Hot runner systems Stack and multi-component moulds: Mastering the challenge of medical technology
Modern medicine wouldn't be anywhere near what it is today without plastics. The technology expert in hot and cold runner technology Günther Hot Runner Technology can point to extensive expertise in medical technology, as the following examples demonstrate.
The pandemic has shown that today’s healthcare systems are no longer able to do without plastic. The reason is that plastic is largely responsible for ensuring that hygiene standards are maintained and infections and bacterial transmissions are prevented. It is the same with medical devices. Plastic is highly beneficial since equipment such as dialysis machines wouldn’t really be able to work without the material. Plastics are used in virtually all areas in medicine today in items such as sterile packaging, syringes, catheters, hose systems, implants, artificial replacements or drug-release dosage forms. Single-use products offer advantages, first and foremost, for hygiene and, as a consequence, increasingly replace reusable parts made of glass or metal.
Reliable, efficient manufacturing processes are required wherever plastic products are used in medicine. Günther’s hot runner technology ensures maximum process reliability for complex assemblies and increasingly smaller moulded parts. A current example are the two casing components for lateral flow tests. The test device features a cassette, whose upper and lower plastic casings enclose a composite pad strip on which various chemicals are impregnated. In March 2020, a UK manufacturer was awarded a contract to produce an eight-drop test tool and the corresponding 32-drop series production tool for the upper and lower cover sections. The project not only required a technically skilled mould maker but also an innovative hot runner supplier. This is where Günther Hot Runner Technology became involved since they had already undertaken such special medical industry projects in the past and knew that valve gate technology was ideal for this project.
The requirements for the hot runner system comprised a clean gate point, uniform filling of all parts and, above all, reliability. 6NMT valve gate nozzles with a small nozzle head were used to ensure the required low cavity spacings could be featured. This valve gate nozzle with a conventional heating element comprises a two-part shaft made of titanium alloy and stainless steel, which guarantees a highly homogeneous temperature profile throughout the nozzle's entire length thanks to its featured heating element. The material is thus gently guided into the drop. During the shut-off movement, the needle is first led over a cone up to the cylindrical pre-centring device for precise immersion into the cylindrical gate point. The Günther needle actuator used provides precise, intelligent needle control for simple installation and connection technology. Uniform opening of the individual valve gate nozzles ensures a reliable injection process, even with the smallest shot weights. The ANEH stroke mechanism is used in both the eight-drop test tool and the 32-drop series production tool.
Intuitive operation of temperature control units
Easy-to-clean hot runner system for disposables
Special multi-component applications or family moulds are highly challenging, such as the one such used to manufacture a locking handle with an injection needle where the metal needle is overmoulded. Since the part design for the locking handle had been changed, the mould needed to be newly designed from scratch. Logic dictated that the mould design needed to be created in such a way that savings would also be achieved on injection times. However, balancing the different part volumes caused problems. This is where Günther Hot Runner Technology came into with its electric drive for valve gate systems.
The electric drive offers significant advantages due to its precise control. Exact, intelligent operation of the valve gate technology is necessary to ensure a reliable injection process. Electrically driven valve gate systems provide a variable, yet precise setting of the needle position and the stroke length in more than two positions. Synchronized needle movement also ensures high reproduction precision. You can open the needles with a time delay for this purpose and balance the mould so that both parts are filled uniformly. An electric valve gate hot runner system with stepper motors was used in this case. The SMA 10 E02 stepper motor ensures high precision combined with optimal force-displacement behavior. Precise valve gate control is guaranteed as the motor is used together with the corresponding DPE control unit. The needle can also be adjusted within the 1–100 mm range. The valve gate nozzle featuring the KA needle guide with a Blue Flow heater and the two-part shaft make this clean room-compatible application complete.
The key issue in this project was to balance the different parts with different volumes, so the control was linked to Priamus temperature and internal pressure sensors. This ensured that balancing takes place based on data from the temperature sensors that are installed at the end of the flow path. When the melt reaches the sensors, the control system retracts the needle valve. The internal pressure sensor is then used to change over to holding pressure. The temperature and internal pressure sensor system also offers the advantage that it communicates excellently with the Günther valve gate system via the associated control technology.
Stack moulds can help to reduce costs
Ongoing research and market changes such as rising labor costs or demographic shifts are forcing hospitals and other medical facilities acquire single-use instruments to replace reusable instruments, which have preparation costs at around 2.50 to 3.50 euros per instrument and cycle. This is because procurement costs for single-use products are sometimes significantly lower than those for preparing reusable instruments. If healthcare organizations only use new instruments, they will not incur repair costs either.
Manufacturers, however, are forced to increase their production capacity to supply these single-use instruments at a cost-efficient price. This is why stack moulds are in demand. Manufacturers like to use them in the injection moulding process to keep the size of their injection moulding machine relatively small, yet still increase the volume of parts produced. From a financial standpoint, a significantly increased production quantity is manufactured for the machine hour rate. As a result, the machine produces the planned batch size in almost half the time while creating additional production capacities.
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However, in the case of stack moulds, close co-ordination between the toolmaker, injection moulder and the hot runner manufacturer is vital since the layout of the hot runner system is highly complex as the example of a 16+16-drop mould for a special medical technology component shows. The part to be produced is made of polyolefin and has a volume of 10.5 cm3.
The following information needs to be obtained first: How should the part be gated: with an open nozzle or with a valve gate? The nozzle type is then selected based on the plastic type and the part geometry. The type of drive for the valve gate design then depends on whether individual valves or plate drives are used. Answers are required to questions such as: will the parts be placed back to back or staggered? Will the melt be transferred into the moving mould area through a long spout or via a split transfer path? What is the expected injection time or material throughput per time unit. The mould size and the possible system height can then be determined as a final step.
Fitting and removal in the parting line
The nozzle types selected to gate the part generally depend on the required gate point quality. Only a valve gate system will achieve completely fiber-free gating. If it is decided to use open hot runner nozzles due to cost concerns or restrictions on the mould height, it must be ensured that sufficient decompression can be achieved during the process. If machine decompression is not adequate, the melt may run into the parting line when the mould is opened. This excess will lead to a cold slug during the next cycle, which is subsequently either found in the part or prevents the gate from opening during injection. Suitable decompression units can be fitted to the manifold to aid decompression. If the decompression issue cannot be clearly settled in advance, it is advisable to include the space required for the relief valves in the structural design. If valve gate nozzles are used, the melt cannot run out of the gate points if the needles are closed at the right time. Fitting and removal options must be taken into account when choosing the nozzle.
The STT or STF nozzles for open systems and the NTT or NTF nozzles for the valve gate available from Günther are designed in such a way to allow fitting and removal in the parting line. This means the rest of the hot runner system does not need to be dismantled. The nozzle can also be changed with the mould clamped in place if needed. The nozzles are equipped with plug connections to connect power and heating. Time and effort are saved as there is no need to solder the wiring up to the attachment housing when replacing the nozzles. The injection moulding machine's downtime is reduced as a result. The nozzle is positioned using two dials on the head. The upper dial also provides leak protection. Routing the cables through the nozzle holding plate towards the parting line prevents thermal overload in the cables. The 6NTT3-150VA valve gate nozzle with a gate point diameter of 1.2 mm was used for the 16+16-drop mould. The VA needle guide type is made of a powder-metallurgical steel and ensures a long service life and an excellent gate close to the needle guide thanks to a hardness of around 60 HRC.
How innovative hot runner technology uses material more efficiently
The mould height reduced
The hot runner manifold is designed to transfer the plasticized melt from the connecting nozzle to the hot runner nozzle. In the case of the 16+16-drop mould, the runner diameters in the manifold, the hot runner nozzles and the connecting nozzle were calculated using the simulation software Sigmasoft. The manifold's thermal properties are calculated in addition to computation results on pressure loss and material shear. Distribution now takes place to four eight-drop sub-manifolds via a straight two-drop main manifold. The melt channel diameter starts at 16 mm at the manifold inlet and is reduced to 6.2 mm in the outlet hole to the valve gate nozzle. The staggered arrangement of the nozzles in the stack mould makes it possible to work with a smaller number of melt manifolds. This reduces the system height and also the weight of the mould.
The melt transfer in hot runner systems for stack moulds is either continuous with a long, end-to-end connecting nozzle or with a split transfer path. In our application, a split transfer was needed because the mould could not be clamped as a whole. The fixed nozzle side, the moving center block with a hot runner and the moving ejector side are clamped separately and mated on the injection moulding machine. The melt channel in the connecting nozzles is 16 mm in diameter. An open nozzle is fitted on the fixed nozzle side and a connecting nozzle with a valve gate on the moved center block. This valve gate ensures no melt can penetrate the parting line when the mould is opened. The gate for the connecting nozzle is driven by a single pneumatic cylinder. The back-to-back solution with EEV single pneumatic valves was used in the 16+16-drop application. The air supply holes to the valves need to be well balanced in this case. Balancing and sufficiently large hole diameters are responsible for ensuring flawless operation of the single valves. The temperature of the die plates where the EEVs are mounted should not exceed 100 °C during operation. This was not a problem for this application as the general mould temperature is 20 °C during the process.
The examples show that the increasing cost pressure in healthcare poses extreme challenges for manufacturers to increase their production capacity and adopt cost-effective work practices. It is thus recommended to seek close coordination with the toolmaker and hot runner manufacturer at an early stage when designing stack and family moulds or multi-component applications to ensure such projects are successful.
* Horst-Werner Bremmer is Sales Director of Günther.