Rapid control of mold temperature during injection molding process

Rapid control of mold temperature during injection molding process

Sara Liparotia, Tsang Min Hunagb, Andrea Sorrentinoc, Giuseppe Titomanlioa, Mukerrem Cakmakb

aDepartment of Industrial Engineering. University of Salerno Via Giovanni Paolo II, 132, 84084 Fisciano (SA), ITALY

bDepartment of Polymer Engineering, The University of Akron, Akron, OH 44325, USA cInstitute for Polymers, Composite and Biomaterials (IPCB) - CNR, P. Enrico Fermi 1, 80055 Portici (NA), ITALY

Abstract. The control of mold surface temperature is an important factor that determines surface morphology and its dimension in thickness direction. It can also affect the frozen molecular orientation and the mold surface replicability in injection molded products. In this work, thin thermally active films were used to quickly control the mold surface temperature. In particular, an active high electrical conductivity carbon black loaded polyimide composites sandwiched between two insulating thin polymeric layers was used to condition the mold surface. By controlling the heating time, it was possible to control precisely the temporal variation of the mold temperature surface during the entire cycle. The surface heating rate was about 40°C/sec and upon contact with the polymer the surface temperature decreased back to 40°C within about 5 secs; the overall cycle time increased only slightly. The effect on cross section sample morphology of samples of iPP were analyzed and discussed on the basis of the recorded temperature evolution.

Keywords: injection molding, iPP, heating surface layer. PACS: 83.50.Uv

INTRODUCTION

Injection molding is one of the most widely used process for mass production in the polymer industry to produce complex three dimensional parts, due to its relatively short molding cycle time, high automatization, high dimensions accuracy, and flexibility. Mold temperature is a key parameter in this process because it has a direct influence on the part quality and molding cycle time. Generally, the mold temperature is controlled by adjusting the rate and the temperature of the coolant that moves inside the mold channels. In literatures [1-3] is well documented how a high mold temperature has a great positive effect on the surface appearance, strength, size, and shape accuracy of the molded part, moreover, it also decreases the required injection pressure and clamping force of the molding machine [4]. However, the corresponding long cooling time, resulting from the high mold temperature, decreases the productivity and increases the costs. For this reason, it is generally preferred to set a low mold temperature in spite of the poorer qualities of the molded parts.

In recent years, high mechanical strength, close dimensional tolerances, high shape accuracy, low residual stress, good appearance, and excellent surface texture are required for plastic products. Therefore, mold surface rapid heating technologies have been proposed to obtain a fast increase of temperature, just before the filling phase followed by a fast cooling stage. These technologies could eliminate the frozen layer resulting from the low mold temperature and greatly improve the flowability of the polymer. At the same time, rapid cooling can keep the whole cycle time at an acceptable level.

Different techniques have been proposed to reach this objective, such as induction coils [2], radiation source [1] or proximity heating [5] for the local heating of mold surface. These solutions require additional design and tool cost.

Jansen and Flaman [6] developed a multilayer mold consisting of two insulation layers with a resistance layer in between to reduce the molecular orientation and residual stresses in injection molded products by rapidly heating the mold cavity surface.

In this work, a thin heater film is proposed and tested to maintain surface temperature relatively high before and during the filling stage. The device is composed by a carbon black loaded polyimide layer, that is the active layer, and by two insulating polyimide layers that are used to electrically insulate the mold from the heater.

In this work, the effect of the pulse duration that is the time interval during which the heater is kept active during the mold cycle on the morphology of the molded sample was investigated.

MATERIALS

The polypropylene (i-PP) grade adopted in this study was supplied by Montell (now Basell), and its commercial name is T30G (non-nucleated, Mw=376 000, Mw/Mn=6.7, tacticity=87.6%): it was extensively characterized as far as rheology and crystallization kinetics (7-10).

The polymer used to produce conductive layer is a poly(amide-imide) solution (PAI) (provided by Akron Polymer Systems, APS). The solvent used is Dimethylacetamide (DMAc - Sigma Aldrich, synthesis grade); the concentration of the solution is 8 wt%. The high conductive carbon black (CB), ENSACO 260 is a product of TIMCAL (Switzerland), where the particle size is below 60 nm, the apparent density is 170 g/L, the specific surface is 70 m2/g, the specific volume resistivity is 5.0 Ωcm.

EXPERIMENTAL PROCEDURES

Conductive film production

The casting solution was prepared by mixing PAI with CB (8 wt% of solid content) in a Thinky Mixer at 1000 rpm for 20 min. Optical micrograph of the casting solution, containing 30 wt% of CB, is shown is FIGURE 1.

20 22 24 26 28 30

1E-5

1E-4

1E-3

0,01

cond

uctiv

ity lo

g V

(S/c

m)

CB loading (w) (%) FIGURE 1 Micrograph of the PAI-CB (30%) solution. FIGURE 2 Plots of conductivity (log σ) versus

CB weight fraction at room temperature

FIGURE 1 shows that CB is well distributed in the polymeric solution. In the casting procedure, the polymer solution is introduced inside the reservoir of the Dr. Blade casting system.

The speed on the motorized coater can be adjusted and finally the moving bar is set in motion. As the bar moves, it pushes the Dr. Blade on the glass substrate depositing desired thickness coating (gap distance set at 0.7 mm). Coated glass substrate is then placed in a vacuum oven for 24 h, at 120°C. This casting method is found to be very stable and it provided an excellent repeatability. The electric conductivities of the polymer films were measured by four point probe technique with a Keithley 2400 source/meter (Pro4, Lucas Labs). Electrical percolation behavior for PAI loaded with a series of CB concentrations is shown in FIGURE 2

The conductive layer used for the following experiments has a CB loading of 30 wt% and a conductivity of 0.0056 S/cm. It has a smooth surface and a thickness of 50 μm.

Atomic Force Microscope (AFM) was used to investigate the surface of the conductive film, whereas, the distribution of the electrical charges was characterized using Tunneling Atomic Force Microscope (TUNA-AFM). FIGURE 3 shows the micrographs related to the produced conductive film.

FIGURE 3 AFM micrograph of the CB-PAI film (sx) height characterization; (dx) electrical characterization by TUNA-AFM.

FIGURE 3(sx) shows the surface characteristics of the CB-PAI film. The size of the embedded carbon particles defines the lower limit of the surface roughness. Tunneling AFM (FIGURE 3 (dx)) allows to detect electrical charges on the film surface; in that case, the brighter areas have a higher conductivity than the brown ones, probably those areas are related to CB particles that are present on the polymeric surface. In any case, the CB conductive particles seems uniformly distributed on the polymeric surface.

Injection molding apparatus

A 70-ton Negri-Bossi reciprocating screw, injection molding machine was used for the experiments. The polymer was injected into line gated rectangular cavities having length L=70 mm, width W=20 mm, and thicknesses S=1 mm. FIGURE 4 shows the cavity used for the injection experiments.

-2 -1 0 1 2 3 4 5

0

100

200

300

400

500

600

700

PACKING

P (b

ar)

time (sec)

P0

P1

P2

P3

FILLING

FIGURE 4 Geometry of the cavity adopted in this work FIGURE 5 Experimental pressure curves.

The molding machine and the mold were equipped with four piezoelectric transducers: one in the injection chamber (P0), one just before the gate (P1) and two in the cavity (P2 and P3), located in the non-moving part of the mold (15 and 60 mm downstream from the gate position). Moreover, two temperature sensors were located in the cavity, positions P2 and P3. The heater occupies 700 mm2, and half of the cavity length. The power supplied is 0.07 W/mm2. The injection molding experiments were carried out adopting 2.3 seconds as injection time, a melt temperature of 230°C and a mold temperature of 23°C. FIGURE 5 shows experimental pressure curves in four transducer positions for one representative experiment. The curves were translated respect to the time at which the polymer goes in contact with the transducer in pos. P2, this time correspond to zero. During the filling, pressure inside the injection chamber (P0) slowly increases in the first stage, during which the polymer fills the sprue and the runner, after that, when the polymer enters the cavity, a fast pressure increase takes place, due to the area constriction. During the packing phase, pressure is maintained at 270 bar for 2 seconds. Pressure evolution downstream the gate indicates that packing time is sufficient to have a complete solidification in position P2.

Thin slices were cut along the flow direction and parallel to the flow-thickness plane from the two samples. The slices were oriented so that the flow direction was aligned along polarizer direction; other micrographs were taken rotating the sample of 45° with respect to polarizer direction. Normally, the change of brightness during a 45° rotation is an index of the material orientation level.

ANALYSIS OF TEMPERATURE PROFILES

The main objective of this work is to increase mold surface temperature during the filling stage. Surface temperature rise and temperature pulse duration (heating time) determine the amount of the melt orientation by effect of the flow and stress relaxation during cooling. Different experiments have been performed by changing the time (tp) of the surface heating after the contact with the polymer.

-2 0 2 4 6 8 10 12 1420

40

60

80

100

120

140

160

180

200

T (°

C)

time (sec)

tp=8" (pos. P2)

tp=0,5" (pos. P2)

mold withoutheater (pos. P3)

tp=2''

(a) -2 0 2 4 6 8 10 12 14

20

40

60

80

100

120

140

160

180

200

ta=0,7'' pos. P2

T (°

C)

time (sec)

ta=1,4'' pos. P2

mold without heater pos.P3

(b) FIGURE 6 Experimental temperature profiles. Comparison between temperature profiles obtained with and without the heater, at

different heating time tp (a) or different starts heating time ta (b).

Three injection experiments are considered in FIGURE 6(a), each of them was carried out with different value (0,5; 2 and 8 seconds) of the time tp, in all cases the surface resistance was activated 2 seconds before the contact with the polymer (time ta). In this figure, it is also shown temperature profile in the unheated region (pos. P3); temperature profile in pos. P3 is shifted to bring at zero second the temperature peak and in order to compare the cooling time of all the curves.

In position P3, without any surface heating, when the hot polymer reaches the thermocouple a thermal peak of about 80°C is measured (see FIGURE 6(a) for temperature profile in pos. P3). Applying the heating layer at the mold surface (pos. P2), it is registered a first temperature increase (up to 0 secs), the temperature increase rate depends on the contact between the heater and the mold (if the heater is not closely in contact with the mold a faster temperature increase is observed). Afterwards, when the hot polymer reaches the position P2, temperature quickly increases up to about 100°C. The surface heating time tp strongly affects the cooling step. When the heating was not applied at the mold surface (see curve P3 in FIGURE 6(a)), temperature reaches 40°C after 2 secs; whereas, turning on the heater, 5.5 and 10 secs are necessary to reach 40°C if the surface heater is activated for 0.5 and 8 secs respectively. As a first approximation, the cooling time increased is not proportional to the time that the heater was activated.

FIGURE 6(b) shows temperature profiles obtained at tp=1.2 seconds and changing the time ta, that is the time at which the resistance is activated before the polymer arrives on the thermocouple positions.

Temperature profile, during the cooling phase, does not change for different times ta, because that profile is only influenced by time tp.

The heating time also influences the molded morphology, as shown in FIGURE 7. Micrographs reveal a morphology distribution typical of an injection molded semi-crystalline samples, often

referred to as skin–core morphology, characterized by the presence of a series of distinct regions: a thin, oriented skin layer (of the order of 10 micron); an oriented non-spherulitic zone (the ‘shear layer’, the dark zone); a spherulitic core with essentially no preferred orientation. In a previous work [4], results on the same iPP grade showed that at low mold temperatures (25°C), the layer closest to the mold surface has a lower orientation than the internal layers as it is formed due primarily biaxial deformation at the end of flow front due to fountain effect that is rapidly cooled upon contact with cold surface. Vice versa, at higher mold temperatures, which, in our case, is represented by the activation of the heater, the orientation of the layer closest to the mold surface becomes higher, due to the shear effect. Furthermore, the shear layer thickness significantly decreases as the time tp increases (see FIGURE 7(a) and (b)). However, at high time tp a significant increase of cooling time takes place.

FIGURE 7(c) and (d) shows the micrographs related to the samples produced at tp=1.2 seconds and at different times ta. The samples always show the skin-core morphology, however, it is not evident a significant effect of time ta on the thickness of the shear layer.

FIGURE 7 Micrographs in polarized optical light of samples obtained at different times tp (a) tp=0.5 secs (b) tp=8 secs; and

different time ta (c) ta=1.4 secs (d) ta=0.8 secs

CONCLUSIONS

Injection molding tests were performed by changing the mold temperature on one side. The results clearly show that the time at which the mold surface is kept heated, after the contact with the polymer, strongly influences the thickness of the oriented region. In particular, the shear layer thickness was found to decrease with the contact time on the heated side and to increase about of the same amount on the other side, where the heater is not present.

Furthermore, the morphology on the sample skin is strongly influenced by the heating time, also setting a low contact time on the heated side a strong reduction of the oriented layer is observed. The time at which the heater is activated before the contact with the polymer has only a limited influence on the sample morphology.

REFERENCES

1. S.C. Chen, Y. Chang, Y.P. Chang, Y.C. Chen, and C.Y. Tseng, International Communications in Heat and Mass Transfer, 36, 1030-1035 (2009).

2. S.C. Chen, W.R. Jong, and J.A. Chang, Journal of Applied Polymer Science, 101, 1174-1180 (2006). 3. G. Zhao, G. Wang, Y. Guan, and H. Li, Polymers for Advanced Technologies, 22, 476-487 (2011). 4. R. Pantani, I. Coccorullo, V. Speranza, and G. Titomanlio, Progress in Polymer Science, 30, 1185-1222

(2005). 5. D.G. Yao, T.E. Kimerling, and B. Kim, Polymer Engineering and Science, 46, 938-945 (2006). 6. K.M.B. Jansen and A.A.M. Flaman, Polymer Engineering & Science, 34, 894-897 (1994).