Co-extruded Cap Layer Peeling: When the Interface Never Welded

A decorative or protective cap layer that lifts at a cut edge is a sign of poor interfacial welding, not of a weak base compound.

What the failure looks like on the line

Coextruded and laminated sections fail at the interface, and the symptom is usually attributed to the visible layer rather than the bond.

On a co-extruded profile, the thin cap layer separates from the core at cut ends, corners or punched holes, and can be peeled back as a continuous film. Adhesion is often acceptable in the middle of a length but poor near start-up or after a grade change.

Two melts join inside the die or in a feedblock and must interdiffuse at the interface before the laminate is frozen. If the melts arrive at widely different temperatures, or a compatible tie layer is missing, the contact plane stays sharp and carries little entangled network. Adequate interface temperature and contact time build the bond; mechanical keying alone cannot recover a cold interface.

The useful question is not whether the part conforms, but which condition was never written into the specification. On a buyer's specification this distinction matters: a part can measure inside tolerance and still fail in use, because the drawing records geometry while the complaint records behaviour. Interface melt temperature is usually the first value both sides compare, and it is also the one most often quoted without the conditions attached to it.

The process window that decides it

Four variables move this result more than the rest. The ranges below are illustrative engineering guidance for a first discussion; confirm them on your own tool, material grade and part geometry before they become contract terms.

Table 1. Starting window for a joint process trial. Values are engineering guidance, not a measured survey.

Interface melt temperature is typically held at 190-220 C. A cold interface freezes before the two melts interdiffuse Record it together with the measurement point and the moment in the cycle; a single number without a location is not a process setting.

Melt temperature mismatch is typically held at < 20 C between layers. A large gap leaves the cooler layer unable to weld to the hotter one When this drifts, the symptom usually appears at one cavity, one sheet position or one lane before it appears across the whole batch.

Tie-layer thickness is typically held at 30-80 um. Too thin a tie layer cannot bridge incompatible resins A change here is cheap during a trial and expensive after tooling release, so treat it as a controlled variable with a documented re-approval.

Contact time in die is typically held at 2-8 s. Short contact time leaves insufficient interdiffusion depth Put it on the change-control list, otherwise an unannounced material or setting substitution will be mistaken for ordinary process noise.

Reading the response curve

The left panel shows peel strength (relative) as interface melt temperature (c) increases: the response rises from 28 to 90 on the illustrative 0–100 scale. The dashed line at 55 marks a proposed acceptance limit, not a universal standard. Move that line only after the test method and the sample plan have been agreed, otherwise two suppliers can both be compliant and still deliver different parts.

Read the shape rather than a single point. A curve that changes quickly near the limit is more sensitive to ordinary process noise than one that approaches it gradually, and that difference should be reflected in how tightly the setting is controlled and how often it is checked.

Where the causes actually sit

The right panel of Figure 1 splits audited failures of this type into five groups. The percentages are illustrative engineering guidance for prioritising an investigation, not a measured survey and not a prediction for your line.

Table 2. Cause split and the corresponding first check. Shares are indicative and sum to 100.

Cold layer interface accounts for roughly 33% of the illustrative split. Peel a cut end by hand and note whether separation is interfacial or cohesive. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Large layer temperature gap accounts for roughly 24% of the illustrative split. Log both layer melt temperatures at the feedblock at start-up and steady state. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Missing or thin tie layer accounts for roughly 19% of the illustrative split. Compare peel strength at the head, middle and tail of a production length. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Checks before sign-off

Work through these in order; measuring a later variable before an earlier one hides the cause rather than finding it.

1. Peel a cut end by hand and note whether separation is interfacial or cohesive.
2. Log both layer melt temperatures at the feedblock at start-up and steady state.
3. Compare peel strength at the head, middle and tail of a production length.
4. Confirm the tie-layer grade and thickness against the running recipe.
5. Re-run after a stabilised warm-up before changing the base resin.

Keep a retained reference sample beside every result. When a batch is disputed, the comparison that settles it is almost always between the retained part and the current one under the same method, not between either part and a number on a drawing. Record the instrument, the operator, the conditioning time and the position on the part or sheet for each reading.

Writing it into the specification and the evidence pack

Peel strength between cap and core, measured by the agreed peel method on conditioned samples, shall not fall below the stated minimum at any sampled position along the length.

Write the requirement as a behaviour at a stated condition, not as a brand of material or a machine setting. State the test method, the conditioning, the sample plan and the rule for deciding a pass. Where a document is needed, ask the supplier for documentation that covers the actual construction and check its scope against your own market before approval; a document for the base material alone does not describe the finished part you are buying.

At first-article approval, freeze the drawing revision, the material and colour identification, the process window, the tool and cavity identification, and the inspection method. Any later change to grade, site, tool, or window should trigger a new first article rather than a paper waiver. Keep retained samples from the approved lot and from each production lot so that a future comparison is possible.

Table 3. Minimum evidence pack for a first-article decision and a later batch dispute.

The values quoted here are illustrative engineering guidance, not a measured survey. Agree the test method, the sample size and the acceptance rule before comparing numbers across suppliers.