Streamoulding of PVC. Giza 41. As mentioned in section 2.1.1, PAs are critical in the foaming process of PVC [10]

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Christina Morgan
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1 Streamoulding of PVC 1 Experimental 1.1 Materials BH-04 3 grades of UPVC were used and are shown in Table 3.1. These grades were pelletized and are generally used for injection moulding in building products. Vinakon XL80241 Giza 41 Giza 41 Mod 1 (Mod 1) Giza 41 Mod 2 (Mod 2) Material Coding A B0 B1 B2 Supplier Ineos Known information Contain TiO2and Pa TPV Contain 1phr of TiO2 No PA 1 phr of PA 2 phr of PA Table 1.1: PVC used As mentioned in section 2.1.1, PAs are critical in the foaming process of PVC [10] 1.2 Summary During this injection moulding trial, 2 objectives of moulding were investigated namely; weight saving and surface appearance. Although unrecommended injection moulding machine and mould design was used to process PVC, foamed kitchen handles were produced. However, due to these unrecommended setting, staining of the kitchen handle was observed and was determined to be residue from the injection moulding machine. 1

2 Variation in weight and dimensions for both materials are small with the maximum percent variation for both PVC A and PVC Bs being at 12% for weight, 4% for thickness, 0.7% in width and 0.4% for length. The amount of water that is being injected into the process was believed to be one of the factors that will affect the surface finish. Very good surface finishing can be obtained from this process; however, further trial is required to confirm this due to insufficient material made available during the initial trials. It is noticed that samples made from PVC Bs are actually more yellowish in nature as compared to samples made from XL This is not a problem with the process but it is an inherent problem due to the difference in formulation (Ca:Zn ratio in stabiliser system) in all four PVC grades. This has been confirmed when the PVC pellets were compression moulded, from the initial colour on the heat stability chart. Based on the results obtained from heat stability charts, it would appear that PVC A has poorer heat stability than PVC Bs. However, if colour is a concern, PVC A has better long term colour (whiteness) as compared to PVC Bs (which turn yellowish very quickly). It can be related to the EDX scan performed showing that PVC A would appear to have co-stabilisers such as zeolites or hydrotalcitewhich helps to improve the initial colour of the PVC. However, PVC A turns from white to brown very rapidly which is a distinct characteristic of a stabiliser system which has a higher ratio of Zn:Ca in the stabiliser systems. At the same time, an increase in PA seems to affect the rate at which yellowing occurs in PVC. An increase in PA caused a slight increase in torque as a result of increased melt viscosity. This increase in melt viscosity cause a higher temperature to be registered on the degradation test plot due to higher shear heating in PVC sample but PA is required to ensure good fusion of material and provide enough melt strength to prevent the bubble cell from collapsing and forming a more uniform cell size distribution in the final product.[10] 2

3 In bending test, the stiffness of all foamed handles made from PVC A is lower than that of solid handles made from PVC A with increasing amount of weight reduction which was expected and in agreement with all the literature review. However, the stiffness per unit weight of the foamed sample is much lower to that of the solid sample when weight saving was about 40%. If weight saving were to be at set about 10% or less, the stiffness per unit weight should not varies too much from that of the solid handle since at 15% weight saving, the difference in stiffness per unit weight is about 20%. It would also seem that reducing the weight saving to below 10% might actually cause a higher strength in the material. Under flexural bending, most of the foamed handles made by PVC A had undergone brittle failure while solid handles made from PVC A had undergone ductile failure. This is due to the fact that foamed samples are essentially solid samples with numerous voids. However, samples made in TPV-0 and TPV-1 had undergone mixed failure mode while samples made in TPV-2 did not break but just yielded. Furthermore, it is noticed that samples made in TPV-0, TPV-1 and TPV-2 have comparable stiffness per unit weight and energy absorption per unit weight as solid PVC A handle. This could be a result of difference in formulation or just the result of thicker skin and less foaming. As no solid samples were made from PVC Bs, the increased in stiffness per unit weight due to difference in formulation could not be established. Although thicker skin is noticed in some of the moulded handles made from PVC Bs, the increase in stiffness and energy absorption as a result of the formulation could not be ruled out. By doing microscopy, it is seen rather clearly that the cells are closed cell and that a wide cell size distribution is observed. At the same time, it can also be seen that the skin of the sample is actually made up of collapsed foam although some of the bubbles can still be seen under high magnification. Having a closed cell property gives rise to better mechanical property foamed product. 3

4 EDX was performed on the tainted surface of the PVC samples and the discolouration was found to be iron oxide or more commonly known as rust (Fe2O3). This confirms that the colour found on the surface of the PVC samples were not a result of degradation but the wearing and corrosion of the machine parts in injection machine. Fluorescence microscopy was also used to double confirm the staining on the surface of the PVC handles was the result of rust being deposited onto the surface as the PVC melt abrades the rust off the surface of the mould tool. From the cell diameter (taken using Feret s diameter), an increase in PA reduces the amount of variation found in the cell diameter. DSC was performed to show the processing temperatures which each PVC handle had experienced. What was noticed that all the PVC handles has seen a temperature of at least 195oC while majority has reached 200oC. Some of the PVC handles may have undergone excessive shear heating or conduction from the Streamoulding nozzle which had been heated above 200oC and therefore, the second endotherm could not be detected and thus the % fusion was assumed to be 100%. It was also determined that all the kitchen handles made had about 95% fusion. This proves that the wrinkles found in the injection mould kitchen handle to be due to excessive foaming. Finally, it would seem that a short duration of processing at 200 oc would not affect the colour of the PVC that is being injected significantly. In addition, processing at such a high temperature is only beneficent if high fusion of PVC is desirable since a higher fusion in PVC translate to higher stiffness or strength in the product. 4

2 Work completed and implications for industrial practice for PVC For a technology to be successful, it must be able to be implemented into the industrial world.

5 2 Work completed and implications for industrial practice for PVC For a technology to be successful, it must be able to be implemented into the industrial world. As such, the industrial implication of Streamoulding will be studied. Versatility of a process is one of the major factors that is always been looked at when determining the feasibility of the process. Up to the point where this report is being written, trials with other polymers such as Polypropylene (PP), Glass reinforced (GF) Nylon, Poly(ether ether ketones), various thermoplastic elastomers and PS have been on-going or done. These polymers have been said to be foam-able by using Streamoulding making this process very versatile. Two examples that have been performed by the R&D factory will be highlighted here. Firstly, a 35% GF-Nylon 6,6 (Ultramid A3WG7) has been moulded using this process and it is shown in Figure 5.1. It can be seen that although surface is slightly blemished, the product has been formed without any problem although mechanical properties were not tested to the author s knowledge. As this sample was made prior to the injection trial, the surface appearance of this material could be further enhanced. Figure 2.1: Samples made from 33% GF-Nylon 6,6 5

Secondly, food containers (made from PP) have also been made from Streamoulding and were found to be successful. Figure 5.2 shows food tubs made from the process and Figure 5.

6 Secondly, food containers (made from PP) have also been made from Streamoulding and were found to be successful. Figure 5.2 shows food tubs made from the process and Figure 5.3 shows the cross section of the thin wall using a handheld microscope where foaming can seen clearly. Figure 2.3: Cross-section of tub under handheld microscope where wall thickness is 0.7mm Figure 2.2: Tubs made from streamoulding top: Solid (clear) middle: foamed bottom: foamed with colour masterbatch From these two examples, we can see that Streamoulding has the versatility to make products of both thick and thin and various polymers. However, in both cases, the mechanical property of the samples is unclear. At the same time, because the Streamoulding nozzle can be retrofitted; a large capital is not required to buy a new machine to suit this process. The fitting of the Streamoulding nozzle can be performed very quickly thus minimizing downtime of machine during manufacturing should the company decide to take up this process.[12] Even the amount of water that is being injected into the process is very minute (from the author s understanding, the R&D factory is planning to install a small reservoir in the control plinth to reduce the need of finding a water source). 6

7 Given that the diffuser used in the process can be changed, the process itself can be changed to accommodate the different types of polymers used. E.g. In the case of PVC, a larger diffuser diameter can be used to reduce shear heating while for a smaller diameter diffuser could be used for high melt flow material. The changes made to the diffuser diameter could also be performed to reduce the pressure drop across the injection moulding to ensure complete filling. Given that a weight reduction can be found in this process, a cost reduction should also be present. However, as the actual cost of the polymer is unknown, an actual cost reduction could not be calculated. A direct estimation would be the cost reduction would be proportional to the weight reduction. However, the cost per product is determined by various parameter and it is unfair to just take the weight reduction to be directly related to the cost saving. This effect was not investigated as insufficient information was obtained. However, it would be not be incorrect to say that a reduction in cost per product should be observed since a reduction in viscosity will result in a reduction in energy consumption as lower pressure is required. As mentioned previously in section 4.5, it was noticed that no foaming was observed at areas near the injection gate. This could be useful where the product could be design to allow foaming to be limited to one area while the rest of the product remain solid to achieve desired property. Moreover, processing conditions could be varied to obtain the desired foaming and density profile. The improvements that could be made to the material and process can be found in section 7. 3 Conclusions Different formulation of PVCs was used in this work to assess the feasibility of foaming PVC using Streamoulding. Weight and dimension variation obtained from the kitchen handles are also considerably low which proves that this process is a viable process to allow weight saving while minimising variation between each cycle. The resulting kitchen handles were tested in bending to determine its stiffness and the foam cell structures were investigated. It was noticed that foaming is possible 7

8 with all the PVC grades that was used. Weight saving of up to 20% is feasible but not recommended as the stiffness per unit weight dropped significantly (up to 40%). Recommended weight saving should be capped at 10% to ensure that stiffness of the product does not differ too much from that of the solid product. Although the temperature experienced by the PVC during process has reached above 195oC, no degradation was found on the moulded kitchen handle. At the same time, as a result of high temperature, the injected PVC samples have very high fusion (>95% fusion) which is desired if high strength or stiffness is required. Next, it has been found that the staining found on the skin of the PVC kitchen handle was the result of rust from the injection machine being abraded and deposited on the surface using fluorescence microscopy and EDX and not due to degradation of PVC. However, a wide distribution of cell size was found in the sample using OM which is undesirable if one wish for good mechanical property. Although, up to date, getting a single cell size distribution throughout a product has yet to be achieved. The cell structures found in the process were closed cell which gives better mechanical property and thermal insulation. The cell sizes distribution that was obtained could be modified with various processing condition to obtained desirable cell size distribution and foaming. From all the results obtained, it is possible to foam PVC with Streamoulding while obtaining weight saving, good surface finish and mechanical property. Although the injection machine was not made specifically for PVC, PVC degradation has been avoided and good product has been obtained. Therefore, if proper equipment that has been used to produce PVC products, the result should be better than what is being achieved in this study. Finally, a low water dosage should be used to obtain good surface finish of the product. Low water dosage could also produce less foaming and thus a thicker skin which imparts better mechanical properties to the final product. 8

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