbo, China) was used in this study, of which the morphology and crystallinity and its spatial variations could be strongly influenced by the molding process. Performance parameters of this iPP were as follows: melt flow index (MFR) of 2.5 g/10 min, melting point of 167°C, density of 0.91 g/cm³ and an isotactic index greater than 94%.

Electric heating rods were employed to heat one side of the RHCM mold, which was discussed in detail in an earlier study (33). The mold temperature was measured with a K-type thermocouple (Beijing CHN-TOP Machinery Group Co., Ltd., Beijing, China), and the heating temperature was displayed and regulated by an MTS-32II controller (Beijing CHN-TOP Machinery Group Co., Ltd., Beijing, China). Electrical heating rods and cooling tunnels were deployed at the stationary (fixed) mold half, which could be rapidly heated by electric heating before being filled and cooled by circulating water after filling. Meanwhile, the moving mold half only had regular cooling tunnels. The asymmetric molding surface temperature was intended to introduce different degrees of crystallization and lamellae orientation in the RHCM molded iPP.

An RHCM part was produced under the following process conditions: heating mold surface temperature of 120°C, melt temperature of 220°C, injection pressure of 90 MPa, packing pressure of 50 MPa, cooling time of 30 s, and coolant temperature of 25°C. An CIM part was produced under the same condition except the absence of the heating mold surface temperature.

2.2 Specimen preparation

In order to investigate the variation of crystallization and lamellae orientation in the multilayered structure of the RHCM and CIM parts, specimens were sliced as shown in Figure 1 based on the traces of flow front advancements, and were then investigated by SEM and WAXD.

Figure 1:

Schematic of part and locations of tested specimens.

2.3 Microstructure investigation

SEM was employed to observe and estimate the multilayered structure and morphology of the cross sections of the samples. The sliced splines were first quenched in liquid nitrogen, dried after 15 h of etching with xylene, and then surface-coated with gold. The splines were then observed with a HITACHIS-4700 field emission scanning electron microscope (Hitachi Limited, Tokyo, Japan) with an acceleration voltage of 15.0 kV.

WAXD was employed to determine the morphology, crystallinity, and orientation variation on the cross section at five sites along the each flow direction (cf. Figure 1). Specimens were polished layer by layer using metallographic sandpaper with particle sizes of 300 #, 600 #, and 1500 # on a grinding and polishing machine. WAXD was carried out on an X′ Pert PRO X-ray diffraction instrument (PANalytical B.V., Almelo, The Netherlands), with an X-ray source of Kα radiation from a Cu target (λ=0.154056 nm), a voltage of 40 kV, and a current of 40 mA. Its diffraction angle 2θ ranged from 10° to 40°.

In this study, the crystallinities of different layers were calculated based on a graphical method, given by (34),

where I₁₁₀, I₃₀₀, I₀₄₀, I₁₃₀, I₁₁₁ and I₁₃₁ are the diffraction intensities corresponding to each crystal phase, while I_α corresponds to the diffraction intensity of the amorphous phase.

The orientation degree f of lamellae was obtained based on the relative intensity ratio of two crystal diffraction peaks in the diffraction pattern. For iPP, the orientation degree f was the ratio of I[040] and I[110] (34). The value of f was used as an index in this paper to quantitatively study the variation of lamellae orientation throughout the multilayered structure.

3.1 Multilayered structure

The multilayered structure was divided into the skin layer, shear layer, and core layer depending on the morphology. In particular, the crystal was highly oriented in the skin layer, there were a large number of fibrous shish-kebab crystal structures in the shear layer, and there were spherulite crystal structures in the core layer.

The multilayered structure on the cross section of the injection molded part reflected the thermal and shear histories of the polymer. The final multilayered structure of the part was determined by the shear stress, temperature and cooling rate of the polymer during the molding process. The thickness difference of the multilayered structures, crystallization and lamellae orientation at different sampling sites or molding and between RHCM and CIM processes also reflected the variations of the temperature and shear gradient during the filling process.

The main difference between RHCM and CIM was the different temperature histories in the mold, which introduced various thermal and shear histories to the polymer. Accordingly, the multilayered structures differed from each other, which could be observed from the SEM pictures of the sampling sites L11 (Figure 2). Note that L11 was the common sampling point on the RHCM and CIM specimens.

Figure 2:

Multilayered structure at L11 in parts: (A) CIM, (B) RHCM. The heating surface of the RHCM part is on the bottom side.

Figure 2A shows the multilayered structure of the CIM part. The thicknesses of the skin layers were almost the same on both sides as expected [34]. However, the multilayered structure of the RHCM part was different from that of the CIM part. The thickness of the skin layer next to the rapidly heated stationary mold half was noticeable smaller than that of the opposite side next to the movable mold half. To more closely examine the skin layer, additional SEM analyses were conducted on the area close to the stationary half of the CIM and RHCM parts and the results are shown in Figures 3 and 4, respectively. It can be seen that the thickness of the skin layer decreased gradually along with the flow length, as shown in Figure 3. The thickness of the RHCM skin layer next to heated stationary half was less than that of CIM, as shown in Figure 4. The difference between them was marked as the region ΔS in Figure 4.

Figure 3:

Thickness variation of the multilayered structure next to the stationary half in the CIM part with an average skin layer thickness of 87 μm.

Figure 4:

Thickness variation of the multilayered structure next to the stationary half in the RHCM part with an average skin layer thickness of 30 μm.

The different multilayered structures were due to the different temperature gradients and shear variations during the filling stage resulting from the different molding processes employed. During the filling stage, the polymer melt flow was considered to be a creeping laminar (Hele-Shaw) flow. In the flow front area, the melt at the center with higher speed flowed toward the wall, forming a complex “fountain” flow.

In CIM, temperature distributions on the movable and stationary halves were almost the same, and the melt front filled the cavity in a typical “fountain flow” manner, as shown in Figure 3. However, in RHCM, temperatures on the stationary and movable halves differed from each other, which resulted in different temperature gradients between the melt close to both halves, and thus led to asymmetric “fountain flow” behavior in the forefront flow, as shown in Figure 4.

During the filling stage of CIM, the melt on the cavity surface of the flow front was rapidly cooled by the cold mold, and solidified to form a thin frozen layer between the mold surface and the melt, forming the skin layer of the molded part. The more solidification that happened during the ensuing flow thickened the solidified layer with frozen-in molecular elongation and lamellae orientation. The subsequently solidified polymer layer in the molded part constituted the shear layer with a highly oriented structure. During the packing and cooling stage, the packing pressure pushed the remaining polymer melt to continue to flow in order to compensate for the volume shrinkage due to the cooling, solidification, and crystallization. As the rates of cooling, crystallization and solidification of the remaining melt were much slower in the core region, the oriented polymer had time to greatly disorient via relaxation before completely solidifying, which introduced the lower lamellae orientation disappeared in the shear or skin layers. The temporal and spatial changes of flow, shear stress and cooling led to the variation of the morphological distribution from a highly oriented structure in the shear layer to a less-oriented structure in the core layer, as shown in Figure 3.

In RHCM, the mold is heated rapidly before injection and cooled during the cooling stage. Because only the stationary half of the RHCM mold was heated in this study, the melt next to the stationary half flowed under a higher temperature, which introduced a thinner skin layer. The two surfaces of the part were subjected to different temperature and shear fields. The different thermomechanical histories induced the thickness variation of the multilayered structure on the different sides, especially in the regions next to the mold wall, defined as ΔS of the skin layer in Figure 4.

3.2 Crystallization and lamellae orientation of layers at the same location

The aforementioned study suggested that the thicknesses of multilayered structures differ between parts molded by RHCM and CIM. The different thicknesses were caused by the variation of temperature and shear fields due to the different mold surface temperatures. The different temperature and shear fields also led to the variations in the crystallization and lamellae orientation at each layer in the multilayered structure. Hence, further investigation was conducted on the crystallization and lamellae orientation of the multilayered structure at the skin layer, shear layer and core layer at the same CIM and RHCM sampling location. WAXD was conducted on different layers of the samples sliced from the CIM and RHCM parts and the resultant X-ray diffraction patterns are shown in Figure 5.

Figure 5:

WAXD patterns at different layers of CIM and RHCM parts: (A) skin layer, (B) shear layer, and (C) core layer.

Based on Figure 5, the data of crystallization and lamellae orientation of each layer were obtained through measurements and calculations and were plotted in Figures 6 and 7. According to Figure 6, the crystallinity in all layers of sampling points of the RHCM part was higher than its CIM counterpart, and the variation trend of crystallinity in the thickness direction of the RCHM part differed from that of the CIM part. The crystallinity distribution was centro-symmetric in CIM, but not in RHCM. The variation trend from the skin layer next to the moving half to the core layer of the RHCM part was consistent with that of CIM, but the variation trend from the core layer to the opposite skin layer was very different from that of CIM. The crystallinity of the CIM part increased from the skin layer to the core layer; it was a minimum at the skin layer and increased gradually inward until it reached a maximum at the core.

Figure 6:

Crystallinity W_{c, x}(%) at different depths at sampling points in CIM and RHCM parts.

Figure 7:

Orientation degree f of different depths at the sampling points of the CIM and RHCM parts.

The crystallinity increased from the skin layer next to the moving half to the core layer continuously in the RHCM part; this trend was similar to that of CIM, as shown in Figure 6. However, the crystallinity was relatively uniform without obvious variation from the skin layer next to the heated stationary half to the core layer in the RHCM part. This relative uniformity was due to the fact that the skin layer next to the stationary half was adjacent to the heated surface. The heated mold surface improved the temperature and fluidity of the melt next to the mold surface, and reduced the gradient of temperature and stress throughout the region from the skin layer next to the stationary half to the core layer. The higher temperature of the polymer melt induced the higher crystallinity, and the lower gradient of temperature and stress of the polymer melt facilitated the more uniform distribution in crystallization.

The degree of lamellae orientation of the sampling points in the thickness direction of CIM and RHCM parts also differed, as shown in Figure 7. In the CIM part, the degree of lamellae orientation was also roughly centro-symmetric, which increased and then decreased from the skin layer to the core layer. The degree of lamellae orientation reached a maximum at the shear layer and reached a minimum at the core layer. For the RHCM part, the degree of lamellae orientation of the different layers varied slightly, and the degree of lamellae orientation of all layers were less than those in the CIM part. The maximum degree of lamellae orientation was at the skin layer next to the moving half, and it decreased slightly to a minimum at the core layer. Then it increased slowly until reaching the skin layer next to the heated stationary half, at which point the degree of lamellae orientation was less than that of the opposite skin layer. The variation of lamellae orientation was attributed to the different flow and temperature histories of the RHCM and CIM molding processes.

In CIM, the polymer melt front was quenched by the cold mold wall during filling and the melt adjacent to the cold mold wall was frozen with a high degree of elongation, which introduced oriented lamellae when the polymer crystallized. But there was not enough time for the polymer to form oriented lamellae because of the low temperature. Therefore, the degree of lamellae orientation was relatively small at the skin layer. The flow behind the forefront was laminar, and the polymer melt next to the frozen layer solidified slowly under high shear conditions during filling, which introduced the high lamellae orientation. The degree of lamellae orientation reached the larger orientation degree of 1.68 at an X depth of 625 μm. Beyond the shear layer and close to the core, the polymer melt did not solidify during the filling stage, and there was enough time to release the shear stress and relax the polymer molecular chains. Thus, the degree of lamellae orientation declined again from the shear layer to the core layer.

In RHCM, the heated stationary half kept the polymer temperature high in the mold cavity. The higher polymer temperature increased the fluidity of the polymer melt and reduced the cooling rate of the polymer during filling, which led to a relatively even distribution of temperature and less shear stress in the polymer. For orientation, temperature and stress were the most important influential factors. The stress mainly affected the formation of the lamellae orientation, while the temperature mainly influenced the relaxation of the lamellae orientation. Thus, there was less lamellae orientation in the RHCM part than in the CIM part.

In summary, the crystallization and lamellae orientation in the RHCM and CIM parts were distinctly different resulting from the different temperatures of the mold surface during molding. The higher mold surface temperature in RHCM introduced different temperature and shear histories of the polymer melt than in CIM, which resulted in different degrees of crystallization and lamellae orientation. In CIM, the orientation degrees of the skin and the shear layer were higher, while the crystallinity of the core layer was the largest. However in RHCM, the lamellae orientation and crystallization showed much less variation throughout the thickness and the various layers.

3.3 Crystallization and orientation of the skin layer at multiple sites

As mentioned previously, the crystallization and lamellae orientation were dependent on the layers in the molded parts. They were influenced greatly by the temperature and shear histories of the polymer caused by the different molding processes. The different temperature and shear histories were not only caused by the molding process, but also related to the location in the mold cavity which mainly depended on the filling direction, flow distance, and local melt front speed.

The results presented previously showed that there were more differences in crystallization and lamellae orientation in the skin layer between the RHCM and CIM parts. A previous study (33) also showed that the thicknesses of the skin layer varied the most in the molded parts. In short, the differences of the microstructure between the RHCM part and CIM part were most obvious in the skin layer. Thus, the crystallization and orientation in the skin layer at multiple sites along three different flow directions of the RHCM part will be presented in this section. The sampling sites and three flow directions are shown in Figure 1.

The crystallinity of the skin layers at the multiple sites are shown in Figure 8, where the direction from point L11 to point L55 was defined as the angle direction; the direction from point L11 to L15 was defined as the horizontal direction; and the direction from the point L11 to point L51 was defined as the vertical direction. There were distinguishable differences between both skin layers in these three different flow directions.

Figure 8:

Crystallinities at the both skin layers in three flow directions of the RHCM part.

The crystallinity of the skin layer next to the heated stationary half was greater than that of the opposite skin layer, especially next to the gate location. The higher crystallinity was because the skin layer next to the stationary half was heated before filling, and the heated mold surface thinned out the skin layer and improved the fluidity of the melt. Therefore, the temperature field and stress field gradients were reduced throughout the region from the skin layer next to the stationary half to the core layer, which provided favorable temperature and shear conditions for crystallization.

The variations of crystallinity in the corresponding skin layers were mainly consistent in the three directions, but there was an obvious distinction between the variation tendency of the crystallinity in the skin layer next to the heated stationary half and the opposite skin layer. The crystallinity in the skin layer next to the heated stationary half decreased slightly before it dropped greatly at a sampling point 34 mm away from the gate inlet. Meanwhile, the crystallinity in the skin layer next to the moving half increased continuously, reaching a maximum at X=34 mm, and then decreased continually.

The crystallinity was dependent on the combination of crystal nucleation and crystal growth, and was mainly dependent on the shear and thermal histories in the polymer (35). For the polymer next to the gate location, the stronger shear accelerated the crystal nucleation but reduced the crystal growth, while the higher temperature reduced the crystal nucleation but enhanced the crystal growth. For the polymer far from the gate location, the polymer shear and temperature deceased gradually so that the speed of crystal nucleation and growth varied correspondingly.

For the skin layer next to the stationary half, the heated mold surface reduced the temperature gap between the melt and the mold surface and induced a relatively uniform temperature distribution across the polymer. The shear history thus played a more important role in the crystallinity than the temperature. The shear decreased as the flow distance increased because of the increasing melt front area. Correspondingly, the crystal nucleation decreased and the crystallinity diminished along the flow distance.

For the skin layer next to the moving half, the polymers along the flow path went through greater differences because the mold surface was much colder than the polymer melt during filling. Next to the gate location, the flow speed was very high, so the crystal growth was restricted and the influence on the crystallinity of the shear history was greater than that of the temperature history. Hence the crystallinity was not high next to the gate location. For the polymer slightly away from the gate location, the flow speed and temperature decreased and the crystals were easier to grow, although the crystal nucleation rate continued to decrease. Hence the crystallinity reached a maximum at around 34 mm before cooling and the weaker effect of shear took effect. Thus, the crystallinity at a greater distance from the injection location would be lower due to rapid cooling and weaker nucleation.

The lamellae orientation of the skin layers in the RHCM part is shown in Figure 9. The lamellae orientation didn’t vary as much as the crystallinity along the flow distance. The degrees of lamellae orientation of the skin layer next to the stationary half varied consistently; their values increasing to a maximum at X=18 mm, and then decreasing continuously thereafter. The degrees of lamellae orientation of the skin layer next to the moving half were higher than that of the opposite skin layer in the three directions and varied from each other, but the overall variation tendency was in accordance, which decreased slightly as the flow distance increased and the shear effect decreased.

Figure 9:

Degree of lamellae orientation of skin layer in filling direction of RHCM part.

The skin layer next to the stationary half was adjacent to the heated mold surface, and the lower cooling rate and higher fluidity of the polymer lowered the gradients of the temperature field and the stress field of the polymer from the skin layer next to the stationary half to the core layer. While the skin layer next to the moving half was adjacent to the unheated mold surface, the shear stress of the polymer was always higher. Therefore, the lamellae orientation of the skin layer next to the moving half was higher than that of the opposite skin layer.