How an Injection Molding Machine Works
The full cycle explained in plain language — plasticizing, injection, holding, cooling and ejection — with a short video from the AMAN workshop.
Watch: the working principle
The injection molding cycle, step by step
1. Plasticizing. Plastic granules fall from the hopper into a heated barrel. A rotating screw conveys them forward, and the combination of barrel heat and shear from the screw melts the material into a uniform melt cushion in front of the screw tip.
2. Mould closing and clamping. The clamping unit closes the two mould halves and holds them shut with a force rated in tons — this is the machine size you see in model names. The clamp must beat the pressure the melt will exert inside the mould, otherwise the mould flashes.
3. Injection. The screw stops rotating and rams forward like a plunger, pushing the melt through the nozzle, down the mould’s runner system and into the cavities in a fraction of a second.
4. Holding and cooling. The screw keeps packing pressure on the melt while the part starts to solidify, compensating for shrinkage. Cooling channels in the mould carry the heat away — cooling is normally the longest part of the cycle and is governed by the thickest wall of the part.
5. Ejection. The mould opens and ejector pins push the finished parts off the core. A robot or conveyor takes them away, the mould closes, and the next cycle begins — fully automatically, around the clock.
How machine and mould are matched
Three numbers drive the choice: clamping force (from the part’s projected area and cavity count), shot weight (total grams of plastic per cycle, which sizes the screw and barrel), and tie-bar spacing (the mould must physically fit between the tie bars). AMAN supplies the machine and builds the matching injection mould in-house — caps, closures, PET preforms, jars, crates and household parts — so the two are engineered together rather than reconciled after the fact. For PET bottle programmes this is a dedicated PET preform injection molding machine.
FAQ
How does an injection molding machine work?
An injection molding machine melts plastic granules in a heated barrel with a rotating screw, injects the melt under high pressure into a closed steel mould, holds pressure while the part cools and solidifies, then opens the mould and ejects the finished part. The cycle repeats automatically.
What are the main parts of an injection molding machine?
Four systems: the injection unit (hopper, heated barrel, reciprocating screw and nozzle), the clamping unit (platens and toggle or hydraulic clamp that holds the mould shut), the mould itself, and the control system that sequences temperatures, pressures, speeds and times.
What does clamping force (tonnage) mean?
Clamping force is how hard the machine holds the mould closed against injection pressure, quoted in tons or kN. As a rule of thumb you need roughly 2 to 5 tons per square inch of projected part area; a part that spreads over a larger area, or runs with more cavities, needs a bigger machine.
What determines the cycle time?
Cooling usually dominates the cycle and depends mainly on the thickest wall section of the part. Injection and mould movements add seconds more. Thin-wall packaging parts can cycle in a few seconds; thick technical parts can take a minute or more.
How many cavities can one mould have?
From a single cavity for large parts (crates, pails) up to 96 or more for small parts like caps, closures and PET preforms. More cavities multiply output on one machine but need more clamping force and a hot-runner system to feed every cavity evenly.
Injection molding vs blow molding - which do I need?
Injection molding makes solid parts: caps, closures, preforms, boxes, cutlery, fittings. Blow molding makes hollow parts: bottles, jerry cans, drums, tanks. Many packaging lines use both - an injection machine for the cap and preform, a blow molding machine for the bottle. If your part is hollow, start with blow molding; if solid, injection.
Sizing a machine for your part?
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No drawing yet? Photos of a sample part work too. This is all we need to match the machine and quote it:
- ✓Part drawing, 3D file, or photos from all sides
- ✓Part size and weight
- ✓Material, e.g. PP, HDPE or PET
- ✓Pieces per hour, or the tonnage you have in mind