Injection molding makes complex three-dimensional plastic parts by forcing molten thermoplastic into a closed two-part mold, holding it under pressure while it cools, then opening the mold and ejecting the part. A small workshop can do it with a heated steel tube, a plunger driven by a lever or screw press, and a mold clamped shut, and the mold is the most critical and most expensive part of the whole system.
Part of Plastics and Polymers
Injection molding is the forming of thermoplastic parts by injecting a melt into a closed mold cavity, where it solidifies into the cavity’s shape.
Use It When A Part Has Shape, Not Just Length
Extrusion makes anything with a constant cross-section. Anything with ribs, hollows, bosses or threads needs a mold: containers, tool handles, fittings, gears and housings. Plastics Manufacturing lists these as the products of injection molding.
For solid thermoset parts such as Bakelite switch bodies, the simpler route is compression molding, because a thermoset cannot be melted and pushed through a runner.
What You Need
- Thermoplastic, identified and dry. See Sorting and Identification.
- An injection cylinder: a steel tube in which the plastic melts, with a plunger.
- A mold in two halves, with a gate, runner and ejector pins: Mold Design.
- A way to clamp the mold shut against the injection pressure: C-clamps, bolts or a lever press.
- A plunger drive: a lever, a screw (Screw Mechanisms) or a hydraulic jack (Hydraulic Press).
The Cycle Has Five Steps
The carrier’s low-volume method:
- Heat plastic in the steel tube to melting temperature.
- Clamp the mold halves together tightly.
- Force the melt into the mold with the plunger.
- Hold pressure while the plastic cools; the carrier gives 30 seconds to several minutes depending on part size.
- Open the mold, remove the part, and trim the gate and runner.
The details of each step, and what goes wrong, are in Molding Process.
Worked Example: Clamp Force
The mold wants to open while plastic is injected. Force on the mold halves is the pressure in the cavity times the projected area of the part.
- Take a part 100 mm × 60 mm in plan view: 6,000 mm² = 6 × 10⁻³ m².
- Assume a cavity pressure of 20 MPa (an assumption, not a corpus figure).
- Force = 20 × 10⁶ Pa × 6 × 10⁻³ m² = 120,000 N = 120 kN.
That is about 12 tonnes-force, far more than a pair of C-clamps holds. Either lower the pressure by heating the plastic to flow more easily, use a smaller part, or build a proper press. A mold that opens under pressure squirts hot plastic.
Common Mistakes
- Too little clamp force — plastic seeps out along the parting line as flash, or the mold bursts open.
- Molding thermosets this way — they cure in the cylinder.
- Using a wooden mold for hot plastic — it chars; wood is only for low-temperature plastics such as starch or casein, per the carrier.
Hot melt under pressure
Molten plastic sticks to skin and burns deeply, and a mold that fails under injection pressure sprays it. Wear gloves, eye and face protection, stand to the side of the parting line, and never open a mold before the part has set. Do not inject plastic that has not been identified: see Fume Safety.
What’s Next
- Learn next: Mold Design — cavity, runner and gate
- Combine with: Remelting — salvaged plastic as the feed