Breaking the mould26 October 2018
Medical device manufacturers are under pressure to produce innovative, high-tech products in increasingly greater volumes. Injection moulding represents an indispensable part of the manufacturing process, and due to the specialist skills and equipment required, it is highly likely to be outsourced. Medical Device Developments explores how selective laser melting can improve the efficiency of the injection-moulding process, without sacrificing quality.
American inventor John Wesley Hyatt, together with his brother Isaiah, patented the first injection-moulding machine in 1872. The device was relatively simple compared with those used today; it worked like a large hypodermic needle, using a plunger to inject plastic through a heated cylinder into a mould. Over the next few years, the technology progressed slowly, producing products such as collar stays, buttons and hair combs. Today, injection moulding is used for creating products across multiple fields, including electronic, automotive and home appliance, as well as the medical device industry.
Due to its ability to produce large volumes of parts, the injection-moulding process is crucial to the medical device industry, which is a sector that works on impressive scales. The process is well suited to manufacturing parts with precision, meaning the final product can be as thin, light and sleek as required.
In a previous interview with Medical Device Developmets, Gus Breiland, customer service engineering manager at Protolabs, talked of the benefits of injection moulding for the medical device industry.
“It offers a wide range of opportunities for medical device companies to produce more ergonomically correct components,” Breiland said. “And it allows them to use a large variety of antimicrobial materials.” As a result of the improvements in technology, the technique has become particularly popular in recent years. Breiland continued, “The injection-moulding materials and manufacturing processes are much better than they’ve ever been.”
Better technology is improving countless lives and creating new opportunities for medical device manufacturers. In order to keep up with demand, companies depend on extremely high annual production rates. Injection-moulded plastic parts are quickly replacing traditional materials in medical devices, not only because of their wideranging material advantages, such as sterility and design flexibility, but also because of the cost and speed at which parts can be manufactured.
Due to the high degree of specialisation required for injection moulding, beyond the scope of most small-to-medium-sized medical device companies, the process tends to be outsourced to one or more contract manufacturer. When outsourcing, it can be challenging to maintain the quality and speed required of medical parts. Injection moulders that need to maintain this level of quality at an output of severalhundred- million plastic parts a year from a single, high-cavity mould often turn to highperformance hot runner systems.
A hot runner system is a molten plastic conveying unit used within an injection mould. The benefits of hot runner systems were emphasised in a previous interview with Harald Schmidt, of Mold Hotrunner Solutions, for Medical Device Developments.
He said, “Hot runner systems offer high rates of efficiency and productivity. Most notably, they dramatically cut production costs.” Many medical applications employ custom-engineered polymers, which means the excess material is a major cost factor. Exotic thermoplastics are often priced at thousands of euros a kilogram. Sometimes, the extra material can be mechanically reground using auxiliary equipment and then reprocessed. As most medical applications permit only the use of virgin material, a hot runner is valuable to ensure the moulding cell remains ‘clean’, because the mould produces only parts and no waste, dust or other particle contamination.
Due to the sensitivity of the materials offered by suppliers and the small processing window available, these materials are inherently challenging for injection moulding. When these materials are susceptible to heat or shear, extra consideration has to be given to the design of the melt path, or the distance the material spends travelling to the mould cavity and how long it takes. The primary function of a hot runner is to accurately control the flow of plastic using heat zones, shut-off valves and cooling lines, making it a good fit for these considerations.
Up to speed
Cycle time is a hugely important aspect of injection moulding. One of the most effective ways to improve productivity is to make parts faster, but this is easier said than done. There are two main ways to speed up production. The first is to increase the number of cavities, either in a single mould or through additional moulding cells. Adding more injection-moulding machines to the factory floor is an easy but costly solution in terms of energy and resources.
The other option for reducing cycle time is to run the injection-moulding machine faster, but this is not always possible due to the inherent physical limitations in how fast a part can be made. Depending on the size of the part, the time it takes to fill the cavity is finite. For example, parts with a greater wall thickness are slower to fill. The material also needs enough time to cool before the part is ejected from the mould. Optimised cooling in the mould can help to significantly lower cycle time.
The cooling stage is one of the lengthiest aspects in the overall cycle time of injection moulding due to its strong influence on a plastic part’s final properties. During this stage, parts are cooled by building channels throughout them. Typically, these channels are straight lines, resulting in uneven cooling.
Researchers from the Polytechnic Institute of Leiria emphasise the importance of these channels for the cooling process, saying, “To achieve an efficient cooling, the channels should reflect the best practices concerning channel spacing and channel distance to the moulding surface.”
By using conformal cooling channels, which conform to the shape of the part, efficient cooling can be achieved, reducing the overall cycle time. But these types of channels are challenging to produce by conventional methods.
The researchers noted, “Conventional manufacturing methods can only produce linear channels and planar cooling circuits which, depending on a part’s geometry, prevents cooling channels in order to keep constant distance to the moulding surface, sometimes resulting in unavoidable hot spots.” 3D printing is an appealing alternative to conventional methods, as it provides the ability to build conformal cooling channels in injection-moulding tools.
Adding another dimension
The researchers from the Polytechnic Institute of Leiria have developed a 3D-printing technique called selective laser melting (SLM) to build conformal cooling channels. SLM is a technique that generates complex 3D parts by selectively melting successive layers of metal powder on top of each other, using the thermal energy supplied by a focused and computer-controlled laser beam.
This technique has already shown promise within the moulding industry more generally, as researchers highlighted, “Its use on the mould-making industry has provided excellent results due to its free-form capacity to build conformal cooling channels to optimise the injectionmoulding process.”
Researchers aimed to produce tooling for a support for pipette tips used in the medical industry. Conventional production of this part is associated with a long cycle time as a result of cooling difficulties on the thickest areas of the part. Metallic tool inserts were produced by conventional manufacturing and SLM technology, in order to compare the performance of the injection-moulding process and its economic impact on the production process.
The plastic part researchers created for the study was a support for pipette tips, a rectangle with 12×8mm housings for the tips, divided by thin walls. The part was designed to be stacked up with equal empty parts to save storage space, meaning the outer walls are stronger. Researchers first conducted injection-moulding trials with conventional manufacturing techniques. Certain thick spots at the intersection of the inner walls and the outer walls caused hot spots, where the material cooled slowly, which caused sink marks and warping on the inner walls.
Researchers then re-engineered the mould using selective laser melting in an attempt to reduce the cycle time and prevent warping. Using this method enabled them to reduce the cooling time from 35.5 to 18 seconds, a reduction of almost 50%, and the overall cycle time from 38 to 25 seconds, a reduction of just over 34%. Researchers also managed to prevent warpage by reducing the temperature difference on different areas of the part to a maximum of 10.6°C.
The economic feasibility of SLM is an important consideration. The manufacturing costs of SLM tools compared with conventional tooling are significantly different. “Concerning the production process itself, the new cycle time enables important savings,” researchers explained.
Taking into consideration all the operations involved and the raw materials required, researchers reported that SLM presents processing costs that are four to five times the costs of conventional tooling.
However, researchers highlighted that mould tools were built separately, which was not great in terms of costs, and that alternative approaches could yield more savings, saying, “These manufacturing costs could be optimised if the cavity and core inserts were built in a single process, or built on top of a previously machined part, reducing its economic impact on the mould’s initial costs.”
When considering the economic impact, it’s important to remember that the reduced cycle time of injection moulding produced by SLM in itself provides significant savings compared with conventional methods. Researchers calculated the time it took to produce one million parts and found that SLM achieved this over 600 hours faster. Even considering a continuous 24-hour production shift, this corresponds to 150 days, enabling a significant time-to-market reduction for these parts.
Researchers noted that parameters such as mould cost, the raw material processed and the processing cost, including injection machine and energy, also need to be considered for a more detailed economic analysis. As a result of the production time savings for the whole production, SLM also reduces the carbon footprint significantly compared with conventional methods.
“On the energy efficiency context, pointing at the production time savings for the whole production, a large energy saving is achieved,” researchers stated.
Although injection moulding is an effective manufacturing process, there is always room for improvement. Whichever tools are used, they must be reliable under extreme heat and pressure, over millions of cycles. Designing and building these tools requires collaboration between expert teams with experience in processing highvolume medical applications.
“Always be collaborative, and always try to leave some room in your design for modifications, to make manufacturing easier,” Breiland advised. “But communication is key. In real estate it’s ‘location, location, location’ and in manufacturing it’s ‘communication, communication, communication’. That’s the biggest driver of success, and the biggest hindrance to success is making assumptions and not communicating with your manufacturing group.”
With injection moulding now firmly established as an indispensable part of the manufacturing process, these words are set to remain pertinent for some time to come. In light of the continual, rapid developments in technology, particularly computer-aided design, injection moulding is likely to become increasingly efficient over the next few years. Of course, such efficiencies need to be accompanied by an ability to maintain the high quality required for medical devices. Effective communication between medical device manufacturers and contract manufacturers can ensure that these dual needs are met.