Bent wood technology

Bent wood technology : 

Bent wood technology N.KANAGARAJ BSF-06-007 BSC[FORESTRY]

Bent wood technology : 

Bent wood technology

content : 

content Introduction Type of bending process Laminated Bending Kerfing Chemical softening used in bending process Alkali softening Compression bending Cold bending Internal heating / Microwave heating External heating Hot plate method Boiling method Steaming method Bending and ductility of wood material The suitable material for bending Equipment used for bending process Bending procedures Problems during the bending process Reference

Introduction : 

Introduction wood bending is one of the techniques that is necessary in the furniture making and also in its design strategy, use make and design furniture accompanied with a new value-added elemnt. Ornamental Ash Bend

The suitable material for bending : 

The suitable material for bending Material with good grain condition (with a little shake) High ductility in characteristic. Use materials that are free of defects such as knots and checks

Slide 6: 

Bending method Laminated Bending Solid wood bending Kerfing Compression bending chemical Bending Hot bending Ammonia treatment Alkali treatment Internal heating External heating Direct heating method

External heating : 

External heating a. Direct heating method b. Hot plate method c. Boiling method d. Steaming method High pressure method Low pressure method

Laminated Wood Bending : 

Laminated Wood Bending Laminating wood to bend it is a fairly simple process. need a form or mold but you won’t need to build a steam box or risk burns with steam. Thin pieces of wood bend easier than thicker ones, but aren’t as strong. So by laminating thin strips of wood together in a mold or form adds strength and when the glue dries it will hold the shape also.

Laminated Wood Bending : 

Laminated Wood Bending

Kerf-cut Bending : 

Kerf-cut Bending Kerf-cut bending is done by using your saw to cut slots across the stock. This will allow the stock to be bent because the kerfs can compress together. The kerfs are usually only used on the inside of the bend. then you would use a veneer you cover the entire piece.

Kerf-cut Bending : 

Kerf-cut Bending

Kerf-cut Bending : 

Kerf-cut Bending

Chemical softening used in bending process : 

Chemical softening used in bending process Ammonia treatment Chemicals such as urea, urea-adehyde and ammonia have been used to soften the matrix that holds wood fibers together. Wooden strips approximately 6mm by 600mm were covered for about 35 minutes. The anhydrous ammonia was then drained onto the ground and allowed to evaporate. Specimens could be bent within one hour after removal from the soak tank.

Alkali softening : 

Alkali softening Natrium hydroxide (caustic soda), caustic potash and etc are the chemicals used to plasticize the material. All these chemicals are used in a solution form. Moreover, once the material is applied with the chemicals, the softness (ductility) of the material is maintained as long as the water (moisture) is there. Even, when it becomes dry, adding water would make the material to become soft again.

Cont.. : 

Cont.. The weakness of this method is that the chemical changes the color of the material. The thickness of material suitable for this method is between 10-20mm, means that the range of its use is limited.

Preparing the solution : 

Preparing the solution make a solution of the caustic soda with a concentration between 10-15%. Dissolve 10-15 parts of caustic soda to 90-85 parts of water in weight proportion. Immerse the material into the solution to penetrate the chemical. Bleach. Take it out from the alkali solution, put it into the 3-5% hydrogen peroxide solution and bleach (could be skipped). Wash it with water. Start the bending until the drying process

Compression bending : 

Compression bending Firstly, the material is pressed along its length direction (along the grain) before it is bended. The structure of the fiber becomes like a bellow, making it easier to bend. It is weaker in term of the strength. The method used is to use a device that hold both the ends of the material firmly. wood is inserted into a casing that just fit its size, and using a hydraulic cylinder the material is pressed along its length for about 10%, and then it is returned back to about 5%.

Compression bending : 

Compression bending

Cont… : 

Cont… the process is complete when the steamed material is pressed along its grain with the hydraulic pressing device.By applying a pressure, a pleat occurs at the wall surface of the fiber (cellulose), and leaving it in several minutes would make it easy to bend even in the cold condition

Advantages : 

Advantages The compression has several advantages e.g.: It can be bent and formed after the cooling for a long time It is able to resist major deformations, much more than without compression The deformation much less determined by the orientation like usual in case of woods The bending is more simply and performed by less energy It can produced by environmentally friend process without any chemical additions After the drying the retention of shape is good After the compression several mechanical properties are improved

Cold bending : 

Cold bending Cold bending is a method where water is applied to the material before it is put into the mold and bended. The material becomes soft when it is applied with water, it could be bended sufficiently. However, this method needs to use a good material with no shake compare to the one used in hot bending. After it is bended, apply a heat treatment would stabilize the form of the material. In Japan, this method is widely used

Cold bending : 

Cold bending

Microwave heating : 

Microwave heating Bending small pieces of wood becomes a very simple process with the use of a microwave; Microwave bending is also steam bending but in a much quicker, more manageable way. To generate steam when microwaving, wrap the wood in a piece of wet paper towel, place it in a microwave oven and microwave it on high. The frequency of microwave in the microwave oven used at home is about 2450MHz (245 millions cycles per seconds). The exact amount of time needed to heat a piece of wood will vary depending on the size and the thickness of the wood.

Internal heating / Microwave heating : 

Internal heating / Microwave heating

Merits : 

Merits Heating process can be done to the internal side in a short time. Shortening the time needed for heating process. Energy efficiency is high.

External heating : 

External heating Hot plate method This type of method is to apply a fire to heat the plate (former), where the material is placed and bended on it. It also can be used to make the bended material becomes straight again. The material is placed and fixed on the plate until it is cold.

Bended wood : 

Bended wood

Boiling method : 

Boiling method This type of method is to apply a heat and moisture to the material by boiling it in a hot water. Except for the boiling process, compare to the steaming method, it is all the same in term of material consideration and bending process. However, compare to the steaming method, more time is needed for the drying process after the bending is done. This method is suitable for a partial bending, or a small component such as the spindle.When we want to bend just some part from the whole material, we just boil it in the hot water. Small component such as a spindle usually just need a short time in about 30 minutes, the drying process also do not take too much time and not much problem is expected.

Steaming : 

Steaming

Steaming : 

Steaming High-pressure method. The concept of device for the high-pressure method is same as a pressure pot used at home. Pressure between 3 to 7 atmosphere is applied during the steaming process. Since the materials is steamed in a high temperature and high pressure condition, the process finish in a short time, and becomes easy to bend.

Steaming : 

Steaming

low-pressure method : 

low-pressure method For the low-pressure method (at normal pressure), the method is same as a method of cooking using a steamer. The device used for low pressure steaming is simple and very suitable for a small quantity bending. Moreover, it has a high safety.

Equipment used for bending process : 

Equipment used for bending process 1.Steamer 2.Bending strap 3.Former 4.Shim

Steamer : 

Steamer The material from a wood like the water-resistant plywood is the best material to make the steamer. The reasons are as below. A good heat-retaining characteristic, make it hard to condense in the cold region. It can be made easily to any size. Cheap.

Steamer : 

Steamer Dimension of steamer225(h) x 260(w) x 1625(l)(mm)

Steamer : 

Steamer

Slide 38: 

Bending strap The function of the bending strap during the bending process is to absorb the stress caused by the action of pulling. Former Steamed material is bended along the former. The former itself is made using the plywood, or a metal in the case of mass production.

Slide 40: 

Shim Shim is inserted between the material and the end block of the bending handle, functioning to adjust the gap.

Bending procedures : 

Bending procedures Softening process Bending process Drying Bending machine

Softening process : 

Softening process While softening we should consider moisture content, processing temperature and processing period. Regarding the period of the softening process, the practical standard used is for the air-dried timber, for every inch of the scantling material, 1 hour is needed to steam at normal pressure. To be more specific, it is good to use a material at its saturation point (moisture content of 30%).

Bending process : 

Bending process After the material is taken out from the steamer, set it right away to the bending handle and insert the shim with a suitable thickness. Start to bend the material promptly, same to both right and left side, and clamp it so that it would not straightening out by itself. Together with the former, take them off from the workbench and dry it.

Bending process : 

Bending process

Drying : 

Drying In the bending process, after the material is taken out from the former, it is important to make sure the material has a little spring back and a little varying in property. For that reason, after the bending is finish, the material is dried in a high temperature without taking it out from the former.

Problems during the bending process : 

Problems during the bending process Collapse of compression side Factors : Softening process that is not enough, high moisture content, the use of material with holes and the use of material that is not flexible. Split of draw side Factors : Expansion caused by the bending strap that is not strong enough, softening process that is not enough and small curvature of the material.

Bending Machine : 

Bending Machine Bending Machine This machine applies top pressure, bottom pressure, and end pressure while bending the wood. This machine bends 8 chair bows at a time. It uses hydraulic power to bend the wood around the form

Chair Bows This is a photo of commercially bent chair bows. : 

Chair Bows This is a photo of commercially bent chair bows.

Ornamental Ash BendThis creative bend was done with one ash board. : 

Ornamental Ash BendThis creative bend was done with one ash board.

Bent Baseball Bats The photo here is of a real baseball bat with a curve. : 

Bent Baseball Bats The photo here is of a real baseball bat with a curve.

referance : 

referance http://www.solidwoodbending.net/photos.htm www.woodweb.com » www.woodweb.com/knowledge_base/Steambending.html - 37k www.thefreelibrary.com/Anhydrous+ammonia+helps+bend+wood.-a020133971 - 29k www.taunton.com/finewoodworking/materials/materialsarticle.aspx?id=29137 - 37k www.ejpau.media.pl/volume7/issue1/wood/art-02.html - 28k