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Crushing and Chipping

Crushing is the first phase of the shredding process of large “green” wood, that still has a certain percentage of water and resins inside. It applies to both virgin wood i.e. wood coming from wood felling and recycled wood from the so-called “urban forests”. Whole pallets, large reels and portions of doors can go into primary crushers.

Chipping is always a form of primary grinding, but it is based on knives instead of hammers as in the case of shredding. Wood chips are a product reduced to workable sizes using cutting (instead of crushing) to be subsequently processed for the production of wood panels of the PB, LSB or MDF type. Chipping is also applied to wood logs or, rarely, to recycled wood and may also be applied to primary wood of smaller size as opposed to wood shredded using crushers.

Refining is the process that reduces material into chips, particles or flakes, i.e. particles with a well-defined thickness, sizes suitable for the creation of layers of particleboards, pallet blocks or also pellets.


How a crusher operates

Crushers shred wood through rotors which in most cases are fitted with clubs. This is the reason why they are also called “hammer mills”.

Crushers may be equipped with one rotor only (single-rotor) as in the case of the TIGER, or with two rotors as in the case of the ALLIGATORThey may also be fitted with blades to shred brushwood that would be difficult to reduce in size using hammers. This is the case of the TIGER-K crusher (where K stands for Knife).


How a chipper operates

Through knives, chippers reduce wood logs into a myriad of small pieces of rather regular sizes, typically rectangular with a relatively constant thickness (depending on the feed of the incoming material).  This is the reason why they are also called “knife mills”.

Chippers may be mobile as in the case of the MOBILE CHIPPER MCU  operated by a diesel engine, or fixed but still knife-based as in the case of the DRUM CHIPPER MTG.

The RMG re-chippers are a separate category compared to chippers, as they are in fact small chippers that recover oversize pieces from recycled material.


How a refiner operates

In refiners, wood chips (generally sized 50 mm) are further chopped into sizes deemed workable for panels (typically less than 10 mm).

The use of a hammer refiner such as the HAMMERMILL FALCON or of a knife refiner such as the RCG or SRC  machines depends on the product needed, i.e. the accuracy of the shape and the thickness required. Through a knife-based cutting process, the RCG or SRC KRFs, thanks to a precise gap, generate an elongated particle (flake) with a reduced and constant thickness that is optimal for particleboards. The FALCON, through hammers and fine mesh screens, generates a less elongated particle with varying thickness that is not optimal for PB, but excellent for pellets and pellet blocks (isotropic materials without particular mechanical flexural properties).

There are also other particular machines that generate fibre from wood chips called defibrators and the STRANDER SDG that, through a disc, generates scales for OSB from logs.


Difference between crushing, grinding and refining

A crusher reduces through hammers, a chipper through knife-cutting. However, the common feature is that they are unable to generate small-sized material suitable for resin coating and panel formation from very large material (preferably processed by hammer mills) in just one single step. In this case, in fact, refiners are used, i.e. special knife mills also called Knife Ring Flakers.

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