Graham Webster, Director of Plastic-IT, discusses the injection molding pathway to fabricate a high quality part for medical devices.
At first glance, the injection molding process is seemingly simple: make a mold, force molten plastic into it and let it cool, open the mold and ship the product. However, those reading this will know that this is an oversimplification – so how complex is it?
Let’s first see why we use this process, which today is the largest manufacturing method in the world. The common denominator is that it’s done for profit – at least that’s the original goal. The “long and winding road” is full of problems that detract from this main objective, and here we try to identify them and provide knowledge-based information on how they can be avoided. Whether we’re making a small, close-tolerance part out of an “exotic” engineered material or high-volume packaging or a disposable medical product, every facet of the process must contribute to creating a “margin” – financial or altruistic.
At the start of this journey, many difficult decisions must be made. The first is to determine which of the more than 10,000 commercial grades of polymers we should use for the application we are considering. All injection moldable thermoplastics melt between about 90 ° C and 400 ° C, so they generally cannot compete with metals on a thermal basis. Most are good electrical insulators, while metals are mostly good conductors. Thermoplastics can be very rigid especially when combined with glass or carbon reinforcing fibers (19,000 MPa – 60% glass-filled polyamide 6) or very flexible and even elastic. Some are completely transparent and others are completely opaque. Some burn easily, others go out on their own if the ignition source is removed. Most can be colored – some better than others. Some are UV and ozone resistant (weather resistance) while others are poor.
Determine what is the optimal polymer – the quality of this polymer and other additives such as lubricants, flame retardants, etc. to choose from, is a technical and commercial minefield for anyone who is not a specialist.
Until the product performance criteria are determined, it is not possible to specify the type and grade of the polymer. Therefore, the design of the part’s geometry for its fit, function and aesthetics must be done in concert with the choice of polymer. This should be the area of expertise of the product designer, but as has been illustrated, it is a complex task that few people can undertake with certainty. Once a mold is made, it is possible to mold different materials for evaluation, but due to the likelihood that these polymers will shrink by different amounts during the molding process, dimensional accuracy will be compromised.
Eventually, after several iterations, a CAD emerges, and a type and grade of polymer is identified – what next? This is often a call to a mold maker because surely all that remains to be done is to machine the part and mold it? This is often what happens, but the result is hardly ever right the first time. Therefore, the costs will begin to increase as the mold trials and subsequent mold restart cycle develop.
The best way is to prototype it. Today, additive manufacturing can produce a 3D object directly from CAD into a polymer something “similar” to the intent of injection molding. However, it offers little more from a production engineering perspective as at this point there has been no evaluation as to whether the geometry can be satisfactorily molded. To do this, we turn to a CAE mold simulation product such as Moldflow.
Comes another knowledge-based technology that baffles the majority. You might be surprised to find that the technology has been around for over 40 years. In expert hands, it will simulate the entire molding process and determine the final sizes and shape of the part after molding using the CAD model and a very comprehensive database of polymers. If you had a crystal ball that would point you to the six numbers for next Saturday’s lottery, surely you would use it. CAE tools, expertly used, are like crystal balls – telling you precisely how well the CAD would mold – without having to make a mold.
Due to the aforementioned complexity of developing a geometry that can be used (which meets the requirements for fit and function in an economically viable polymer that can be cast), DO NOT take advantage of the scientific predictions of a CAE tool in 2021 is madness.
Frequently we see this application from CAE called the DFM report when, of course, “design to manufacture” (DFM) is, by definition, a “design process” and not an error checking process to be used just before that the steel is cut and executed. by the mold maker.
Let’s go back for a moment to understanding the product development process. It is unlikely that the exact Widget we want to create as an injection molded part has not been made before, so there may not be any pre-existing knowledge of the details of how it was made to begin with.
It is impractical to expect one person to know so many diverse technologies, so it is best to be a collaborative process of teaming up with experts in their field as early as possible in the life of the project. Instead of being a serial process, today with CAD, CAE, and online meetings, the iterative process of moving from art to piece can now easily be a true team effort.
The process should start with the product designer as it is normal here that the brief or requirement is distilled by the end user. These are still “fit and function” specifications and probably also aesthetic aspects. Minimizing costs is also a prerequisite. Today, the process should be a collaborative process between the product designer, materials expert, tooling engineer, and manufacturing engineer who also collect data, advice and feedback from CAE engineers who can assess strength, dimensional accuracy and moldability. Thirty years ago the term “Concurrent Engineering” was coined; today it is called Concurrent Knowledge based Part Production or CKPP for short. This is a specific methodology following simultaneous engineering concepts specifically for plastic injection molding. Once adopted, the result is the right parts the first time at the optimum cost in the shortest possible time.
Shouldn’t that be the methodology you adopt on every project?