Teningen/Waiblingen, July 2023 − THIEME GmbH & Co. KG, a specialty machine builder for industrial printing applications, has developed a highly automated printing machine that prints four-colour designs onto chainsaw guide bars for STIHL AG. One of the main challenges was to time the printing process cycles to mesh perfectly with the current production process.
Established in 1926, family company STIHL is a global market leader in chainsaws for forestry as well as horticulture and landscaping. Chainsaws contain a replaceable tool unit consisting of a guide bar and a cutting chain that runs around the bar. To visually emphasize STIHL’s technological leadership, the company decided to print an attractive four-colour design onto the guide bars instead of the previous single-colour STIHL logo. To make this happen, THIEME developed a high-output, application-specific digital printing system. Several machines in this series are already in use at various production locations. They replace existing printing lines that were only capable of printing a single colour (STIHL logo).
The entire printing process, including supplying and removing the guide bars, is fully automated. Once the pre-coated guide bars are positioned and aligned, they are primed, printed with the four-colour image and exposed to UV light inside the machine in order to cure the coating. Each guide bar then receives a laser marking. The current machine configurations can print multiple guide bars simultaneously. The guide bars are printed with a slight bleed to ensure full coverage; the excess paint particles are suctioned away. The printing process uses a multi-pass approach – the printing unit makes several passes over the substrate with a minimal offset between each pass. This process hides the kind of nozzle defects that can occur in digital printing and produces very consistent, high-quality printing.
Optimization at the THIEME Technology Center
The printing machine is equipped with Ricoh printheads. STIHL, following an extensive sourcing process, selected printing inks that work well within its production processes. Months of printing trials were then conducted at THIEME’s Technology Center in order to nail down the ideal combination of productivity and print resolution. The results of the printing process have very stable delta E values and so can be reproduced with a high degree of accuracy. STIHL was very keen on high colour accuracy in order to ensure that the guide bars would have a consistent, high-quality appearance.
Alongside print quality, another major focus was coating durability – an obvious consideration given the use case. The printing process was developed jointly with THIEME in order to meet the high quality standards.
Seamless Integration in the Production Process
The new printing station was incorporated into an existing production line, where it replaced a single-colour printing machine. It prints multiple guide bars simultaneously in order to achieve the required cycle times. It now takes only a few seconds to print both sides of a guide bar – and that includes drying and laser marking. The machine is fully enclosed; filtered air streams into the system from the top and exits at the bottom. This clean “print island” approach avoids possible quality problems caused by factors such as dust from the production environment.
The material handling is fully automated as well. Handling robots and servo drives supply, remove, transport and rotate guide bars of different lengths (from 300 to 1300 mm). High-precision linear motors move the substrates up and down all the print axes.THIEME developed different machine designs for STIHL’s Light and Strong product lines: Two printing stations were placed in series in order to coat the shorter Light bars (300 to 700 mm in length), while a two-tier system was built for the 1.30-meter-long Strong bars. Once the first side is printed, the bars pass through the upper tier and then return to the same printing station, which then prints the second side. This approach made it possible to design a highly compact machine with a small footprint.
Automatic Data Transfer
The printing system is controlled by a Siemens PLC. It is operated using touch panels, with users receiving different authorisation levels (for workers, maintenance, etc.). When a print job is selected, the THIEME digital printing system retrieves the associated image right from the STIHL print data server. That way, different designs can be quickly and easily implemented for specific batches, such as special editions for specialty retailers. Set-up times are minimal since settings such as bar size do not have to be manually configured. However, it takes different knowledge to operate this system than the previous printing process – knowledge that THIEME teaches in intensive courses.
Once the print job has been selected and the image imported, the system automatically sets itself up on its own. In fact, after changing products, all the operators have to do is restart the process. If necessary, they can request sample bars for a visual check. The system then ejects one set of guide bars onto a roller conveyor for inspection. In all other respects, the printing process is fully operational without requiring any additional intervention. The machine even cleans the printheads autonomously as needed. It automatically stops, flushes the printheads and restarts at the end of the cleaning process – no start command is needed.
The printing system is also connected to a master production control system that can pass on production information such as system status, error messages, printing mode, productivity, etc. This information allows rapid responses to departures from the normal production process. The printing machines used at STIHL offer a high level of technical availability on par with the remainder of the automated production line. In addition, THIEME provides different after-sales service packages, including 24/7 service with three-hour response times.
Custom Development to Customer Specifications
These extraordinary digital printing systems were developed specifically for and with STIHL (but informed by THIEME’s decades of experience in building printing machines for industrial applications). The partners engaged in a joint optimisation process that yielded a high-output, high-availability system design that considers all of STIHL’s preferences and requirements with regard to functionality, flexibility and productivity. These are customized printing systems that integrate perfectly with the existing production environment. The machines, which are already running in three shifts 24 hours a day, print millions of Light guide bars every year.
Additional two-tier systems for the strong product line are currently under construction. Once they are finished, all production capacity will be able to transition to digital printing. That means even more STIHL guide bars will feature a striking four-colour design that stands out in the industry and underscores STIHL’s role as a technology leader. The STIHL application also clearly demonstrates how highly automated printing processes can be seamlessly integrated into industrial production processes.