The Injection Moulding Process
Injection moulding is a repetitive process in which melted (plasticized) plastic is injected into mould cavity / cavities where it is held under pressure until it is removed in a solid state after cooling. It is a very important process in manufacturing of plastic products. Injection moulding is primarily used where mass manufacturing is needed at a low cost. The process is highly efficient for fast production and can be used for complex part design. Of the total raw plastic consumed worldwide, injection moulding occupies a lion’s share. Most of the children’s toys, plastic medical equipment, home products like jars, bins, lunch boxes, car windshield accessories, etc. are made by the injection moulding process.
Most thermoplastics (plastics that can be melted and recast) can be processed by injection moulding. A few raw materials that are used in injection moulding include: nylon, polycarbonates like polypropylene and polyethylene and Acrylonitrile -Butadine-Styrene (ABS). All of these are non-Newtonian fluids. We will talk more about non-Newtonian fluids and the simulation challenges they pose shortly.
There are three main parts to an injection moulding machine - the feed hopper that provides the raw plastic, the screw and a heated barrel. The screw is rotated to melt plastic introduced from the hopper and to accumulate molten plastic in front of the screw. The molten plastic is then injected through the heated barrel, and passed through the nozzle into a mould cavity. A choice of cold or hot runner systems can also be selected for carrying the plastic into the mould cavities.
An injection moulding machine essentially performs the following functions:
Once ejected, the object / part is ready for use.
Injection Moulding Challenges
Injection mould design is a complex and iterative process. The development stages of injection moulded parts are often handled separately. A designer needs to concentrate on the desired functions and behaviour of the completed part / object. More often than not, the designer has limited or no knowledge of mould processing and material properties. A mould developer inherits the design and creates the mould. Unfortunately, the mould designer too has limited understanding of the processing and material properties needed for the processing. The process engineer or the injection moulder then inherits the mould and has to figure out a process that can produce the required part. The challenges can compound if the mould designing company is different from the mould processing company. To add to these complexities, most of the thermoplastics that are used for injection moulding are non-Newtonian fluids.
Newtonian and Non-Newtonian Fluids
What are non-Newtonian fluids any way and what challenges do they pose? To understand that, let us first define viscosity. Viscosity is the quantity that describes a fluid’s resistance to flow. A fluid with high viscosity resists motion, while a fluid with low viscosity flows relatively easily. Newtonian fluids are those that change their viscosity as per the temperature. As the temperature of a Newtonian fluid increases, its viscosity decreases and it flows more smoothly. Water, oil and honey are a few examples of Newtonian fluids. However, there are certain fluids that do not decrease their viscosity as the temperature rises. These are termed as non-Newtonian fluids. The viscosity of a non-Newtonian fluid will change due to agitation or pressure - technically known as shear stress. However, shear stress will not affect the viscosity of a Newtonian fluid. Thermoplastics, the raw material most popularly used for injection moulding, are non-Newtonian in nature due to the complex bonding in their polymers. Unless and until you have an advanced simulation software (like Altair Inspire Mold), it is extremely difficult to predict its flow.
Simulation Software for Injection Moulding
Sans software, process engineers are forced to use trial and error methods to choose the proper material. Naturally, manufacturers cannot afford to go into production mode based on such trial and error methods. To mitigate the designing and processing issues associated with production, the injection moulding manufacturers have been using CAD / CAM / CAE technologies since the last 20 years or so. Creating an injection mould is an expensive affair because of the complex geometry required to be created. More often than not, designers do not give considerations to the engineering feasibility of the finished product. This is because manufacturers encourage designers to be creative, in order to be innovative. However, since making a moulding is machine is expensive, and any change in the final product will entail a good of extra money, manufacturers look at CAD / CAE solutions to evaluate all aspects of the product.
Before launching a new product, it is extremely essential to study its feasibility, durability and longevity. All this requires subjecting the product to various stress tests that involve thermal testing, fatigue testing, resilience testing, and testing for many other engineering properties. If injection moulding is carried out the traditional way, the process of designing and building a mould, and moulding the first plastic parts can take up to five months. Only then would it be able to evaluate the mechanical properties of the object under consideration. Having recognized that quality can be achieved by overall optimization from the very beginning of a design concept through to production of injection-moulded parts, manufacturers have turned to geometry-based CAD systems of today in order to create a favourable environment for mould designers. Since the past 20 years or so, plastic injection moulding manufacturers have relied on CAD / CAE software to move designing from 2D drafting tools to truly spatial 3D mathematical modelling tools. As new simulation packages evolved, they allowed 3D modelling, visualisation, finite element analysis (FEA) and simulation of objects. This advanced software has benefitted injection moulding manufacturers in many ways:
Mould Design Challenges
Plastic mould design is a complex task. Their brittleness, flammability and thermal expansion properties present design challenges. Only an efficient mould flow analysis software (like that from Altair) is really useful for mould design. Since plastics are non Newtonian fluids, the flow of material in the mould too present design challenges. They can be solved properly using AcuSolve, a leading software from Altair for the analysis of flow of molten material. Challenges faced during entry point of molten material, runners and cooling channels can be handled properly by PTC Creo.
But be Cautious...
Although CAE based optimisation and simulation models provide the mould developers with effective tools, it is necessary to understand that they are not a foolproof solution. The underlying assumptions and simplifications in these models can sometimes lead to discrepancies between the real optimal scheme and those obtained from the models. While the advanced features of today’s leading simulation software like Altair Inspire Mold provide better results than ever, adequate training is necessary for the proper use of these tools.
Increasing demand for high quality plastic products have forced injection moulding manufacturers to adopt sophisticated CAD / CAE solutions in order to optimise production. A high quality part or object can be obtained only when the material and process selection is correct. In this highly competitive field, wasting time and money in trial and error solutions is not an option anymore. Simulation software is the only recourse savvy injection mould manufacturers have to optimise their production. However, proper training is necessary to leverage the true utility of such simulation tools.