Media
Article 10-05-06
Blister Package Inspection
By David Lieberman, Vision Systems Design Magazine
November 2006
Untold millions of medicinal tablets and capsules are produced every year, and many of the over-the-counter varieties are encased in blister packs for protection. Existing manual methods for inspecting these packages are subjective and lack repeatability, while electronic techniques have to date been stymied by the great variation in blister packages.
In both of these cases, vision alone is not sufficient to meet the QC needs of pharmaceutical packaging, whether that vision is human or machine. To fill the gap, Packaging Technologies & Inspection (PTI) has developed a technology that combines vacuum-leak testing with electronic visual inspection, combining the strengths of both techniques in a single nondestructive test machine.
How important is the physical integrity of a blister pack? “Any permeation will affect the shelf life of a tablet or capsule,” said PTI spokesman Michelle Wolf, adding that FDA recalls because of package integrity issues are very common. “That’s a very costly event.”
INSPECTION ALTERNATIVES
Perhaps the oldest technique for blister pack inspection is the water bath test: a sample package is immersed in water, a vacuum is applied and the operator looks for bubbles and makes a judgment. “You don’t get a statistical pass or fail result,” said Wolf. The dye penetration approach in turn, in which the packs are immersed in dye, is “unreliable and messy.” Further, both of these are destructive tests, so repeat testing on flawed packages is not possible and, worse, good product is trashed by the inspection process.
More sophisticated means of gauging cover seal integrity have been successfully applied, however, such as vacuum leak testing. But while this approach works very well for testing the integrity of the blister pack as a whole, it cannot identify which blister chambers are flawed. (Blister cavities, after all, have very little area and very little air within their confines.) As a result, the feedback available for improving QA and QC is very limited.
As for inspection techniques based on laser beams and photosensors, these are very sensitive to differences in the surfaces they inspect and, for blister packs, variety in covers is the norm. Some covers are made of paper, some of foil, some of plastic; some are light and some are dark, while others have alphanumerics and/or graphics printed on them. “The result will vary depending on the surface,” Wolf said. “A method may be good for some surfaces but not for others.”
INTO THE BREACH
The VeriPac 225/BLV (see photo) is a nondestructive tester and, according to PTI president Tony Stauffer, it is “the first inspection method for blisters that is able to test the integrity of the entire package and identify leaks within the individual cavities without destroying the product or package.” It does this by combining vacuum leak testing with machine vision: if the former reveals a flawed blister pack, the latter then hones in to find the flawed blister chamber(s). The stainless-steel machine measures 36 (H) x 25 (W) x 25 (D) inches.
A VeriPac 225/BLV operator first logs into the equipment by means of its touch screen, which provides both the (touch) output and (display) input for the equipment’s interactive operator interface. He identifies himself and then selects the ID code of the type of blister pack being tested and enters whatever other information the particular application requires, such as tablet type, tablet manufacturer, etc.
Next, the operator opens the test drawer on the front of the equipment to expose the test chamber, inserts the pack in its test fixture into the test chamber and closes the drawer, sealing the chamber. The test fixture contains an embedded ID code, captured by the internal camera, which confirms the format and other characteristics of the blister pack type being tested. When the operator pushes the START soft button on the screen, the vacuum leak testing subsystem goes into action.
The key to the integration of vacuum leak testing and vision is a flexible structure the company alternately calls a bladder, a mat and a wall. In operation, the system brings the flexible structure into contact with the blister pack cover, gently compressing it which, according to Applications Engineer Oliver Stauffer, increases measurement sensitivity.
A vacuum is then pulled, and transducers measure the absolute and differential change in the vacuum in millibars per second to determine if air is seeping out from flawed blister chambers. The multipurpose flexible structure supports the blister pack cover to prevent the vacuum from adversely affecting blisters with good seals.
The vision system then comes into play to pinpoint which specific blister cavities are defective. The vacuum testing process has caused faulty blisters to deflate, suffering a minute collapse compared to blister chambers without flaw. This creates a slight spatial displacement—-a dip, imprint or impression--in the flexible structure over blister chambers whose seals have not held. There is a discernable visual difference in appearance between good blister sites and sites that have been breached. Stauffer notes a “consistent profile difference” between the two. At sites that have not leaked, the flexible structure is “smoother in appearance,” he said.
The flexible structure in the VeriPac nondestructive inspection system also serves to block the variations of material, print, etc., of blister pack covers that have limited the breadth of use for visual blister-pack inspection systems to date.
Figure 2 shows an example test screen on the PTI inspection system of a blister pack that has failed. Good and bad blisters are made clearly apparent to the operator: bad cells are shown in red and good ones in green.
SYSTEM COMPONENTS
The three main electronic components of the VeriPac 225/BLV are a digital camera from Cognex, a PLC (Programmable Logic Controller) from Direct Logic, and a PanelPC from Aaeon Technology Inc. PanelPCs, available from many computer board suppliers, combine an SBC (Single-Board Computer) with an LCD display within a housing to create an intelligent, interactive terminal, frequently used as a networkable human-interface engine. As with the vacuum leak testing subsystem and machine vision subsystem, the PC and PLC complement each other in the system, each assigned to tasks that suit its native strengths.
The camera is a Cognex In-Sight® 5400. It’s based on a 1/3-inch CCD with a 640 x 480-pixel VGA format and recognizing 256 gray levels. Packaged in a 1.62 x 4.91 x 2.43-inch, NEMA 6/IP67-rated, die-cast aluminum housing, it features progressive scan, software-controlled gain/offset, and a selectable shutter speed between 16 microseconds and 1 second. The VeriPac 225/BLV does not place great demands on its imaging subsystem. Only a single captured image, for example, is required “to determine the package quality and location of defects” in a blister pack, Stauffer said.
The PLC, in turn, is a Model DL06 from DirectLogic’s Micro Programmable Logic Controller family, capable of managing between 36 and 100 I/O points. It features a 229-item instruction set and PTI puts it to work handling all the inputs to and outputs from the test chamber.
A 9.09 x 4.61 x 2.56-inch device, the PLC controls or monitors all the switches, valves, transducers, relays, etc. in the system, the vacuum generator and the rest. All in all, the VeriPac requires about 20 inputs and 20 outputs, according to Stauffer. The modular expansion capabilities built into the PLC allowed PTI to configure it exactly for their interface needs. The PLC has two built-in serial communications ports and accommodates an Ethernet expansion module.
As for the PanelPC, it’s a Model APC-8152 from Aaeon Technology Inc. which supports a Socket 478 microprocessor, such as a Pentium 4 or Celeron, and up to 1 Gbyte of DDR (Double Data Rate) DRAM. Measuring 13.2 (H) x 17.5 (W) x 14 (D) inches, the PC runs the custom front-end operator interface while providing the local storage for test results and images, as well as a side-mounted Ethernet interface to the customer’s host computer. Test results and images are automatically stored on the APC-8152’s 2.5-inch IDE hard disk drive, along with statistical analyses of the prevailing inspection trends.
The PanelPC incorporates a 15-inch-diagonal active-matrix LCD with a 1024 x 768-pixel XGA format. The LCD has a typical brightness of 250 cd/m2 (candelas per meter square), supports a 256,000-color palette and has a wide viewing angle: roughly 140 degrees off center in both horizontal and vertical directions. The display is fed by a 4x AGP graphics interface built into the PC. An 8-wire resistive touch-screen serves as the input medium of the operator interface. It supports a matrix of as many as 2048 x 2048 touch points.
PUTTING IT ALL TOGETHER
What ties the VeriPac 225/BLV together is Ethernet, which interfaces the PanelPC, PLC and camera to each other and the PanelPC to the outside world. The Ethernet port available at the side of the housing allows the operator to export data to the host system and access other nodes on the network. The Ethernet port likewise provides remote access to the VeriPac inspection system for other devices on the network. In addition to managing the operator interface, the PC in the PanelPC also runs the custom image analysis software that determines the integrity of the seal on individual blisters.
Being able to isolate seal problems to specific blister sites is critically important for the pharmaceutical packaging industry, said Wolf, as well as the up and coming nutraceutical business in vitamins and other dietary supplements. In both, “Statistical inspection with a high degree of repeatability is a huge advantage,” she said.
“If you’re producing millions of blister packs and consistently have defective pockets in certain areas, you have a problem,” said Wolf. “And if you know where the problem is, you can go back to your [packaging] equipment and rectify the problem. But you can’t fix a problem if you don’t know it exists.”