Precise and efficient cutting (sawing) of lightweight core materials
In the light of the goal of reducing CO2 emissions and energy consumption in conjunction with growing mobility, the demand for lighter, more robust materials is constantly increasing. Lightweight core materials play a central role in meeting this demand. This article has been published in JEC Composites Magazine N°155.
The proportion of lightweight core materials is increasing, especially in sectors such as aerospace, transport vehicles and wind power. Processing them requires special cutting technologies to achieve maximum precision with minimal material loss.
Applications of lightweight core materials
Honeycomb core materials, based on aramid, aluminium or glass fibre, are used in areas such as interiors, winglets, spoilers and many other parts for commercial and military aircraft, helicopters and similar (Figure 1).
Wind turbine blades are becoming ever larger to increase energy efficiency. This places higher demands on lightweight materials. Rotor blade core materials include (recycled) PET, PVC rigid foams and balsa wood (Figure 2).
Rigid foam panels (e.g. PET, PU) and honeycomb structures (e.g. PP) are used in transport vehicles, trains and ships, for components such as interior fittings, side walls, roofs, floors, partitions and sanitary fixtures. Typically, these lightweight core materials are further processed into a sandwich panel structure by reinforcing them with glass or carbon fibres, for example, to increase their stiffness.
Sawing process
Requirements
Through various cost-intensive processes, these lightweight core materials (be they aramid honeycomb, rigid PET foam, PP honeycomb or balsa wood) are initially used to produce a high-value raw block. Throughout the value chain, horizontal sawing is one of the main processes used to produce these end products. To save as much of the high-quality materials as possible during sawing, the focus is on a very thin kerf with a high degree of precision and productivity. The thinner the kerf, the less raw material is lost during sawing. Special horizontal band saw machines and saw blades are used to produce the above-mentioned components, which fulfil these requirements.
When sawing aramid honeycomb with a cutting width of more than 2 m using horizontal band saws, customers achieve kerfs smaller than 1 mm, for example, depending on the workpiece type. To be able to achieve considerable material savings in the long term, every tenth of a millimetre of kerf counts. At the same time, tight thickness tolerances of less than ±0.15 mm must be achieved when using extremely thin saw blades (Figure 3).
In addition to thickness tolerances (in the ±0.3 mm range), the resin uptake is also important when using PET rigid foam panels for rotor blades for wind power plants; this in turn depends on the surface quality after sawing. Different objectives –such as optimum surface quality with high productivity and the thinnest possible kerf– place high demands on the sawing process and on the selection of suitable saw blades.
Saw blades and objectives
A range of objectives can be considered in different orders of priority when sawing these lightweight core materials, depending on the application and material. Typical objectives for aramid honeycomb include a high degree of precision in conjunction with very low thickness tolerances of just ±0.05 mm. Next comes the requirement for maximum productivity with feed rates of up to 15 m/min for PET rigid foam. Further demands placed on the sawing process include a defined surface quality – e.g. minimum fibres in aramid honeycomb, prevention of scratches and optimised resin uptake in PET and balsa wood. Thanks to high-precision cutting technologies, material can also be saved by avoiding a trim cut after each saw blade change. High machine availability resulting from less saw blade changes, longer saw blade lifetimes and repeatable cutting results are also essential.
Using suitable saw blades with the appropriate cutting parameters plays a crucial role to achieve these objectives. A large range of different process options makes it easier to optimise these targets according to the application and priority.
Suitable saw blades for the respective applications are tested in HEMA’s test centre in collaboration with the customer. Hereby, a variety of saw blades are used, from extremely thin ones (e.g. 0.5 mm thickness) to strong blades (e.g. 1.1 mm thickness and widths up to 34 mm). The type of saw blades varies as well: hardened teeth, bi-metal, stellite-tipped or carbide-tipped teeth, to name just a few.
In addition to using the right saw blade, special technologies are needed to achieve the targets set out above. These include high-precision, dust-protected, easily-adjustable saw blade guides. Furthermore, it is important to achieve high saw blade speeds of over 4,000 m/min.
Intelligent blade tensioning plays an essential role as well. The saw blades are automatically loaded with the calculated tension by specifying the desired saw blade tension on the control panel. The saw blade is positioned to the desired cutting height with a positioning tolerance of ±0.05 mm, in a highly precise and dynamic way. Also important is precise workpiece fixing with automatically-adjustable vacuum zones depending on the workpiece size.
Turnkey solutions including material handling
In commonly-used band saw solutions, the raw blocks are loaded with a special block handling system, and the sawn sheets are removed manually.
However, turnkey solutions are increasingly often required from a single source. The block feed to the saw takes place automatically via buffer stations. Then, the sawn sheets are automatically removed by a robot or portal handling system and sorted if required (Figure 4, previous page). The sheets typically run through the cleaning station and onto quality control (weight, dimensions, ultrasonic testing, etc.), then they are printed and stacked. Increasingly often, the entire manufacturing process is implemented using a complete production line manufactured from a single source.
The digital cockpit
Digitally depicting the sawing process maximises productivity. The band saw operation is monitored with various apps and supported by subsequent data analysis.
This takes place both with the help of real-time data and based on aggregated historical data, which is stored continuously.
The monitoring app records data such as, for example, the axis speeds and current power consumption of the drive axes, as well as position data from the machine axes and much more. Each data record can be limited with target and actual values. If a value exceeds a predefined limit, a corresponding alarm is generated via the alarm application. Production management can then intervene immediately and analyse the anomaly.
A deep understanding of the cutting process for each application enables the user to consciously set the appropriate parameters to achieve the desired results (Figure 5). Machine builders strive to offer their customers the widest possible process window to achieve the required objectives, while still tolerating deviations from the parameters set by the operator.
FOCUS : Heermann Maschinenbau GmbH (HEMA)
HEMA, a company with over 100 years of experience, develops, manufactures and distributes band sawing systems and cutting solutions for different materials and applications. A particular specialisation of its portfolio is cutting solutions for lightweight materials, as well as construction and insulation materials. The company supplies band saw and cutting solutions ranging from single machines to complete block processing lines including loading, sawing, trimming, handling, transporting, sanding, etc. focusing on flexible, high-precision customer-specific solutions. HEMA exports to Europe, the U.S. and Asia, with an export ratio of over 90%. HEMA has belonged to the Austrian Wintersteiger Group since January 2022. The development and production of saw blades are carried out within the group together with Wintersteiger Sägen GmbH in Arnstadt (Thuringia). Automation expert VAP-Wintersteiger GmbH in Mettmach, Austria –with over 20 years of experience in the automation of production lines– is also on board. Like Wintersteiger, HEMA is a specialist, technology and quality leader in a niche sector.
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