Blow moulding rejects: wall thickness, weight and handle defects

A tally of rejects per shift tells you the size of the problem. It does not tell you the cause. Ask the operator to pick a defect from a short list at the machine, and to note the mould and the cavity that made it.

Keep the list short enough to use at the machine. Ten or twelve entries cover most bottle, can and drum faults. A free text box invites vague notes. Avoid it.

Add the clock. A reject at the start of a run, after a colour change or near the end of a resin bag points in a different direction each time. The time belongs on the sheet.

Wall thickness on a blown container depends on the parison. If the programme controller is set wrong, the shoulder or base thins while the middle is fine. Sectioning a sample and measuring at fixed heights shows the pattern.

Write down the die gap, mandrel position and the programme points used on the run. When a thin base returns two weeks later, you can compare the settings that were in place.

Melt temperature matters as well. A colder melt sags less but shows weld lines. A hotter melt sags and thins the top. Record barrel zones when you change them.

Each bottle has a target weight and a tolerance. Weigh a few pieces every hour, and chart them against the shot. A slow drop usually points to a worn screw or a filter that is clogging, and a sudden step to a hopper or dryer change.

Heavy bottles waste resin. Light ones fail on drop or top load tests. Both are rejects, and both need the same log entry.

Check the resin lot against weight. Melt flow changes between lots, and a lot at the edge of the specification can move weight without any setting touched.

Handle bottles fail where the handle meets the body and at the pinch off. A weak handle weld, a hole at the pinch, or a handle that fills short each trace to cooling, blow pressure or mould wear.

Look at the mould before the settings. A worn pinch off blade or a blocked vent shows on one cavity only. If two cavities fail together, the cause is more likely upstream, in the melt or in the blow air supply.

Keep a photo of each handle defect against the entry. A photo of a short fill teaches a new operator faster than a paragraph. Keep them.

Once a week, sort rejects by defect, then by mould, then by resin lot. A defect that follows one cavity is a mould job. A defect that follows a lot is a supplier job. A defect that follows a shift is a training or setpoint job.

Bring the sorted page to the Monday meeting and name one cause for action. Fix it, then watch the next week’s log to see whether it moved.

Begin with a printed checklist at the machine this week. Review it every Friday with the shift leader and keep the sheets in a folder.