This is the second in a series of blog articles intended to educate users new to flow wrapping and serve as a quick reference for experienced users. It explains the sealing fundamentals of horizontal flow wrapping. This is useful in selecting the general type of packaging material and determining the machine settings that can be adjusted to influence the seal quality. (Read the first article in the series “Principle of Operation of a Horizontal Flow Wrapper“)
Generally speaking, two types of film can be distinguished: (1) heat seal film and (2) cold seal film. The choice of using heat or cold seal film depends on a variety variables, such as:
- Product characteristics
- Material costs
- Required capacity/speed
- Necessary barriers (Ultraviolet, Oxygen, etc.)
- Hermetic seal strength or integrity
Cold seal films consists of a base material (carrier) and a thin layer of glue. The sealing of this type of film is achieved by pressing the two layers of glue together. One of the advantages of this film is that it can be used at higher film speeds than heat seal films. It also requires no heat to seal, so the lack of hot machine surfaces may be beneficial when dealing with temperature sensitive products, such as chocolate. However, there are some disadvantages. Primarily that they can never offer the same protection and hermetic seal qualities as heat seal film. They also need to be stored in a climate controlled room and lastly, the cost is generally higher than that of heat seal film. Therefore this type of film is less common than heat seal film.
Heat seal jaws cut and seal packages on a Pack 101 flow wrapper.
Heat seal films are often multi-layered and consist of an outer coating that prevents the film from melting, the core of the film such as
polypropylene, and a sealant layer on the interior which has a lower melting temperature and bonds to seal the package together. When high integrity seals are required, heat seal film offers the best alternative. Generally, it is also less expensive than cold seal films. Heat seal films are also offered in a wide range of laminations so that barrier properties can be optimized to meet product protection requirements.
The three important factors that should be kept in mind and controlled when producing a seal are:
(1) Heat: Temperature of the sealing surfaces (heat is not necessary with cold seal film)
(2) Dwell: Amount of time the machine jaws are in contact with the sealing material
(3) Pressure: Amount of pressure applied to the sealing material
Whenever a change is made in any one of these factors, one or both of the other two factors must be adjusted to compensate for this change. For example, when machine speed increases significantly, the dwell time will drop, so the temperature (and sometimes pressure) may need to be increased to compensate for this.
More details on optimizing sealing parameters and an overview of different packaging films and their uses can be found in Bosch’s Guide to Flow Wrapping. It’s available for free download under the “Links and Downloads” section on the Pack 101 flow wrapper webpage on the Bosch Packaging website. This guide contains other sections to assist users in initially setting up their wrapper, changing over to new products, or solving problems common in flow wrapper operation.