Processes involving thermoplastics are always carefully studied by CD Automation in order to provide partners with customised power quality projects aimed at the efficiency and greater sustainability of such processes.
In this article we will try to look at a further plastics processing operation, which makes it possible to produce a variety of hollow-body items: blow molding. In more detail, we will describe the stretch blow moulding process and the solutions offered by CD Automation for this process.
What is blow moulding?
Blow moulding is a technique used in the processing of plastics, derived from glassblowing. The first dedicated blow moulding machine was designed by Enoch Ferngren and William Kopitke and sold to the Hartford Empire Company in 1937, the year in which the technology became commercially widespread. Glass, too fragile for such processing, was soon supplanted by plastic, and in 1939 the first mass production of bottles of this material was made in the USA.
How the blow-moulding process works?
Blowing is a fairly simple and intuitive operation; however, it consists of several steps:
- First, the raw material in the form of granules must be introduced into a heated cylinder.
- The high temperatures make the material pasty and ready to be mixed, via a screw, and extruded or injected to form a tubular plastic preform, the parison.
- The parison is clamped in a special mould, which is shaped according to the final product shape.
- Air is blown into the preform which inflates the plastic, causing it to adhere to the walls of the mould.
- Once the thermoplastic material has cooled and thus hardened, the mould opens and the part is ejected.
What are the advantages of this technology?
Blow moulding provides a wide range of benefits, including:
- short production times
- possibility of blowing complex parts and adjusting the weight of the container;
- opportunities to incorporate handles into packaging design.
Such processing is also ideal for producing multi-layer packaging and containers with a visibility window.
Focus on stretch-blow moulding
Blow moulding is carried out according to various techniques. One of the most effective, particularly in the beverage sector, is stretch-blow moulding. This process is in fact well-known for the production of PET bottles. Used since the early 1970s, the process spread widely in parallel with the growth in demand for carbonated beverage bottles
The benefits
One of the main advantages of stretch-blow moulding is the ability to stretch the preform in a biaxial direction to increase the tensile strength, barrier properties, transparency and top load of the containers. These increases lead to a reduction in the overall weight of the containers of 10/15% compared to other processes.
Single-stage or two-stage, how to choose?
Stretch blow moulding processes can be divided into two categories: single-stage and two-stage. The former uses the extruder to inject a preform into a mould, in which the plastic is rapidly cooled, forming the parison. Once softened, it extends to about twice its original length. The operation then continues, in the same machine, as described above for blow moulding. This technique is effective in applications where particularly high production speeds are not required.
Two-stage stretch blow moulding, on the other hand, is used for high production volumes. It differs from the previous technique in that it uses preforms that have been previously manufactured. This feature allows companies to produce or purchase their own preforms.
In this process, the machine injection moulds a preform, which is then transferred to another machine, where it is blown and ejected. Two-stage stretch blow moulding usually requires large production runs to justify the heavy expenditure on the injection moulds needed to create the preform and the blow moulds needed to obtain the container.
The above-described independence of the injection moulding process from the blow moulding process allows both processes to be optimised separately. This advantage means that preforms can be stored, shipped long distances and used when required.
Application in the beverage sector
Stretch-blow moulding, as anticipated, is part of the two-stage production process of Polyethylene Terephthalate (PET) bottles, the most widely used material in beverage packaging. For this application, stretch blow moulding consists of two stages: in the first, injection moulding, the machines produce parisons in the shape of a vial or test tube. Their necks are completely finished, but the dimensions of the preforms are considerably smaller than those of the finished product, which is only obtained through the heated stretch-blow moulding phase.
The function of infrared lamps
The last indicated stage is provided by heating modules equipped with infrared lamps. This is a delicate moment in the process. It is indeed difficult for the different preforms to spend the same amount of time inside the ovens, due to the way they are indexed. And this problem is exacerbated as the number of mould cavitation increases. Furthermore, the heat emitted by an infrared lamp is not constant along its entire length. In fact, its central point emits 10-20% more heat than its extremities.
These disadvantages prevent the preforms from being heated uniformly. This in turn leads to differences in the distribution of the bottle walls in the mould.
CD Automation solution
CD Automation, with over 30 years of experience, has developed the REVO PN, multi-channel power unit for the efficiency and sustainability of stretch blow moulding processes and the control of heating elements and infrared lamps. This unit is capable of handling applications with many power zones in a small footprint. Thanks to the power distribution algorithm, with REVO PN it is possible to balance and limit the peak current demand so as to maintain a high quality of the current drawn, limiting losses and consumption.
REVO PN role
The use of REVO PN in stretch blow moulding processes allows power peaks to be significantly reduced, keeping them below contractually agreed levels and avoiding penalties and unexpected expenses.
The unit is capable of handling up to 24 heating elements. By synchronising the switching on of all channels, it is possible to avoid voltage peaks, while keeping the power factor close to 1. And each module can communicate autonomously with the most common field buses and diagnose heater element failures and short circuits on the thyristors at an early stage.
Last but not least, REVO PN’s ‘Dynamic burst firing‘ mode reduces flickering in short-wave infrared lamp applications.
The solution for controlling infrared lamps
For applications requiring the use of infrared lamps, CD Automation offers a complete, integrated panel comprising thyristor unit and temperature or humidity controller with automatic/manual control mode. Temperature detection can be done by thermocouple or pyrometer, with the option of using a speed input as a power setpoint, thus ensuring precise and reliable process control
Do you want to find out how to make your stretch blow moulding processes more efficient and competitive? Contact us and book a free consultation session with one of our experts. You will receive strategic and customised proposals according to your needs.
