Thermoforming of Medical-Grade Packaging

Medical Plastics and Biomaterials Magazine | MPB Article Index

Originally published September, 1996

BRETT BAKER

Along with injection molding and extrusion, thermoforming is one of themajor manufacturing technologies for polymer-based medical devices andpackaging. The essence of the thermoforming process is that a sheet of plasticis heated to a working temperature and then formed into a finished shape bymeans of vacuum or pressure. For the production of thermoformed medicalpackaging, however, this simple description refers to a variety of complex processes demanding superior technical finesse with extremely tight control and stringent documentation.

This article will review some of the internally developed practices commonlyemployed by thermoformers conversant with medical applications, with theintention of offering device manufacturers an overview of the level of expertiseand kinds of procedures and techniques they should encounter when dealing with aquality medical thermoformer.

PRODUCT DEVELOPMENT

A well-organized system for product development that conforms to GMPs and therequirements of ISO 9001 is essential for effective medical thermoforming. Atypical product development process can be divided into six steps or phases: (1)new-item phase; (2) planning phase; (3) equipment, tooling, and material phase;(4) trial phase; (5) R&D to production validation phase; and (6)final-packet-specification approval phase. Once a project is under way, progresscan be categorized in the appropriate phase according to guidelines set up by the thermoformer to optimize work flow.

New-Item Phase. It should be the goal of the thermoformer to be asfamiliar with the product to be packaged as is the customer requesting thatpackage. Accordingly, at the start of any new medical packaging application, thethermoformer should provide the device manufacturer with a product developmentquestionnaire seeking essential facts about the project. Information shouldencompass product specifications for all components to be packaged, details ofthe manufacturing and distribution environments, and drawings. For example, itis important for the thermoformer to know as much as possible about the liddingmaterial, sealing equipment, and sealing processes to be used, as these willdirectly impact thermoforming flange tolerances. Other critical informationincludes the test criteria that will be used to validate the package and,especially, the chosen method of sterilization.

When the product information has been received, the thermoformer files it in acomputerized logbook that will serve to track the phase and status of theproject from start to finish.

Planning Phase. The decision to proceed with a project should comeonly after a careful feasibility study has convinced the thermoformer that itcan design and manufacture the package with quality and consistency. This willinclude checking the material availability and verifying the capacity of itscurrent equipment. Once the customer has approved the thermoformer's engineeringpart prints and issued a purchase order specifying price, quality, and deliverytime, prototype molds can be designed. A software design program such asProEngineer (Parametric Technology) can be used to generate solid models thatcan be plotted or reproduced with a thermal wax printer, which enables minormodifications to be made quite rapidly. The solid models can then be downloadedto a machining-center computer for manufacture of prototype molds via Smartcamand Freeform Surfacing software.

The preparation of prototype molds is an essential step in the planning phaseof developing a thermoformed medical device package, allowing for closesampling and analysis of the finished part. Part fabrication with prototypemolds enables the

thermoformer to stage preliminary sealing, sterilization, and mechanicalstability tests, and permits changes to be made at minimum cost beforeproduction tools are built.

Equipment, Tooling, and Material Phase. Also known as the buildphase, the next step in the development process represents the point at whichthe plan becomes tangible and the equipment, tooling, and material are ordered.A build phase can actually occur more than once during the development cycle;for example, when the design of a prototype is altered or when productiontooling is modified during the trial phase.

Trial Phase. During the trial phase, the thermoformer conducts a trialrun of the prototype tooling and sends prototypes to the customer forevaluation. If they are accepted, final approval can then be given for theprocurement of production tooling, material, and equipment. If modificationsare required, the thermoformer

returns to the planning phase and repeats the process until trial runs of themodified mold produce new prototypes for the customer to evaluate.

R&D to Production Validation Phase. When the device manufacturerapproves the prototype package, the thermoformer is ready to enact a completevalidation run of the production tooling. All of the conditions normally presentduring actual production--personnel, procedures, work environment,inspection--should be in place for verification. A report on the validation runshould be reviewed and signed by managers responsible for engineering,production, and quality assurance, and by the thermoformer's general manager orpresident. The parts, along with a summary specification document, are then sentto the customer for final approval.

Final-Packet-Specification Approval Phase. The last phase of thedevelopment process is when the end result of the validation run is officiallydesignated as a production item and is registered as part of the thermoformer'sproduct line in a master record. From this point on, the product can only bemodified through an official "request for process change."

MATERIALS

A review of all of the material considerations for the wide variety ofavailable medical packages would require a presentation beyond the scope of thisarticle. Therefore, the discussion that follows will concentrate on materialsused in the thermoforming of rigid medical device trays that undergopostsealing.

Resin Types and Grades. Materials currently employed to produce rigidmedical trays include copolyesters (PET, PETG, APET), polystyrene (HIPS),polypropylene, polyvinyl chloride (PVC), polycarbonate and other engineeringresins, and acrylic multipolymers.

PETG offers good clarity and rigidity, and has become something of a favoriteamong medical device manufacturers for their thermoformed trays. Though PETG ismore expensive than some other resins, the added cost is often justified by thematerial's desirable properties. One disadvantage of PETG used for postsealedpackages is that a silicone coating must be added as an antiblocking agent toaid in denesting stacked trays.

High-impact polystyrene (HIPS) provides many of the same properties as PETG. Itis also less expensive, and does not require an antiblocking additive. However,one cannot achieve the same degree of clarity as with PETG. This is an importantconsideration in the medical market, where it is almost always required that aproduct be clearly visible and where a tray serves not only as a sterile barrierbut also as a logical means of presenting a device in settings such as theoperating room.

The use of PVC in thermoformed trays has declined somewhat in recent years. Onereason is that the material will sometimes develop pinholes in deep-drawapplications. Some hospitals are also disinclined to consider PVC as a materialof choice for incineration or recycling.

Polycarbonate is an excellent material for high- temperature applications,although it is somewhat expensive. Like other engineering resins, it can alsobe rather difficult to thermoform and is generally reserved for specialtyapplications.

Acrylic multipolymers feature good forming and barrier properties. They alsooffer excellent mechanical strength, but are less than ideal for recycling.

Material Specifications and Inspection. Material purchasespecifications should be prepared by the thermoformer in coordination with thematerial supplier, in a joint effort to satisfy customer requirements. Toprocure the best available material, it is advantageous for the thermoformer toknow as much as possible about the quality of the resin and the sophisticationof the material supplier's manufacturing process. Whenever feasible, the resinproducer and converter should be experienced in meeting the demands of themedical packaging market. The purchasing specification should be controlled as amaster-record document and have a revision and approval log.

Incoming material inspection can be quite time- consuming during the initialqualification of a resin supplier. Once a vendor has met the qualificationrequirements, however, it should be relatively simple for the thermoformer tocontrol quality as a standard operating procedure without spending an undue amount of time inspecting.

TOOLING

Quality molds and dies are a critical part of any thermoforming project, andare especially important in medical applications. For many thermoformers, thepreferred material for making prototype molds is an aluminum-filled epoxy resin.Stable and capable of being polished or filled for modification purposes ifrequired, this material works well for resins that do not require moldtemperatures above 180°F, such as polycarbonate or polysulfone. At highertemperatures, however, the considerable expansion ratio of the epoxy needs to betaken into account.

When building production molds, a thermoformer should always select the highestquality material that is feasible for the part being formed--which means usingmachined 6061 aluminum whenever possible. This material polishes well and isextremely predictable and stable. Because good mold-temperature control isessential, flood-cooling designs that maintain temperature to ±2°F arerecommended.

Steel-rule dies are the least expensive and most common form of tooling used totrim thermoformed trays. Shortcoming of steel-rule dies includeless-than-optimal accuracy, die life, and punch-through ability, along with atendency to produce "angel-hair" scrap that must be vacuumed off theparts. The dies of choice for an increasing number of thermoformers are forgedhigh dies, which cost about twice as much but require minimal cleaning and offeraccuracy approaching what can be achieved with matched metal punch dies, alongwith good punch-through ability and a die life at least three times longer thanthat of a steel-rule die. Matched metal punch dies are the cleanest and mostaccurate and long-lasting dies, as well as the most expensive--five to six timesmore than forged high dies. The press used for matched metal punch dies must beflat and parallel and capable of holding tight x- and y-axis tolerances.

The integrity of a medical package is greatly influenced by the quality of thetooling and by the degree of care taken in applying proper thermoforming designprinciples. Any modification to the tooling must be controlled by arequest-for-process-change document in the master record file so as to ensurethat proper validation and testing are performed and that all personnel involvedare notified.

EQUIPMENT

Widespread advances in thermoforming equipment have contributed to theproduction of more accurate, cost-efficient packages. Device manufacturersshould consider whether a thermoformer works with the most recent technology,which can help to enhance product reliability and cleanliness.

Controls. The increasing demand for innovative plastic parts has led topart designs that require sophisticated and accurate process control. In thelast 20 years, process controls for thermoforming machines have improved by anorder of magnitude, with development of the microprocessor sparking a veritablechain reaction in peripheral device improvements. For example, the advent ofmicroprocessor control has led to interfaces between computers and programmablelogic controls that can signal or monitor the response of motors, relays, andtimers; and to the deployment of servo- controlled stepping motors with encoderson in-line thermoformers that can index thermoplastic sheet to within ±0.015in. Tool and setup menus can be stored for fast downloading, and reportsdocumenting running conditions can be generated automatically to ensureconsistent processing from setup to setup. With the aid of a modem and atelephone line, diagnostic technicians from the equipment manufacturer can helpdetermine problems and reprogram machinery.

Effective medical thermoforming also requires close monitoring of the systemsproviding compressed air, vacuum, electricity, and water. It is important thatthermoforming equipment always have a sufficient supply of air, ascompressed-air "starvation" can induce difficult-to-detect productflaws that can cause the failure of a sterile package in the field. Reservoirsurge tanks can be used to help ensure an adequate air supply. The air must alsobe kept clean through the use of dryers and oil and particulate filtersequipped with moisture-discharge valves.

A well-maintained vacuum system is critical, especially with high-volume moldsrunning at high cycle speeds. Vacuum volume rate and timing must be controlled,and pump discharge processed in a clean manner. It is a good idea for thethermoformer to use large vacuum lines with no sharp bends and to locate thevacuum supply as close to the demand as possible.

The thermoformer's supply of electricity must also be controlled, in two ways.The first, and most important, concerns the regulation of the main power-supply line that powers heaters, servo drives, and other auxiliary equipment.Capacitors used for power-factor correction should be supplemented through theaddition of a tuning reactor, forming a "filter" that avoids adverseharmonic resonance points while cleaning up existing levels of harmonic currentsat the tuned frequency. Second, computers should be supplied power throughuninterrupted power supplies (UPSs) to protect against outages. It is criticalthat the thermoformer's power systems be designed by certified electricalengineers.

Consistent process control for medical-grade thermoforming requires clean heattransfer and mold cooling. A thermoformer's water supply should be treated viaclosed-loop processes to inhibit the proliferation of scale, corrosion, andmicrobial growth--all of which can impair effective heat transfer.

Although most modern thermoforming machines no longer incorporate hydraulics,any hydraulic pumps, lines, and controls that are used should always be keptbelow the sheet line to lessen the risk of part contamination.

Calibration and Setup. Maintaining the accuracy of thermal controls,timing devices, and mechanical actuators demands regular calibration of thisequipment. The thermoformer should have in place a preventive maintenanceprogram designed to control, maintain, and document equipment accuracy. Thoroughprocess and product validation requires that windows for equipment capabilitiesand maintenance procedures be formally established as quality standards, assuggested by GMPs and required by ISO 9002 prior to certification.

FACILITIES AND PERSONNEL

One of the cardinal rules in medical thermoforming is that the manufacturingarea must stay clean. Positive air pressure should be maintained, and any makeupair must be filtered. Air quality and contamination can be monitored andreported in parts per million with the aid of a laser particle counter. Toreduce dust, floors should be covered with vinyl, epoxy, or urethane coatings,and walls with a two-part epoxy coating. All material handling should be donewith pneumatic or electric power. In order to stabilize humidity and lessen theburden of static charge, static controls in the form of deionizing bars andair-wash systems are recommended. Clothing for personnel should be antistaticand non-lint-producing. Good lighting is also imperative if a thermoformerexpects to produce high-quality parts, and lighting systems have been developedrecently that minimize eye strain and facilitate on-line inspection. Cleanlinessis important as well in the materials- receiving area or warehouse, which shouldideally be kept at the same temperature as the manufacturing floor. Documented training programs for all personnel should accord with GMP and ISO9002 criteria.

DISTRIBUTION

Thermoformers packaging medical trays for distribution are strongly urged touse black antistatic bags, which reduce contamination and guard againstultraviolet exposure that can lead to photodegradation of some thermoplasticparts. Many medical device manufacturers require that trays be delivereddouble-bagged and labeled. Corrugated containers used to transport finishedproduct are a major contributor to particulates in the manufacturingenvironment, and should be eliminated in favor of plastic totes.

CONCLUSION

Medical thermoformers dedicated to quality consider the package to be asimportant as the product it is designed to protect. In the medical industry thisis literally true, since a defective package can compromise the sterility andthus the functionality of a device. A successful medical thermoformer is onethat can plan with intelligence and foresight, process with precision andcontrol, and validate and document with care, objectivity, and integrity.

Brett Baker is a process engineer at Sabin Corp.(Bloomington, IN), where he specializes in the development of thermoformedmedical trays and turnkey packaging systems. A member of the Cook Group, Sabinproduces extruded, insert- and injection-molded, and thermoformed devicecomponents.