ZAE206 Advanced Manufacturing

Injection molding process entails four major steps. The initial step is preparing and gathering tools used for injection (Baranov, 2017). In this stage, thermoplastic materials are melted and thermoset materials mixed. The next step is filling; this is injection of molten polymer in the mold cavity. The third step is holding; this is compaction of the polymer in the mold with maintained pressure. The final step is ejection and cooling; Here polymer is left in the mold until it solidifies. A cooling system which is independent may be installed at this stage.

During the ejection and cooling steps, a screw is retracted to fill the barrel again with the polymer (Baranov, 2017). Holding and filling are the most significant stages in the process because they have an impact on the final product. Also, filling step influences polymer orientation inside the cavity. Temperature, cycle time and pressure settings have an effect on mechanical properties.

Pressure is a significant factor in process control. Maintenance of equipment to give a required water pressure is a necessary. Optimum temperature is required in the process (Baranov, 2017). The temperature should be set such that materials melt systematically. Cooling lines should be placed in a manner that they are spaced and set for maximum heat removal. Cycle time should be sufficient in each processing step for the machines to arrive at equilibrium and adjust.

Viscosity is the tension in an individual ideal flow field by the degree of distortion in a period of shearing (Felix, 2017). Melt viscosity can determine the filling of a mold cavity. High viscosity is the difficulty in flow through higher injection and thin-section fill pressure. Shear and temperature rates are important critical parameters in considering the viscosity of the fluid polymer. Flow-lines and sink marks could occur if viscosity is not adequately controlled.

Flow lines are lines, patterns or streaks which are majorly off-toned and appear on the prototype region. The defect is caused by variation in the speed in which molten plastic moves or flows round mold tool. Sink marks are depressions or tiny craters that form in thicker places of the molded injection prototype in a situation where shrinkage has developed in the inner portions of the end products. Other errors include surface delamination, weld lines, short shot, warping, burn marks, jetting and flash.

Sink marks defects result from the deficiency of cooling mechanism or time used by plastic to maximum cure or cool when it’s still in the mold (Felix, 2017, PP.95). Vacuum voids- they are air trapped pockets close or within an injection molded surface. Vacuum voids are a defect which results from uneven solidification within or between the inner section and surface of a prototype. 

Barrel temperatures during mold injection should range from 190 – 200 degree Celsius. Optimum barrel temperature profile is determined by various variables; cycle time, screw design, machine capacity ratio and mold design (Baranov, 2017). Barrel controller temperature should be set such that materials melt systematical and should be of PID type with hotter and cooler zone temperature on the front and rear parts. In the case of vented machines, temperature profile, which is flat, is advisable to ensure that polymer melts at the point it arrives at the event zone. Reverse profile of temperature is installed to minimize screw design which is improper.

Mold temperature should range from 7 to 16 degrees Celsius (Baranov, 2017). To obtain consistent tolerance in the cycle time, removal of heat uniformly is a major factor. Use of controller to maintain mold temperature will reduce temperature variations. For efficient heat removal, cooling lines must be spaced and placed correctly in the surrounding parts.

For maximum melting of the polymer, screw speeds are recommended. Screw speeds should range from 40 -140 rpm (Baranov, 2017). Rotation of screw should take place through the whole process of cooling cycle. This can be achieved by establishing a balance between back pressure and screw speed.

The process of Injection molding occurs in two stages. The initial stage is injection and then packing (Baranov, 2017). During the filing stage, machine pressure is recommended. It is set to its maximum settings with the maintenance of speed (ram) and velocity control.

Recommended back screw pressure should range between 50 -150% psi. It is a primary necessity to use back pressure to ensure that polymer melt uniformity is maintained for maximum performance (Baranov, 2017).

This is dependent on section and the machine particulars. Varying speed of injection may result in faults, thus, dose rates are recommended. Polymer thickness is reduced by elevating the shear speed, allowing materials’ smooth flow and having the thin walled parts filled (Baranov, 2017).

The role of melt cushion is to ensure inoculation of sufficient material to the cavity as it contracts during the packing phase (Baranov, 2017). Gathering for the contraction, the cavity gate is frozen off by added material supply.

Shrinkage occurs in the molding process due to varying of polymer density. When mold injection is taking place, shrinkage variation results to internal stresses. Shrinkage can occur from walls that are intersected and with lack of uniformity in thick walls (Felix, 2017). Examples include bosses, ribs or nominal wall projection. Thicker walls have the potential of solidifying slowly; this leads to shrinkage at the area of nominal walls while projections shrink. Other causes of shrinkage are low injection pressure, high melting temperature and low holding pressure.

Week 10: Composite Vacuum Infusion

The mold, which is of high quality is prepared to be used in vacuum infusion. The mold must be rigid with a high-gloss finish (Menta, 2014). Reinforcement material such as fiberglass is selected. Fiberglass does not allow the resin to flow smoothly. Flow media is selected to aid the movement of resin.

Before closing the vacuum bags, vacuum and resin lines are installed. Vacuum lines are selected to absorb resin and access move space across the laminate (Menta, 2014).

A vacuum bag is built as soon as materials dry (Menta, 2014). The excess resin in the bag can lead to improper fusion. Resin trap equipment is used at this step to force resin flow to the laminate. Vacuum pressure is a necessity which aids in the drawing of resin.

The vacuum pump is attached to help in expediting the infusion. A quality and proper space is selected and applied to avoid leaking of resin (Menta, 2014). Pressure is necessary for vacuum pump functioning.

A quality resin to be used is selected (Menta, 2014). Recommended resin is the one with lower viscosity. The resin bucket is set and used in transferring of resin.

Immediately after everything has been prepared and ready, the resin is mixed. The flow regulator is removed to open the resin inlet allowing them to be sucked quickly throughout the tube into the laminate (Menta, 2014). Pressure is a necessity in the sucking of adhesive to the laminate. Clamp-off is turned on to prevent bubbles entering after resins are no longer needed to complete infusion.

Helpful supplies are gathered for vacuum infusion. Temperature is necessary because it informs one when resins start to cure and when the laminate has turned to room temperature (Menta, 2014). The thermal gun is used to check temperature level. The stop watch is used to monitor resin flowing rates.

Vacuum setup is complicated. Although time is limited, vacuum bagging necessitates the placement of vacuum tubing only. Placement of the resin lines and space varies from one part to the other, and there is no other way to set them well (Menta, 2014). These considerations must be fixed and evaluated before lay-up to avoid ruin.

Another defect is that it is easy to destroy. Once infusion starts, little can be done to mend the mess or errors, that is, if leaking occurs, the slightest air entering the parts poses a mortal danger to the region (Menta, 2014). The best correction of this defect is to have a right and proper plan of the process.

Another defect is that Vacuum infusion is seen as trial and error act. This is having a mindset that few parts or many places will be destroyed before the end of the process (Menta, 2014). The best solution here is documenting every attempt so as to learn from trials. Another one is keeping track with the resins flowing rate.

Process parameters include pressure, viscosity, temperature, volume fraction, filling time and permeability (Subbiah, 2017). Another parameter that must be taken into consideration independently comprises nozzle attack from the angle, fiber orientation, shear rates and flow paths and stratification. To obtain quality end products, combination and maintenance of the above parameters should be taken into consideration.

Baranov, AV (2017), 'Non-isothermal Modelling and Optimisation Methodology of the Injection Moulding of Rubber Products', Polymers & Polymer Composites, 25, 2, pp. 153-160, Academic Search Premier, EBSCOhost, viewed 25 May 2017.

Felix, M, Perez-Puyana, V, Romero, A, & Guerrero, A (2017), 'Production and Characterization of Bioplastics Obtained by Injection Moulding of Various Protein Systems', Journal Of Polymers & The Environment, 25, 1, pp. 91-100, GreenFILE, EBSCOhost, viewed 25 May 2017.

Menta, V, Vuppalapati, R, Chandrashekhara, K, & Schuman, T (2014) 'Manufacturing of Transparent Composites Using Vacuum Infusion Process', Polymers & Polymer Composites, 22, 9, pp. 843-850, Academic Search Premier, EBSCOhost, viewed 25 May 2017.

Subbiah, R, Tjong, J, Nayak, S, & Sain, M (2017) 'Effect of Natural Fiber Network on Permeability and Tensile Strength of Composites through Vacuum Infusion Process', Journal Of Natural Fibers, 14, 2, pp. 278-286, Academic Search Premier, EBSCOhost, viewed 25 May 2017.