Methods of producing wood pulp

Mechanical pulping

Advantages

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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.
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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. 
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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.
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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.
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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.
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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.
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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.

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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.
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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.
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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.
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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.
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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.
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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.
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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.
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