Introduction To Injection Moulding Process: Steps, Parameters And Shrinkage

Injection Moulding Process

Question:

Discuss about the Injection Moulding Polymers for Thermal Analysis.

The process of injection moulding is a famous method of manufacturing for various reasons. This process is mainly used for producing different parts by injecting the molten material within a mould. This process could be performed by using a host of different materials that would mainly include metals, elastomers, confections, glasses and the most widely used thermosetting and thermoplastic polymers (Dang 2014).

The process of injection moulding comprises of four major steps. The first step involves the basic preparation and gathering of the tools that would be used for the injection process. During this stage, the thermoplastic materials are being melted and hence the thermoset materials are mixed. The next step includes the filling, which involves the injecting of the molten polymer in the cavity of the mould. The third step includes the holding, which involves compacting the polymer in the mould with the maintenance of the pressure. The final step is ejection of the thermoplastic material and cooling them. In this process, the polymer is left within the mould until it gets solidified (Madan et al. 2015).

During the process of ejection and cooling, a screw would be withdrawn in order to refill the barrel with the polymer. The most important stages in this process are filling and holding as they have a major impact upon the final product. The filling step has an influence on the orientation of the polymer within the cavity. The settings of the pressure, time of cycle and the temperature have a significant effect on the mechanical properties.

The effect of pressure also plays a major role within the process control. The maintenance of equipment in order to provide a required pressure to the water is also necessary. An optimum amount of temperature is also required within the process. This temperature should be set in such a way that the materials should melt in a systematic way. The lines of cooling should also be placed in such a manner that they are spaced and set for the maximum removal of heat. The cycle time should also be sufficient enough for each step of processing in order for the arrival of the machines at an equilibrium position and then adjust themselves.

The Melt Flow Index (MFI) is a measurement related to the easiness of the flow of the melt that is in relation with a thermoplastic polymer. The MFI could also be defined as the total mass of a polymer. It could be measured in grams. The melt could flow through a capillary tube having a specific length and diameter by a certain amount of pressure (Sun et al. 2014).

The Four Major Steps

The producers of polymers and the manufacturers of the polymer products would use the results in order to ensure that a material would be able to perform as per intended. The MFI based on a polymer is an extremely useful information in order to estimate the behavior of a material during the process of injection moulding. Polymers that possess longer chains or have greater branches would take a longer time to pass through the capillary. The mechanical properties of the product would be affected and as a result of this, the MFI would increase. Different imbalances within the process would have an adverse effect within the mechanical properties and the results of the melt flow (Abbas-Abadi 2013).

Low MFI based polymeric materials are not used in the technique of injection moulding. Instead, low viscous materials are used. The molecular is directly proportional to the viscosity and is inversely proportional to MFI. The fact depicts that polymers that have a high molecular weight would be mainly used in the process of extrusion moulding as compared to the process of injection moulding.

The different parameters for the processing for the process of injection moulding would provide good tensile strength products with the study based on polypropylene. The primary parts of the process of manufacturing of the different parts of polypropylene are intrusion moulding and extrusion (Fei, Mehat and Kamaruddin 2013).

The mechanical properties of polymers could be specified with many of the similar properties, which are mainly used for metals as modulus of electricity, tensile and many others. The tensile strength could be defines as the point of fracture and it could be lowered than the yield strength. The input parameters for injection moulding mainly includes melt temperature, mold temperature, cooling time, packing pressure, injection speed and packing time.

Based on various studies that were conducted, it was seen that the behavior of polycarbonate was directly linked with the various processing parameters of injection moulding. The study also showed that the tensile stress would increase with the temperature of the melt and the temperature of the mold. This would help the polymer in order to set a higher orientation of the molecules and thus have lower residual stress. With the increase in the temperature of the mould, the rate of cooling also lowers. The pressure of packing and the speed of injection are not a significant factor for the strength of the polymers (Azaman et al. 2013).

Input Parameters

The process of shrinkage is essential within the process of injection moulding. This phenomenon occurs as the density of the polymer normally varies from the temperature of processing to the ambient temperature. During the process of injection moulding, the variations in the shrinkage would be able to create internal stresses. In the process of injection moulding, it would be possible to acquire a molded product that would have the desired dimensions with the use of the phenomenon of mould shrinkage. The mould shrinkage is the phenomenon in which the volume of the molten plastic would be filled within the cavity of a mold that would be shrinking at the time of cooling and solidifying (Annicchiarico and Alcock 2014).

The extent of the shrinkage would be referred as the “moulding factor of shrinkage”. If the moulding shrinkage factor could be known accurately with the preparation of the mould, then it would be possible for the formation of the moulded item such that it would have the intended dimensions.

The shrinkage could occur from intersected walls and thick walls that would have a lack of uniformity. The examples would include ribs, projection of nominal wall and bosses. The walls that are thick have the potential of slow solidifying, which would lead to shrinkage at the portion of nominal walls while the projection at the walls would shrink. Uneven thickness of walls would bring various kinds of challenges. At the intersection of the walls, the shrinkage would occur. This is due to the fact that thick walls cool slowly and thus a great amount of shrinking would occur. Thin walls have the capability of cooling faster and thus cause less shrinkage. Various other causes of shrinkage include higher melting temperature, lower pressure of holding and lower pressure of injection.  

The two methods that could be used in order to reduce the shrinkage for a provided component of geometry are:

1. Modification of the parameters of die casting machines – With increasing of the pressure of the work, it would have a positive effect on the issue of shrinkage porosity. The modification of the temperature and the type of the cooling fluid would be possible to cool the zones of porosity.
2. Modification of the Mould – The change in the mould morphology would mean to avoid the issues of the shrinkage porosity (Cheng, Liu and Tan 2013).

References

Abbas-Abadi, M.S., Haghighi, M.N., Yeganeh, H. and Bozorgi, B., 2013. The effect of melt flow index, melt flow rate, and particle size on the thermal degradation of commercial high density polyethylene powder. Journal of Thermal Analysis and Calorimetry, 114(3), pp.1333-1339.

Annicchiarico, D. and Alcock, J.R., 2014. Review of factors that affect shrinkage of molded part in injection molding. Materials and Manufacturing Processes, 29(6), pp.662-682.

Azaman, M.D., Sapuan, S.M., Sulaiman, S., Zainudin, E.S. and Abdan, K., 2013. An investigation of the processability of natural fibre reinforced polymer composites on shallow and flat thin-walled parts by injection moulding process. Materials & Design, 50, pp.451-456.

Cheng, J., Liu, Z. and Tan, J., 2013. Multiobjective optimization of injection molding parameters based on soft computing and variable complexity method. The International Journal of Advanced Manufacturing Technology, 66(5-8), pp.907-916.

Dang, X.P., 2014. General frameworks for optimization of plastic injection molding process parameters. Simulation Modelling Practice and Theory, 41, pp.15-27.

Fei, N.C., Mehat, N.M. and Kamaruddin, S., 2013. Practical applications of Taguchi method for optimization of processing parameters for plastic injection moulding: a retrospective review. ISRN Industrial engineering, 2013.

Madan, J., Mani, M., Lee, J.H. and Lyons, K.W., 2015. Energy performance evaluation and improvement of unit-manufacturing processes: injection molding case study. Journal of Cleaner Production, 105, pp.157-170.

Sun, L., Huang, W.M., Lu, H., Lim, K.J., Zhou, Y., Wang, T.X. and Gao, X.Y., 2014. Heating?Responsive Shape?Memory Effect in Thermoplastic Polyurethanes with Low Melt?Flow Index. Macromolecular Chemistry and Physics, 215(24), pp.2430-2436.