Exercises: Woods, metals and plastics

Match each material to its family: (a) pine, (b) plywood, (c) steel, (d) aluminium, (e) PVC. Families: 1. non-ferrous metal; 2. natural wood; 3. plastic; 4. manufactured board; 5. ferrous metal.

According to their origin, materials can be natural, derived (processed from a natural material) or synthetic (man-made). Classify these materials: pine wood, plastic, steel, cotton, glass. Explain the criteria you followed in each case.

The figure shows the classification tree of materials according to their origin, with examples in each branch, but one card is covered with a question mark, as shown in the diagram.

What example of a synthetic material could be hidden behind the covered card? Explain what each of the three branches means and add a new card, with its example, to each branch.

True or false? Justify each answer: (a) aluminium is a ferrous metal; (b) most plastics are obtained from crude oil; (c) wood is a renewable material if forests are replanted; (d) all metals oxidise with the same ease.

Look around your classroom: identify the main material of five objects (the desk, the chair, the ruler, the bin, the window) and classify it as wood or manufactured board, metal or plastic. Which of the three families is the most common in your classroom?

A natural wooden board and a piece of plywood can be distinguished at a glance. Explain what differences can be seen (continuous grain versus thin glued layers) and why plywood is considered a processed wood material and not natural wood.

Match each item of waste with its destination: (a) a plastic bottle, (b) an old newspaper, (c) a glass jar, (d) a broken wood and metal chair. Destinations: 1. green bin; 2. blue bin; 3. yellow bin; 4. household recycling centre. Explain why the chair does not go into any of the first three.

Window frames are usually made of aluminium or PVC, rarely of steel. Explain which properties make steel impractical for this use (weight, rusting/corroding with rain) and what each of the two materials that are used contributes.

Metals are divided into ferrous (containing iron, like steel) and non-ferrous (like copper or aluminium). Compare their typical properties (hardness, oxidation, electrical conductivity, weight) and match each metal with a common use: steel, copper and aluminium with tools and structures, electrical cables, and cans and window frames.

The figure shows two production chains going from the raw material to the finished object, but one link is covered, as shown in the diagram.

Which link is missing in the second chain? Explain the difference between raw material, material and object, and build a completely new chain starting with 'sand' and ending with 'window'.

Manufactured boards (plywood, MDF, chipboard) have replaced solid wood in most furniture. Explain two advantages of manufactured boards (price, large stable panels, use of wood waste) and one disadvantage (they get damaged by moisture, joints hold less weight).

Choose the most suitable material and justify it using two properties: (a) a garden fence: treated wood or cardboard; (b) a bicycle frame: aluminium or lead; (c) a squeeze bottle for a hiking backpack: flexible plastic or glass.

A simple pen combines several different plastics: the rigid body, the soft grip section and a transparent part to see the ink. Explain why a single plastic is not used for everything and what property each part aims for.

An old mobile phone contains plastic, glass, copper, aluminium and even a pinch of gold. Explain why it should not be thrown in the normal bin, what a household recycling centre is, and which materials you think can be recovered and for what purpose.

Electrical cables are almost always made of copper rather than steel, even though steel is cheaper and stronger. Reason which properties win out in this decision (copper conducts electricity much better and is ductile: it can be drawn into thin wires) and why price is not everything when choosing a material.

A table can be made of pine (softwood) or oak (hardwood). Compare the price, hardness and ease of working with each wood, and choose one for the workshop workbench, which will suffer knocks and scratches daily. Justify your choice.

Analyse a typical classroom chair: tubular steel frame, polypropylene (a plastic) seat and backrest, and rubber feet. Explain what each material contributes to the structure and reason what would happen if the entire chair were made from a single material.

The local council is going to install a park bench: it will be outdoors, in the rain, and subject to heavy use. Evaluate the options of treated wood, painted steel and recycled plastic against the criteria of moisture resistance, maintenance and sustainability, and recommend one with justification.

A packaging company analyses two options for distributing juice within a 40 km radius: returnable glass bottles (360 g each, with an estimated lifespan of 25 washing and reuse cycles) versus single-use PET plastic bottles (28 g each, intended for the recycling bin). Interpret these data considering transport weight, water and chemical consumption at the washing plant, and end-of-life waste management. Justify which of the two options has a lower environmental impact for this local distribution, and predict how your decision would change if the product were distributed at a distance of 1.500 km.

A carpenter states: "To build the tables for the Technology workshop-classroom, it is always better to use solid pine wood than melamine-faced chipboard, because natural materials always provide higher quality and durability". Technically refute this statement: analyse how each material responds to changes in ambient humidity and continuous scratching in a workshop, and explain the advantages that chipboard provides in terms of sustainable use of forestry resources and economic cost.

A company markets a cleaner in a container consisting of a PET plastic bottle, a polypropylene cap, and a PVC wrap-around label attached with permanent adhesive. Although all three plastics are theoretically recyclable separately, in the recycling plant this container is almost impossible to recover and is downgraded to low-quality waste. Explain why this happens on a technical level compared to recycling a steel can, and redesign the container by applying ecodesign criteria to ensure it can be recycled with high quality.

In the workshop classroom, you have wooden slats from discarded pallets and copper pipe sections left over from a construction site. You want to design a sturdy wall-mounted coat rack to hang backpacks weighing up to 7 kg. Develop a detailed design (using a dimensioned sketch or technical description) indicating what function each material performs according to its mechanical properties, how you will securely join them, and what essential prior checks or treatments you must carry out on these used materials before machining them to prevent accidents or breakage.

A shredded mixture of steel cans (density: 7,8 g/cm³), aluminium cans (density: 2,7 g/cm³), and polyethylene bottle fragments (density: 0,95 g/cm³) arrives at a sorting plant. An electromagnet separates the steel pieces in a first step, but lets the aluminium and plastic pieces pass through together. Justify why this happens based on the magnetic properties of metals, and design a continuous technical system that allows aluminium to be separated from plastic by taking advantage of water (density: 1,0 g/cm³) and the differences in density between both materials.

Following the ban on single-use plastics, a fast-food restaurant replaces its cups with untreated cardboard and its forks with uncoated wood. Customers complain that the cups become soggy and leak a few minutes after being filled with soft drink, and that the forks splinter and absorb grease from the food. Explain what technical and hygienic properties have failed in these materials, propose what technical solutions the industry applies to resolve this, and analyse what new environmental problem is created when applying these solutions when it comes time to recycle them.

Unit challenge. Imagine you work as an ecodesign consultant. Your mission is to analyze the use of wood, metal, and plastic in your home and propose a more sustainable alternative for an everyday object.

1. Home inventory: Select $3$ objects in your home (one made of wood, one of metal, and one of plastic). Measure their dimensions with a ruler to estimate their approximate volume and note two key technical properties of each (hardness, conductivity, lightness, elasticity...).

2. Official data search: Go to the INE website (or the Ministry for the Ecological Transition) and check the recycling rate for plastic and metal packaging in Spain (for example, rates close to $51,5\,\%$ compared to $85,0\,\%$). Write down the official data and compare them.

3. Redesign proposal: In a LibreOffice Calc spreadsheet, compile the data into a comparison table and write a short proposal: which material (certified wood or recyclable metal) would you use to replace the analyzed plastic object to improve its ecological impact without losing functionality? Justify it based on its mechanical properties.

Assessment criteria:

• Correct identification and technical characterization of the three materials.

• Rigor in taking direct measurements and consulting official statistics.

• Coherent justification of the redesign based on physical properties and recyclability.

• Clarity and neatness in the document created with LibreOffice.