In Situ Time-Resolving Small-Angle X-ray Scattering Study of the Injection Moulding of Isotactic Polypropylene Parts ^†

Injection moulding is the most common fabrication technology used to shape plastics. In this technique, molten plastic is injected into a metallic mould under high pressure where it rapidly cools to a solid, thereby preserving the shape and it is then ejected from the mould. Industrial scale injection moulding can be automated to produce several parts per minute. This high throughput has led to its widespread use, although the time from concept to real parts is quite long as it requires a specific mould to be machined from steel or aluminium to define the shape of the product. Complex moulds such as the facia for an automobile may well cost over 1 M Euros. The properties of the part critically depend on both the polymer used and the complex processes of flow and cooling within the mould. The analysis using X-ray scattering technique after moulding reveal complex behaviour [1] which would be easier to develop and understand, if the data were obtained in real time. This would enable the different processes to be observed separately rather than superimposed as in the final product. This is the objective of the current work. For an amorphous polymer the material solidifies by cooling below the glass transition but for a semi-crystalline polymer, solidification involves crystallization which is strongly affected by the flow and cooling processes within the mould. Some researchers have set out to understand the influence of flow and cooling patterns on crystallization but simple flow devices including those developed by one of the authors [2] and others are not able to replicate the high shear rates or the high rates of cooling experienced during injection moulding. To develop an understanding of these processes we have set out to design a realistic replica of an industrial injection moulding system, but which would fit on the ALBA NCD-SWEET beam line to allow us to use in situ time-resolved small-angle X-ray scattering techniques to follow the development of structure and morphology of the polymer following the injection stage. There are many challenges in preparing a successful design. Foremost is the restricted space available on the beam line and the need to protect sensitive parts of the beamline equipment from the high temperatures of the mould during the cycle. A second restriction is the weight of the system. The sample translation stage of the NCD-SWEET beamline has a maximum load of 100 kg.

In order to maintain the equivalence of the mould to industrial practice we used industry standard mould frames and inserts made from an aluminium alloy AW6082. To provide relatively non-absorbing windows, the inserts were milled to provide a series of six 1.5 mm diameter blind holes, with window thickness of 0.08 mm. The inserts were mounted in the mould frames and aligned to provide an X-ray path through the mould. The design of the part for the mould is relatively simple (Figure 1). It is a rectangular sheet 50 × 21 mm with a thickness of 1 mm. These mould elements were mounted on parallel sheets with an aperture for the X-rays to pass and were fitted with motorized screws to enable mould opening and closing. We used a commercial injection unit from BabyPlast. In this the controls and power supply are mounted in a floor-based unit and only the injection unit complete with a funnel for pellet feeding was mounted above the mould unit. The mould was maintained at constant temperature using an industrial scale closed recirculation unit from Tool Temp.

This work is focused on the moulding of polypropylene parts. The material used was A SABIC Polypropylene PP595a which has been specially developed for use in automotive compounding. It exhibits a melt flow index of 47 g/10 min.

We performed in situ time-resolving small-angle X-ray scattering measurements using the NCD-SWEET beamline at the ALBA Synchrotron Light Source in Barcelona The beamline contains facilities for measuring both SAXS and WAXS simultaneously. In these experiments the geometry did not permit the collection of WAXS data. SAXS patterns were taken in a Q-range from 0.0017 Ᾰ⁻¹ to 0.125 Ᾰ⁻¹. The SAXS Detector was a DECTRIS Pilatus3S 1 M system which is a hybrid single photon counting system. The Pilatus system is built up of a number of silicon sensors and as a consequence a small portion of the detector is not sensitive (~7%) which appear as black stripes in the recorded intensity images. This detector has a pixel size of 172 × 172 micrometres with a dynamic range of 0–1,048,576. To stop saturation of the detector by the zero angle X-ray beam, an absorbing beam stop is placed in front of the detector to absorb the transmitted beam. The sample to SAXS detector distance was 6.0 m with an incident X-ray wavelength of 1 Ᾰ. The detector orientation and sample to detector distance was calibrated using the well-known standard silver behenate.

An important aspect of the design of the mould is the capability to contain the molten plastic at the moulding pressure and temperature. Previous example of in situ X-ray scattering with injection and micro injection moulding have utilised diamonds as windows mounted in a metallic mould. We sought to produce a mould which replicates industrial practise and accordingly we used the insert material as the window, machining the insert to provide an X-ray window of 0.02 mm. One can see that the SAXS pattern for the empty mould was relatively clean and contains some scattering around the beam stop which is isotropic and arises from the precipitates in the aluminium alloy. The hardening of the aluminium alloy from which the inserts were made was studied by Rowolt et al. [3]. This scattering is relatively straightforward to extract from SAXS patterns obtained during the injection moulding cycle. The temperature cycle which the inserts experience during the injection moulding experiments does not affect the level of precipitation in the alloy which could alter this “background scattering”. The other critical factor is the level of absorption in the windows and whether this compromises the data collection cycle time or introduces an unacceptable level of noise in the data.

Injection moulding was performed by using the Babyplast controller together with a time-resolving readout of the mould pressure measure at the junction between the cold runner and the mould cavity. We performed a simple injection moulding cycle in which the mould was preheated to the operation value, typically ~50 °C. The volume of the plastic to be injected was selected together with the injection time and pressure. We held the plastic in the mould for 290 s before ejecting the part. The final version of the injection moulding system is shown in Figure 2 mounted on the ALBA NCD-SWEET Beam-line.

Small-angle X-ray scattering data were recorded on a ~1 s time cycle. Figure 3a shows a pattern recorded as the part started to crystallise. This pattern was recorded 64 s after the start of injection. The central part of the pattern is dominated by the scattering from the mould windows. The ring of scattering around the beam stop is typical for semi-crystalline polymers and arises from the contrast between the chain folded lamellar crystals and uncrystallised polymer melt. If the intensity around this ring is constant, then the lamellar crystals are randomly arranged in the irradiated volume. In Figure 3a, we can see that the scattering is more intense in the horizontal section parallel to the injection direction. This suggests that under the condition of moulding used, some of the longer chains are extended in the flow conditions and serve as row nuclei to initiate crystal growth in the perpendicular direction to the extended chains, hence the high intensity along the axis of flow (horizontal). Figure 3b shows the SAXS pattern taken at the end of the mould cycle. Now the overall intensity is greater, and the level of anisotropy is reduced through the crystallisation of isotropically distributed crystals. There is still a suggestion of higher intensity along the horizontal flow axis.

We designed and implemented an injection moulding system, based on industrial practice which can be mounted on the NCD-SWEET beamline at the ALBA Synchrotron Light Source in Barcelona. This system is able to obtain useful quantitative data on the injection moulding of isotactic polypropylene. We were able to observe directly the value of the time-resolved data in differentiating between the different stages of the development of the solid polymer morphology. The mould cavity has a volume of 5.6 cm³ and the system has a minimum charge of pellets for successful operation of ~12 g.