Wood, metals, and plastics
In the technology shop, almost everything we use is wood, metal, or plastic. At home and in your neighborhood it's the same: the carpenter, the blacksmith, and the plastics stand at the market all work with one of these three families. Where does each one come from, what techniques are used to work with them, and what do we do with them when they're no longer useful? Let's find out.
Where materials come from
The criterion is simple: ask how much the material has changed from how it is found in nature. A log cut into boards is still natural wood. If you break it down into thin layers and glue them together, it's now derived. And if what you have came out of an oil refinery, it's synthetic.
Renewable and non-renewable resources
Another way to classify raw materials is to ask whether they get replenished. A renewable resource is produced again within a reasonable time: wood, if new trees are planted where others were cut down (managed or reforested forests), or cotton, which is planted every year. A non-renewable resource takes millions of years to form, so what is used is not replaced: petroleum and the minerals that metal comes from.
Careful: wood is only renewable if the forest is replanted. If trees are cut down faster than they grow, the forest runs out. That is why plastics (from petroleum) and metals (from minerals) are recycled so much.
According to their origin, materials can be natural, derived (processed from a natural material) or synthetic (man-made). Classify these materials: pine wood, plastic,…
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 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.
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…
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'.
Woods
Look at the edge of a board: natural wood shows continuous grain and rings, while plywood shows thin layers glued with wood glue, each with its grain crossing the one before. That's why plywood is a derived material: it comes from wood, but someone transformed it.
Boards are cheaper, come in large sheets, and hardly warp. Particleboard, for example, is made of wood particles pressed and glued together, sometimes with a decorative layer on the outside (veneer). Its weak point is moisture: it swells and comes apart if it gets wet. Also, screws hold worse in it than in solid wood.
Among natural woods there are softwoods (pine: cheap and easy to cut and sand) and hardwoods (oak: more expensive and resistant to blows, but harder to work with).
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…
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.
Manufactured boards (plywood, MDF, chipboard) have replaced solid wood in most furniture. Explain two advantages of manufactured boards (price, large stable panels, use…
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).
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,…
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.
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…
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.
Metals: ferrous and non-ferrous
| Metal | Group | Typical properties | Common use |
|---|---|---|---|
| Steel | ferrous | hard, strong, heavy, rusts | tools and structures |
| Copper | non-ferrous | very good electrical conductor, ductile | electrical wires |
| Aluminum | non-ferrous | lightweight, hardly rusts | cans and window frames |
Techniques they share
Different materials are worked with similar techniques: measuring, cutting, sanding or filing, drilling, bending, and joining. Metals are cut with a hacksaw, filed, drilled, and joined with screws or welding.
The trades in your community show this well:
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The blacksmith cuts, bends, and welds iron to make gates and railings.
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The carpenter measures, cuts, sands, and joins wood with wood glue and screws.
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The potter shapes clay, lets it dry, smooths it, and fires it in the kiln; Oaxacan black clay is polished and fired with great care.
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The leather worker measures, cuts, sews, and joins leather to make belts, saddles, and huaraches.
Even though the material changes, the same steps keep repeating: measuring, cutting, shaping, and joining.
Match each material to its family: (a) pine, (b) plywood, (c) steel, (d) aluminum, (e) PVC. Families: 1. non-ferrous metal; 2. natural wood; 3. plastic; 4. manufactured…
Match each material to its family: (a) pine, (b) plywood, (c) steel, (d) aluminum, (e) PVC. Families: 1. non-ferrous metal; 2. natural wood; 3. plastic; 4. manufactured board; 5. ferrous metal.
Metals are divided into ferrous (containing iron, like steel) and non-ferrous (like copper or aluminum). Compare their typical properties (hardness, oxidation,…
Metals are divided into ferrous (containing iron, like steel) and non-ferrous (like copper or aluminum). Compare their typical properties (hardness, oxidation, electrical conductivity, weight) and match each metal with a common use: steel, copper and aluminum with tools and structures, electrical cables, and cans and window frames.
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…
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.
Heavy or light: density
A rock and a piece of Styrofoam of the same size don't weigh the same. The rock is more dense: it has more matter in the same space. That difference is useful for recognizing materials and also for separating them.
| Material | Density (g/cm³) |
|---|---|
| Steel | 7.8 |
| Aluminum | 2.7 |
| Water | 1.0 |
| Polyethylene (plastic) | 0.95 |
The rule is: a material denser than water sinks, and one less dense floats. A solid piece of aluminum (2.7) goes to the bottom; a piece of polyethylene (0.95) stays on the surface.
A shredded mixture of steel cans (density: 7.8 g/cm³), aluminum cans (density: 2.7 g/cm³), and polyethylene bottle fragments (density: 0.95 g/cm³) arrives at a sorting…
A shredded mixture of steel cans (density: 7.8 g/cm³), aluminum 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 aluminum 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 aluminum to be separated from plastic by taking advantage of water (density: 1.0 g/cm³) and the differences in density between both materials.
Plastics
Plastics are also worked with techniques similar to those used for wood and metals: they are cut with a hacksaw, sanded, drilled, and joined with glue. Many can be bent or molded with heat, just as the potter shapes clay or the leather worker shapes leather.
True or false? Justify each answer: (a) aluminum is a ferrous metal; (b) most plastics are obtained from crude oil; (c) wood is a renewable material if forests are…
True or false? Justify each answer: (a) aluminum 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.
Window frames are usually made of aluminum or PVC, rarely of steel. Explain which properties make steel impractical for this use (weight, rusting/corroding with rain)…
Window frames are usually made of aluminum 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.
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…
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.
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…
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?
Choosing the right material
Choosing a material is not a matter of taste: you compare what the object needs with what each material offers. You look at two or three key properties, the price, what is available in your area, and what will happen to the object when its useful life ends.
Think of a classroom chair: the steel tube gives strength, the polypropylene seat is lightweight, cheap, and easy to clean, and the rubber tips keep it from slipping and scratching the floor. If the whole chair were steel, it would be heavy and cold and uncomfortable; if it were all plastic, it would bend under weight.
A park bench is chosen like this: treated wood withstands rain if it is maintained; painted steel is sturdy but rusts if the paint gets scraped; recycled plastic needs almost no maintenance and puts waste to use, although it costs more up front. Each option wins on a different criterion, and that is why you have to justify the decision.
Choose the most suitable material and justify it using two properties: (a) a garden fence: treated wood or cardboard; (b) a bicycle frame: aluminum or lead; (c) a…
Choose the most suitable material and justify it using two properties: (a) a garden fence: treated wood or cardboard; (b) a bicycle frame: aluminum or lead; (c) a squeeze bottle for a hiking backpack: flexible plastic or glass.
Analyse a typical classroom chair: tubular steel frame, polypropylene (a plastic) seat and backrest, and rubber feet. Explain what each material contributes to the…
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 municipality 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…
The municipality 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.
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…
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.
Waste and recycling
In most communities in Mexico, trash is separated into organic (food scraps, peels, leaves) and inorganic. Among the inorganic, the recyclables (PET, cardboard, paper, glass, cans) are collected at collection centers, and the non-recyclables go in the trash. Objects with many mixed materials, like a chair made of metal and wood, or electronic devices, don't go in the trash can: they are taken to a collection center where they are taken apart and sorted.
For example, an old cell phone has plastic, glass, copper, aluminum, and a little gold. If it is thrown in the trash, those materials are lost and some contaminate the soil. At an electronic waste collection center it is taken apart and the metals are recovered to make new things.
Reuse before recycling
Recycling requires melting and manufacturing again. Reusing is simpler: the same object serves many times. A single-use container is thrown away after one trip; a returnable one is washed and refilled, for example the glass soda or milk bottles that are returned to the store.
To compare two containers, look at the material used per use, and think about three things: weight (how much it costs to transport), washing (water and chemicals), and the final destination of the waste.
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…
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 center. Explain why the chair does not go into any of the first three.
An old cell phone contains plastic, glass, copper, aluminum and even a pinch of gold. Explain why it should not be thrown in the regular trash can, what a household…
An old cell phone contains plastic, glass, copper, aluminum and even a pinch of gold. Explain why it should not be thrown in the regular trash can, what a household recycling center is, and which materials you think can be recovered and for what purpose.
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…
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 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.…
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.
Back to the beginning: wood, metal, and plastic come from different places, are worked with similar techniques, and each one is reused, sorted, and recycled in its own way.