Changing the game: How Schӧttli is increasing efficiency in injection moulding

Stefan Schneidmadel, development engineer from Schӧttli, a Husky Company, explains why Schottli believes it is increasing the efficiency of injection moulding.

Market requirements

The medical product industry represents an exciting and challenging market for manufacturers. This is because many products will be in direct contact with human organs, and therefore requirements set by regulating authorities place high demands on the productivity and quality of the plastic parts. These requirements have led to the use of specialised multi-cavity moulds within the medical sector, in order to ensure consistent component quality at the lowest cost of ownership across millions of injection moulding cycles.

In addition to the legal requirements, manufacturers also face challenges at the production level. The majority of the product range is made up of thin-walled products with a long flow path and a shot weight of just a few grams. Long flow paths generate higher injection pressure, which results in a higher force to keep the mould closed during injection. This creates the requirement for injection moulding machines with a higher tonnage.

At the same time the injection units of bigger tonnage machines are often larger than necessary, which results in increased residence time of the melt within the injection unit. This can lead to production problems when using heat-sensitive plastics. Figure 1 shows the typical clamping force and injection volume based on the tool clamping surface. According to the graph, multi-cavity moulds lie outside of the machine manufacturer's standard product portfolio as they require a higher clamping force with a lower shot volume. The customer must therefore use larger injection moulding machines for this type of mould.

Schӧttli has recently developed an advanced 8-cavity side injection nozzle, aimed at offering its customers a competitive advantage. This nozzle works by increasing the number of mould cavities with approximately the same tool dimensions, thereby improving the productivity of the tool and making more effective use of existing installation space.

Design of the hot runner system

A test tool was produced as part of the development project. Figure 2 shows the basic design of the system. The liquid melt is injected into the hot runner block from the injection moulding machine through the injection unit.

The side injection nozzle is an open system, which means that the system is fully relieved after the holding pressure time. From the heated hot runner block, the melt is injected into the pre-chamber via the nozzle holder and the nozzle channels, before it is then injected into the mould cavity. Previously, the projected surface of the pre-chamber contributed significantly to the lifting force, whereby the existing clamping force of the machine was crucial for the number of usable cavities in the mould.

With the new 8-cavity side injection nozzle with pre-chamber relief, the pre-chamber no longer contributes to the lifting force and the mould can therefore have a higher number of cavities with reduced clamping force.

Mechanical design of the components

In order to meet the customer's requirements regarding consistency, a complex measuring system was installed that records the melt pressures at different positions within the system. The purpose of the measurement is to obtain an accurate indication of the prevailing pressure at the machine nozzle, in the melt pre-chamber, and in the mould cavity. The measured values are used in the mechanical simulation to ensure that the design of the mould components is suitable for the durability as well as to detect any deformation of the mould inserts. This means that when designing new moulds, Schӧttli has a reliable data set that includes not only process parameters such as melt temperature and injection speed, but also the effects of the injection moulding material. For new developments, the design of the mould components can thus be made more efficient in terms of production time and economic costs.

Temperature equilibrium in the pre-chamber

The design of the pre-chamber poses a particular challenge when designing the hot runner nozzle. Here, the manufacturer is faced with the task of finding the optimum thermal equilibrium for the mould. The pre-chamber surface and the distance between the nozzle and the nozzle tip determine the thickness of the insulation layers. High demands are also placed on the gating area. Although one advantage of thick insulation is a high nozzle temperature with a low output on the heater band, there may be problems when the mould part is torn off the melt. The nozzle tip also has a significant impact on the tear-off behavior. If the nozzle tip is positioned too far inside in relation to the gating area, the injection point will be shut off. If the nozzle tip is positioned too far from the gating area, this can result in unacceptable gate quality on the plastic part. For the design of the pre-chamber geometry, Schӧttli uses thermal simulations that reflect the temperature behavior of the pre-chamber and hot runner nozzle during heating and over several injection moulding cycles. This ensures that the hot runner nozzle is in the optimum position, and that the thermal aspects of the pre-chamber geometry are suitable.

Typical medical applications

Injection moulding tests were carried out using various standard thermoplastics from the medical industry. Schӧttli can now offer its new star-shaped nozzle for polypropylene, polyethylene and polystyrene as well as acrylonitrile-butadiene-styrene. Typical mould parts include pipettes, needle holders for insulin pens, and conventional needle holder applications.

Other materials such as methacrylate-butadiene-styrene have also been successfully tested. Depending on the customer's requirements, the prevailing pressures can be documented during injection moulding tests in order to ensure the new mould is designed in the most reliable way. Due to interchangeable inserts it is also possible to create specific parts for pre-series according to customer requirements with short lead times. The findings from the tests are then incorporated into the development of the production mould, which mitigates risk and ensures a faster time to market.

Conclusion

A multiple cavity side-injection nozzle allows customers to make efficient use of existing installation space, whilst a greater number of cavities with the same mould size ensures economical production processes with higher productivity.