5. Resultados
5.1 Sección I: Caracterización Sociodemográfica
Screw extraction tests took place at an external facility at KU Leuven Technology Campus in Ostend, Belgium. Due to the significance of the test regarding this investigation, the procedure is described in detail below.
• Screw samples were collected by stopping the injection moulding process on the Arburg Allrounder 270C injection moulding machine after 10 parts were produced and the process was stable. Once the screw had been pushed back, and cooling begun, the injection unit was left in its forward position before the machine was switched off and left to cool overnight.
• Once the injection moulding machine had cooled, the injection unit was moved to its most backward position. Zone 3 (screw heater band located closest to the screw) and the nozzle were heated to 70oC to allow for easier untightening of the nozzle. The nozzle heater band was then removed along with the bolts securing the injection moulding machine to the mould (Figure 3.21 (a) and (b)).
• The safety switch (Figure 3.21 (c)) was then disconnected from the nozzle safety guard and reconnected to override any errors that may arise later in the screw extraction process when the screw is pushed out.
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• The whole injection unit was then carefully pulled back away from the mould (Figure 3.21 (d)). The corresponding heat sensor for the nozzle was then removed and the nozzle was gently unscrewed with a wrench (Figure 3.21 (e)). Ideally, a glass fibre polypropylene nozzle sample would be isolated which would then be detached. Again, silicon grease was sprayed inside a clean nozzle beforehand to help prevent the polymer from sticking inside and copper grease applied along the thread of the nozzle to facilitate screwing and unscrewing of the nozzle.
• Any excess virgin material was removed from the hopper so as not to interfere during the push out of the screw. The heating zones along the barrel are then set to 160oC. This was found to be the ideal temperature for the screw pull out test.
• Once the barrel has been reheated to the desired temperature level the safety guard on the back of the injection moulding machine is removed exposing the motor (Figure 3.21 (f)). The screw was then ‘unclipped’ from its electric motor drive (Figure 3.21 (g)) and was then pushed forward before the drive was brought back to its original position without pulling the screw back.
• This allows space for the placement of steel rods which can be positioned behind the screw (Figure 3.21 (h)) and pushed into the barrel one at a time carefully, leading to the slow push out of the screw.
• Once the screw has been pushed out sufficiently as pictured in Figure 3.21 (i), it can be simply removed from the barrel by hand.
• The screw was then placed in a large Venticell drying oven (Figure 3.21 (j)) at an optimum temperature of 170oC as shown in Figure 3.21 (k) for a short period of time to allow for successful sample removal.
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• The screw was then positioned on a wooden palate with predetermined measurements marking out the different zones along the screw. Sample extraction was then carried out along the screw by first taking a 24 mm specimen, measured along the helical direction of the screw, from the first available section of material in the feeding zone. This sample was named F1. Subsequent 24 mm samples are then isolated, cut off and put into sample bags until the next selected sample location to be evaluated for fibre length analysis is reached (F2). Samples between each marked location are recorded and placed in sample bags too. F2 marks the mid- point of the screws feeding zone apart from one screw test which consisted of a shorter feeding zone where F2 was taken to be the location of the first available section of material to be removed. F3 indicates the end of the feeding zone and samples taken from C1 are the beginning of the screws compression zone. Sample C2 is the mid-point of the compression zone with C3 being the end of the compression zone. Samples M1 and M2 are each taken from the beginning and end of the screws metering zone, respectively. An example of sample locations is shown in Figure 3.8.
• Once all samples have been documented the screw was cleaned using a blow torch and a brass brush. The screw was then returned to the injection moulding machine which was set up as it was originally ready for the next screw pull out test.
• Due to the time consuming nature of the screw extraction and the fact this investigation is centred on the screw itself, if a nozzle sample was not successfully removed (shown in Figure 3.21 (l)) there was no opportunity to repeat the test.
• A range of temperatures were tested for optimum screw pull out before 160oC was found to the most effective. Any less and the screw could not be pushed out, anymore and the polymer matrix would begin to melt exposing glass fibres on the outer surface of the polypropylene which would then be trapped back along the screw as it was pushed out, possibly
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leading to further unnecessary fibre attrition as showcased in Figure 3.21 (m).
• Different temperatures were also examined when the screw was placed in the drying oven before sample removal. A temperature of 170oC was chosen because this allowed the material to be peeled off relatively easily off the screw without it being too hot for it to stick to the screws surface or too cold, consequently leading to difficult extraction of screw samples.
• As the screw cooled down, the samples would become increasingly difficult to isolate, thus the screw was returned to the drying oven for a minimal amount of time before being returned for the continuation of sample removal.
• Screw pull out tests were originally intended to be performed on an Arburg Allrounder 320S Injection moulding machine with a 25 mm screw. However, upon screw extraction, the glass fibre reinforced material appeared to not be feeding correctly as the screw was not filled along the compression and feed zones. However, the screw was filled with material in mixing zone. This is depicted in Figure 3.21 (n).
• Numerous tests were carried out to find a solution but action was taken due to the limited time available, to concentrate and complete as many screw tests on the 20 mm screw for the duration of the research placement.
• It was originally intended for more screw tests to be performed to evaluate the scatter of results for each test, however due to the availability of the machine being off-site (Belgium), time constraints at the location as well as the time required for each measurement analysis, repeat tests were not performed.
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(a) (b) (c) (d) (e)
(f) (g) (h) (i) (j)
(k) (l) (m) (n)
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