Blister Packaging Design: Cavity, Card Seal and Retail Handling

What a blister actually has to do

A blister pack does four jobs at once, and most design arguments come from two people optimising different ones.

It has to hold the product so it cannot move, rotate or rattle. It has to show it, because the pack is the only advertising a product gets on a shelf. It has to protect it through a supply chain that includes a pallet drop. And it has to resist being opened and resealed, because a pack that can be opened in store is a theft problem.

The cavity does the first two, the card does the third and fourth, and the seal decides whether any of it works. A pack that is designed beautifully and sealed badly arrives on the shelf with the product loose in the bottom of the carton.

The three materials, and why PVC still dominates

Blister cavities are made from a small set of materials, and the choice is driven by the seal more than by anything else.

Table 1 — Blister materials. PVC dominates because it heat-seals to a coated card without any surface treatment.

The PVC dominance is not sentiment. PVC softens at a low enough temperature that the seal can be made without distorting the cavity, and it bonds to the solvent-based coating on the card by dissolving into it. That combination is difficult to reproduce with a polyolefin, which is why PP blisters need a special coating and still seal less reliably.

The environmental pressure on PVC is real and it has moved a large share of the market to PETG, which is a copolyester that forms and seals almost as well and is mechanically recyclable. The cost is 15 to 30% higher and the seal needs a coating formulated for polyester rather than for PVC, which means the card supplier has to be told.

Cavity design: the product has to be held, not just covered

The most common blister design fault is a cavity that is a loose box around the product. The product rattles, rotates, and in transit it works its way to the edge and punctures the seal.

A good cavity locates the product on at least three features and holds it against the card. That can be ribs that contact the product, a shaped pocket that matches its outline, or a small undercut that the product snaps behind.

Table 2 — Cavity geometry. The draw ratio limit is what constrains tall or bulky products.

Holding features are worth designing deliberately. A rib that contacts the product on two sides plus a shaped base that stops it sliding gives a pack that survives a drop with a thin cavity wall. A cavity that is 2 mm larger than the product on every side needs a much thicker wall to achieve the same protection, which costs material and makes the pack harder to form.

For a heavy product — a tool, a bottle, anything over about 300 g — the cavity needs a mechanical stop. The product's weight will bear on the seal in a pallet drop, and a seal that is asked to carry 300 g in shear will peel. A ledge in the cavity that the product rests on transfers that load into the blister body instead.

The seal: where blister packs fail

The seal is a heat-sealed bond between the blister flange and the coated card. It is 3 to 8 mm wide, it is made in under two seconds, and it is the single point of failure for the whole pack.

Table 3 — Heat seal parameters. All seven interact, which is why a seal problem is rarely one number.

The failure modes are distinctive and each points at a different cause.

Peels cleanly with no fibre tear. The bond never formed. Either the temperature was too low, the coating was wrong for the blister material, or the coating had been contaminated.

Tears the card fibres. The bond is stronger than the card, which is what you want. A peel that takes paper with it is a good seal.

Seals on part of the perimeter only. Pressure is uneven, usually because the platen is not parallel or the flange is not flat. A wavy flange from a cold mould is the usual cause.

Seal looks good and fails in the cold chain. The coating became brittle at low temperature. Solvent-based coatings are more cold-flexible than some water-based ones, and the difference only shows at −20 °C.

Table 4 — Seal failures. The first row is the one that gets misdiagnosed as a machine fault.

The flange is the part of the design that most affects seal reliability and gets the least attention. It has to be flat, at least 3 mm wide, free of radius at the corners, and formed at a temperature that does not leave it wavy. A flange that is 2 mm wide with a 1 mm radius at each corner has almost no seal area at the corner, which is exactly where a peel starts.

Card design and the retail interface

The card is not just packaging — it is the only place the product can be described, and it has a physical job as well.

Table 5 — Card specification. The coating row is the one that has to be agreed with the blister material.

The hanger hole is where a surprising number of packs fail. A 400 gsm card with a 6 mm hole and no reinforcement will tear through in a week on a peg, and the pack ends up on the floor. A doubled card, a plastic reinforcement ring, or a hole placed with 12 mm of material above it all solve the problem, and all cost a fraction of a cent.

The card also has to be big enough for the legal content — warnings, ingredients, manufacturer details, recycling marks. On a small hardware item this is often the constraint that decides the pack size, not the product.

Hanger holes, stacking and the shelf

A blister pack has to work on a pegboard, in a shelf-ready tray, or in a shipper, and each of those imposes a different requirement.

On a pegboard the pack hangs from the hole, so the card has to be stiff enough not to curl and the hole has to be strong enough not to tear. Packs are hung two or three deep, which means the front pack carries the weight of the ones behind it.

In a shelf-ready tray the packs stand on their base edge, so the card has to be stiff enough to stand up and the base edge has to be flat. A pack that curls forward will not stand, and the whole tray looks untidy.

In a shipper the packs are stacked flat and the product's weight bears on the blister of the pack below. This is where a cavity that is too deep, or a product that is not located, produces crushed blisters in the middle of a case.

Child resistance and tamper evidence

Some product categories need the pack to be difficult to open, which is the opposite of the usual retail requirement and changes the design substantially.

A child-resistant blister is usually a peel-and-push design: the user peels a corner of the lidding foil and then pushes the tablet through a tough film. The blister is made from a laminate with a paper or PET layer that resists pushing until it has been peeled, and the seal is stronger than a normal retail seal.

For consumer goods, tamper evidence is usually achieved by a seal that cannot be reopened without visible damage — a card that delaminates when peeled, or a blister flange that tears the card. This is a property of the coating and the board, not of the forming, and it has to be specified at the same time as the seal strength.

The tension is that a seal strong enough to be tamper-evident is also strong enough to be annoying, and a pack that customers damage while opening generates returns. The usual resolution is a peel tab: a small unsealed area at one corner that gives the user somewhere to start.

Defects and where they come from

Table 6 — Blister defects. Four of the seven are decided at the design stage rather than on the machine.