Injection moulding operator: shift and part defects

An injection moulding operator spends most of the shift inspecting moulded parts, taking them off the machine and packing them, while the process parameters are set by the setter (ustawiacz). When flash, a short shot, a sink mark or streaks appear on a part, the operator sets the faulty piece aside and reports the problem instead of changing the settings alone.

What does an injection moulding operator do during a shift?

The machine works in a cycle: plastic in the form of granulate is melted in the barrel, injected into a closed mould, cooled and ejected as a finished part, known as a moulding. An operator's shift usually follows a fixed order:

1. Taking over the station from the previous shift: which part is running, whether there were problems and what was reported.
2. Checking the first piece against the reference sample or the station instruction.
3. Collecting the mouldings, visually checking every piece or a sample, and removing sprue remnants if the instruction requires it.
4. Packing into containers and labelling them according to the instruction.
5. Writing down in the station card: number of pieces, rejects, downtime, reported defects.

The work is repetitive and its pace is set by the machine cycle. We describe the organisation of work on the shop floor in more detail in the article on what work in production looks like. Work at injection moulding machines is often shift-based, so it helps to know how to prepare for shift work.

Operator or setter: who changes the parameters?

Temperatures, pressures, holding and cooling times are set by the setter or the process engineer. The operator knows these names because they appear on the machine panel, but does not change them unless the station instruction clearly allows it. The reason is simple: one defect can have several causes, and changing a parameter by eye can remove one problem and cause another.

The most common moulding defects and what they mean

The operator does not need to know the cause of a defect, only to recognise and describe it. The causes below are general principles of the technology, and the setter decides on the specific one.

• Flash: a thin film of plastic on the edge of the part, at the mould parting line. The cause may be too little clamping force, too high a filling pressure, a low-viscosity material, or a worn or dirty mould parting line.
• Short shot: the mould did not fill completely and a fragment of the part is missing, most often at the end of the plastic flow path.
• Sink mark: a depression on the surface in a thicker area of the part, usually when packing pressure is too weak.
• Streaks: dull, elongated marks aligned with the direction of melt flow. A common cause is moisture in the granulate (insufficient drying) or water from a leaking mould cooling system; the plastic can also degrade at too high a shear rate.
• Cold slug mark: a portion of plastic that solidified in the runner system or nozzle and ended up in the part. It is usually visible near the sprue.

With streaks, the operator can check what is at hand: whether the granulate dryer is working and whether there is any water leaking near the mould. The result of this observation goes to the setter.

What to do when a moulding is faulty?

1. Put the faulty piece in the reject container so that it does not end up among the good ones.
2. Check several following mouldings: does the defect repeat or was it a one-off?
3. Report the problem to the setter or the shift leader and say what you see and since when.
4. If the instruction says so, stop the machine until a decision is made.
5. Write the event in the station card.

A precise description ("flash on the left edge since 10 o'clock") shortens the search for the cause. In the automotive industry, where customer quality requirements are especially strict, this procedure is followed very closely, as we describe in the topic on work in the automotive industry.

Is the work of an injection moulding operator dangerous?

The greatest risk is the mould area, because remnants of the moulding sometimes have to be removed from it. Accidents in this place have happened when an operator reached into the mould during an automatic cycle and a hand was caught by the closing mould. The causes of such incidents include a poor condition of the guard, controls unsuited to the risk, lack of inspections, fatigue and little operator experience.

The rules that follow:

• The guard of the mould area is linked to an interlock and the controls, and the cycle may start only when the guard is closed and locked.
• Do not reach into the mould during an automatic cycle. Remnants are removed in the mode and in the way described in the station instruction.
• Report a damaged or non-closing guard immediately, before working further.
• The barrel and nozzle are hot, so do not touch them without need and use the protective equipment required at the station.

Frequently asked questions

Do you need a course to become an injection moulding operator?

We do not know of a regulation that would require separate qualifications to operate an injection moulding machine. The requirements are set by the specific job offer: some positions are for people without experience, and operation is learned on site.

How long does induction at the station take?

It depends on the part and the number of defects to recognise. With simple mouldings the basics of operation are learned faster, and with parts under strict quality standards learning to assess them takes longer.

Can the operator correct the parameters alone if they know what to do?

Only if the station instruction clearly allows it. Otherwise the defect is reported to the setter, who is responsible for the process parameters.

Where do streaks on mouldings come from?

Most often from moisture in the granulate or from water getting in from a leaking mould cooling system. The operator checks the dryer and the mould and passes the result to the setter.

This text describes the general rules. What you are entitled to depends on your type of contract: an employment contract (umowa o pracę) or a contract of mandate (umowa zlecenie). Your own contract and payslip are what count.