Injection molding is arguably the most successful area of modeling and simulation for any polymer forming process. This is demonstrated by the number of companies devoted to development of software for molding simulation – like 3D Timon’s 3D Injection Molding CAE System for example. While plastic injection molding remains one of the most popular ways of manufacturing plastic, the actual process is complicated and fraught with risks. In the first place, there are many design constraints that need to be tackled. For example, the wall thickness of a plastic part or object needs to be even. An uneven wall thickness can result in shrinking and warping issues later on. The internal and external corners should be smooth as well; sharp corners can crack as they take more time to cool than the rest of the component body. While this seems like an easy task, it is not. Resins come in various compositions and viscosities, and it is an engineering challenge to ensure that the resin spreads throughout the mold uniformly to ensure product consistency. And this is where moldflow analysis plays an important role.
Mold Flow Analysis
In a typical injection molding process, the screw is pushed axially, and plays the role just like that of a piston in a car by injecting molten polymer into the mold cavity at a controlled speed. A suitable high pressure is maintained till the mold cavity is filled up with the molten polymer and an additional quantity is injected into the cavity to counter the effects of heat shrinkage. After allowing cooling down to a suitable temperature, the mold is opened and the part ejected; preparing the cavity for the next batch. Mold flow analysis is the study of these and other parameters that affect the quality of the object / part / component being molded.
Here are some important engineering considerations in the moldflow process.
Gate Location: As the name implies, a gate in injection molding is a small opening through which the heated molten plastic enters mold cavity. There are different types of gates, and choosing the right one and determining its location is a critical aspect of the product success or failure. Additionally, depending upon how complex the part is, there may be more than one gate involved in the mold injection process.
Screw design: The screw, located in the plasticator barrel (where the raw material is melted or plasticized) propels the melt towards the mold cavity. It is the job of the screw to keep melt uniformity, minimize temperature variation and maintain uniform speed.
Mold: This is actually where the part / component / object take shape. The hot melt moves into the mold under pressure, and is allowed to cool down. It is necessary to design the mold properly, so that the desired shape is achieved, and the air allowed to escape properly.
Shear flow: As far as a fluid like molten polymer is concerned, shear flow is the rate at which layers of fluid slide over each other. It is the flow induced by a force gradient, and is dependent on the viscosity of the fluid. Greater the viscosity, higher the shear force. In molds, shear flow can occur in the channels and between parallel plates.
Elongational viscosity: In simple terms, when a fluid flow is deformed as a result of a stretching motion, the resulting phenomenon is called elongational viscosity. As far as mold flow fluids are concerned, there are three types of elongational viscosities to tackle - uniaxial, biaxial and pure shear elongational viscosity. In a moldflow process, it is encountered on the axis of convergent or divergent geometry.
Melt temperature: Since the molten resin needs to be transported from the hopper to the mold cavity, another important mold flow analysis parameter is the melt temperature. Too much temperature can alter the molecular structure of the raw material, and too less temperature can result in increased viscosity and shear / elongational forces.
Cavity filling time: Another important parameter that affects the part / component quality is the mold cavity filling time and injection speed. Too fast or too slow injection speed can result in uneven part thickness and uneven properties.
Mold cycle time: For any manufacturer, time in money. In the injection molding process, cooling time is important as it occupies about 80% time of one mold cycle. And naturally, lesser the cooling time, the better the productivity. However, moldflow involves a tremendous amount of heat, and each mold needs to be cooled properly for defect free part / product.
Challenges in Mold Flow Analysis
Apart from these engineering parameters, there are other considerations that affect the product quality. There are literally thousands of plastics to choose from. Each of the plastics has different thermal, tensile, fluid and other properties. In addition, most products use a compounding of different amounts and combinations of additives, fillers and reinforcements to improve their attributes like tensile strength, heat resistance, colour, finish, etc. If the proper raw material and the proper process parameters are not chosen, the product can result in defects like warpage, molded-in stresses, flashing (excess plastic that accumulates on the surface of molded parts), and other related issues. Before the advent of sophisticated computer programs (like 3D Timon, for example), it was virtually impossible to arrive at the correct mold flow process easily. Manufacturers were forced to use trial-and-error methods to determine the correct parameters and product mix. Today however, advanced moldflow analysis software or plastic flow simulation software like 3D Injection Molding CAE System from 3DTimon allow the engineering team to simulate the proposed mold design and melt material mix in order to ensure that the end product / part that emerges from the mold is virtually defect free. Such mold flow analysis software works by optimizing the resin mix, melt temperatures at different locations within the injection molding machine, the shear and elongation rates, the melt pressure and temperature, cooling time and other information pertinent to the flow analysis. There are many moldflow analysis software or plastic flow simulation solutions in the market today; it is essential to select the software carefully as each software incorporates different mathematical models and techniques.