Blow-Moulding Defects: What Each One Tells You About the Process

A defect on a blow-moulded bottle is not a cosmetic detail. It is a message about the process, and it usually points at one of four places: the parison, the mould, the cooling cycle, or the resin. Reading that message correctly is the difference between a supplier who fixes a problem and one who just sorts more bottles into the reject bin.

Eccentric Wall Thickness

What you see: one side of the bottle noticeably thicker than the other. The bottle feels heavier than it should, and the thin panel is where it will fail first.

What causes it: uneven parison temperature, a misaligned die head, or the parison being inflated off-centre. It is a process and tooling issue, not a resin issue.

Why it matters commercially: the thin panel never appears in an average wall measurement. Specify minimum wall thickness at defined points on the drawing, or you have no enforceable standard.

Thin Base

What you see: normally invisible — until a drop test. A thin base is the single most common reason a filled bottle cracks on impact.

What causes it: the parison is stretched too far during closing, or the pinch-off is too aggressive and thins the material at the base seam. A related defect is a rough or partially open pinch-off line.

The fix: adjust parison programming and closing speed. It is almost never fixed by ordering a thicker overall wall, which adds cost everywhere instead of thickness where it matters.

Flash

What you see: a thin fin of plastic along the mould parting line, or around the neck.

What causes it: worn or poorly fitted mould halves, insufficient clamping force, or too much material in the parison. Small flash is trimmed; persistent flash means the mould needs maintenance.

Sink Marks and Shrinkage

What you see: a shallow dimple, usually on a flat panel or above a thick section such as a handle or a moulded-in boss.

What causes it: thicker sections cool more slowly, and the material contracts inward after the surface has set. Often a design issue rather than a process issue — uniformly redistributing wall thickness is the real fix.

Black Specks and Contamination

What you see: dark specks embedded in the wall or sitting on the inner surface. Sometimes the specks are burnt resin; sometimes they are degraded material from a previous run.

What causes it: degraded material in the extruder, contaminated regrind, or a dirty hopper. This matters more than appearance — for a product with no fragrance or colour to hide it, a visible speck is a customer complaint.

The commercial issue: contamination from regrind is the reason to ask what percentage of regrind a supplier is running, and whether it changes between your first order and your fifth.

Weld Lines

What you see: a visible line where two flows of material met during inflation, typically near a mould feature or an insert.

What causes it: two fronts of material meeting at a temperature too low to knit fully. The weld line is a weak point, and it is where a crack will start if the bottle is exposed to stress cracking.

Ovalisation and Warping

What you see: the bottle is no longer round in cross-section, or it rocks on a flat surface. Sometimes it only becomes visible after the bottle has been filled and stacked.

What causes it: uneven cooling, insufficient cooling time before ejection, or handling the bottle while it is still soft. If it appears only after filling, the cause is usually insufficient top-load strength rather than a moulding defect.

Surface Defects that Come from Decoration

Scratches, inconsistent ink coverage, incomplete foil transfer, colour mismatch, or bubbling under a label. These originate in the decoration step, not the moulding step — but they are usually found at the same final inspection, and it is worth knowing which supplier in the chain owns them.

How to Handle a Defect Claim

1. Describe the defect and where it occurs on the bottle, with photographs.
2. Give the rate. Four in 200 and 40 in 200 are different problems with different urgencies.
3. State the acceptance criterion it breaches — with the drawing dimension, not the word "quality".
4. Send samples back if the batch is disputed. A photograph cannot be measured.

The Short Version

Eccentric walls, thin bases and weld lines come from the process. Flash comes from the mould. Sink marks come from design. Black specks come from material handling. Knowing which is which tells you who should be paying — and it turns a complaint into a corrective action with an owner.