A science teacher runs a gardening club and wants students to grow plants from seed in a small greenhouse. Seedlings often fail because the humidity inside ...

(a) Four functional points [4 marks]

1. Transparent lid for light transmission — Seedlings need light for photosynthesis. The lid must be made from a clear or translucent material that allows sunlight to reach the growing plants while retaining warmth and moisture inside the tray. An opaque lid would starve seedlings of light and prevent healthy growth. [1]
2. Watertight tray base — The tray must hold moist compost and contain irrigation water without leaking onto the greenhouse bench. Any seams or joints in the base must be sealed. The material must not degrade when in continuous contact with damp soil. [1]
3. Fine ventilation control — Humidity inside a sealed propagation tray can reach 100%, which encourages damping-off disease (a fungal infection that kills seedlings). The ventilation mechanism must allow gradual adjustment from fully closed (maximum humidity for germination) to fully open (lower humidity for hardening off), enabling the student to manage the transition progressively. [1]
4. Easy to clean and reuse — A school gardening club reuses propagation equipment across multiple growing seasons. The tray and lid must be made from materials that can be washed, disinfected, and dried without degrading. Smooth surfaces with no hidden corners make thorough cleaning practical. [1]

(b) Two ventilation mechanisms [4 marks]

Mechanism 1 — Sliding panel vent: A section of the lid (approximately 100 mm wide, running the full length) slides along grooves to create a variable-width opening. When fully closed, the panel overlaps with the fixed lid section to seal the tray. When slid open, a gap of up to 100 mm allows air exchange. The guide tracks prevent the sliding panel from lifting in a breeze. This provides continuously variable ventilation from zero to maximum. [2]

Mechanism 2 — Hinged flap with notched prop stick: A rectangular flap (approximately 100 mm x 200 mm) is cut from the lid and re-attached with a small piano hinge along one edge. A thin prop stick (3 mm acrylic or metal rod) with notches at intervals slots into a bracket on the lid frame. Each notch holds the flap at a different fixed angle (e.g., 15, 30, 45, and 90 degrees), providing stepped ventilation options. The flap can be propped fully open or left closed for maximum humidity. [2]

(c) Three design ideas [12 marks]

Idea 1 — Flat tray with domed lid and sliding vent strip: A shallow rectangular tray (400 mm x 250 mm x 50 mm deep) with a clear domed lid that rises to 100 mm at the centre, giving seedlings room to grow upward. A sliding vent strip runs along the ridge of the dome, providing variable ventilation. The domed shape sheds condensation droplets to the sides rather than dripping onto seedlings. The dome requires thermoforming, which is available in most school workshops. [4]

Idea 2 — Deep box tray with hinged lid and adjustable louvres: A deeper tray (400 mm x 250 mm x 80 mm) with a flat hinged lid. Two sets of adjustable louvres at each end of the lid can be rotated from closed to fully open using a thumb wheel. The louvres provide precise airflow control. The flat lid is simpler to manufacture than a dome, but condensation may drip directly onto seedlings rather than running to the sides. The louvre mechanism adds complexity. [4]

Idea 3 — Modular cell tray with clip-on cover and rotating disc vents: A cell tray (individual compartments for each seedling) with a flat clip-on acrylic cover. Three circular disc vents (30 mm diameter) are set into the cover. Each disc rotates to align holes in the disc with holes in the cover, varying the open area. This design suits individual seedling management, and the clip-on cover is easy to remove for watering. The small vent openings provide limited total airflow compared to a sliding panel. [4]

(d) Evaluation and justification [8 marks]

Idea 1 (Domed lid + sliding vent) — Strengths: domed shape gives seedlings room to grow; condensation runs to the sides rather than dripping on plants; sliding vent provides continuously variable airflow. Weaknesses: the dome shape requires a vacuum-forming mould.

Idea 2 (Flat lid + louvres) — Strengths: flat lid is simpler to make; louvres provide precise control. Weaknesses: flat lid means condensation drips onto seedlings; louvre mechanism is complex to manufacture accurately.

Idea 3 (Cell tray + disc vents) — Strengths: individual cell compartments suit different seed types; clip-on cover is easy to remove. Weaknesses: rotating disc vents provide limited total airflow; the small openings may block with condensation.

Chosen idea: Idea 1. The domed lid with sliding vent provides the best growing environment for seedlings: the dome gives headroom for growth, condensation sheds to the sides (reducing the risk of damping-off), and the sliding vent provides smooth, continuously variable airflow adjustment. The vacuum-forming process is standard in most school workshops. [8]

(e) Full solution — construction details [12 marks]

The solution is drawn in isometric projection with a cross-section through the tray and lid showing the seal, drainage, and vent mechanism.

• Tray base: Vacuum-formed from 1.5 mm HIPS (high-impact polystyrene). External dimensions: 400 mm x 250 mm x 50 mm deep. The base includes four raised ridges (5 mm tall) running the length of the tray to create drainage channels. Six 3 mm drainage holes are drilled in the lowest points between the ridges.
• Lid: Vacuum-formed from 3 mm clear acrylic into a dome shape (100 mm tall at the centre). The lid rim overlaps the tray rim by 10 mm on all sides, sitting on a silicone rubber sealing strip.
• Sliding vent: A rectangular panel (300 mm x 30 mm x 3 mm clear acrylic) runs along the apex of the dome in two guide tracks (3 mm grooves routed into the dome surface). The panel slides to expose a ventilation slot up to 300 mm x 30 mm. A small tab at one end provides a finger grip.
• Handles: Two moulded finger recesses on the lid rim (formed during vacuum forming) allow the lid to be lifted off for watering and transplanting.

(f) Materials [4 marks]

1. Clear acrylic sheet (3 mm) for the lid. Acrylic transmits over 90% of visible light (more than glass), maximising the light reaching seedlings. It is lightweight, rigid at 3 mm thickness, and can be thermoformed into a dome shape using a vacuum-forming machine. It is waterproof and easy to clean with mild soap and water. [2]
2. High-impact polystyrene (HIPS, 1.5 mm) for the tray base. HIPS is rigid, waterproof, and can be vacuum-formed to include integral drainage channels and raised edges without additional assembly. It is food-safe (suitable for growing edible plants), resistant to soil acids, and can be cleaned and reused across multiple growing seasons. [2]

(g) Manufacturing the tray base (vacuum forming) [6 marks]

1. Make the mould: Shape a wooden mould of the tray form (400 mm x 250 mm x 50 mm) on a disc sander and with hand tools, including raised ridges for the drainage channels. Smooth all surfaces and apply a release agent (wax or petroleum jelly). [1]
2. Prepare the plastic: Clamp a sheet of 1.5 mm HIPS into the vacuum-forming machine frame, ensuring it is held securely on all edges and the sheet is level. [1]
3. Heat: Switch on the heater and heat the plastic sheet evenly until it sags by approximately 20-30 mm, indicating it is soft enough to form (approximately 170 degrees Celsius for HIPS). [1]
4. Form: Lower the heated sheet over the mould (or raise the mould into the sheet) and switch on the vacuum pump. The atmospheric pressure differential draws the plastic tightly around the mould shape, replicating the drainage ridges and tray walls. [1]
5. Cool and demould: Allow the formed tray to cool completely (approximately 60 seconds) before releasing the vacuum and carefully lifting the tray from the mould. Premature removal would cause the tray to distort. [1]
6. Trim: Trim the excess plastic (flashing) from the tray edges using a craft knife or band saw. Smooth the cut edges with fine glasspaper (240-grit) to remove sharpness. Drill the six 3 mm drainage holes using a hand drill. [1]