1.5 The use of finishes

Study notes

  • Core Learning Outcomes

    • Understanding how materials (paper, boards, polymers, metals, and woods) are finished to enhance aesthetics and function.

    • Knowledge of different printing processes and their suitability for various production scales.

    • (A-level only) Knowledge of acrylic spray paints, TPE, and methods to prevent corrosion and decay in metals (sealants, preservatives, anodising, plating, coating, and cathodic protection).

  • Categorization of Material Finishes

    • Self finished: A material that possesses an acceptable finish immediately after the manufacturing process.

    • Self coloured: A material that has an acceptable colour after processing, without additional pigment application.

    • Applied finish: A substance applied to the material surface after processing to provide properties the material lacks.

  • General Aims of Applied Finishes

    • To prevent the material from absorbing moisture.

    • To protect the material against decay or corrosion.

    • To protect against insect attack.

    • To enhance the aesthetic appearance of the final product.

Paper and Board Finishing Techniques

  • The Importance of Finishing in Graphic Design

    • Finishes protect surfaces from liquid spills or dirt and allow for material enhancement to attract consumers to specific features.

  • Laminating Methods

    • Lamination via encapsulation: Performed using a desktop laminator where the paper is enclosed by a polymer pouch or film roll. It is heat-sealed, creating a border around the edges. The polymer used is typically a mixture of PETPET and ethylene-vinyl acetate (EVAEVA).

    • Lamination via a surface coating:

      • Liquid lamination: Applied via roller or spray. Used for signage to protect against moisture, dirt, and UV light (preventing colour fading).

      • Film lamination: Can be hot or cold. Used for menus and business cards. Generally made of PPPP (polypropylene) with an adhesive. The laminate is fed from a roller and pressure is applied to stick it to the paper.

  • Embossing and Debossing

    • Embossing: Creates a raised design for visual and tactile effects. Common on greeting cards, invitations, and luxury packaging like chocolate boxes. It uses two dies (male and female) made from stainless steel or brass. Pressure is held until the shape forms. A "blind emboss" refers to a raised area with no ink or foil.

    • Debossing: The opposite of embossing; it creates an imprinted depression sitting below the surface. It follows the same two-die process and is used similarly for decorative text or sections in cards and packaging.

  • Foil Blocking

    • Involves heat and pressure to transfer metallic paper (foil) onto a substrate to create depth and texture. Foils come in metallic, gloss, matt, clear, and holographic varieties.

    • A heated die stamps the design through the foil. If the heat is too high or pressure is held for too long, the foil will bubble and peel. If insufficient, it will not adhere.

  • Varnishing and Specialty Inks

    • Standard Varnish: A clear, non-pigmented ink used on pre-coated papers (to prevent soaking/swelling) to enhance colour and protect against dirt/fingerprints. Available in gloss, matt, satin, and tinted finishes.

    • UV Varnishing: Provides an exceptionally smooth finish. The ink is applied via rollers and cured instantly under UV light. It is high-gloss or matt and resistant to abrasion and chemicals.

    • Spot Varnishing: Applied to specific areas rather than a flood coat. It utilizes plates to press the varnish onto a specific spot and can be done with standard or UV varnish.

Paper and Board Printing Processes

  • Purpose and Terminology

    • Printing communicates info (barcodes, safety warnings, ingredients) and provides aesthetic appeal.

    • Substrate: The term for the material onto which the ink is applied (e.g., paper or board).

  • Screen Printing

    • Used for small print runs (posters, display boards, t-shirts). It has minimal setup costs.

    • Process: A mesh screen/stencil is held in a frame. A different screen is needed for each colour. A squeegee (flexible polymer blade) forces pigmented ink through the mesh onto the substrate.

  • Flexographic Printing

    • Uses a four-colour process: Cyan, Magenta, Yellow, and Key (Black) (CMYKCMYK). These overlap to create required colours.

    • Registration System: Precision marks (QCQC check) outside the print area ensure colours line up and are not blurry.

    • Advantages: Fast-drying water-based inks; least expensive; ideal for long, continuous runs (uses rolls/webs of substrate).

    • Process: Flexible plates on rotating cylinders. An anilox roller ensures uniform ink thickness. The substrate passes between the plate and impression cylinders and then into an overhead dryer.

  • Offset Lithographic Printing

    • Extremely versatile; can handle up to ten rollers (for CMYKCMYK, metallics, varnishing, and duplex/double-sided printing). Used for medium to long runs (1,000+1,000+ copies) of books and magazines.

    • Scientific Principle: Oil and water do not mix. Plates are treated to be oleophilic (oil-absorbing) in image areas and hydrophilic (water-absorbing) in non-image areas.

    • Process: Plates are made via Computer to Plate (CTPCTP) lasers. A dampening roller applies a fountain solution (mostly water) to the hydrophilic areas. Ink is then applied to the oleophilic areas. The plate cylinder transfers the image to a blanket roller, which then transfers it to the substrate.

  • Digital Printing

    • Operates similarly to laser printers/photocopiers. It allows for mass customization (e.g., specific names on drinks cans) and different designs on every page. Fast-drying inks make it efficient for both low and high-volume promotional materials.

Polymer Finishing and Overmoulding

  • Self-Finishing Properties

    • Polymers are generally self-finishing as the mould produces the texture and pigments provide the colour. Example: A ketchup dispenser with a green injection-moulded top and a red blow-moulded base is ready for assembly immediately upon removal from the machine.

  • Pigmentation

    • Pigments are tiny particles added to polymer granules in the hopper or during the manufacture of the stock form.

    • Smart Pigments: Thermochromic or phosphorescent (e.g., "glow in the dark" stars) can be added during injection moulding.

    • Laminated GRP: Pigments can be added to the polyester resin gel coat during the lay-up process for products like boat hulls.

  • Acrylic Spray Paints (A-level Only)

    • Fast-drying, water-soluble when wet but water-resistant when dry. Used in the automotive industry for mass customization (colour-coding wing mirrors and bumpers). It protects against UV light and weathering.

  • Overmouldings (A-level Only)

    • Moulding a second polymer (usually TPETPE) over parts of a product for grip or texture (e.g., toothbrush handles).

    • Method 1: Using two separate injection moulding moulds.

    • Method 2: Twin shot injection moulding. A single machine with two cavities rotates 180∘180^\circ to apply the overmould in a continuous cycle.

Metal Finishing

  • Natural Protection

    • Most metals form an oxide layer. Copper forms a bright reddish layer that eventually becomes a green patina (e.g., St Paul's Cathedral dome). Steels are the exception; they form a porous oxide layer that allows moisture in, leading to crumbling rust.

  • Cellulose and Acrylic Paints

    • Provides colour and a corrosion barrier. Used on low-cost metals like steel. Surface must be degreased for the primer (e.g., red oxide primer) to "key" or grip.

  • Electroplating

    • Coating a base metal with a donor metal (silver, gold, zinc, copper, tin) via an electrolyte solution and direct current. Examples: Silver-plated teapots, chromium-plated kitchen taps.

  • Dip Coating

    • Polymer dip coating: Product is heated to 230 ∘C230\,^\circ C and dipped into a fluidization bath of polymer powder. Air makes the powder set evenly. Examples: Wire coat hangers, dishwasher racks.

    • Metal dip coating: Steel is cleaned and dipped in molten metal.

      • Tin plating: Molten tin at 320 ∘C320\,^\circ C for food cans.

      • Zinc plating (Galvanising): Molten zinc at 460 ∘C460\,^\circ C for industrial/agricultural use and car bodies.

  • Powder Coating

    • The product is statically charged (negative), and thermoset polymer resin (positively charged) is sprayed. It is then baked in an oven to melt the powder into a hardwearing coating. Used for white goods (washing machines, fridges).

  • Varnishing, Sealants, and Preservatives (A-level Only)

    • Metal Varnish: Clear finish for expensive metals (aluminum, brass, copper).

    • Sealants: Polymer/Silicon-based coatings for automotive protection (e.g., road salts, insects). Cures for 15 minutes15\,minutes and is buffed with a cloth.

    • Preservatives: Wiped on or sprayed to protect moulds and dies from fingerprints or minor corrosion.

  • Anodising (A-level Only)

    • Used for aluminum to enhance the oxide layer for hardness/toughness. Involves a sulphuric acid electrolyte. Current flows from the part (positive anode) to a negative cathode. Often used for mobile phone casings or torches.

  • Cathodic Protection (A-level Only)

    • Controls corrosion by making the metal the cathode of an electrochemical cell.

    • Impressed Current: Component is connected to an electrical power supply.

    • Sacrificial Anodes: A more active metal (e.g., Zinc or Aluminum) is wrapped around a less active one (e.g., Steel in ship hulls or pipelines). The sacrificial metal corrodes instead of the base metal.

Wood Finishing

  • Natural Degradation Factors

    • Wet Rot: Alternating wetness/dryness causes fungi to break down fibers into a soft, wet state.

    • Dry Rot: A fast-spreading fungus that turns wood into a dry, powdery state.

    • Insects: Furniture beetle (woodworm) attacks hard/softwoods; Deathwatch beetle attacks hardwoods. Larvae tunnel through the wood and exit via flight holes.

  • Surface Preparation

    • Surface must be smooth (planed/sanded). Sanding must be done in the direction of the grain using progressively finer grades of glass paper.

  • Wood Finishing Methods Table

    • Polyurethane Varnish: Tough, heat/waterproof. Gloss, satin, or matt. Clear finish shows grain.

    • Acrylic Varnish: Water-based; provides protection and colour.

    • Water-based Paints: Used for softwoods; makes them tougher and moisture resistant. Requires knotting treatment first.

    • Stains: Water or spirit-based. Enhances grain but offers no specific protection.

    • Colour Wash: Adds colour/vintage style while showing grain.

    • Wax: Increases surface hardness; can be buffed to high gloss.

    • Pressure Treating: Wood is placed in a vessel with copper sulphate and preservatives. Forced into fibers via vacuum/pressure. Protects for up to 50 years50\,years.

    • Yacht Varnish: High gloss/satin for outdoor items (boat parts, window frames).

    • Danish Oil: Maintains natural appearance; builds a water-resistant finish with regular application. Wait 5 minutes5\,minutes then rub excess away.

    • Teak Oil: Used for naturally oily woods like teak. Improves weather and fungal resistance.