Methods of producing wood pulp
Mechanical pulping
Advantages
- Created by: Alex Stephens
- Created on: 27-04-13 14:44
Mechanical pulping advantages
Mechanical pulping
Advantages:
- Provides a 90% yield from the pulpwood as it uses the whole of the log except for the bark.
- Investment costs for mechanical pulp mills are relatively low in comparision with other types of mills.
- Well suited for 'bulk' grades of paper, i.e. newsprint and packaging boards.
- Can be bleached to produce higher value-added products.
Mechanical pulping disadvantages
Mechanical pulping
Disadvantages:
- Lower strength characteristics than softwood chemical pulps.
- Paper can 'yellow' when exposed to bright lights due to high lignin content.
Chemical pulping advantages
Chemical pulping
Advantages:
- Higher quality wood pulp produced with longer, stronger fibres that contain fewer impurities.
- Produces 'chlorine-free' disposable products.
- Wasre lignin from the process can be burnt as a fuel oil substitute, often supplying power to the national grid or steam to local domestic heating plants.
Chemical pulping disadvantages
Chemical pulping
Disadvantages:
- Lower yield than mechanical methods as the lignin is completely dissolved and seperated from the fibres.
- No chemical pulp is produced in the UK, therefore it must be imported.
Waste pulping advantages
Waste pulping
Advantages:
- Makes use of recycled papers, which are a sustainable resource.
- Well suited for 'bulk' grades of paper, i.e. newsprint, tissue and packaging boards.
Waste pulping disadvantages
Waste pulping
Disadvantages:
- Cannot be recycled indefinitely as pulp loses quality - virgin pulp needs to be added.
- Does not save any more energy in processeing than other methods.
- Requires considerable processing and additives to produes good-quality paper.
Fourdrinier process
The Wet end starts with the wood pulp diluted to 99 per cent fibre to form a slurry that is held into the head box. A continuous stream of slurry is pumped through the head box through an adustable slit (called the slice) onto a moving gauze wire belt that vibrates to drain off some of the water and allow the fibres to interweave. Raised patterned formed in the gauze create the watermark - a feate in many high quality writing papers.
The Press section uses a system of nip presses or rollers that wirings out the majority of excess water from the pulp and stretches it out into a rough paper. It is at this stage that the thickness of the paper/board is determined. The gap between the press rollers is adjusted to allow for differing thicknesses such as card. The term card usually refers to paper which has a density greater than 160gsm.
The Dryer section dries the paper using a series of stem-heated rollers by removing the moisture. The resulting paper has a water content of 4-6 per cent and sizing agents, starches and resins can be added to enhance the paper's properties.
The Calendar section comprises a series of rollers through which the paper is fed in order to smooth it out and give it a uniform thickness. The pressure applied to the paper by these rollers determines the finish of the paper.
The paper is then wound onto a roll after calendaring (known as a web) and stored. It can be placed onto a precision cutting machine to product the desired size of shipped to a printer for web-fed printing.
Blow Moulding
Blowing Moulding
Advantages:
- Intricate shapes can be formed.
- Can produce hollow shapes with thin walls to reduce weight and material costs.
- Ideal for mass production - low unit cost for each moulding.
Disadvantages:
- High initil set-up costs as mould expensive to develop and produce.
Applications:
- Plastic bottles and containers of all sizes and shapes, e.g. fizzy drinks bottles and shampoo bottles.
Injection moulding
Injection moulding
Advantages:
- Ideal for mass production - low unit cost for each moulding for high volumes.
- Precision moulding - high-quality surface finish or texture can be added to the mould.
Disadvantages:
- High initil set-up costs as mould expensive to develop and produce.
Applications:
- Casings for electronic products, containers for storage and packaging.
Vacuum forming
Vacuum forming
Advantages:
- Ideal for batch production - Inexpensive.
- Relatively easy to make moulds that can be modified.
Disadvantages:
- Mould needs to accurate to prevent webbing from occurring. Large amounts of waste material produced.
Applications:
- Chocolate box trays, yoghurt pots, blister packs, etc.
Offset lithography
Offset lithography
Advantages:
- Good reproduction quality, especially photographs.
- Inexpensive printing process.
- Able to print on a wide range of papers.
- High printing speeds.
- Widely available.
Disadvantages:
- Colour variation due to water/ink mixture.
- Paper can stretch due to dampening.
- Set up costs make it uneconomic on short runs.
- Can only be used on flat materials.
Applications:
- Business stationary, brochures, posters, magazines and newspapers.
Flexography
Flexography
Advantages:
- High-speed printing process.
- Fast-drying inks.
- Can print on same presses as letterpress.
Disadvantages:
- Difficult to reproduce fine detail.
- Colour may not be consistent.
- Set up costs high and would rarely be used on print runs below 500,000.
Applications:
- Packaging, less-expensive magazines, paperbacks and newspapers.
Screen printing
Advantages:
- Stencils easy to produce using photo-emulsion technique.
- Versatile- can print on virtually any surface.
- Economical for short, hand-produced runs.
- Fully automatic methods capable of producing large volumes.
Disadvantages:
- Generally difficult to achieve fine detail (photographic screens able to reproduce fine detail).
- Print requires long drying times.
Applications:
- T-shirts, posters, plastic and metal signage, promotional items, e.g. pens and mugs.
Gravure
Advantages:
- Consistent colour reproduction.
- High-speed printing process.
- Widest printing presses.
- Ink dries upon evaporation,
- Variety of in-line finishing operations available.
- Good results on lower-quality paper.
Disadvantages:
- High cost of engraved printing plates and cylinders.
- Only efficient for long print runs.
- Image printed as 'dots' that are visible to the naked eye.
- Very expensive set-up costs, so only used on large print runs.
Applications:
- High quality art and photographic books, postage stamps, packaging, expensive magazines.
Related discussions on The Student Room
- Biology - Energy transfers »
- Selling books »
- Kingston School of Art (KSA) - Workshop Information »
- Name a film without the letter 'E' in the title »
- Make it More Evergreen-ey !! »
- help with chem aqa rp4 method »
- What film (real or imaginary) would you hate to miss? »
- calculating rate of reaction »
- Make it More Tree-ey !! »
- Music »
Comments
No comments have yet been made