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Blister Packaging using Thermoforming Technique

Blister pack is a term for several types of pre-formed plastic packaging used for small consumer goods,foods, and for pharmaceuticals.

The primary component of a blister pack is a cavity or pocket made from a "formable" web, usually athermoformed plastic. This usually has a backing of paperboard or a "lidding" seal of aluminum foil or plastic.A blister that folds onto itself is often called a Clamshell.

Thermoforming is a manufacturing process where a plastic sheet is heated to a pliable forming temperature,formed to a specific shape in a mold, and trimmed to create a usable product. The sheet, or "film" whenreferring to thinner gauges and certain material types, is heated in an oven to a high-enough temperature thatit can be stretched into or onto a mold and cooled to a finished shape.

In its simplest form, a small tabletop or lab size machine can be used to heat small cut sections of plasticsheet and stretch it over a mold using vacuum. This method is often used for sample and protoype parts. Incomplex and high-volume applications, very large production machines are utilized to heat and form theplastic sheet and trim the formed parts from the sheet in a continuous high-speed process, and can producemany thousands of finished parts per hour depending on the machine and mold size and the size of the partsbeing formed.

Thermoforming differs from injection molding, blow molding, rotational molding, and other forms of processingplastics. Thin-gauge thermoforming is primarily the manufacture of disposable cups, containers, lids, trays,blisters, clamshells, and other products for the food, medical, and general retail industries. Thick-gaugethermoforming includes parts as diverse as vehicle door and dash panels, refrigerator liners, utility vehiclebeds, and plastic pallets.

In the most common method of high-volume, continuous thermoforming of thin-gauge products, plastic sheetis fed from a roll or from an extruder into a set of indexing chains that incorporate pins, or spikes, that piercethe sheet and transport it through an oven for heating to forming temperature. The heated sheet then indexes

into a form station where a mating mold and pressure-box close on the sheet, with vacuum then applied toremove trapped air and to pull the material into or onto the mold along with pressurized air to form the plasticto the detailed shape of the mold. (Plug-assists are typically used in addition to vacuum in the case of taller,deeper-draw formed parts in order to provide the needed material distribution and thicknesses in the finishedparts.) After a short form cycle, a burst of reverse air pressure is actuated from the vacuum side of the moldas the form tooling opens, commonly referred to as air-eject, to break the vacuum and assist the formed partsoff of, or out of, the mold. A stripper plate may also be utilized on the mold as it opens for ejection of moredetailed parts or those with negative-draft, undercut areas. The sheet containing the formed parts thenindexes into a trim station on the same machine, where a die cuts the parts from the remaining sheet web, orindexes into a separate trim press where the formed parts are trimmed. The sheet web remaining after theformed parts are trimmed is typically wound onto a take-up reel or fed into an inline granulator for recycling.

Most thermoforming companies recycle their scrap and waste plastic, either by compressing in a balingmachine or by feeding into a granulator (grinder) and producing ground flake, for sale to reprocessingcompanies or re-use in their own facility. Frequently, scrap and waste plastic from the thermoforming processis converted back into extruded sheet for forming again.

Vacuum forming (vacuuforming), is a simplified version of thermoforming, whereby a sheet of plastic isheated to a forming temperature, stretched onto or into a single-surface mold, and held against the mold byapplying vacuum between the mold surface and the sheet. The vacuum forming process can be used to makemost product packaging, speaker casings and even car dashboards.

Normally, draft angles must be present in the design on the mold (a recommended minimum of 3°), otherwiserelease of the formed plastic and the mold is very difficult.

Material Handling Trays

Vacuum formed trays from Thick Plastic

sheets are used by customers to transport

products within their factory or from one

factory to another

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Vacuum forming is usually - but not always - restricted to forming plastic parts that are rather shallow indepth. A thin sheet is formed into rigid cavities for unit doses of pharmaceuticals and for loose objects that arecarded or presented as point-of-purchase items. Thick sheet is formed into permanent objects such asturnpike signs and protective covers.

Relatively deep parts can be formed if the form-able sheet is mechanically or pneumatically stretched prior tobringing it in contact with the mold surface and before vacuum is applied.

Suitable materials for use in vacuum forming are conventionally thermoplastics, the most common and easiestbeing High Impact Polystyrene Sheeting (HIPS). This is molded around a wood, structural foam orcast/machined aluminum mold and can form to almost any shape. Vacuum forming is also appropriate fortransparent materials such as acrylic which are widely used in applications for aerospace such as passengercabin window canopies for military fixed wing aircraft and "bubbles" for rotary wing aircraft.

Usage

Unit dose packaging of Pharmaceuticals: Blister packs are commonly used as unit-dose packaging for pharmaceutical tablets, capsules orlozenges. Blister packs can provide barrier protection for shelf life requirements, and a degree of tamperresistance. In India, blister packs are mainly used for packing physician samples of drug products, or forOver The Counter (OTC) products in the chemist shops. In other parts of the world, blister packs are the mainpackaging type since pharmacy dispensing and re-packaging are not common.

A series of blister cavities is sometimes called a blister card or blister strip as well as blister pack. Insome parts of the world the blister pack is known as a Push-Through-Pack (PTP). The main advantages ofunit-dose blister packs over other methods of packing pharmaceutical products are the assurance ofproduct/packaging integrity (including shelflife) of each individual dose and the possibility to create acompliance pack or calendar pack by printing the days of the week above each dose. Blister packs also hinderthe use of OTC drugs in the manufacture of illegal drugs.

Blister packs are created by means of a form-fill-seal process at the pharmaceutical company or designatedcontract packer. A form-fill-seal process means that the blister pack is created from rolls of flat sheet or film,filled with the pharmaceutical product and closed (sealed) on the same equipment. Such equipment is called ablisterline.

Consumer goods: Other types of blister packs consist of carded packaging where goods such as toys, hardware, and electricalitems are contained between a specially made paperboard card and clear pre-formed plastic such as PVC. Theconsumer can visually examine the product through the transparent plastic. The plastic shell is vacuum-formedaround a mold so it can contain the item snugly. The card is brightly colored and designed depending on theitem inside, and the PVC is affixed to the card using heat and pressure to activate an adhesive (heat sealcoating) on the blister card. The adhesive is strong enough so that the pack may hang on a peg, but weakenough so so that the package can be easily opened (in theory). Sometimes, with large items, the card has aperforated window for access.

Clamshell: A hinged blister is known as a clamshell, used for a variety of products. It can be used as a securitypackage to deter package pilferage for small high-value items, such as consumer electronics. It consists ofone sheet folded over onto it and sometimes fused at the edges. They can be securely heat sealed, makingthem difficult to open by hand to deter tampering. A pair of scissors or a sharp knife is often required to openthem (although these are often sold in similar packages). Care must be used to safely open some of thesepackages.

Medical Blister:Medical Blister trays differ from Pharmaceutical blister packs in that these are not push-through packs. Thethermoformed base web is made of a thicker plastic sheet, generally between 500µ to 1000µ and cannot becollapsed, thus forming a solid tray. The lidding film provides a peel-open feature and is generally porous toallow sterilization. Such medical blister packs are used for medical devices, used in hospitals.

Raw Materials Use

PVC (Polyvinyl Chloride):The most basic material for the forming web is PVC or Polyvinyl Chloride. The principal advantages of PVC arethe low cost and the ease of thermoforming. The main disadvantages are the poor barrier against moistureingress and oxygen ingress; moreover PVC has a negative environmental connotation due to its chlorinecontent.

In the case of blister packaging the PVC sheet does not contain any plasticizer and is sometimes referred to asRigid PVC (RPVC). In the absence of plasticizers, PVC blisters offer structural rigidity and physical protectionfor the pharmaceutical dosage form.

On the other hand, the blister cavity must remain accessible by the push-through effect and the formed webmay not be too hard to collapse when pressed upon; for this reason the PVC sheet thickness is typicallychosen between 200µ to 300µ depending on the cavity size and shape.

Most PVC sheets for pharmaceutical blisters are 250µ or 0.250 mm in thickness. Typical values for the WaterVapor Transmission Rate (WVTR or MVTR) of a 250µ PVC film are around 3.0 g/m2/day measured at38°C/90%RH and the Oxygen Transmission Rate (OTR) is around 20 cc/m2/day. In order to overcome thelack of barrier properties of PVC film, it can be coated with PVDC or laminated to PCTFE or COC to increasethe protective properties.

Multi-layer blister films based on PVC are often used for pharmaceutical blister packaging, whereby the PVCserves as the thermoformable backbone of the structure. Also, the PVC layer can be colored with pigmentsand/or UV filters.

The European Pharmacopoeia (EP) references the requirements for PVC blister packs for pharmaceuticalprimary packaging in the monograph EP 3.1.11 "MATERIALS BASED ON NON-PLASTICISED POLY(VINYLCHLORIDE) FOR CONTAINERS FOR DRY DOSAGE FORMS FOR ORAL ADMINISTRATION".

In order to be suitable for pharmaceutical blister packs, the PVC formulation also needs to comply with the USPharmacopoeia <661>; EU food legislation; US 21.CFR and Japanese food contact requirements.

PET (Polyethylene terephthalate): PET is a thermoplastic polymer resin of the polyester family and is used in synthetic fibers; beverage, food

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PET is a thermoplastic polymer resin of the polyester family and is used in synthetic fibers; beverage, foodand other liquid containers; thermoforming applications; and engineering resins often in combination with glassfiber.

Because PET is an excellent barrier material, plastic bottles made from PET are widely used for soft drinks(see carbonation). For certain specialty bottles, PET sandwiches an additional polyvinyl alcohol to furtherreduce its oxygen permeability.

Biaxially oriented PET film (often known by one of its trade names, "Mylar") can be aluminized by evaporatinga thin film of metal onto it to reduce its permeability, and to make it reflective and opaque (MPET). Theseproperties are useful in many applications, including flexible food packaging and thermal insulation, such as"space blankets". Because of its high mechanical strength, PET film is often used in tape applications, such asthe carrier for magnetic tape or backing for pressure sensitive adhesive tapes. Non-oriented PET sheet can bethermoformed to make packaging trays and blisters. If crystallizable PET is used, the trays can be used forfrozen dinners, since they withstand both freezing and oven baking temperatures.

When filled with glass particles or fibers, it becomes significantly stiffer and more durable. << Back

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