Coolants in the composite machining process

Composite materials such as carbon fibre reinforced plastic (CFRP) and glass fibre reinforced plastic (GFRP) are important raw materials for many industries due to their special properties; however, they are difficult to machine. Drilling and machining on these materials are usually dry operations. However, the workpieces often pose a challenge to production managers due to their poor quality and excessive waste. Lubricant manufacturer Rhenus Lub is proposing a problem-solving and forward-thinking approach to working with CFRP: using specially developed coolants in the machining process.

The lighter the better
The automotive industry and aerospace industry, for example, rely on materials that are lightweight, but also high-strength and reliable. The reasons for this are obvious: When a process requires acceleration and speed, weight reduction is quickly followed by a noticeable reduction in operating costs. In addition to high-strength light metals such as aluminium, magnesium and titanium, fibre-reinforced polymers (FRP) in particular are playing an increasingly important role. Carbon fibre-reinforced plastics are particularly prevalent. Not only are they exceptionally light, they are also extremely sturdy.

Dry machining is put to the test with disappointing results
Composites are in high demand, but the process of dry machining—widely used for this innovative material—is highly inefficient. Although components made of CFRP are already manufactured in near-net shape and often only require supposedly simple machining operations such as drilling or milling, dry machining can result in serious problems. This is an open secret among many materials experts. However, as there was no real alternative to the conventional dry process, these problems and the resulting increase in costs have been more or less accepted up to now.

This remains a critical issue. Processed workpieces add a great deal of value, yet come at a significant disadvantage. This is because components are often inadequate in terms of quality due to delamination and split fibres despite the fact that the speed and feed rate during processing are very low. The tools are also under remarkable stress: The fibres contained in the composite can cause drilling, milling and grinding devices to wear extremely quickly, resulting in frequent tool changes. To counteract this heavy wear, high-quality tools made from polycrystalline diamond or tools with diamond-like coatings are generally used. The sheer magnitude of the resulting tool costs can have a significant impact on the competitiveness of the pioneering material.

However, dry machining of CFRP also raises a number of questions with regard to health and safety. The fine dust released during cutting is a particular cause for concern. High quantities of fine particles can have a negative effect on the health of the workers employed in the production area. To avoid endangering employees, employers must install ventilation systems and filter systems in the workplace. This equipment comes at a high cost, but is mandatory to provide adequate protection against fibre dust.

Taking into account the considerable costs and the potential health risks, it begs the question: Why rely on dry machining of fibre-reinforced plastic in the first place?

Forward-thinking solution from Rhenus Lub
Since dry machining these high-strength materials is difficult to justify both economically and in terms of health, alternative procedures may be the key to improvement. Lubricant manufacturer Rhenus Lub, which specialises in high-performance lubricants and greases, is all too familiar with processing exceptionally demanding materials and the associated challenges this brings (increased tool wear, reduced workpiece quality, health risk). The Mönchengladbach-based company is offering a promising solution for the sustainable machining of fibre-reinforced plastics: the use of coolants specially developed for this field.

The highly specialized development department has already been testing the machining of CFRP with coolants for some time and joined forces with strong partners from the outset of the research project to generate synergies for this innovative project. The Mönchengladbach team is carrying out testing on newly developed coolants assisted by the Research and Transfer Centre at the University of Applied Sciences Zwickau. Tools and workpieces are inspected for wear and machining quality during suitability tests carried out in a production environment. The company also works closely with the German Aerospace Centre (Deutschen Zentrum für Luft- und Raumfahrt, DLR), carrying out material tests and compatibility tests on fibre-reinforced plastics.

Logical evolutionary step
The promising alternative to the conventional machining method may surprise one or two market operators. However, for Rhenus Lub, with its extensive market knowledge, years of research expertise and chemical proficiency, this is the logical evolutionary step, as Dr Hans-Jürgen Schlindwein, Head of Coolant Quality Control R & D at Rhenus Lub knows well: "With our product and service portfolio, we aim to bring a wide range of our customers' manufacturing operations ever closer to perfection — and this is precisely what the use of special coolants can do for CFRP machining, as our large-scale practical tests have proven. To continue using the same methods would be a real barrier to innovation". The first fully developed products are the two special coolants rhenus XY 190 FC and rhenus XT 46 FC. "The coolants are suitable for the machining of fibre-reinforced plastics such as carbon fibre or glass fibre and cast serious doubt over the efficiency of dry machining processes under certain conditions", explains Schlindwein.

Practical results are convincing
Industrial field operations with selected customers from the automotive industry show that using special coolants to machine CFRP and GFRP has a direct and positive impact on the quality of the cuts and drillings, the cost per drilling, the process costs and costs of waste, as well as on other key process figures.

The direct comparison in the industrial field test (1) shows the advantages of CFRP machining with coolants in contrast to conventional dry machining: In the tests carried out with coolant, tool life was up to four times longer. Production times were significantly reduced thanks to increased feed rates (up to 60% faster). With its high degree of heat dissipation and excellent lubrication performance, the special coolant produces cutting data that would be inconceivable in dry machining.

Table Cutting Data

(1) Cutting data comparison (without coolant and with special coolant)

Machining accuracy and dimensional accuracy also improved when using the special coolants, as shown in (2) and (3). When using the special coolants rhenus XY 190 FC and rhenus XT 46 FC, the tools performed much more accurately compared with dry machining and the dimensional stability fell more frequently within the defined tolerance field. 

Chart Accuracy

(2) Machining accuracy of CFRP drillings

Chart Drillings

(3) Dimensional stability of CFRP drillings

The improvement in the quality of the drillings manufactured with special coolant during test runs was clearly discernible even to the naked eye: (4), (5), (6) and (7).

Delaminations

(4) Inlets and outlets when drilling — fewer delaminations with special coolant

Tool Life

Left: no coolant; total tool life: 42 m
Right: with rhenus special coolant; total tool life: 150 m
(5) Industrial field test shows better quality results when machining with coolant

Drilling Wall

(6) Smooth and shiny drilling wall when machining with coolant

Feed Stroke

(7) Surface quality when milling: better results using special coolant (in comparison: dry, rhenus XY 190 FC, rhenus XT 46 FC, each with 17-m and 48-m feed stroke)

Fewer deviations and increased dimensional stability not only results in a smaller number of reject components, but also improved quality of components. Delamination is avoided and fewer parts need to be reworked.

Despite the higher feed rate, faster cutting speed and more accurate machining, tools like drills and milling machines were handled with greater care during field tests (8). Positive result: a longer tool life and lower costs through reduced tool wear.

Test Werkzeugverschleiss

Without coolant                                                               With special coolant

(8) Industrial field test: tool wear during CFRP machining with and without special coolant

With the help of the German Aerospace Centre, the interactions between the coolant and the CFRP were studied in a series of tests carried out under worst-case conditions (storage of the CFRP for a week at 60 degrees Celsius in the coolant, tests up to -50 degrees Celsius). The result is positive: The short residence time to the coolant during machining will not change the material properties of the CFRP and there were no signs of the coolant penetrating the composite material.

Using coolants binds the dust together perfectly, improving health and safety conditions and ensuring maximum acceptance of the process among process owners and machine operators. At the same time, the special flushing properties of the coolants keep the machine rooms cleaner.

A pioneer in FRP machining with special coolants
Meinhard Kiehl, Director of Marketing and Product Management at Rhenus Lub, sees his company as a pioneer, with its innovative process and the associated high-performance coolants: "With the advances we have made in machining carbon and other composite materials with specially developed coolants, we are heralding a new era. This method is not only appealing to process owners, who up to now have regularly encountered the potentially dire results of dry carbon machining. Even companies that currently use coolants but do not yet machine carbon can now seamlessly process the high-strength material using their existing machinery thanks to our specially designed fluids for fibre composite materials, thereby expanding their service portfolio. This opens up great opportunities for new business. "When it comes to economical machining of FRP without posing risks to health or the sustainable competitiveness of these promising materials, we must accept that in the long term, coolants are the only alternative.

www.rhenuslub.de/en/

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