Avoiding Stressed and Underformed Blisters

How to make sure that your thermoforming processes do not lead to future packaging failures.

By Greg Young, Manager, Technical Services, VPI Mirrex (Delaware City, DE)

The continued growth of unit-dose and OTC blister packaging has put increased pressure on packaging engineers to improve equipment efficiencies and increase output. But in order to produce packages that pass integrity tests and perform well in the marketplace, engineers must avoid forming blisters at too-cool of a mold temperature or with processes that put an undue amount of stress on the blister film itself.

This article will point out some of the mold and forming conditions of blister manufacturing that can lead to stressed or underformed packages. If left ignored, these problems can lead to further stress and ultimately cause the blister packages to crack, split, or separate.


By using polarized polyester filters, stress or molecular orientation becomes visible in formed blisters. The intensity of the color will change from brown to blue and then to more brilliant colors as the degree of orientation increases. The molecules are changed during thermoforming from a random relaxed state to a stretched, spring-like chain.

The temperature at which the stress is locked into the blister is often the same temperature that is required to activate the heat-seal coating of most lidding materials. As heat is applied during sealing, the stress in the blister begins to relax. This causes the sealing flange of the blister to shrink and pull away from the adhesive coating of the lidding. The sealing area is decreased as the blister deforms, causing weakened seals and high failure rates.


Over the years of pharmaceutical blister packaging, two primary mold designs have dominated the industry. They are the female cavity mold with plug assist and the female cavity mold without plug assist. Both mold styles normally clamp the preheated material between a top and a bottom mold and use air pressure to form the blisters.

The upper mold has typically been a mirror image of the lower mold (see figure). This is critically important, because the blister cavity is fully clamped and therefore each blister is formed using only the material directly over each female cavity. The sealing flange is never stretched, thinned, or stressed. Female rotary molds using a vacuum to form the blisters are also normally stress-free in the sealing flange.

Comparison of a standard top mold and a universal top mold.

However, a third type of mold, often called a universal top mold, began to be used primarily for physician samples. This style of tooling is less expensive and allows several configurations of lower female molds to be interchanged without removing the top mold. The universal top mold does not clamp around each blister cavity, but only around the outer edges of the lower mold. This outside clamping allows the material to be stretched, thinned, and stressed in the sealing area of the blister flange, causing sealing failures.


Even a correctly built mold can cause a stress-formed blister. The two most common problems are incorrect timing of either a plug assist or the forming air pressure. In both cases, the plug assist or forming air is activated slightly before the upper and lower mold have fully closed. This begins to push material into the female cavity before it has been clamped around each cavity. Such incorrect timing can cause stretching, thinning, and stressing of the sealing flange.

In addition, slowing down total line speed without resetting the plug or forming air can cause timing problems. One sign of incorrect timing is a rippled edge along the blister film after forming.


Extremely cold molds are another problem that causes not only stressed blisters but also underformed blisters. Even small pinholes can be caused by overchilling the mold. Mold temperature should be adjusted up or down depending upon the material type and thickness and the equipment speed. It is important to control the cooling-water flow rate and to know the exit water temperature of a mold.

In most cases, a slightly warmer mold allows for easier forming, sharper definition, and less stress. Molds that get too warm cause postshrinkage of the blisters and lead to alignment problems in the sealing area.

Again, changes in the equipment speed can cause either problem if the mold temperature is not changed.


Most pharmaceutical blister lines in use today feature similar preheating areas—similar both in design and in mistakes. Contact heat is most often used with both an upper and lower heating plate. But one length of heater does not work for all applications.

Too often a standard heater length is used on the forming equipment, regardless of the index length. Instead, a minimum ratio of 3:1 heating-to-forming length should be used to preheat most materials. The use of a standard heater length often causes overheating of the leading edge of each index, while at the same time underheating the back half.

This problem also causes both stressed and underformed blisters. Pinholes are often seen because of underheating or overheating the blister material. Correction of this problem cannot only improve product quality, but it can increase production and forming rates.


Machine engineers often find that to avoid underforming they must either slow their equipment forming rates or deal instead with other problems associated with running higher forming temperatures. Higher forming temperatures increase the tendency of materials to stick to the lower heater and then lose width in the transverse direction. The loss in web width can lead to poor seals and breakage of the scrap web.

Higher forming temperatures can also exacerbate the problems associated with worn or dirty Teflon coatings on the heating plates. Forming materials with lower temperatures helps avoid sticking or elongation in the machine direction.

Drawing two parallel lines on the unformed material and then rechecking the spacing between the lines after forming can help to quantify the elongation. This will also identify the need to clean or replace the heating plates.


Using a material that has been engineered and processed for your particular type of forming equipment is your best choice. The ability of a material to uniformly soften using contact-style heaters requires that the manufacturing process hold tightly to standard operating conditions.

All materials have some degree of shrinkage and orientation locked in during their manufacturing process. Materials with a high degree of machine-direction shrinkage require additional time and heat to reach their softening points. Materials with a low degree of machine-direction shrinkage are quicker to relax and are therefore quicker to lie down on the lower heater, allowing a more uniform heating.

Highly strained material can add to the problems of cold forming and can also cause underformed blisters and random pinholes due to underheated areas of material.

Young presented these recommendations at the Healthcare Compliance Packaging Council's Showcase 2000 held in September 1999.

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