Buyers usually arrive with a machine size in mind and a mould in the corner of the workshop. It is worth reversing the order: start from the mould and the part, and let those decide the machine.

Clamping force follows the projected area

The clamping unit has to hold the mould closed against the pressure inside the cavity. The rough rule used on the shop floor is to multiply the projected area of the part and runners by the required cavity pressure, then add a safety margin. Thin-walled and long-flow parts need markedly higher pressure than thick, simple geometries, and glass-filled materials need more than unfilled ones.

The practical consequence: a machine that clamps comfortably for a chunky technical part may flash badly on a thin-walled lid of the same footprint.

Shot volume: stay inside the comfortable band

A screw that is asked for almost its entire stroke on every shot leaves no cushion and no room for variation. A screw that only uses a small fraction of its capacity keeps material sitting hot in the barrel for too long, which shows up as degradation and colour streaks. Aim for the middle of the range rather than either edge.

Screw diameter and the material

  • Larger screw diameter — more volume per stroke, lower injection pressure available.
  • Smaller screw diameter — higher pressure, better for thin walls and long flow paths.
  • Check that the screw and barrel suit your material: abrasive glass-filled compounds and PVC need appropriate metallurgy.

What to check on a used machine

Ask to see the machine running, then look at the parts of it that cost money to fix:

  • Wear on the screw and non-return valve — ask when they were last replaced.
  • Platen parallelism and tie-bar condition, or the frame on tie-bar-less machines.
  • Heater bands and thermocouples on every zone, and whether the zones hold temperature.
  • Hydraulic leaks around the injection unit and the clamp.
  • Control battery, parameter backup and mould data still stored in the control.
Bring a mould you actually run and make a real part during the inspection. Nothing else tells you as much in half an hour.

Do not forget the periphery

A machine alone does not make parts. Material dryer, mould temperature controllers, granulate feeding, a robot or at least a safe drop area, and cooling water with sufficient capacity all belong in the budget. Second-hand machines are often sold with some of this equipment included — it is worth asking explicitly what stays with the machine.

Browse the injection molding machines currently listed, or tell us the part and material and we will suggest a size range.