Exercises: Manufacturing techniques

Topic challenge. Match each industrial plastic-forming technique with its typical product and justify one of the pairs.

Techniques:

1. Extrusion

2. Blow molding

3. Injection molding

4. Vacuum forming

Products:

A. Bottles and jerrycans

B. Trays and blister packs

C. Pipes and profiles

D. Housings and precise toy parts

What you must do:

1. a) Write the four pairs (technique number and product letter), for example: «1 - C».

1. b) Choose one of the pairs and explain in 2 or 3 lines why that technique is the right one to manufacture that product (how the plastic is shaped).

What will be assessed: that the four pairs are correct, that the justification describes the process of the chosen technique and that you use technical vocabulary (mold, molten plastic, pressurized air, vacuum, nozzle…).

Explain extrusion using the analogy of an icing syringe or a toothpaste tube: what comes out when the molten material is pushed through a shaped die, and why extrusion products are always 'long with a constant profile'.

The figure shows the four steps of blow moulding a bottle, but the third step is covered.

Deduce what happens in the covered step by observing the change in the component between the second and fourth panels, explain the role of compressed air, and why the inside of the mould has the exact shape of the bottle.

Explain what plastic injection molding consists of and reason why it is the technique chosen to manufacture millions of identical and precise parts, even though the mold is very expensive.

Hint. Think about what happens to the plastic inside the mold (what state is it in when it enters and when it leaves?) and about how the cost of the mold is spread across all the parts that are manufactured.

You will be assessed on: describing the stages of the process (melting the plastic, injecting it under pressure into a closed mold, cooling it and ejecting the part) and justifying the choice with at least two advantages (for example, speed, repeatability and precision of the parts, or the low cost per part in large production runs).

Describe vacuum forming using a muffin tray: what happens to the plastic sheet when heated, what the mould does, and what role the vacuumed air plays. Why do the corners of the tray turn out thinner?

Topic challenge. You are going to build a wooden box in the workshop and you need to choose the right tools.

These are four workshop tools (numbered) and four tasks (lettered):

1. Tenon saw

2. Drill

3. File

4. Clamp (or vise)

A. Making holes

B. Holding the pieces in place while they are glued

C. Cutting in a straight line

D. Roughing down and smoothing edges

Answer:

1. a) Match each tool with its task. Write the pairs in number-letter format.

1. b) Choose one of the tools and explain how it is used and what safety precautions you must take.

1. c) Which tool would you use first when making the box? Justify your answer.

What will be assessed: that all four pairs are correct, that the explanation is clear and uses technical vocabulary, and that the work order you chose is justified.

Topic challenge. Write three safety rules for drilling plastic in the workshop and explain the accident that each one prevents.

1. a) First rule and the accident it prevents.

1. b) Second rule and the accident it prevents.

1. c) Third rule and the accident it prevents.

You will be assessed on the three rules being different, being related to the use of the drill, and each accident explained making sense with its rule.

After cutting acrylic with a saw, the edge is left with burrs and rough edges. Explain why filing is necessary after cutting, the difference between filing and sanding, and the order in which you would use them to achieve a smooth, transparent edge.

Choose the appropriate technique for each job and justify it considering the shape of the part and the quantity: (a) ten thousand half-litre bottles; (b) fifty trays for a model; (c) six-metre irrigation pipes; (d) one hundred thousand remote control casings.

The figure shows a diagram of injection molding: the pellets (small plastic granules) fall from the hopper and travel through a covered zone before filling the mold. The covered zone is marked with "?".

1. a) Explain what happens to the plastic in the covered zone.

1. b) Explain why the mold must be very strong.

1. c) Explain what advantage there is in manufacturing millions of identical parts this way.

Order the steps for building an acrylic box in the workshop and justify why filing comes before gluing: joining the sides with adhesive, measuring and marking the sheet, cutting with the saw, drilling the intended holes, filing the edges, finishing the surface.

Extrusion produces 'continuous' parts (cut to the desired length) and injection moulding produces 'closed', fully shaped parts. Explain the limitation of each technique and why you cannot make a bottle by extrusion alone and why a six-metre profile is not viable by injection moulding.

A workshop wants to manufacture only three prototypes of a casing. Reason why injection moulding is not viable (the mould costs thousands of euros) and compare two more sensible alternatives for so few units: 3D printing or vacuum forming over a simple mould.

Compare three ways of joining plastic parts in the workshop: contact adhesive, cyanoacrylate (superglue), and nuts and bolts. Indicate a situation where you would choose each one and explain when it is useful for the joint to be removable.

Topic challenge. When you drill a plastic part at high speed, the drill bit sometimes leaves the material half-melted and the hole comes out deformed.

1. a) Explain why this phenomenon happens. In your answer, relate the movement of the drill bit, the heat that is generated and the properties of plastic.

1. b) Suggest two specific measures that can be applied when drilling to prevent the plastic from melting and the hole from coming out deformed.

What will be assessed: that you correctly identify the cause (where the heat comes from and how the material responds), that the two measures are different, realistic and related to that cause, and that you briefly explain why each of them helps.

Hint: think about what happens when you rub two surfaces against each other, and about what happens to the chips if they stay inside the hole while the drill bit keeps spinning.

The diagram matches four techniques with their products, but one is missing.

State which product is missing in the blow moulding row, explain why that technique is suitable for it (thin-walled hollow part), and describe in your own words how it is produced.

An intern suggests to a factory that they manufacture five hundred thousand threaded bottle caps using extrusion and five hundred thousand drinking straws using injection moulding, claiming that "both machines melt plastic and push polymer through a nozzle". Technically refute their proposal: explain what geometric limitation prevents manufacturing the caps by extrusion, why manufacturing straws by injection moulding would be unfeasible in terms of cost and time compared to continuous extrusion, and what the correct technique is for each product.

A classmate tried to make a mobile phone stand in the workshop by bending a $4\ \text{mm}$ acrylic sheet using the direct flame of a blowtorch after cutting it out with a jigsaw at maximum speed. The result was disastrous: melted, chipped edges, and a bend area full of whitish bubbles and microcracks.

1. Technically explain what causes these two defects in acrylic (during cutting and during bending).

2. Design the corrected process sheet to manufacture the piece successfully: an ordered list of stages, the appropriate tool or machine for each one (avoiding the previous mistakes), and the mandatory safety measures.

A catering company orders the design of one thousand reusable and sustainable plastic trays. The design team is debating between two thermoforming options:

• Option A: Plain polypropylene sheet $2,5\ \text{mm}$ thick.

• Option B: Polypropylene sheet $1,2\ \text{mm}$ thick with geometric reinforcing ribs on the base and a folded perimeter rim.

Analyse and compare both options in terms of structural stiffness, raw material consumption, and energy use during the heating cycle. Determine which is more sustainable for the batch of one thousand pieces, and propose two specific workshop guidelines for managing the offcuts or scrap left after thermoforming.

Many battery packages use the blister pack system (a transparent plastic dome heat-sealed to a printed cardboard backing).

1. Deduce which industrial forming process was used to manufacture the transparent blister and justify why that method is ideal for thin sheets.

2. Analyse the two major problems with this packaging: user frustration/cut risk when opening it, and the impact on the recycling plant due to the heat-sealed bond between two different materials.

3. Design an alternative packaging that protects the batteries and allows the product or its details to be seen, but makes it easy to open without tools and ensures mono-material or easily separable recycling.

You need to cut rectangular pieces of $30\ \text{cm} \times 40\ \text{cm}$ from polystyrene sheets of $100\ \text{cm} \times 120\ \text{cm}$.

1. An operator suggests placing all the pieces in the same orientation (for example, aligning the $30\ \text{cm}$ sides along the $100\ \text{cm}$ side). Calculate how many pieces fit and how much surface area is wasted with this method.

2. Design a mixed layout (nesting combining orientations on the same sheet) that achieves $100\ \%$ material utilisation (zero scrap). Draw or describe with exact dimensions how the blocks of pieces should be arranged.

3. Explain why, in the plastics industry, computer-aided nesting saves large amounts of money and energy.

A company must choose how to manufacture the casing for a new sensor and is hesitating between two technologies:

• Option 1 (Fused deposition 3D printing): does not require a mould ($0\ \text{€}$ initial fixed cost), but each casing costs $8,50\ \text{€}$ to manufacture.

• Option 2 (Injection moulding): requires manufacturing a steel mould that costs $15.000\ \text{€}$, but the injection cost for each casing is only $1,00\ \text{€}$.

• Calculate from how many units produced injection moulding becomes cheaper than 3D printing (break-even point or mould amortisation).

• If the sales forecast is $1.200$ casings in the first year and $1.800$ in the second year before redesigning the model, which manufacturing method would you recommend financially? Justify your answer.

In an automotive component injection moulding plant, it is detected that a series of polypropylene bumpers breaks easily during impact testing. Upon inspecting the feed hopper, they discover that the operator mixed $80\ \%$ regrind material from scrap with only $20\ \%$ virgin pellets, exceeding the $15\ \%$ limit set by engineering.

1. Explain at the molecular level (polymer chain length and thermal degradation) why a thermoplastic loses mechanical properties when it is shredded and remelted multiple times.

2. Why is it physically impossible to apply this shredding and remelting process to parts made of polyester resin or Bakelite? Justify your answer by explaining the chemical bonding structure of thermosetting plastics.

In the workshop, $25$ leftover strips of transparent acrylic sheet, $3\ \text{mm}$ thick and measuring $5\ \text{cm} \times 20\ \text{cm}$, have accumulated.

Design a useful product for a desk (for example: a ruler stand, a note lectern/holder, a cable organiser...) that makes use of at least one of these strips generating virtually no new waste.

1. Draw a dimensioned sketch of the object indicating the position of cuts, drill holes, and bend lines.

2. Describe the technical sequence of operations you must follow in the workshop.

3. Explain the exact thermal procedure for bending the strip: what tool must be used, what state the plastic must be in before applying force, and how to prevent it from burning or bubbling in the curved area.

Unit challenge. Have you ever noticed the marks that industrial processes leave on everyday objects? Become a polymer expert by analyzing your own surroundings in three stages:

1. Household sampling: Select $5$ different plastic objects from your home (packaging, electronic components, textiles, or housewares). Locate their identification code ($1$ to $7$), if they have one, and classify them according to their family: thermoplastics, thermosets, or elastomers.

2. Reverse engineering: Inspect physical marks on each part (flash, mold parting lines, injection points, or wall thicknesses under $2,5\text{ mm}$). Deduce and argue which shaping technique was used to manufacture it (injection molding, extrusion, blow molding, thermoforming, calendering, or compression molding).

3. Technical report: Record the data in LibreOffice Calc, generate a bar chart showing the identified families, and write a proposal to replace one of the non-recyclable objects with a sustainable alternative or reduce its mass by $15,5\,\%$.

Assessment criteria:

• Accuracy in the identification and classification of polymers.

• Thoroughness in justifying the manufacturing techniques based on real geometric features.

• Proper presentation of the table and chart in LibreOffice Calc.

• Technical feasibility of the environmental improvement proposal.