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Modularity of eCONTROL 8611 with Buerkert instrumentation
Modularity of eCONTROL 8611 with Buerkert instrumentation
Photo: 1234567
Meeting the increasing requirements concerning surface quality and moulding accuracy represents a challenge in injection moulding. Non-tension and non-deforming work pieces are demanded—featuring a high degree of moulding accuracy and a seamless surface. Innovation is driven mainly by the automotive and consumer goods industry. Products like headlamp housings or flat-screen televisions require a very neat-looking surface. Flow irregularities or seams need to be avoided. Apart from surface quality and moulding accuracy, the efficiency of the injection moulding process plays a central role. 70 per cent of the production time is taken up by cooling of the moulded part. Speeding up the cooling could lead to more efficient production.

These three challenges— seamless surface, shorter injection cycles, and a high degree of moulding accuracy—require a special elaborate cooling technology for injection moulding tools. One of these is variothermal temperature control, which has become much more widespread in recent years. .

Variothermal temperature control of tools


Variothermal temperature control means that the mould temperature is not constantly the same, but is controlled according to a defined temperature profile during one injection cycle. At the time of injection the temperature is about 10-30°C higher than the glass transition temperature of the injected material. After injection, the temperature is quickly brought down to de-moulding temperature. With this technology it is possible to produce plastic parts with high-glossy surface without any flow lines, and no deforming. There are several technologies available for achieving variothermal temperature control:
Fast heating of the tool by
· inductive heating,
· infra-red heating,
· electro-thermal heating or
· heating with hot water.
All of these technologies feature fast cooling by means of liquid cooling down to de-moulding temperature.
In view of the requirement for short cycle times, heating based on CPH technology[1] with high-performance ceramics as a heating element or heating with steam have proven to be the most effective methods.

Implementation of such a dynamic process requires temperature channelling that is close to the mould’s outlines, to guarantee quick heating-up and cooling-down. The fact that different zones within the work piece conduct heat in different ways because of diverging wall thicknesses also has to be taken into account. The more cooling channels that run through the mould and the closer they are to the cavity, the more precisely the cooling process can be tuned and regulated. As a precondition, however, every cooling channel needs its own temperature control.

Especially for temperature control in the injection mould with hot and cold water there is now a new type of valve block consisting of 6 individual valves in a modular assembly. (see photo). In combination with one hot and one cold water temperature control unit, this can be used to easily implement a thermal cycling system for variothermal temperature control of injection moulding tools. The 6 pneumatically actuated valves are controlled by a pilot control block which switches the valves between warm and cold based on the injection cycle. The advantage of this solution is a very compact design with the option of integrating the temperature control block in the temperature control units.

Cavity-close temperature control

Variothermal temperature control is partially contradictory to the demand for short cycle times. But this conflict can be resolved by new tooling technologies like the „ Segmented Tool Temperature Control“[2] or Lasercusing[3]. With these technologies, cooling channels are led very close to the mould cavity. Thus it is possible to implement three-dimensional channel structures within the mould und thereby achieve an ideal configuration of the cooling channels.

Accurate temperature control is essential in injection moulding. It depends on the mean temperature of the mould wall inside the injection mould’s cavity. Temperature control can be accomplished inside the mould near the cavity or by measuring the cooling fluid’s temperature in the return line. The most important criterion for reproducibly achieving the correct mould wall temperature is measuring the flow rate of the cooling fluid. The flow rate is measured by using very compact impeller sensors which can be integrated in customized systems and which transmit the current flow rate to the controller. It controls the cooling fluid flow rate of each single channel and corrects the flow rate to the correct rate within fractions of a second by means of innovative proportional valve technology. This type of demand-actuated cooling significantly improves surface quality and considerably reduces cycle times.

Innovative valve technology

The new development of proportional solenoid valves enabled the implementation of multi-loop parallel-working flow rate regulation circuits. The advantage of these new valves lies in the frictionless bearing of the magnetic core ensured by special form springs that avoid stick-slip effects. This is reflected in an excellent performance record comprising paramount response sensitivity (0.1 per cent of the final value), minimal reversal errors and excellent regulation performance. The measuring range of the new solenoid valves is 1:100, which makes it possible to adjust to even the slightest fluctuation in the flow rate by very precise corrective movements of the valves.

The decisive pilot value is the return temperature and flow of effluent water which provides for measuring the cooling process of the work piece. This helps to regulate cooling of the work piece and the mould in an absolutely reliable and accurate way. ”The flow rate has to be measured for every single cooling channel, which makes it necessary to have a valve and a sensor in each conduit,” explains Dr. Egon Hüfner, process engineer at Bürkert Fluid Control Systems. To be able to measure the extracted heat for the cooling effect, the temperature in the forward flow and return flow is also needed. State-of-the-art sensors detect flow changes within 100 to 300 milliseconds and control valves adjust the flow rate within 0.3 to 0.5 Seconds. Impeller sensors particularly qualify for deployment because of their small frame size and excellent response time. For nominal diameters of 6 millimetres and above, magnetic inductive sensors are also used.

Actuator engineering

The choice of the valves depends on the flow rate and the degree of contamination of the cooling fluid. “For high flow rates and dirty liquids, direct-acting valves are recommended. Up to 200°C Celsius, pneumatically operated on/off or regulating valves are appropriate,” says Dr. Hüfner. On/off temperature control in the cooling phase of an injection moulding cycle is applicable if valve opening times are controlled according to the dissipated amount of heat.

For low flow rates and very clean cooling fluids, i.e. if there is a central water treatment unit involved, and for water temperatures below 120°C Celsius, servo-assisted electromagnetic valves are perfect. In spite of a small bleed hole they provide nominal diameters of up to 20 mm. The latest on/off valve generation of Bürkert’s valve technology even manages without bleed holes in the diaphragm and therefore is less sensitive to polluted cooling fluids.

With multiple-channel temperature control that involves more or less cooling, depending on the nature of the work piece, magnetic regulating valves, direct-acting or servo-assisted, provide a clear advantage. Their fracture opening can be swiftly regulated to any value between 0 and 100 per cent. This dynamic regulating valve technology permits the use of flow rate profiles that are pre-programmed and tailored to the work piece.
For high medium pressures, high temperatures and high flow rates, pneumatically actuated valves, available starting at a diameter of 4mm, are ideal. To adjust a particular fracture opening, a supplementary electronic control is necessary. With proportional solenoid valves this is done by pulse with modulation (PWM). With normal process valves it is executed by a positioner that is assisted by an integrated valve position control. It adjusts the piston of the process valve to a particular fracture opening.

Decentralised temperature control

The usual integrated flow system consists of a flow sensor (paddlewheel or ultrasonic principle) a proportional valve and a flow rate controller with cascaded temperature control that can be combined with various valve types. The controller plays a decisive role within the system as it has to be able to process different sensor signals such as temperature, pressure or flow rate. And it must be capable of addressing pneumatic and electric control systems. Bürkert’s new universal controller eCONTROL features all of these qualities.
Among its most important features are:

- Temperature regulation
- Pressure control
- Flow rate control
- Control of on/off magnetic valves, proportional valves, process control valves and electromotive regulation valves
- Inputs for normal sensor signals (standard signal 4-20 mA / 0-10V, frequency, PT 100)
- Communication to central control units with 4-20mA, 0-10V for set point and process value feedback
- Easy start-up according to the selected application (flow, temperature or pressure control)
- Data of most Buerkert valves and sensors are stored in memory

eCONTROL is also available as a panel version with 1/16 DIN cut-out that can be integrated into an existing control cabinet.

Decentral regulation offers the following advantages in particular:
· Sensor, valve and regulator work together perfectly
· Machine controls can focus on their proper tasks.
· It also facilitates retrofitting. It requires only simple optimisation measures to considerably shorten cooling cycles and significantly optimise work piece quality.


Notes:

[1] Dynamic temperature-controlled precision tools with CPH High performance ceramics, GWK information brochure

[2] ibid.

[3] LaserCUSING is a patented production process from CONCEPT Laser GmbH


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