The technical paper session of the PMA meeting is always lively, Sarah Houlton reports on some of the key papers
The first technical paper, presented by managing engineer Sara Sina and chief executive Linda Katz of Wisconsin-based Molded Dimensions, looked at the work of the company’s cross-functional Urethane Restoration Team in reducing scrap.
The team includes a wide group of people from across functions. ‘We’re not just looking for a quick fix, but at the root cause of problems,’ Katz explained. ‘There are lots of tricks involved in making a good part, and it is a long training process. The aim was to remove the need for these individual tricks.’
One issue that commonly causes problems is mould temperature variation, Sina said. ‘It causes defects in parts such as air cracks, low durometer and uneven surface finish,’ she said. It turned out that the electric hot tables for open casting at her firm had up to a 30°C temperature variation across them, and casters would instinctively know which part of the table to use. The hot table covers were not always insulated, and some were in poor condition.
To address the problems, they replaced most of the electric hot tables with water-based ones that had much thicker platens to maintain the heat, and built new, insulated hot table covers that were lifted using a winch-and-pulley system to protect them from damage. ‘There is now a 1°C variation across the table,’ she said.
The number one cause of scrap, Sina added, is entrapped air, and the incidence of air bubbles increases in summer with higher humidity. The firm also found that the vacuum mixer was not reliable at degassing. ‘Vortex mixers provided a solution, but size constraints meant the largest buckets cannot be degassed this way,’ she said.
Probably the largest and most expensive improvement made was to the building’s climate control and humidity monitoring. The humidity was kept low, and the HVAC set to 27°C. A custom plastic mould was built for a new bucket size with the maximum possible volume for vortex mixing.
‘We have had great results,’ Katz said. ‘In 2009, scrap, as measured by cost, was close to 4%, while thus far this year it’s just below 1%. We have changed nearly every piece of equipment and improved methods for nearly every process but it’s been really successful.’
In a spin
Jamie Schmidt, vice president of manufacturing at USA Drives, gave an overview of the spin casting technique, and how recent developments have driven improvements. Spin casting offers multiple advantages over other methods, including the formation of parts without any visual bubbles of trapped air, and filling intricate cavities that cannot be filled consistently via gravity, compression or vibration.
It is also able to make very thin layers of materials, lay down varying hardnesses depending on the surface, and incorporating different additives in different layers. Further possibilities include the ability to fill multiple moulds at once, moulds that are very small, or those that require rapid fills because of quick gel times. Finally, it allows specific reinforcements, splices or inserts into products.
‘At USA Drives, we have had very good success in processing materials from 10 [Shore]A all the way to 75D,’ Schmidt said. However, he added, in order to use the spin cast method, a very well balanced machine is required for rapid spinning. ‘Out-of-balance machines can walk off the floor very quickly, much like an overloaded washing machine,’ he said.
Another important consideration is whether the spinning will be horizontal or vertical, or a combination of the two. ‘There are very low-cost processes that will spin slowly, producing tubes 3-4m or longer in pipes that are rotated on a horizontal plane,’ he said. ‘Some machines are filled vertically, and then tipped over to the horizontal plane so they can spin and be inside an oven. Others are vertical spinning machines, which allow for the filling of the mould from outside to inside.’
Cast PU or TPU?
An overview of the similarities and differences between cast and thermoplastic polyurethane, and where cast urethane could be used instead of TPU, was given by Scott Archibald, R&D manager at Coim USA. In conjunction with his colleagues Raimondo Baldassarri and Andrea Donghi, TPU experts at Coim in Italy. Archibald tested cast and TPU samples of similar hardness and chemistry, to understand their advantages and differences.
Cast polyurethane is a thermoset material, the resin is formed into the final shape before and cured. Thermoplastics like TPU are pre-polymerised and typically come as pellets that can be reversibly melted and shaped via extrusion or injection moulding.
‘TPU moulds are a lot more expensive, typically 10 times the cost of those for cast urethane,’ Archibald said. However, the cost-per-mould falls as the cast urethane moulds are reused, making them more cost-effective. ‘Some TPU parts, especially when injection moulded, have built-in stresses, and require post-curing.’ A benefit of TPU is that scrap can be reprocessed, which is not the case for cast urethane.
However, the use of prepolymers to make cast urethane introduces more flexibility, he said, as different diisocyanates, curatives and stoichiometries are possible. ‘Similar prepolymers can also be blended to produce multiple harnesses of polyurethane.’ In contrast, the chemistry of TPU is limited to MDI and 100% theory, he added, and different resins must be used to achieve different hardness ranges.
Some of the performance advantages are constant across both types of material. These include tear and abrasion resistance, toughness and load-bearing ability. The limitations are the same, too, including high temperature service, chemically reactive environments, and a higher cost than other polymers such as polypropylene.
He then gave a range of dynamic mechanical analysis results for both types of material. ‘If you are looking to displace TPU from an application, [this gives information about] how, why and where you should compete.’