Sheet Sag on Large Machine Covers: When Heater Zones Drift Apart

Wide covers sag before forming when heater zones fall out of balance; diagnose the temperature spread and set a sag window the tool can still form.

What the failure looks like on the line

Machine covers are large, lightly stiffened panels; their problems show up as fit, rattle or waviness rather than as a failed tensile test.

On long covers the heated sheet droops into a visible hammock between the clamp frames, and the formed part shows a thin crown with heavy material at the edges.

Large sheets spend long enough in the oven for gravity to act while the polymer is soft; any zone running hotter softens its band earlier and that band sags first. The sag redistributes material before the draw starts, so the final wall distribution no longer matches the tool geometry.

Start with the failure as it is reported, not with the parameter that is easiest to measure. 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. Zone-to-zone temperature spread 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.

Zone-to-zone temperature spread is typically held at Within 5-8 C across the sheet. A wider spread makes one band sag earlier than its neighbours Record it together with the measurement point and the moment in the cycle; a single number without a location is not a process setting.

Sheet surface temperature at draw is typically held at 150-175 C (ABS/HIPS). Overheated sheet loses melt strength and sag accelerates When this drifts, the symptom usually appears at one cavity, one sheet position or one lane before it appears across the whole batch.

Free span between clamp frames is typically held at Keep under 1.2 m where possible. Longer unsupported spans sag more at the same temperature 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.

Heating time is typically held at Roughly 25-45 s per mm of gauge. Extended heating raises core temperature and increases sag 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 mid-span sag (mm) as zone-to-zone temperature spread (c) increases: the response rises from 2 to 30 on the illustrative 0–100 scale. The dashed line at 12 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.

Heater zone imbalance accounts for roughly 36% of the illustrative split. Log each heater zone temperature on a calibrated pyrometer before the cycle. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Sheet overheated accounts for roughly 24% of the illustrative split. Measure mid-span sag against a fixed height gauge at a defined time in the heating cycle. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Unsupported span too long accounts for roughly 18% of the illustrative split. Compare sag readings between the centre and the clamp-frame edges. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Checks before sign-off

In practice the sequence below is cheaper to run than a root-cause debate after a rejected delivery.

1. Log each heater zone temperature on a calibrated pyrometer before the cycle.
2. Measure mid-span sag against a fixed height gauge at a defined time in the heating cycle.
3. Compare sag readings between the centre and the clamp-frame edges.
4. Verify clamp-frame tension is even around the full perimeter.
5. Recheck the sag window after any heater element or reflector change.

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

Mid-span sag measured at the specified point in the heating cycle shall stay within the agreed band, with the measurement time and sheet surface temperature recorded alongside it.

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.