Seal of Approval
The market for sterile pouches is growing along with the demand for improved strength, puncture resistance, and barrier properties of new materials used to make the pouches. As medical device companies develop new products requiring different levels of protection, the selection of a medical pouch sealer becomes more critical.
Companies want a strong, repeatable seal that won’t expose it or its products to the risk of an FDA recall. A consistent sealing process that is easily validatable; a seal that is affordable to produce and won’t result in costly rejects; and a sealer that requires less maintenance, less downtime, and a lower total cost of ownership (TCO) is desired. Is throughput a key factor? The ability to seal a range of materials? Each company will have a different set of criteria that may impact which sealer is most appropriate.
Types of Medical Pouch Sealers
There are three major medical pouch sealing technologies—impulse heat, constant heat, and continuous band sealers. All sealers work by either activating an adhesive layer or melting the pouch material under pressure to form a seal.
The most important considerations are the quality/consistency of the seal, the repeatability of the process, the ability for the process control system to be calibrated, and the sealer and process to be validated. To ensure that the seal is formed under the proper temperature and pressure, sealers should operate within a temperature and pressure alarm range.
Once sealed, the pouch is subject to manual inspection; it’s important that the pouch not only be sealed, but that the seal look perfect in order to avoid rejection. Rejection can occur for incomplete sealing, wrinkled seals, channels/gaps in the seal, or seals that have been burned through. While high rejection rates cost the manufacturer money, the failure of a seal that has avoided rejection, and a potential subsequent FDA recall, can cost millions.
Impulse Heat Sealers
Impulse heat sealers were the “original” heat sealers used for sterile packaging in the 1970s. Impulse heat sealers run a timed impulse of electricity through a nichrome wire element that heats up rapidly and seals the pouch. The heating process can take anywhere from a fraction of a second to a few seconds, depending upon the thickness and temperature requirements of the material being sealed.
|SencorpWhite 12-IH/2 Impulse Heat Sealer||SencorpWhite 12-P/2 Constant Heat Sealer|
Impulse sealers can be relatively inexpensive to purchase and can handle most popular materials (single layer films and most laminations).
Medical grade impulse heat sealers utilize either thermocouple-based or current/resistance based control systems. On thermocouple-based systems, the thermocouple construction and placement is critical, and needs to be extremely fine for a rapid response. In simple terms, an impulse sealer responds like a toaster—it heats up and cools down quickly.
Thermocouple-based impulse sealers always overshoot the temperature set-point and require that the temperature controller has a rapid sampling rate to minimize this tendency. An alternative control technology for impulse sealers features TOSS technology, which uses current and resistance to control the temperature of the heating element, resulting in greater repeatability without temperature overshoot.
Other important considerations for impulse sealers include:
- Total Cycle Dwell Time – This can change as the residual temperature of the die experiences “heat-creep”; ramp up gets shorter and cool down gets longer. This results in the pouch being exposed to heat for varying periods of time.
- Seal Pressure – It’s critical that the die not achieve full pressure until the wire has completely expanded.
- Wire Design – Wire width is usually limited to 3/8 in. Wire ends should be copper-coated to reduce hot-end syndrome. End connections should be spring loaded to accommodate wire expansion as it heats.
- Cantilevered dies should be avoided because it is difficult to maintain parallelism between the dies.
Impulse sealer dies can be water cooled to help minimize heat creep. However, the residual temperature of the seal die still increases. As the residual temperature increases, its “heat sink” effect on the seal wire reduces, resulting in slowly increasing peak temperatures with more “heat/energy” transferred into the pouch over time. Total cycle time can also increase as cooling compensates for the increase in temperature. The result is that seal strength and appearance can change over time (without any adjustments being made within the control system) as production rates and operator rhythms vary. This can make validation a significant challenge.
Impulse pouch sealers offer the following advantages:
- Typically lower cost of purchase
- Ability to handle a diverse range of pouch materials, including PTFE
- Quick start-up
As the residual heat in the die increases over time, the amount of heat transferred to the pouch changes. When working with Tyvek/film pouches, a process that begins with a good seal/appearance can end up with a transparent appearance of the seal and loss of peelability. The result is process inconsistency, lack of repeatability, and difficulty validating the process. Cycle time and production rates can be adversely affected by residual heat buildup. The operator can adversely affect the process—changing throughput rates can have a potential negative effect. The potential for an FDA recall due to process and seal inconsistency should be an unacceptable business risk.
High maintenance requirements will affect system efficiency and production rates. Systems are subject to frequent band and/or thermocouple replacement, which increases the maintenance costs and downtime. Band replacement and die maintenance requires a “mini-requalification” to ensure the correct temperature. The average ongoing maintenance, downtime, and qualification costs are significantly higher than a constant heat sealer. Calibration is also a greater challenge.
There is a potential low voltage electrical shock hazard with metalized film pouches. The conductive material can transfer the electrical impulse used to generate the heat back to the operator if any breach exists in the barrier material covering the seal wire. This also exposes the product to potentially damaging low voltage if the product is electrically sensitive.
|Pouches for sterile applications|
Constant Heat Pouch Sealers
As its name implies, a constant heat pouch sealer generates a constant and consistent heat across the seal die. A high quality medical grade constant heat sealer is initially more expensive than most impulse sealers but has distinct advantages.
A constant heat sealer clamps the pouch between either one heated die and one non-heated silicone rubber pad or between two heated dies, up to one inch wide.
The heater should be custom-wound to match the heat-sink characteristics of the die in order to provide an even temperature profile. Temperature stability is achieved by heating the entire die mass. A thermocouple is used to control the temperature of the seal die. Cycle speed is always faster and dwell time is always repeatable compared to an impulse sealer because there is no need for a cooling cycle.
Constant heat sealers can be used with all materials except PTFE. Recent advances make it easy to use a constant heat sealer when working with LLDPE bags and header pouches. Constant heat pouch sealers are ideal when working with metalized films, coated papers, Tyvek®, and other plastic pouch materials. They form consistent, repeatable seals, the process is easily validated and the process controllers can be easily calibrated.
Constant heat pouch sealers require minimal maintenance. In addition they have a long lifecycle cell over 15 years in most cases. While the initial equipment cost is typically higher, the TCO over a five year period could be as little as 50% of that of an impulse sealer.
With its closed-loop temperature control system and the stability of a heated die mass, the constant heat sealer is not influenced by the operator. The process and machine are easy to validate and the control system is easy to calibrate, so providing a process audit trail to meet FDA approval is easily accomplished. Many companies who start with impulse sealers eventually end up with constant heat sealers to make validation easier and to eliminate the cost and effects of downtime.
Constant heat sealers produce a repeatable seal without wrinkles, gaps, or other inconsistencies. The final seal on the pouch is actually made using the same sealing technology that was used to construct the pouch. The seal easily passes visual inspection; reject rates are significantly less than with a continuous band sealer or an impulse sealer.
Throughput is also greater than an impulse sealer as the constant heat sealer can run faster and eliminates “heat creep” to keep cycle times repeatable.
The biggest challenges for constant heat sealers deal with volume and materials. If an application requires high speed/volume, an impulse sealer or constant heat sealer will not be satisfactory. Band sealers provide the best solution for high volumes at faster speeds.
Also, if working primarily with PTFE materials, an impulse sealer will offer a better solution because it can achieve higher seal temperatures.
Continuous Band Pouch Sealers
Continuous band sealers are used in high volume applications and can seal most materials. They are called band sealers because the top, or “mouth,” of the pouch is captured between two bands/belts. The operator feeds the pouches into the bands, which come together to form the seal. Band sealers can be horizontal or vertical and provide very high throughput.
The bands are typically heated by hot air or heated plates. Sealing pressure is spring/compression controlled and frequently degrades over time. Validation and calibration are typically more challenging. Dwell time is determined by speed/throughput.
The quality and appearance of the seal made by a continuous band sealer can be significantly affected by the size/weight of the product and the width of the pouch. Conveyors can be used to support a pouch containing a heavier product. Large and/or heavy products frequently result in misaligned seals and wrinkles.
The primary advantage is high throughput. Continuous band sealers work best with narrow pouches that contain flat/lightweight products. They can handle a wide range of pouch materials including Tyvek and clear films.
Continuous band sealers are operator sensitive. The operator controls the orientation of the pouch, and ultimately the quality of the seal.
Typical problems include wrinkled/crooked seals due to creep or poor alignment by the operator when feeding the sealer. While throughput is really unmatched by other technologies, the rejection rate can also be very high (20% is not uncommon).
Because of the degree to which the operator can affect the process, validation and calibration are extremely difficult. This technology is a poor choice for heavy/bulky products as it may result in poor quality seals with a high possibility for wrinkles in the seal.
Picking the Appropriate Sealer
As new materials or new uses for older materials continue to impact pouch construction and sealing, each of these technologies will be pressed to provide high quality seals that can be readily validated. When selecting pouch sealing equipment, companies are most concerned with avoiding risk due to poor quality seals. The cost of a recall can be devastating, both in terms of dollars and corporate reputation.
Selecting the appropriate sealer requires a close look at materials, throughput, and process validation. Where validation, repeatability, and consistent high-quality seals are the overriding factors, constant heat sealers are becoming increasingly popular.
With a combined 33 years of experience at Sencorp, Lynne H. Barton and Kent A. Hevenor are senior account executives responsible for the sales and marketing efforts for the CeraTek product line for SencorpWhite. They also participate in numerous industry technical and professional organizations on behalf of SencorpWhite. Barton can be reached at LBarton@sencorpwhite.com and Hevenor at KHevenor@sencorpwhite.com.