Until the late 1970s to early 1980s blow molding machines were primarily delivered with contractor control. This type of control offered the advantage that the electrical components could be checked by appropriately trained maintenance personnel and easily replaced in the event of a fault. Changes in the machine control system could be made with the help of simple auxiliary devices. However, the disadvantages were the susceptibility to faults and short service life due to wear of mechanically moving parts, the large number of components (up to 150 contactors, 12 temperature controllers, etc.), the size of the components, and how these elements were connected. This type of control was called “hard-wired programmed logic controller”.

As early as 1978, a control system for blow molding machines was introduced that avoids these disadvantages. This was the programmable logic controller (PLC) for the automatic function sequence of the blow molding machine.

In contrast to the old hard-wired control systems, the logical connections of the machine using a programmable logic controller are not in the wiring of the individual components, but instead stored in a memory (EPROM).

In addition to the pure sequence control of the machine (movements of the clamping unit, ejection of the accumulator head, opening and closing of the blow mold, moving the blow pin in and out, start of pre-blowing, start of blowing, blowing time, exhaust, etc.), the PC control takes over several other functions:

• Temperature control of all heating zones (extruder, head, die)
• Extruder speed
• Digital stroke measurement of blow mold and mandrel movements (wall thickness control)
• Speed control of the blow mold and mandrel movements (wall thickness control)
• Wall thickness control (formerly mostly 64, 128, or more points; today mostly freely selectable) for continuous and discontinuous extrusion
• Parison length control
• Process data acquisition and monitoring
• Fault diagnosis, including remote diagnosis and troubleshooting via web-based remote control
• Weight control

The machine operator controls all the settings of the machine via a user interface which is connected to the real-time-capable software PLC. The HMI (Human-Machine Interface) is used exclusively for the visualization of the machine and process as well as for the input of commands by the machine operator and for recipe management.

All machine and product settings can be saved as a recipe. Normally, these recipes can then be managed and backed up independently of the machine using a USB stick (flash drive) or network access. Cloud-based systems for continuous data storage and evaluation are already in the works as well. The machine functions themselves are controlled by the PLC. The software PLC is a system independent of the operating system, which is still functional even if problems occur in the operating PC.

Today, modern machine controls work with so-called Industrial Ethernet protocols such as Ethernet-IP, EtherCAT (Beckhoff), or ProfiNet (Siemens). Here, the signals from the PLC are sent and received from and to the control elements via a bus system. A central unit within the PLC converts the information into signals, which are then sent to a decentralized transfer unit. The individual control elements are connected digitally and via analog input and output units using relatively short 24 V cables. I/O-Link has recently become alternative to the use of analog signal transmitters.

Such a bus control system allows a considerably reduced wiring effort in the machine. In the past, many individual cables had to be plugged or unplugged, whereas today there are only one or two network cables involved. Furthermore, it is much less sensitive to electromagnetic interference (EMI) than the classic solution described above. More recent trends are already using wireless communication paths, for example in Wi-Fi-based robot tools for inserting components into fuel tanks and later removing the finished fuel tanks.