A machine that melts plastic granules and injects them under pressure into a closed mould to form a part.
01 / Definition
What a Injection Moulding Machine is
An injection moulding machine melts plastic and forces it into a closed mould. A screw turns inside a heated barrel, and as granules move forward along the flights they are melted by both the barrel heaters and the shear of the screw itself. The screw retracts as melt collects in front of it, then drives forward like a plunger to inject that shot into the mould.
After filling comes holding pressure, which keeps feeding a little more melt as the part cools and shrinks — this is what decides dimensional accuracy and whether the part sinks. Then the part cools until rigid, the clamp opens, ejectors push it out, and the cycle repeats. Most quality problems come from one of four settings: barrel temperature profile, injection speed, holding pressure and cooling time.
Also called
02 / Anatomy
The parts that matter
The components a technician reaches for when this machine stops working.
Screw and barrel
Melt, mix and inject. Wear on the screw flights and barrel bore shows up as inconsistent shot size.
Non-return (check) valve
Stops melt flowing back over the screw during injection. When it wears, shot weight varies randomly.
Barrel heater bands and thermocouples
Hold the temperature profile along the barrel. A failed thermocouple can overheat a zone while the display looks normal.
Clamp unit
Toggle or hydraulic. Holds the mould shut against injection pressure; insufficient clamp tonnage causes flash.
Ejector system
Pushes the finished part out. Bent pins mark parts and eventually damage the mould.
Mould temperature control
Water or oil circuits that set cooling rate — the largest single influence on cycle time.
03 / Applications
Where it is used
High-volume plastic components for packaging, automotive, electrical and consumer goods.
04 / Failure modes
How it fails
What actually goes wrong on Indian shop floors, and what each fault looks like to the operator.
FM_01
Short shots
Insufficient material reaching the cavity: low shot size, a worn check valve, a blocked nozzle, or melt too cold to fill.
FM_02
Flash around the parting line
Clamp tonnage too low, mould faces damaged or dirty, or injection pressure raised to cure a fill problem that had another cause.
FM_03
Sink marks and voids
Holding pressure or holding time too short, so the part is not fed as it shrinks. Often mistaken for a material problem.
FM_04
Shot weight varying run to run
A worn non-return valve is the classic cause, letting melt escape backwards unpredictably during injection.
FM_05
Burn marks and degraded material
An overheating barrel zone from a failed thermocouple, or material sitting too long at temperature during stoppages.
05 / Onsite
What our technician checks
A methodical path to root cause, not a scattergun of part swaps.
- 01Check the barrel temperature profile with an independent probe against the controller display, zone by zone
- 02Weigh ten consecutive shots and look at the spread — variation points at the check valve before anything else
- 03Inspect the nozzle and sprue bush for blockage or wear and confirm nozzle contact and radius match
- 04Verify clamp tonnage against the projected area of the part before raising injection pressure
- 05Check mould temperature at inlet and outlet on each circuit and confirm the cooling channels are clear
- 06Inspect ejector pins for bending, scoring and free movement
06 / Get it fixed
Related services
The MachineryFix services that cover this equipment.
07 / Questions
Frequently asked questions
Why does shot weight vary when nothing has been changed?+
The non-return valve on the screw tip is the usual culprit. As it wears, a varying amount of melt escapes backwards during injection, so each shot differs slightly. Weighing ten consecutive shots and looking at the spread is the quickest way to confirm it.
What causes flash on the parting line?+
The mould is being pushed open — clamp tonnage too low for the projected area, or damaged and dirty mould faces not closing properly. Raising injection pressure to fix a short shot can cause flash as a side effect, so check why the pressure was raised.
How do I get rid of sink marks?+
Sink marks mean the part was not fed while it shrank. Increase holding pressure or holding time first, and check the gate is not freezing off too early. Thick sections will always be prone to it, which is a design limit rather than a setting problem.
Why are parts burning on one side only?+
Usually a barrel zone running hotter than the display suggests, from a failed or drifting thermocouple. It can also be trapped air in the cavity compressing and igniting as the melt arrives — in which case the fix is venting, not temperature.
Written & reviewed by MachineryFix — last reviewed — questions? Call +91 70133 33007