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Morphology development in the gate region of microinjection-molded
thermoplastics
Chu, Jing-Song; Kamal, Musa R.; Derdouri, Abdessalem; Hrymak, Andrew
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Morphology Development in the Gate Region of
Microinjection-Molded Thermoplastics
Jing-Song Chu,1,2Musa R. Kamal,1 Abdessalem Derdouri,3Andy Hrymak4 1
Department of Chemical Engineering and CREPEC, McGill University, Montreal, Quebec, Canada 2
Micromolding Solutions, Boucherville, Quebec, Canada 3
Industrial Materials Institute, National Research Council, Boucherville, Quebec, Canada 4
Department of Chemical Engineering, McMaster University, Hamilton, Ontario, Canada
The melt experiences extremely high shear rates as it travels through the very small gate, in the microinjection molding process. The combination of the high shear rates, extensive viscous heating, and the large thermal gradients has a profound influence on the characteris-tics of the moldings. However, many of these interac-tions are not clearly understood. In this study, the morphology of moldings in the gate region was observed using a polarized light microscope. Moreover, the specimens from different regions of micromoldings were analyzed using a differential scanning calorimeter (DSC). Two materials were selected for the study: poly-oxymethylene (POM) and high-density polyethylene. Three special morphological features were observed in the gate region for POM but not for polyethylene. The results obtained using the DSC were explained in light of the microstructural features observed using polarized light microscopy. POLYM. ENG. SCI., 00:000–000, 2011.
ª2011 Society of Plastics Engineers
INTRODUCTION
In recent years, the demand for micro- and nanoscale products has made it necessary to develop tools and proc-esses for manufacturing such products. Many plastics microproducts find applications in the biomedical and microelectromechanical systems (MEMS) fields. Microin-jection molding is a new process that attempts to produce repetitively elaborate microproducts in large volumes with high precision and economically. Since at least one of the dimensions of microproducts or devices is in the micro- or even nanoscale, the micro- or nanostructure of the material plays a critical role in its performance and processing
behavior. Since micro- and nanoprocessing technologies and manufacturing systems are new, a substantial amount of scientific and engineering background is required before optimum and effective materials, equipment, and process-ing systems can be developed. Thus, in earlier works [1–6], we have reported results of investigations relating to the characteristics of the microinjection molding process and microinjection-molded products, including their morphol-ogy and mechanical and thermal properties. This work represents a preliminary effort to understand effects of gate dimension and geometry on phenomena that occur during the process and on the morphology of some thermoplastic microinjection-molded products.
The gate serves as the link between the part and the runner system. It facilitates the removal of material solidi-fied in the runners and minimizes outflow, when pressure is released in conventional injection molding. The gates are also designed to control the volume and direction of molten plastic flow in the cavity and to raise polymer temperature by viscous dissipation so that flow marks and weld lines are reduced [7, 8]. However, the effect of shear thinning that occurs in the gate region on flow in the cav-ity is not understood completely, even for conventional injection molding. This is probably because the melt is exposed to high shear rates in the gate for only a very brief period of time, and only the portion of the melt close to the wall experiences the high rates [9]. The importance of the shape and size of the gate should be more pronounced in micromolding because of the extremely high shear rate involved and the very small volume and thickness of microcomponents. Under these circumstances, the oriented polymer chains may not have sufficient time to relax before solidification.
In conventional injection molding, many reports deal with the gate location and number of gates, because they can affect numerous mechanical, dimensional, and
Correspondence to: Prof. Musa R. Kamal; e-mail: musa.kamal@mcgill.ca DOI 10.1002/pen.22143
Published online in Wiley Online Library (wileyonlinelibrary.com).
V
cosmetic qualities of the part. The location and number of gates are closely related to the process parameters, such as pressure to fill, clamp tonnage, and the ability to fill at a desired rate [8, 10, 11]. A few studies have considered the effect of gate geometry on the process and the result-ing microstructure and physical properties [12, 13]. None of these studies deals with the morphology in the gate region and its effects on the subsequent cavity flow.
Boldizar et al. [12] studied three gate geometries with increasing gate cross sections from gate I to gate III. The modulus increased significantly from 4 GPa for gate I to 10 GPa for gate III. The higher stiffness values were attributed to the longer gate sealing time associated with the gate geometry. The higher modulus was probably caused by the greater material movement, as a result of the longer sealing times under high pressure. The greater material movement might have resulted in a larger fraction of crystalline or noncrystalline polyethylene structures to contribute to the mechanical properties. The longer sealing times also counteract relaxation of the interconnecting structural elements, such as tie molecules, during the cooling period of the molding cycle [12].
Pantani et al. [13] studied the effect of both gate and cavity geometries on gate solidification time. Three cavity lengths were used, each in combination with two cavity thicknesses. Several gates were used in the studies, having the same width as the cavity and a length of 6 mm. The thicknesses of the gates ranged from 0.5 to 3 mm. Gate freeze-off time was evaluated by monitoring the weight of the moldings, on increasing holding time. Gate thick-ness was found to be the most important factor determin-ing gate sealdetermin-ing time. An increase in gate thickness resulted in an increase in gate freeze-off time. However, although the effect was strong when the gate was much thinner than the cavity, it vanished when the gate thick-ness approached the cavity thickthick-ness. The cavity geome-try was also quite important. Both cavity thickness and cavity length were positive factors in affecting the gate solidification times.
The above studies imply that the gate thickness has a significant effect on the packing period of the injection molding process. The packing phase is critical, because the pressure profiles inside the cavity can be tuned to obtain desired morphology and subsequently the desired properties of the products, such as Young’s modulus, re-sidual stresses, shrinkage, and thermal properties [14, 15]. In the case of microinjection molding, the effect of the gate could become more important, because of the very small dimensions of the gate and the very small volume of the microcomponents. As mentioned earlier, the portion of the melt that experiences high shear rate in the gate region does not recover rapidly, when the shear rate drops to a lower value in the cavity. Instead, it is likely to freeze quickly because of the fast cooling rate. The frozen molecular orientation or morphology determines the resulting microstructure and properties of the microcom-ponents. Therefore, it is very important to understand the
flow and thermal behavior of the polymer melt, as it passes through the gate.
In this work, a microinjection molding machine was used to obtain micromoldings of polyoxymethylene (POM) to investigate morphology development in the gate region. The morphological features were examined under a polarized light microscope (PLM), along both the trans-verse and injection (longitudinal) directions, on micro-tomed samples covering the whole gate and part of the product. The distribution of thermal characteristics along the injection direction was also evaluated using a differen-tial scanning calorimeter (DSC) and compared with the PLM observations.
EXPERIMENTAL Materials
The materials selected for the present study were: (i) poly(oxymethylene) (POM) homopolymer (Delrin 900P; MFR: 11 g/min; specific gravity: 1.42 g/cm3; Dupont) and (ii) high-density polyethylene (HDPE) (Sclair 2714; MI: 51 g/10 min; bulk density: 0.58 g/cm3, Nova Chemical).
POM has been widely used in microinjection molding studies, because of its processing characteristics, such as low viscosity, fast molding, and good processing stability for deposit-free molding. POM is also noted for its high mechanical strength and rigidity, excellent dimensional stability, natural lubricity, fatigue endurance, high resist-ance to repeated impacts, toughness at low temperature, and excellent resistance to moisture, gasoline, and many other neutral chemicals. POM Delrin 900P was dried at 808C (1768F) for 2 h before use.
HDPE is also a semicrystalline material. The main advantages of this material include its wide processing window, excellent processability for applications requiring good stiffness and toughness, and good cold temperature impact properties. The extensive knowledge available regarding the process–structure-property relationships for HDPE helps in understanding the thermomechanical his-tory experienced by the material during the process and its effect on both the development of microstructure dur-ing the process and, ultimately, on the properties of the molded products. HDPE Sclair 2714 was dried at 658C (1498F) for 2 h before use.
Machine, Mold Insert, and Gate Geometry
A Battenfeld Microsystems 50 all-electric molding machine was used for the micromolding process. The machine features a plunger injection system, which con-sists of a screw plasticizing unit, a metering unit, and an injection unit. An eccentric plate cam mechanism is used to translate circular movement of the cam to a smooth reciprocating (back and forth) motion of the plunger. The packing stage is achieved by optionally specifying the
plunger velocity profile over a distance of 1 mm at the end of the forward plunger course. The machine maxi-mum clamping force, injection speed, and injection vol-ume are 50 kN, 760 mm/s, and 1100 mm3, respectively.
The Battenfeld ‘‘Master Mold’’ concept and a square plate insert of simple geometry were used in the study, as shown in Fig. 1a. Figure 1b displays a three-dimensional (3D) view of the final molding, modeled with a CAD software. The final molding consisted of two parts at each end and a runner system, including a remaining sprue, a gap ring, two blind runners, and two branch runners con-nected with two gates. Figure 1c and d displays a side view and a planform of the gate region, respectively. The gate converges in both the thickness and width directions, from 300 to 200 lm in the thickness direction and from 1780 to 1500 lm in the width direction. The two ribs as indicated in Fig. 1b were designed to avoid deformation of the molding during ejection. They were also used as a positioning tool for mounting the samples accurately dur-ing the test of mechanical properties, which was reported in Refs. 3–5. They also helped to avoid slippage between sample and sample holders during tensile tests.
Morphology and DSC Measurements
The sample selected for morphology characterization included the gate and the 1-mm length of position 1, as shown in Fig. 2a and b. The sample was first separated from the runner and then from the rest of the POM micro-molding using a scalpel. The selected pieces were then cut longitudinally along the center XZ-plane into slices of 10-lm thickness.
The variation of thermal characteristics along the flow direction was studied for microinjection-molded POM.
The sampling positions for thermal properties were selected just after the gate, after the first rib, and in front of the second rib as illustrated in Fig. 2b.
RESULTS AND DISCUSSION Morphology in the Gate Region
Figure 3 displays the morphological characteristics in the gate region for POM Case 4. Three features can be observed. The first feature is the consecutive V-shape bright yellow pattern, which has not been reported previ-ously in the literature. The first stroke of the nonsymmetric V-shape feature, originating from the skin layer, is signifi-cantly longer than the second stroke. The V-shape pattern may have been caused by remelting and flow of the inte-rior part of the skin layer. The skin layer is nonspherulitic, having a high degree of orientation of polymer chains parallel to the injection direction. When the hot melt is forced under pressure into the cooled cavity of the mold, some clusters of entangled molecules will stick to the cold wall and crystallize, while others will flow by, thus provid-ing a mechanism for extension of connectprovid-ing groups of molecules and crystallization of a complex fibril [16]. Under conditions of high melt stress, the overgrowth of folded-chain lamellae will be planar, and the net molecular orientation will be parallel to the flow direction.
Theoretically, the skin layer will continue to grow dur-ing cavity filldur-ing. However, the growth of the crystallized skin layer will slow down the removal of heat from the melt to the cold mold wall because of its low heat con-ductivity. Eventually, the interior part of the skin layer may gain more heat from the material flowing by the crystallizing surface than the heat it loses to the mold wall, especially in areas such as the gate region, where the initial melt temperature is high and viscous dissipation is large because of the high shear rate. As a result, the in-terior part of the frozen skin layer may be remelted, and the remelted material will be dragged forward by the shear flow, which is also influenced by the fountain flow in this region [17, 18], forming the V-shape as observed in Fig. 3. The consecutive V-shape pattern may be caused
FIG. 1. (a) Image of the mold insert and a final molding; (b) 3D view of the whole molding; (c) side view of the gate region (area A); and (d) planform of the gate region.
FIG. 2. Sampling positions for (a) morphology of gate region and (b) thermal properties.
by the shaking movement of the injection plunger as dis-cussed in Ref. 1, and/or the formation of V-shapes may also have contributed to the plunger shaking.
Another possible reason for the V-shape pattern could be slip-stick flow, which was reported at the polymer melt/mold wall interface for the conventional injection molding process [19]. However, this was not supported by the results of the examination of the morphology in the gate region of short shots as shown in Fig. 4. The V-shape pattern did not appear in the morphology of the short shot, so it was most likely formed after the skin layer was developed during the filling stage. The ball-shape material in front of the main flow front was prob-ably formed by the spill of the melt from the center of the flow front because of its inertia, while the flow stopped suddenly at the end of the short shot.
The second feature is the presence of highly oriented fibrillar elements just next to the skin layer, as shown in the zoomed views for position A and position B in Fig. 3. The formation of the skin layer forces the melt to flow in an even narrower channel and increases the shear stress. While passing through the narrowing gate, the polymer melt undergoes both shear flow and extensional flow, so there are two types of molecular chains deformation. The highly oriented polymer chains do not recover instantly when the shear rate drops to a lower value in the cavity. In fact, the high shear stress in the cavity may continue to align the molecules to a higher extent. The oriented
structure in the area B is significantly more intensive than that of area A because of the stronger extensional flow in the tapered surface of the gate as it narrows down to the cavity entrance.
As a result of polymer chain deformation, such as dis-entanglement, slippage of chains over each other, and elongation and alignment, the viscosity of the polymer melt decreases significantly, exhibiting shear thinning. Under the extremely high shear rate in the microgate region, it is not clear how much reduction of viscosity actually occurs in the gate region and how much this influences the cavity flow, especially in the region near the gate.
The flow and heat transfer characteristics reflected in the above two morphological features could explain the big differences between the calculated injection pressure and the recorded experimental injection pressure, as shown in Fig. 5. The injection pressure sensor is installed behind the injection plunger [1]. The simulation was conducted using the estimated experimental percentage flow rate vs. percentage shot volume curve as the filling control profile [20]. The calculated injection pressure features two peaks during cavity filling, corresponding to changes of percent-age flow rate in the simulation. During the cavity filling stage, the calculated injection pressure varied in a range from around 10 MPa to around 100 MPa, while the experi-mental injection pressure varied in a much narrower range from around 43 to 51 MPa. The calculated highest injec-tion pressure was around twice as high as the highest measured injection pressure during the cavity filling stage. The big discrepancy could be attributed to the flow and thermal behavior of the melt, reflected by the above-mentioned morphological features, which are not consid-ered in the current commercial software developed for conventional injection molding. The other hydrodynamic characteristics of microscale and nanoscale polymer flows, such as slippage at polymer–mold, liquid–solid, and
FIG. 3. Morphology characteristics in the gate region of POM Case 4.
FIG. 4. Morphology in the gate region of a short-shot molding (magni-fication: 340).
polymer–polymer interfaces [21], may also have contrib-uted to the big difference between the experimental injec-tion pressure and calculated injecinjec-tion pressure.
Figure 6 displays the calculated extension rate distribu-tion across the thickness in the middle of the gate at 0.0090-s filling time, when the flow front reached one third of the cavity length [20]. The extension rate repre-sents the amount of elongation the polymer undergoes as it passes through a change in thickness. In injection mold-ing, the extension rate is more dependent on flow rate and geometry than the material, which may also influence the extension rate. As shown in Fig. 6, the material experien-ces an extremely high extensional rate, reaching around 20,000 s21. The distribution of the extension rate is not symmetrical because of the converging slope of the upper surface of the gate region, as shown in the zoomed view of the gate region in Fig. 6. The peak extension rate near the upper surface is significantly higher than that near the lower surface. The calculated high extension rates close to the upper surface are in good agreement with the highly oriented molecular structure observed at positions B and C in Fig. 3.
Obviously, the observed skin layer morphology and the remelt and flow morphological features could not be pre-dicted by available commercial software. The prediction of morphology development and the effects of associated processing variables under the extreme strain rates and thermal gradients in the microinjection molding process represent a major computational challenge. This is the subject of a major effort by the group at present.
The third morphological feature is the high molecular orientation at the junction between the gate and the part indicated as position C in Fig. 3. The polymer experien-ces a positive extension in the tapered surface of the gate because of the gradual reduction in thickness along the flow direction and shear flow. This is followed by a nega-tive extension (i.e., expansion) when the melt enters the thicker part of the cavity. The material at position C should exhibit higher orientation than the material in the region to its left. However, the orientation is frozen in the
molding quickly before it relaxes completely because of the large ratio of mold surface area to material at this position.
The above observations of three morphological features were supported by further evaluation of the morphological characteristics in the gate region, under different process conditions, as shown in Fig. 7. The V-shape patterns obtained under conditions of low injection speed, such as in Case 2 and Case 7, are sparser and larger, while those at high injection speed are denser and smaller. Also, the V-shapes obtained at high injection speed appear closer to the gate entrance than those obtained at low injection speed. These observations may be explained by the frozen skin layer being remelted and dragged more easily at high injection speed because of the thinner skin layer, higher shear rate, and more significant viscous dissipation, when the injection speed is set at a high level. The highly ori-ented chain structure elements observed at position B of Fig. 3 is most prominent in Case 4, for which the injec-tion speed is set to a high level, while the mold tempera-ture is set to a lower level.
Also, at the lower packing velocity in Case 4 of Fig. 7, the flow marks for the melt that is pushed into the cavity during packing stage appear to be less prominent than in the other cases, where higher packing velocity levels are employed. This implies that the packing velocity may play an important role in affecting the structure and prop-erties of the micromoldings, especially in the gate region.
The molecular orientation at position C appears to be stronger for process conditions at low mold temperature, such as in Cases 2 and 4, than for high mold temperature, such as in Cases 6 and 7. At lower mold temperature, the material freezes more quickly because of faster removal of heat from the polymer melt. As expected, the highest polymer molecular orientation at position B can be easily observed in Case 4, which has the high injection speed and low mold temperature.
FIG. 5. Calculated pressure at the injection location vs. measured injec-tion pressure.
FIG. 6. Extension rate distribution across thickness in the middle of gate.
Figure 8 displays the morphology in the gate region for microinjection-molded HDPE (Sclair 2714). The V-shaped morphological features of the skin layer observed for microinjection-molded POM do not appear for HDPE. The absence of the V-shape pattern in HDPE samples was most likely due to the observed occurrence of jetting, as the HDPE melt was injected at a high velocity through the gate, which has small cross section, into a
thicker plate cavity, without forming contact with the mold wall [22]. When jetting occurs, the material at the surface of the snake-like stream of polymer melt may undergo cooling and crystallization during jetting. Subsequently, it may be remelted and stretched by the forward or backward filling flow. As shown in Fig. 9, the cross section shows evidences of extrusion instabilities and extrudate distor-tions. The polymer chains at the irregular interface could have acted as the origin of row nucleation.
Highly oriented fibrous structures are observed in the skin layer and at the junction of gate and component. The fibrous structures are more obvious at the tapered surface of the gate and the region immediately after the gate, probably because of the superposition of the shear stress and elongational stress. The bright gold layer just next to the skin layer appears to be similar to the highly oriented chain structure elements observed for POM.
Distribution of Thermal Characteristics
The melting endotherms were determined using the DSC for specimens taken from the three positions shown in Fig. 2b. The measurements indicated that the heat of fusion of the samples near the gate was slightly higher than the sample far from the gate. However, the melting curves of the three specimens along flow direction exhibited signifi-cantly different features, as displayed in Fig. 10.
Ehrenstein et al. [23] attributed melting temperature differences to differences in lamellar thickness. High melting temperatures were associated with the thicker lamellae, while lower melting temperatures were related with the thinner lamellar thickness.
In the present study, it was found that the melting tem-perature peaks may be associated with the morphological features directly. The specimen near the gate exhibited two melting peaks, at 173.18C and 179.48C. The high melting temperature peak may be associated with the fraction of polymer with the observed three morphological features, while the lower melting temperature may be
FIG. 7. Morphology microphotographs of the gate region under differ-ent process conditions (numbers above each micrograph denote the machine setting variables of injection speed (mm/s), packing velocity (mm/s), and mold temperature (8C), respectively, magnification: 340).
FIG. 8. Morphological characteristics in the gate region of HDPE Case 6.
FIG. 9. Morphological characteristics in the gate region of HDPE Case 10.
associated with the fine spherulites as shown in Fig. 3. The high melting temperature is not observed for position 2 and position 3, while a skin layer can be clearly observed there. The specimen of position 2 (the middle position) exhibits two shoulders at 171.48C and 173.38C and a primary peak at 174.88C, which may be associated with 3D spherulites, fine spherulites, and nonspherulitic structure in the skin layer, respectively. The specimen in the far end position exhibits a shoulder at 170. 88C and a primary peak at 175.08C, which may be associated with the skin layer and the 3D spherulites, respectively, according to the morphological observation, as shown in the microphotograph of position F in Fig. 11, which was previously discussed in Fig. 13 of Ref. 2.
Table 1 provides a proposed correlation between the observed melting peaks and shoulders and the morpholog-ical zones at the corresponding positions in the specimens [24–26]. The peaks at lower temperatures (170. 88C and 171.48C) may be related to the melting of kebabs in the skin layer at position 2 and position 3, where the shishes/ fibrils are sparser than those at position 1, because of lower applied strain, and the overgrowth of the lamella is significant. The material melting at 173.18C and 173.38C is probably fine spherulites, which have lower melt tem-perature than 3D spherulites and appear significantly in the gate and middle specimens. The material melting at 174.88C and 175.08C is probably 3D spherulites, which are found in both the middle and end specimens and tend to be thermodynamically more stable than the fine spheru-lites. The high melting temperature of 179.48C may be associated with highly oriented chain structural elements
next to the skin layer, which can only be found in the gate region, as shown in the zoomed views at position A and position B in Fig. 3. The shoulder near the end of the rising flank of the melting peak is shown in the inset view of Fig. 10. The gate specimen exhibits its second peak in this region.
For the end specimen, the heat flow approaches the baseline asymptotically with increasing temperature, whereas for the middle specimen, the heat flow reaches the baseline earlier more directly. This is probably due to the relative amount of the highly oriented molecules in the specimen, since the difference appears in the same temperature range as the second peak of the gate speci-men. This could be regarded as indirect evidence for the existence of shishes in the skin layer, which has been sought by previous researchers [27]. The asymptotic pat-tern implies that the presence of the shishes in the end specimen is sparser than that in the middle specimen.
CONCLUSIONS
Three morphological features were identified in the gate region for microinjection-molded POM. These fea-tures reflect the special boundary conditions controlling microscale melt flow under high shear and high thermal gradients, which do not exist to the same extent in the conventional injection molding process. The V-shape pat-tern, observed in the gate region of microinjection-molded POM (but not with HDPE), was attributed to remelt and flow of the interior part of the skin layer. The highly ori-ented chain structure elements next to the skin layer and at the junction between the gate and part were related to the rapid solidification of the highly oriented melt ele-ments before relaxation under the prevailing high thermal
FIG. 10. DSC melting curves at different positions along flow direction.
FIG. 11. (a) Morphology of XZ-cross sections of microinjection-molded polyoxymethylene (Case 7, at a magnification of 340) and (b) morphology of XZ-cross section with tripled scale of width in Z-direction.
TABLE 1. Assignment of morphological features to DSC characteristic temperatures.
Temperature Structure Specimen position
179.448C Highly oriented chain structure elements and shishes in the skin layer
Position 1, near gate
174.88C and 175.08C 3D spherulites Position 2 and 3, middle and end specimens 173.18C and 173.38C Fine spherulites Position 1 and 2, gate and middle specimens 170.88C and 171.48C Kebabs in the skin layer and less-perfect crystals Position 2 and 3, middle and end specimens
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