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Benefits and Limits of the Controlled Injection Unit

The controlled injection unit has long since ceased to be a special option on diecasting machines but is usually included as a standard feature on high-quality machines. However, there are still many casting shops where conventional machines produce quality castings today. So why should a controlled axis be used? Are all technical possibilities always useful just because they are feasible?

The core question is always: How can an adequate casting quality level be produced at a competitive price? The required casting quality is eventually defined by the end customer. However, the casting producer’s decisions govern the competitive price.

Numerous factors influence the production costs for a casting. Although the investment for a new machine, or a casting cell, comes first, the predominant issues are the cycle times, the scrap rate, the availability, safeguarding and optimizing the quality by the flexibility and stability of the injection unit in conjunction with an excellent process monitoring system.

 


Fig. A: The controlled injection unit provides a freely programmable speed profile
In the first place, a controlled injection unit provides the flexibility for programming any speed profile for the injection piston, both for startup parts and for production. The operator sets the speed in the visualization via the position axis of the injection piston. This makes it possible to design a long startup ramp as well as quick acceleration for the transition to the second phase. It is also possible to program the deceleration at the end of the die filling process.

The performance of today’s injection units permit idle shot speeds of more than 9m/s. This leaves sufficient margin for the dynamic injection force to achieve short die filling times in spite of elevated die filling resistance. The control guarantees a high repetition accuracy for the speed profile even if influence factors related to the die or injection chamber should change. If the controller detects a deviation of the set and actual speeds, an additional control signal is generated on the servo valves to readjust the speed to the set value. Especially with low speeds in phase 1, the solidification conditions in the injection chamber and runner system remain constant since the same distances are always traveled in the same time.

However, a controlled machine enables you to emulate casting profiles of many different conventional machines and in addition respond to special requirements with a maximum of flexibility.

Unfortunately the best casting quality of many castings is in the biscuit, since very often the pressure build-up time in phase 3 is too long. The squeezing process in phase 3 to fill shrinkage cavities cannot be effective in the affected zones because the molten metal has already solidified in the runner system or in the part. The special design of the MW injection unit makes it possible to start phase 3 by automatic detection of the end of die filling, regardless of dosing fluctuations. This is especially useful to realize a very short pressure build-up time in phase 3. All in all, the caster has a maximum degree of flexibility in setting the casting profile. Certainly a conventional 3-phase profile will be sufficient for many casting types.

However, a controlled machine enables you to emulate casting profiles of many different conventional machines and in addition respond to special requirements with a maximum of flexibility.

 


Fig. B: With higher speeds, the efficiency of the controller decreases because of the physical limitations.
Where there is light, there is shadow! This chart shows a casting curve record as a function of time. This depends on the specific casting; especially with high injection piston speeds exceeding 5m/s, the die filling time may be as short as only 25…35ms. However, the acting time of modern servo valves is approx. 12…20ms. With higher speeds, the efficiency of the controller therefore decreases because of these physical limitations of the valves and due to the compressibility of the hydraulic fluid. This makes it even more important to have a good pilot control in order to be able to run any profile also at higher speed levels.

The figure shows how the controller makes the set value for the servo valve deviate from the pilot control in order to reach the set speed. However, the preset speed cannot be fully reached because of high die filling resistance and short acting time.

In summary, we can state:

A good part quality is achieved by a high repetition accuracy of the injection speeds and pressure curves reaching as far as into the limit ranges. The controlled injection unit offers just these benefits in conjunction with convenient machine setup functions and efficient monitoring features.

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