Medical Device Link.

MANUFACTURING

Multicavity moulds

Figure 1: Temperature increase with the system switched off (top) and the system switched on (bottom). (click images to enlarge)

Multicavity moulds with 16, 32 or more impressions are a long established method of manufacturing large production quantities of moulded medical components. These moulds are fed by hot runner systems, often with pneumatically operated valve gates. Valve gates provide more accuracy than other types of hot runner tip because the mould gate can be kept in a molten state until an exact preset time when the gate is closed. Despite this, controlling the accuracy of all the parts moulded in multicavity tools can be challenging and various systems have been developed aimed at improving the process control.

Integral closed-loop control in the mould

Sensor systems for injection moulding have been used for many years for monitoring cavity pressure and temperature or injection pressure at the nozzle of the machine. These systems can be used for quality monitoring or to help optimise the process set up. This type of system can also be used to initiate a switchover to holding pressure that is based on a preset level of mould cavity pressure.

A more recent development is closed loop filling control of multi-cavity moulds within the mould itself, controlled and monitored by an external personal computer (PC) platform or laptop. In this case, the position of the melt front flowing into each of the individual cavities in the mould is linked to the control of the polymer being injected at the gates of each specific cavity.

Figure 2: The thickness measured on the moulded parts near to the gate and at the end of flow. Blue: near gate; red: opposite gate. (system off, top; system on, bottom) (click images to enlarge)

Figure 1 shows the temperature increase that results from the injection of melt across 32 mould cavities with the system switched off (top) and with the system switched on (bottom). This figure illustrates how the control system brings the temperature profiles on filling close together within a fraction of a second for all the cavities.

The system works by using a temperature sensor placed near to the end of fill in each mould cavity and linking this to the temperature control of the relevant hot runner tip. As the injected hot polymer melt reaches the sensor, the temperature rapidly increases. The PC registers the temperature ramp start for each cavity and automatically tells the valve gate heat controller to increase or decrease the temperature of each valve gate. Changing the temperature of a valve gate changes the viscosity of the polymer melt at that point, which fractionally changes the speed at which the polymer flows into the mould cavity. Because the filling speed of each cavity is automatically adjusted, the melt fronts are trained to reach the sensors at exactly the same time.

Figure 3: Thickness difference per cavity (near gate – opposite gate). Red: system off; blue: system on. (click image to enlarge)

Implementing this type of system on multicavity moulds allows the accuracy and repeatability of the moulding process to be dramatically improved. Figure 2 shows the thickness measured on the moulded parts near to the gate and at the end of flow for a medical component. The nominal thickness for both areas of the part is 0.8 mm. By using the closed loop control system for the valve gates, the difference in thickness between these two points is dramatically reduced. Figure 3 shows the difference in thickness for each cavity: the thickness variation has been reduced from between 22 and 54 microns to less than 18 microns, and in some cavities to less than 5 microns.

Figure 4: A part moulded using a system with integral closed-loop control in the mould.

Figure 4 shows a part moulded using this system. The position of the valve gate and sensor are indicated in Figure 5. Here, the mould flow simulation (Moldflow, www.moldflow.com) shows how the temperature sensor is located near the end of flow; in this case, the sensor’s diameter is 1 mm.

Servo-actuated valve gates and synchronised valve stems

The majority of valve gates are pneumatically actuated. In industries in which cleanliness is not critical and where large parts are moulded, some are activated by hydraulic means. Pneumatic systems are popular in the medical device industry because they are relatively clean and easily integrated into a production system. Typically, they provide reliable actuation and effective valve gating technology, and system maintenance is relatively straightforward. The limitations of pneumatically actuated valve gates are in the pressure available to operate the gates; this may necessitate the use of larger cylinders than desired and result in larger valve gate spacing in the mould. The system setup also requires special attention to avoid delays between valve gate pistons resulting in inconsistent cavity filling.

The recent development of electrically actuated valve gates overcomes the pressure and spacing limitations of pneumatic systems and maintains a set up suitable for a clean room environment. There is no dust, debris or oil involved because the electrical power is converted into mechanical movement through a servomotor, bearings and cams. Electrically actuated valve gates arguably provide the best features of pneumatic and hydraulic systems without the associated disadvantages. By using a servomotor to control the valves, the valve stem speed can be adjusted and high stem force can be delivered for good
gate quality.

Figure 5: The valve gate and the temperature sensor, which is located near the end of flow. (click image to enlarge)

It is possible to precisely synchronise the movement of the valve stems by coupling them together in a plate in the mould. This eliminates the lag that is sometimes present in pneumatic systems and ensures that each stem is in exactly the same position during opening and closing of the gates. Coupling the valve stems together in this way and using an electrical actuation system also allows the valve gates to be positioned closely together; for example, valve gate pitch dimensions as small as 18 mm have been used.¹

It is sometimes necessary to individually shut down valve gates. Synchronised valve gates that use plates are designed so that the individual valve stems can be decoupled from the plates when needed. The stems are also designed to automatically decouple at a predetermined force, if any blockage occurs in the gate. The only specific instance in which a synchronised system could not be used is when sequential injection is needed, which would require individually actuated valves.

Improved balancing and thermal control of manifolds

The design of hot runners for technically demanding parts has changed during the past few years to provide better gate balancing, repeatability and thermal control. Gate balancing and repeatability are particularly critical where small components are to be moulded in multicavity systems.

Some materials are particularly heat sensitive or shear sensitive and the design of the hot runner system is a critical factor in how the process performs with these types of materials. Typically, more heater zones are used and the hot runner is designed to avoid hot spots and minimise the temperature deviation across the manifold. Manifolds can be designed to suit specific materials and shot volume or filling speed so that the shear rate is not too high. The overall aim of precise hot runner manifold design is to improve the process window that is available so that the process is more reliable and cost-effective.

Conformal cooling for reduced cycle times

The principle of conformal cooling is to have the cooling water in the mould conform as closely as possible to the shape of the mould cavity to thereby reduce cooling time and provide more uniform temperature distribution and shrinkage. Take up of the technology has so far been limited although it has attracted a lot of interest.

Conformal cooling channels are created by one of three methods.² The oldest is to use a stack-up of plates held together by vacuum brazing, each of which contains individually milled cooling channels. More recent methods are the use of laser sintering to build up a solid steel three-dimensional channel design by fusing together steel powder, or the use of alloys with high thermal conductivity, which are sintered by diffusion bonding into cavities previously introduced into the mould insert.

Using any of these methods makes possible the creation of new cooling channel designs, which could not be manufactured by conventional means. This facilitates more effective cooling including further options for independently controlled cooling channels running close together and the potential to reduce cycle times.

Ongoing developments

Hot runner processes and multicavity moulding would be considered by many to be mature technologies. However, developments in specific areas are continuing to enable advances in terms of quality control, repeatability and reduced cycle times.

Paul Glendenning is Business Development Manager at MicroSystems (UK) Ltd, 101 Golborne Enterprise Park, Warrington WA3 3GR, UK, tel. +44 1942 290 960, e-mail: paulg@microsystems.uk.com, www.microsystems.uk.com