In this post we are going to discuss the structure of die casting cycle time and how its optimization can bring various benefits for the company.
THE STANDARD CYCLE
The term “die casting cycle time” is used to identify the time lapse between the beginning of an injection cycle and the next one. It can be obtained through a summation of the partial times in the die casting process. The standard cycle can be divided in the following phases:
- Injection time:
The piston follows a defined route at 1st phase speed, through which the material is pushed up to the nozzle; it will then be accelerated by a nitrogen expansion in the accumulator in order to reach 2nd phase speed, leading to the filling of the mold cavity.
- Die casting technical time:
It includes both compacting and cooling phases, thus involving metal change of state, from molten to solid. A compacting phase is necessary to compensate the volume reduction caused by phase transection.
- Mold shifting/lubrication time:
It is the sum of mold opening and closing time, extractor’s forward and backward movements and mold lubrication time. The lubrication can be manual, done with a Cartesian system or with fixed nozzles.
- Control time (optional):
It can vary depending on the piece-release system of choice. When the withdrawal is done by a robot, it would be advisable to choose a control system based on photocells. In a free-fall on balance system it will be necessary to set the expected weight on the scale, in order to detect eventual missing parts that could be left inside the mold.
At the end of the die casting cycle, further operations of runners and overflow removal are needed and, for some critical products, further refinishing. Some of these operations can be executed manually by a worker, while others -such as threading- require a dispatching of the raw piece to third-part firms.
Alternatively, it is possible to integrate the production line with anautomated system in line with the casting machine. By using automated machines for scrap removal it is possible to save man-hours, which can be invested in different ways, for example in Kaizen activities.
Another viable optimization can be achieved by integrating automations in the die casting machine for precise machining, such as threading, thus reducing WIP’s transit time in the plant. Thanks to this integrated automatic machineries, capable of complex machining, it is possible to reduce lead time and to fulfill client’s request more easily.
The time necessary to complete the whole automation cycle adds (or overlaps) to die casting cycle time, and together they form the standard cycle time for a determined product.
HOW TO MEASURE COMPANY PERFORMANCES
In order to quantify a company performances, it is possible to rely on 2 indexes:
- The first one is OEE (Overall Equipment Effectiveness), a percentage measure obtained by multiplying availability, performance and quality. The index refers to the productive return of the plant;
- The second one is OTIF (On time in Full), which expresses as a percentage the logistic ability of a company to send the requested quantity of products at the expected time and place;
The optimization of cycle time increases the OEE value by rising the efficiency. Moreover, thanks to a systematic reduction of production time, more machine-hours are available, allowing the company to fulfill a bigger number of orders without changing the plant opening hours.
As stated above, OEE is composed of 3 factors:
- Availability: the relation between available machine time and operating machine time;
- Performance: the relation between produced pieces and producible pieces;
- Quality: the relation between pieces and produced pieces;
Frequently the improvement in die casting cycle time also affects the quality of the casting. A time reduction implies an increased number of injection per hour, which intensifies mold thermal input produced by the molten metal. This leads to an improvement in the surface quality of the casting, since it decreases the number of cold fronts responsible for cold laps on the surface.
There are many advantages achievable by an effective cycle time optimization, that can enhance the whole production. In the next paragraph we are going to explain in detail some techniques for time cycle improvement.
HOW TO OPTIMIZE DIE CASTING AND AUTOMATION CYCLE TIME
First of all, it is necessary to define whether to intervene on the die casting cycle or on the automation cycle. In an ideal production cycle the two coincide, which means that the process are all parallelized and there are no bottlenecks.
In order to define the priority of an intervention a Pareto chart of partial times can be used: the chart is composed of an histogram representing the distribution rate of a phenomenon, displayed in decreasing order so as to permit instantaneous identification of the main factors. After a graph analysis it will be possible to spot the critical points in need of optimization: we are going to choose the highest impact elements.
Once the graph is defined, we need to proceed with a separation between independent processing time and dependent processing time.
The term “independent processing time” is used to define actions that occur in series, one after the other. The reduction of an independent processing time entails an immediate reduction of the cycle time.
On the contrary, dependent processing time define those operations that occurs at the same time as a different one; the reduction of a dependent processing time without a reduction of the main process from which it depends, does not create a variation in the cycle time.
- Revising independent processing time: automation with robotic withdrawal
Once decided to reduce automation time, the next step will be to film the whole cycle of the robot, from withdrawal to dump. Thanks to this shooting, it will be possible to write a complete list of the robot’s movements and operations, along with their exact duration. Starting from these data, it will be defined whether to intervene on the movement speed or on their trajectory.
The movements done by a withdrawal robot through the course of a working cycle are: aligning with the machine columns, entering the press area, withdrawal of the diecasted product, exiting from the press area, transit to the check point and dump.
Before proceeding to the actual optimization, some simulations are run in order to form die casting cycle time hypothesis, and afterwards decide how to intervene: sometimes it is sufficient to alter speed of movement, some others it will be necessary to remove superfluous movements.
- Optimization of die casting time
Die casting process optimization is more challenging, since the majority of the process requires a fixed amount of time, for example in operations like piston compression and casting cooling. Examples of possible improvements in this phase are:
- Cooling time
It can be reduced modifying the mold’s thermoregulation through the use of water/oil cooling units; in other words, a reduction of waiting time of the cast in the mold can be compensated by a reduction of T° in the thermoregulation fluid. - Die casting duration
A prefilling function can be a useful support, since the saving per cycle can be assimilated by the duration of the first phase of injection. The movement of the piston in the 1st phase is then transformed from independent time to a dependent time, bounded to mold closing.
For a better chance of optimization it is possible to intervene on the extraction time (start delay, course speed, return speed, return delay, number of runs), mold opening/closing (opening/closing speed, aperture run) and lubrication (start delay, duration).
When working with an automation system, the necessity of synchronizing movements of the robot to the new schedule should be taken in account. The optimization of cycle time can be highly variable and bring to a 20-30% time reduction or greater, thus leading to a good economical saving for the company.
To learn more about production cycle time optimization, please take a look at our other posts on the topic.
To be always up to date with die casting news, subscribe to our blog.
