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Blister Pocket Depth Variation: Causes and Process Checks
Uneven blister pocket depth is usually a forming-process problem, not a defect that can be corrected by changing one temperature setting. Heat transfer, forming pressure or vacuum, die venting, web tension, material structure, and measurement method can all change the final cavity. A reliable investigation compares pocket depth and wall thickness across the web, then correlates the pattern with forming conditions and material lots. Start with a controlled sample and one verified change at a time.
Alu-PVC blister equipment uses coordinated heating, forming, sealing, and web transport.
What blister pocket depth variation looks like
The visible symptom may be a pocket that is too shallow, a cavity that collapses after forming, or a row whose depth changes from one side of the web to the other. Some defects appear only after the film cools. Others are noticed when tablets do not sit correctly, when the lidding foil contacts the product, or when the finished card shows inconsistent headspace. A single measurement is not enough to identify the cause because a pocket can meet its depth target while its wall has become locally thin.
Define the failure before changing the recipe. Record nominal pocket length, width, and depth, the allowed tolerance, the material structure, and the location of each sample. Measure the same cavity position across several cycles and across the machine direction and transverse direction. If the problem follows one lane, one die row, or one web edge, that pattern is often more useful than the average value.
Start with pocket depth and wall-thickness measurements
Use a calibrated depth gauge, optical measurement system, or validated measurement fixture that matches the pocket geometry. The method should define where the depth is taken: center point, maximum depth, or a specified datum from the flange. For wall thickness, cut or inspect representative pockets at the same locations and document whether the thinnest area occurs at the shoulder, corner, or base. Do not compare values taken with different fixtures or different reference planes.
A useful sampling plan includes the first acceptable cycle after start-up, a stable middle sample, and a sample near the end of the run. Add left, center, and right web positions when the machine allows it. Then place the results beside forming temperature, preheating status, pressure or vacuum trend, line speed, and web-tension readings. This creates a process picture rather than a list of unrelated numbers.
Measurement limit: Pocket depth and wall thickness are material- and tooling-dependent. The correct limits must come from the approved pack design, material specification, measurement method, and validation documentation. Do not adopt a generic depth or temperature range from another machine.
Heat transfer and preheating: the first process branch
Thermoforming film must reach a forming window in which it can stretch into the cavity without losing too much thickness or recovering before cooling. If the web is too cold, it may resist the die and produce shallow or incomplete pockets. If it is too hot, the material may thin excessively at corners, stick to the tooling, or lose shape during transfer. A set-point on the controller does not prove that the film surface has reached the intended condition; heater response, dwell time, shielding, air movement, and material construction all influence the actual web temperature.
Check whether the defect begins after a speed change, a heater-zone change, a splice, or a pause. Compare the forming result with the measured web temperature at a defined location, not just the HMI value. Inspect heaters, reflectors, insulation, and temperature sensors for contamination, loose connections, or drift. If a preheating station is used, confirm that the film enters the forming station with a repeatable thermal profile. Any adjustment should be tested on a defined number of cycles and recorded under the site change-control procedure.
The forming station, tooling, and web path should be assessed together when a cavity profile changes.
Pressure, vacuum, and die venting
Forming pressure or vacuum supplies the force that moves the softened web against the cavity surface. A low, unstable, or delayed force can leave the base or corners under-formed. A leak in a hose, seal, regulator, valve, or manifold can create a lane-specific pattern. On a vacuum-forming station, blocked die vents can trap air between the web and the cavity. The result may look like insufficient vacuum even though the pump reading appears normal.
Confirm the signal at the forming station, the pressure or vacuum at the point of use, and the time relationship between web arrival and forming actuation. Inspect the die face for residue, blocked vent holes, burrs, or damage. Check seals and clamps for flattening or leakage. A pressure or vacuum gauge located upstream of a restriction may not show the condition experienced at the die, so the measurement point matters. If the machine uses positive-pressure forming, verify air cleanliness, dryness, regulator stability, and the condition of the forming plate.
Do not increase pressure or vacuum indefinitely to compensate for a thermal or mechanical issue. Excessive force can accelerate thinning, distort the flange, or stress the tooling. Establish the approved operating window with engineering and validate the change against pocket dimensions, wall thickness, seal area, and downstream fit.
Web tension, alignment, and material-lot effects
Web tension controls how the softened film enters and leaves the forming station. Too much tension can restrict draw into the cavity or pull the formed pocket out of shape during indexing. Too little tension can create wrinkles, lateral wandering, or inconsistent registration. Compare the defect location with the web path: an edge-only problem points toward alignment, guide contact, roll condition, or transverse tension; a machine-direction change may point toward indexing, traction, or thermal history.
Review unwind-brake settings, dancer movement, traction-roll grip, guide alignment, splice construction, and any visible wrinkles before the forming die. A recipe that works for one film lot may not transfer unchanged to another lot if thickness, laminate structure, coating, stiffness, or coefficient of friction has changed. Keep the material combination explicit, for example PVC with the specified backing or laminate, and record supplier lot, thickness, width, and storage condition. The forming behavior of PVC is not automatically representative of PVDC-coated film, PET-based structures, or cold-formed aluminum.
Cause–test–action diagnosis
The table below is a starting framework. It does not replace the machine manual, approved SOP, or a validated troubleshooting protocol. The “test” should be controlled, reversible, and supported by a defined sample size.
Observed pattern
Possible cause
Confirmation test
Controlled action
All pockets are shallow after a speed increase
Reduced thermal dwell or forming timing mismatch
Compare web temperature and forming signal timing at the previous validated speed
Restore the validated recipe or qualify a new speed–temperature window
One side of the web is shallow or thin
Transverse temperature imbalance, die alignment, venting, or web tension
Map depth and wall thickness left/center/right; inspect die vents and guides
Correct the confirmed mechanical or thermal imbalance; do not simply raise the global set-point
Depth varies by cycle and follows an alarm or pressure dip
Unstable pressure/vacuum, valve response, leak, or controller timing
Trend the point-of-use signal against forming-cycle timestamps
Repair the leak or control fault, then repeat the dimensional study
Base forms but corners are thin
Excessive draw, high web temperature, sharp tooling radius, or restricted venting
Measure thickness at base, shoulder, and corner; inspect tooling and vent path
Address the confirmed draw or tooling condition within the approved material window
Defect starts after a splice or material-lot change
Thickness, stiffness, coating, friction, or storage-condition difference
Compare certificates, lot data, sample dimensions, and forming response
Segregate the lot if required and create a lot-specific, approved setup evaluation
A practical troubleshooting sequence
1Contain the affected material. Identify the last acceptable check and hold product according to the site quality procedure. Do not assume that a visual improvement means the whole batch is acceptable.
2Describe the geometry. Photograph the pocket and record depth, width, length, wall thickness, lane, cycle, and machine direction. Include a scale and the material identification.
3Check the simple mechanical conditions. Look for residue, blocked vents, damaged tooling, loose guides, wrinkles, poor web tracking, and abnormal roll contact before changing process values.
4Correlate signals with the event. Review temperature, pressure or vacuum, speed, indexing, tension, alarms, and recent maintenance. Find the first abnormal event rather than focusing only on the final shallow pocket.
5Change one approved variable. Use a defined run, record the original and new setting, and compare both pocket dimensions and wall thickness. If the change affects validation status, follow the site change-control and requalification requirements.
For recurring blister pocket depth variation, a short trend with location and material-lot information is more valuable than repeated operator adjustments. SED Pharma can use those records to help separate forming, tooling, film, and transport causes and to identify what information is needed for a deeper equipment review.
If the defect continues, send the forming film structure, nominal pocket dimensions, a pocket-depth and wall-thickness map, defect photographs, material lot information, and the actual temperature, pressure or vacuum, speed, and web-tension settings. Include the last acceptable sample and any recent change to tooling, film, recipe, or maintenance. Send these details to SED Pharma’s technical team for an initial review and help defining the next controlled check.
Technical limitation: This article provides a diagnostic framework, not a replacement for product specifications, validated process parameters, site SOPs, or engineering approval. Parameter ranges must be confirmed for the machine, tooling, material combination, product geometry, and validation state in use.
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