Nature decided a long time ago that there will be (almost) no wood without faults. Bad knots, resin pockets, splits, broken edges and so forth may be considered ‘natural’ defects; they are nevertheless unacceptable to the producer of multi-layer table tops or plywood panels.

Most companies producing or processing wood based panels therefore must use patching to create a ‘natural’ product with a quite non-naturally high quality. The demanding task is to produce, with high productivity, panels which are both well-closed and visually satisfying for the multitude of later applications such as in construction, buildings, exteriors and interiors and furniture.

The common procedure of manual panel patching is quite expensive and time consuming: typically 12 workers are placed on both sides of the conveyor belt transporting the raw panels. They perform a multitude of tasks simultaneously: visual inspection for detecting and grading faults, cleaning the defects with hand-held drilling equipment and finally closing the holes and splits either by inserting dowels or by injecting putty.

Needless to say, this well-established repair strategy is far from being ideal:

a) The required workforce is important and expensive, the working conditions strenuous and in the long run, prone to generate skeletal and joint problems.

b) Manual patching is only economical for a rather small number of defects per panel – typically less then 8-15 defects per face – and above that limit it is often more economical to just scrap the faulty panel.

c) There is an important waste of material when patching with a fast-drying and quite expensive two-component putty.

d) The quality of a patched panel is highly dependent on the workers, introducing an important subjectivity in the final grading, and thus pricing, of the patched panels.

e) No numerical quality data at all is produced, preventing the automatic sorting of the patched panels into lots of given quality, the real-time generation of commercial data for the actual production lot, a decision on the number of extra panels to be produced as a safeguard for customer complaints, etc.

Here, Baumer Inspection reports on a new technology for the fully automatic and extremely fast patching of large multi-layer wood panels using a combination of advanced optical scanning, knowledge-based patching rules and numerically controlled XYZ patching tools. We also discuss how this system is a proof of the concept of the ‘data driven factory’ through its research on ‘invisible’ patches.

High-speed automatic patching

Baumer Inspection GmbH of Germany, a leader in the automatic visual inspection of panels for the flooring and furniture industry, says it has taken a new approach to the task of repairing wood based panels.

“We have developed, together with the Austrian manufacturer of numerically controlled production equipment FILL GmbH, a highly modular system to automate any type of panel repair technology, including insertion of dowels, injection of one- or two-component putty, or combinations of both,says Robert Massen of Baumer Inspection.

“At the beginning of the patching line, the panels are moved, held on a vacuum conveyor belt, in front of a high-resolution camera-based optical scanner. An advanced image processing and pattern recognition software generates, in real-time, a digital map which describes the detected defects by their position, shape, volume, type and additional appearance features useful for achieving patching with the lowest possible visibility,he explains.

A knowledge-based software module computes a specific patching strategy for every particular panel, following the domestic patching rules of the producer and the specific requirements of the final customer. This software generates all the signals to drive the numerically controlled tools for cleaning the defects and for patching them automatically, either by dowel insertion, by one- or twocomponent putty injection, or by any combination of these.

The basic patching system is designed to perform the patching on a stationary panel. However, the most advanced system operates at significantly higher speed by performing ‘on-the-fly’ patching: all the NC tools are synchronised to the speed of the moving panel and perform defect cleaning, putty injection or dowel insertion while the panel is moving through the patching system.

An ecological productivity boost

“Automating the patching of panels is a very important boost in productivity,says Mr Massen. “With a typical patching speed of 450 faces per hour and a 24 hour/seven day operating scheme, the automatic patcher compares to a workforce of some 50 people.

“In addition to the productivity increase through ‘intelligent’ automation, the high speed, and the sophisticated patching rules used, allow for an economical processing of panels with many more defects than the current 10 to 15 defects per panel face in manual patching lines.

“This is a very positive ecological side-effect, because it gives lower grade wood species a chance to be used for decent panel products instead of being crushed into lower-value fibreboards, or just burned,he says.

“This impressive gain in productivity has convinced our first customer, a leading Austrian panel producer, not to move a planned new facility into the low-wage region of a nearby eastern country, but to build the new plant in the rather high-salary Austrian home ground.”

Mr Massen explains that this plant produces high-quality three-layer table-top panels using automatic patching by insertion of circular and elliptical dowels. It has operated for more than a year near to the existing factory, thus preserving the precious wood processing knowledge within a safe, traditional environment, he says.

“An interesting premium single-face putty-patching system for three-layer plywood panels is shown in Fig 4. The panels to be patched never stop; the NC tools for cleaning the defects and for injecting putty are synchronised to the conveyor’s continuous motion. It performs an interesting putty-patching strategy by using both a low-cost one-component putty injector to close small and narrow defects and a higher-priced two-component injector for the closing of larger defects.”

Plywood is of course prone to quite a substantial number of defects, especially loose, dropped or bad knots. With a total of typically 30 to 40 knots per sheet, the automatic patcher has to be able to patch an average of six to 10 bad knots per sheet.

A data-driven factory

The automatic patching system is a typical approach to what production engineers name a ‘data-driven factory’, explains Mr Massen. Such a factory accommodates raw material of different quality and geometry and automatically performs an identification and specification of its characteristics (scanning of the panel size, the presence, location and identification of defects to be patched).

It uses a knowledge-base which holds the processing steps required for achieving a final product as specified by the customer (the specific patching by dowels or putty etc and the rules to apply in order to obtain the required physical and visual quality).

The knowledge-base knows all the properties, the technical possibilities and the limits of the production tools (the number and type of NC tools; the dynamics and precision of positioning and activating the tools and the specific patching materials to be used, such as dowels of different shape, possibly different colour and wood grain, one- and two-component, possibly with different colour).

The knowledge-base finally automatically optimises the production in terms of productivity and quality goals. This means generation of signals to drive the NC tools with an optimal path strategy and the automatic selection of the type of patching for the best possible closing of the panel surface with the lowest possible visibility.

Beyond the huge increase in productivity, the ‘data driven factory’ also automatically tracks the performance and quality on every processing step and up to the final product; every patched panel comes with a digital file specifying the number of patched defects, their geometric distribution, the visual quality, a quality grade and a pricing key.

A well-designed automatic patching system thus leads the wood processing industry into a new philosophy of manufacturing, claims Baumer, saying that this goes well beyond the crude replacement of human muscle by faster mechanics. The data-driven wood processing factory is both the key for a better usage of raw materials (ecological benefit) and for keeping former personnel-greedy production in high salary countries.

The future: the invisible patch?

Even the best physical patching of a panel, be it by dowelling or by injection of putty, is not able to reconstruct a natural wood appearance at the patch location.

Even a most carefully-patched panel never reaches the visual quality (and price) of a defect-free panel. The patches remain visible due to the local mis-match of wood grain.

This aesthetic down-grading is a serious economic loss which is today taken by the wood industry as unavoidable. It is of course also an ecological loss, as lower quality wood species with a large number of knots, resin pockets, cracks etc cannot be used for high-quality panels for the furniture, flooring and interiors industry.

Baumer Inspection has thus tackled the question: Would it be both economically and ecologically challenging to develop an automatic patching technology which is able to make the patched defects almost invisible?

Mr Massen explains the principles behind such a development.

“This could be achieved by using a local aesthetic reconstruction of the original wood appearance. By analysing the regions around the patched defects with well-calibrated colour cameras and by using machine-learning principles, we should be able to automatically synthesise the lost original wood grain at the location of the patch,he says.

“With an equally carefully-calibrated ink-jet printer head, we could then decorate the patch with this computed synthetic local decor to make it almost invisible to the human eye.

“An early result of this on-going research project (Fig 7 computer simulation) seems to confirm the validity of our approach and promises stunning results. International patents are pending.”


Baumer says the fully automatic patching of wood based panels is economically feasible, even for woods with a large number of defects, through a novel combination of advanced camera technology, pattern recognition, machine learning, knowledge bases and NC tooling. This technology, it says, has been proved by operating for about a year in the patching of multi-layer softwood table-tops by dowel insertion. A second system for the patching of plywood with two putties is currently installed in North America.

“This technology implements the concept of the ‘data driven factory’, ie a production philosophy which is a radical departure from the traditional wood processing using human eye, muscle and brain and leads to a productive and fully automated production of wood based panels,says Mr Massen.

“It handles very varied input materials and qualities, different patching strategies and produces a fully documented and specified panel for a broad range of final applications.

“The very high productivity reduces the pressure to shift wood panel processing factories to low-wage countries.

“Our current research on the automatic local aesthetic reconstruction of the patched defects will lead to a technology which makes the patched defects almost invisible to the human eye. This is a ‘green’ technology, as it allows for far better use of our precious wood stock, including today’s neglected fast-growing species, and woods with a large number of defects.”