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Advances in Mechanical Engineering
Volume 2013 (2013), Article ID 407964, 7 pages
http://dx.doi.org/10.1155/2013/407964
Research Article

Affecting the Ageing Behaviour of Injection-Moulded Microparts Using Variothermal Mould Tempering

Friedrich-Alexander-Universität Erlangen-Nürnberg, Institute of Polymer Technology, Am Weichselgarten 9, 91054 Erlangen-Tennenlohe, Germany

Received 23 May 2013; Revised 2 September 2013; Accepted 3 September 2013

Academic Editor: Gang Wang

Copyright © 2013 Steve Meister and Dietmar Drummer. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The fast cooling of the melt in an injection moulding process for manufacturing polymer microparts can lead to a modified inner structure, resulting in minor mechanical properties. Furthermore, the ageing can be also dependent on the process-induced properties. The results indicate that especially physical ageing processes occur in parts with unpropitious inner properties. Chemical ageing processes seem to occur independently of the process conditions in microparts. Tensile tests indicate that a process-induced favoured morphology can reduce the ageing-based change of mechanical properties.

1. Introduction

Microparts and microsystems technology is reputed as a prospective key technology with an estimated annual growth rate of about 10% [1]. The main fields of application of polymer microparts are seen in the areas of medical technology, as components of optical systems, as microgears in microfluidics, biotechnology, and electronics, or as a microelectromechanical system [2, 3]. The demands on the part quality and reproducibility are also increasing due to the increasing requirements on these microcomponents [4]. Microinjection moulding appears to be one of the most efficient processes for the large-scale production of thermoplastic polymer microparts [5, 6].

Reduced part dimensions cause an increasing cooling affecting the morphological and the resulting mechanical properties of a micropart [7, 8]. In a conventional injection moulding process, the mould surface temperature is far below the melt temperature. This leads to a high cooling velocity and results in a frozen layer close to the mould surface [9] which affects also the filling behaviour due to change in melt viscosity [10]. To counteract this effect, different strategies were developed and investigated to modify and optimize the process parameters. An increasing pressure [1113] or a high shear rate [14, 15] can favour the crystallization which is shifted to a higher temperature. Notwithstanding, the most important process parameters that are discussed to influence the part properties are the temperatures of the mould and the melt, whereas the mould temperature appears to be the key parameter [10, 1618]. In general, with increasing mould or melt temperature the emerging part morphology is favoured, affecting the resulting mechanical properties (e.g., tensile strength) of the part [1923]. In addition, the usage of thermal low conductive mould materials [20, 24, 25] or a dynamic temperature control of the cavity [2629] can influence the cooling rate of the melt.

Ageing effects of thermoplastics are differentiated in physical and chemical mechanisms. Physical ageing influences the physical structure (crystallinity, morphology, and orientations) without changing its chemical structure which can lead to cracks and fractures in the part [30, 31]. Chemical ageing affects the chemical structure of the molecular chains and is dependent amongst others on temperature, oxygen concentration, chemical structure, and the structural part properties ageing [3134]. The effects of degradation on the properties of the aged polymer are versatile. As mentioned by Schnabel [33], the degradation affects the average molecular weight and the molecular weight distribution. Investigations by Valko and Chiklis [35] showed that in the presence of nitrogen the molecular weight of nylon 66 increases due to the cross-linking effect. However, in presence of oxygen it is connected with the simultaneous chain scission due to thermooxidative ageing. The ageing effects can lead to an embrittlement of the polymer part and thus to a reduction in elongation and yield stress [3234]. Polymer parts with smaller dimensions are particularly more affected by ageing as parts with macroscopic dimensions [36].

2. Experimental

The used material was a semicrystalline polyamide 66 (PA66) Ultramid A3 K manufactured by BASF SE. The material was chosen because of its good flow properties and its relevance for the production of common microparts. Characteristic values of the material are shown in Table 1.

tab1
Table 1: Characteristics of the investigated PA66 (manufacturer's data).

For investigations on the influence of process conditions on the long-term properties a scaled tensile bar was used. The dimensions are taken from a normalized tensile bar, according to EN ISO 3167 (type A) and are downscaled up to a ratio of 1 : 8, as shown in Figure 1. Merely the shoulder lengths of the 1 : 8 scaled tensile bar are extended to assure for safe clamping during tensile testing.

fig1
Figure 1: Dimensions of the used 1 : 8 scaled tensile bar in comparison to a standardized tensile bar according to EN ISO 3167 type A (a) and picture of an injection-moulded specimen (b).

The specimen were injection-moulded using an Arburg Allrounder 370U 700–30/30 injection moulding machine, equipped with a position-controlled screw with a diameter of 15 mm. To vary the mould temperature a variothermal process was realized using a variothermal temperature control system (type: SWTS 200, Single Temperiertechnik GmbH). The system employs water as the circulating fluid and has a heating and a cooling circuit-switching device. It allows a fluid temperature up to 200°C. The master mould is maintained at a constant temperature (100°C) for the purpose of process stability, and only the temperature of cavity inserts is actively controlled. These cavity inserts were built up layer by layer from a steel powder using a rapid tooling process (LaserCusing, Concept Laser GmbH). This manufacturing process allows for a complex design of cooling channels whereby an optimized tempering of the cavity can be realized. The combination of insulation from the master mould and conformal cooling channels conduces to particularly rapid temperature changes in the cavity. The mould temperature was measured by cavity near temperature sensors.

In the investigations, mould temperatures of 100°C up to 160°C were used. After reaching the defined mould temperature, the melt is injected and the mould is cooled down. The curves of the temperature development for the different mould temperatures during injection are shown in Figure 2.

407964.fig.002
Figure 2: Mould temperature development during the variothermal injection moulding process at mould temperature 100 or 160°C.

As a consequence of an increasing mould temperature, the cooling rate of the mould after switching to the cold fluid increases too, due to the higher temperature gradient (the temperature of the cold water stays constant). While for a lower mould temperature the average temperature change is around 12 K s−1, it increases with up to 20 K s−1 for a mould temperature of 160°C.

The samples were artificial aged at a temperature of 140°C under ambient air in a hot air oven (type UT 6050 K, Heraeus Instruments). The specimens were taken out after 21 days.

The crystalline morphology was investigated on 10 μm thick cuts using polarised light microscopy. These cuts were taken out of the middle of the test specimen along the injection direction. The cuts of the aged specimen were additionally polished and investigated by incident light microscopy. For the characterization of the crystallinity infrared microscopy (Nicolet 6700 and Nicolet Continuμm, ThermoScientific) was applied. On each part, the crystallinity was measured local resolved across a 10 μm thick cut over the cross-section. As Kohan [37] has verified, the ratio of extinction of the absorbance bands at 1199 cm−1 for the crystalline part and of 1180 cm−1 for the amorphous part describes the degree of crystallinity. The ratio allows an approximate determination of the degree of crystallization by the following equation [38]: Using the infrared microscopy allows also for investigating the local concentration of carbonyl groups in the part. These functional groups arise by reason of thermo-oxidative reactions in the material and give information about the local ageing. In polyamides the thermooxidative degradation leads to radical generation resulting in carbonyl groups like ketone or aldehyde groups. These groups can react further, for example, to carboxyl acids. These reactions can result in chain scission or in cross-linking of the polymer chains [32]. For this, the solution viscosity number was investigated and performed in accordance with DIN EN ISO 307 with sulfuric acid (98%) as solvent. A cross-linking of the polymer chains leads to an increasing of the molecular weight, observable in a higher viscosity. Chain scission results in a lower molecular weight and a lower viscosity.

To determine the mechanical behaviour of the tensile bars, tensile tests according to ISO 527-1 were performed using the tensile testing machine MicroTester (Instron Deutschland GmbH). Due to the dimension of the 1 : 8 tensile bar, testing parameters have to be adjusted, as shown in Table 2. For the measurements of the elongations, a glass scale is used. The characterization of process-dependent mechanical properties, due to modified process-induced morphology, using injection-moulded micro tensile bars was validated by Meister et al. [21, 23]. Due to the influence of water on the mechanical behaviour of PA66 samples, these were conditioned in a vacuum oven at 70°C, to measure the properties in dry condition. The moisture of the PA66 parts was verified before testing by Karl Fischer titration and rendered values below 0.2 wt%.

tab2
Table 2: Tensile test parameters (1 : 1 = standard tensile bar, 1 : 8 = scaled tensile bar).

3. Results and Discussion

3.1. Morphological Structure

The morphological structure of the 1 : 8 scaled tensile bars of the PA66 parts is shown in Figure 3. The influence of the cooling conditions is visible in the polarized transmitted light microscopy of the thin cuts of the nonaged tensile bars (left). A mould temperature of 100°C during injection moulding and immediate cooling leads to a fine morphological structure due to an increasing nucleation effect as well as a fast cessation of spherulitic growth. A higher mould temperature of 160°C delays the cooling of the melt which results in a clearly observable spherulitic structure. However, with increasing of the mould temperature the size of the spherulites increases too. Because the mould temperature is still below the crystallization temperature a visible surface layer arises. Here the spherulite size remains smaller than in the core layer.

fig3
Figure 3: Morphology of the different tensile bars in dependence of process and ageing conditions ((a) polarized transmitted light microscopy on 10 μm thin cuts; (b) incident light microscopy of polished midplane cuts).

The ageing of the tensile bars can lead to a change in the morphological structure. The specimen without a considerably spherulitic structure (injection moulded at 100°C) shows a significant change. The thermal load enables a postcrystallisation process which, in combination with the sufficient high chain mobility, results in a slight growing of spherulites. In the part with higher process temperatures no change in the spherulitic structure can be observed.

A consideration of the polished cuts of the specimens shows a distinct brown coloration over the whole cross-section of the specimens. The nonaged material appears opaque to white. This discolouration is due to a change in the molecular structure of the material as a result of chemical ageing processes. However, no different extension of the discolouration is evident between the different manufactured parts.

3.2. Degree of Crystallinity

The varied cooling conditions affect not only the spherulitic structures in the part but also the degree of crystallinity, Figure 4. As expected, the degree of crystallinity correlates with the observed morphological structures. While the parts injection moulded at a mould temperature of 100°C show a low degree of crystallinity over the complete cross-section, the parts injection moulded at higher mould temperatures reveal an increasing degree of crystallinity. This correlates with the observed surface layer in the morphology structures. The degree of crystallinity in the parts injection moulded at 100°C is about 25%. With higher mould temperature a crystallinity of about 31% is noticeable, where the crystallinity increases also from the surface to the core with a difference of around 4%. Notwithstanding, the degree of crystallinity is for all specimens below a typical value of around 35–45%.

407964.fig.004
Figure 4: Degree of crystallinity of the specimens over the cross-section in dependence of process and ageing conditions.

After the artificial ageing, the specimens show an increasing degree of crystallinity resulting from a postcrystallization process due to the thermal load. Consequently, a physical ageing process occurred in the polymer. The growth of crystallinity takes place all over the part, with an increase of about 13%, whereas with higher mould temperature the change is only 10%, thus, a faster cooling of the mould, and the resulting morphology results in a slightly higher physical ageing.

3.3. Carbonyl Groups

The ratio of the carbonyl groups (carbonyl index) enables information about local chemical ageing processes in the part. As a consequence of thermo-oxidative ageing, the concentration of carbonyl groups increases. The measured carbonyl index is shown in Figure 5. The nonaged specimens (left) possess the same low carbonyl index, independent of the process conditions. The low value can be due to a slight load during the processing of the material.

407964.fig.005
Figure 5: Ratio of the carbonyl groups of the specimens over the cross-section in dependence of process and ageing conditions.

As a consequence of the thermo-oxidative load, a chemical ageing process took place. This is evident in an increasing carbonyl ratio in the specimens. The measurements show that the carbonyl ratio is slightly higher in the surface area as in the core; consequently, the thermo-oxidative ageing occurs initiating from the surface to the core. This is due to the diffusion controlled oxygen concentration which decreases to the core. However, the difference of the carbonyl ratio between surface and core is less significant. So the chemical ageing occurs nearly over the complete cross-section, which is well observable in the part injection moulded at lower temperatures. The higher degree of crystallinity of the parts injection moulded at higher mould temperatures inhibits oxygen diffusion and is additionally more resistant against ageing. However, the difference has a minor significance.

3.4. Solution Viscosity

The artificial ageing of the micro tensile bars leads to a decrease of viscosity number which is due to a thermo-oxidative degradation and an occurring chain scission in the material, Figure 6. The decrease of the viscosity number shows only a slight dependence from the process conditions. Both a low and a high mould temperature (with a favoured morphology) show a decrease over 60%. Nevertheless, the specimens injection moulded at a higher mould temperature have slight higher solution viscosity. While the parts injection moulded at the lower mould temperature show an average solution viscosity of 50 mL g−1, the parts injection moulded at the higher mould temperature have 52 mL g−1. This can be evidence for the influence of the processing on the ageing behaviour of polymers in microparts. In addition, these results go along with the infrared spectroscopic measurements and the analyzed carbonyl ratio.

407964.fig.006
Figure 6: Solution viscosity of the nonaged and aged specimens in dependence of process conditions.
3.5. Mechanical Properties

As a consequence of the different cooling conditions and the affected inner properties, the resulting mechanical properties are affected too. Figure 7 shows the resulting mechanical properties of the different manufactured specimens before and after artificial ageing. The homogeneous morphology and the higher degree of crystallinity support stiffness and strength of the at higher mould temperatures injection-moulded parts. For both, Young’s modulus and the tensile strength an increasing value is observed. An effect on the strain at break cannot be found due to the high standard deviation.

fig7
Figure 7: Young’s modulus (a), tensile strength (b), and strain at break (c) of the nonaged and aged tensile bars in dependence of the mould temperature at melt injection.

The artificial ageing leads to physical and chemical ageing effects, as shown above. As a consequence, the mechanical properties are affected too. Due to the postcrystallization of the specimens Young’s modulus increases in all specimens. Because of the higher postcrystallization of the specimens with the process-induced lower degree of crystallinity, they have accordingly the highest increase in Young’s modulus. The value increases about 9% while the specimens, injection moulded at 160°C, show only 4% increase. The chemical ageing effects are reflected in the tensile strength and the strain at break. Because of the influence on the molecular chains and the resulting chain degradation the material embrittles. This effect is typical for polymer ageing and is especially observable in the significant decrease in the strain at break. All the specimens reach after artificial ageing only an elongation of at most 5%. As a consequence, the tensile strength is also influenced. The low elongation at break limits the bearable load of the material and the specimen breaks. This is observable in the decreasing tensile strength of all specimens. However, the specimens injection moulded with a lower mould temperature are more affected by the ageing. These specimens show lower tensile strength in combination with a significant increasing standard deviation. Both are typical results for occurred polymer ageing. The parts with a process-induced favoured morphology exhibit a slight lower influence on the ageing effects as the average tensile strength decreases less and the deviation is also lower.

4. Conclusion

The ageing behaviour of injection-moulded microparts in dependence of the process conditions was investigated using a variothermal injection moulding process. The results have confirmed that the mould temperature affects the inner properties and the resulting mechanical behaviour as has already been stated [7, 8, 10, 1619]. In addition, ageing of polymer microparts is also dependent on the process inducing inner structure. It has been shown that an unpropitious morphology leads to a more intense physical ageing, that is, postcrystallization of the material. The chemical ageing effects show that the slight dependence on the morphology as the carbonyl ratio increases or the viscosity decreases in the investigated specimens is the same. However, a process-induced favoured morphology can attenuate the ageing effects.

As a consequence of the affected inner properties, the resulting mechanical properties are influenced too. The stiffness increases due to the increasing degree of crystallinity, whereas the tensile strength and the elongation at break decreases as a result of chemical ageing and molecular chain degradation. The change in mechanical properties by reason of physical and chemical ageing effects is more of intense in a polymer micropart if the inner properties are unpropitious.

Further studies have to investigate the influence of time, especially the question when ageing occurs in microparts and how this is dependent on the process conditions. For example, the local resolved ageing in a part is of interest. It is also open to separate the effects of polymer chain degradation and cross-linking on the long-term part properties. As well, it has to be examined if the found process dependency exists for other polymers or other loads, for example, environmental fluids or energetic radiation.

Acknowledgments

The authors gratefully acknowledge the Bavarian Research Foundation for funding the work. They also extend their gratitude to their industrial partners Werkzeugbau Hofmann GmbH, Oechsler AG, Single Temperiertechnik GmbH, Hotec GmbH, Arburg GmbH & Co. KG, and BASF SE for providing equipment and material. They further thank Mrs. Pia Trawiel and Mr. Jürgen König for performing infrared spectroscopy measurements. The authors also acknowledge support by Deutsche Forschungsgemeinschaft and Friedrich-Alexander-Universität Erlangen-Nürnberg within the funding programme Open Access Publishing.

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