Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
1999-09-20
2003-03-18
Yao, Sam Chuan (Department: 1733)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S312000, C156S358000, C156S360000, C156S583500, C264S109000, C264SDIG006
Reexamination Certificate
active
06533889
ABSTRACT:
TECHNICAL FIELD
The invention relates to a method of optimizing the production capacity and the flexibility of the product properties when manufacturing chipboards, fibre boards and the like boards by a continuous process, where a thermosetting binder is applied onto the raw material in form of biomass particles, such as chips, fibres and the like, said raw material being spread on a preforming band to form an endless mat, where said mat is preferably pre-compressed in a continuously operating prepress and finally pressed in a continuously operating hot press in such a manner that said mat is compressed into the desired thickness of the finished plate and the thermosetting binder is hardened.
BACKGROUND ART
Above all, the hot press is essential to the production capacity of an apparatus and for the properties of the product, said hot press having two basic functions viz. to compress a mat of biomass particles glued to the desired thickness of the plate and to heat said mat to a temperature causing a hardening, i.e. a polymerisation/condensation of the binder.
For this purpose, two types of hot presses are used, viz. conventional step presses pressing a section of the mat per pressing cycle and continuously operating through-type presses advancing an endless mat by means of steel bands through a wedge-shaped slot between two pressing planes with the result that said mat is gradually compressed and full-hardened by means of heat from said pressing planes and said steel bands. These modern presses become more and more important and they are expected to dominate the market. The invention is in particular directed towards a use in connection with this type of press.
Below reference is only made to a continuous press, and the capacity of said press depends on
the capacity of the press for transferring heat from the pressing planes to the steel bands. In this connection, the shape of the roller or slide systems between the pressing planes and the steel bands is of decisive importance, and
the transfer of heat from the steel band to the mat of wood particles and through said mat which is to be heated to approximately 105 to 110° C. in the middle in order to harden the binder.
In practice, the heat transfer in the mat turns out to be the limiting factor. The thermal conductivity in the mat is very poor, and accordingly attempts have been made at optimizing the so-called “Dampfstoss-Effekt”, which is a German technical term meaning that the moisture in the surface of the mat evaporates and moves towards the centre of the mat where the steam condenses and releases its evaporation heat.
FIG. 4
shows an example of the temperature course at four different depths of the mat versus the time and consequently the position of the measuring location above the pressing length. The curve segments with a steep temperature gradient represent the “Dampfstoss-Effekt” in the layer in question. The flat temperature gradients represent the heat conducting phase taking over when steam is no longer supplied from the outside.
It appears that the heat conducting phase requires most time and restricts the advancing speed and consequently the capacity of the press.
Thus the “Dampfstoss-Effekt” is the ideal mechanism for transferring heat.
It is, however, subject to limitations because a high steam pressure in the middle layer may cause steam burstings in said middle layer when the plate is leaving the press. The more water/steam that is supplied for heating the mat, the more time the plate must remain under a slight pressure in the press so that the steam can finally condense or escape from the middle layer.
Thus an optimizing of the capacity of the press by means of water/steam dosing presents a compromise between two counter-acting effects.
The conventional method of pressing chipboards or fibre mats in a continuous hot press has, however, not only a limiting effect on the capacity of the press, but also a negative effect on the properties of the product.
The latter situation has been illustrated in the following example showing a conventional pressing of a fibre mat into an MDF plate, cf. FIG.
6
A.
A precompressed 80 mm thick mat of glued wood fibres with a moisture content of 9 to 10% corresponding to a 16 mm thick MDF plate is introduced in a continuous press and subjected to a compressing in the first section of the press by means of a very high pressure, usually of the magnitude of 40 to 50 kp/cm
2
, into a thickness usually being 5 to 10% smaller than the final thickness of the plate, cf.
FIG. 6A-2
.
FIG. 6A-2
shows the distance of the pressing planes, i.e. the thickness of the mat, over the length of the press, and
FIG. 6A-1
shows the specific pressure in the mat over the length of the press.
The high pressure in the first phase and the heating from the press bands (t~200° C. or more) result in a plastifying and compression of the fibres in the outermost layer of the mat into a density often in the range of 1000 to 1100 kg/m
3
for standard MDF-plates.
The pressure is then reduced in the second phase to for instance 1 to 3 kp/cm
2
so as to improve the permeability of the middle layer to the steam penetrating from the heated cover layer. As a result the thickness of the mat increases to approximately 25 mm in the illustrated example.
After the heating of the mat to approximately 100° C., the distance of the pressing planes is adjusted to the final thickness of the plate with the effect that the pressure is increased to for instance 5 to 10 kp/cm
2
so as finally to decrease towards 0 at the termination of the third phase, viz. the calibration phase.
The described method is a method known especially within the MDF industry and it is suited for achieving specific density profiles, cf. FIG.
5
. It is, however, encumbered with a few essential draw-backs which can be avoided by the use of the invention:
The high pressure in the first phase presents very high mechanical requirements to the press, and it involves a risk of band and rollers being damaged when the mat contains foreign bodies, such as compact fibre lumps, glue lumps and the like being undetectable by means of a metal detector.
The very low pressure in the second phase is necessary due to the penetration of steam into the middle layer and the heating of said middle layer, but it implies that the glue full-hardens partially without the particles having sufficient mutual contact.
The terminating compression during the calibration in phase 3 is even worse for the process because the glue bridges established under the low pressure in phase 2 are broken under the higher pressure in phase 3.
All things considered, this method is solely intended for achieving a specific density profile, but it is not suited for achieving an optimum utilization of the binder. Thus the transverse tensile strength of the plate can vary a great deal, and the damage in the middle layer is not always associated with the lowest density, cf. FIG.
7
.
Various suggestions have been made:
A drying of the wood material to a low moisture percentage, such as 5 to 6% followed by a spraying of water on the mat immediately before the press. The latter is in principle an efficient method because the potential amount of steam for the heat transfer is increased without increasing the total amount of moisture and consequently the risk of steam burstings. It is, however, difficult to control the procedure, and in addition it is not possible immediately before the press to apply water onto the bottom side of the mat. The result can be asymmetrical cross sections of the plates and curved plates.
A preheating of the mat by means of high-frequency waves to 50 to 60° C. or more in such a manner that the necessity for a heating in the press is reduced to a level which can be established by means of a moderate “Dampfstoss-Effekt”. The process is difficult to control because even insignificant moisture variations in the mat result in a heterogeneous heating, the dielectric constant of water being approximately 80 times higher than the one for wood. In addition, a heating of the middle laye
Jacobson & Holman PLLC
Wesser & Dueholm
Yao Sam Chuan
LandOfFree
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