Ferrara, typical factory issues you have mentioned in your post.
I believe to do the proper RCA, you should work together with 2-3 key people. These people should be machine operators, maintenance people, QA person and product specialist. Try and brainstorm and gather the variables / things which may be going wrong. Those may be as simple as Operators putting a lot of glue in the applicator immediately causing instant reduction in hot melt temperature OR your glue guns getting stuck (operational practices), to variation in glue quality or machine issues whilst closing the flaps or pressing the flaps or so on.
The gluing or sealing of a carton is the last link in a long chain of operations for converting paperboard into a functional and attractive package. The last link must be as good as the others. Gluing is not difficult but negligence in performing it can be very costly.
You may be having the problems because of
Too little glue
Too short a time for setting under pressure
The glue seam opens up (glue on only one paperboard surface)
Too low pressure
Application temperature too low (hot melt)
Open time too long
The glue seam opens up (glue on both paperboard surfaces)
Too much glue
Too short pressure time
Application temperature too high (hot melt)
Open time too short
Cartons stuck together after side-seam gluing
Too much glue – squeezing out from side seam
Skewed glue seam
Glue line too close to edge of carton
Maintenance of gluing equipment is very important because if a failure occurs here, the production chain stops and the product may be ruined.
The packer’s operation requires proper maintenance. It is expected to run with minimum attention. This is only possible if skilled personnel are there to ensure a safe and reliable function. For example, hot melt glues may be over-heated for long time periods, causing them to oxidise, even carbonise, with the result that they lose adhesive strength.
The gluing operation is very much dependent upon....
Glue type and Description
The type of glue is very important. It must suit the carton blank material and the machine. The glue should not have an odour or affect the packed product. The glue must be able to adhere to the paperboard and must develop tackiness and sufficient bond strength within the brief period of time that pressure is applied to the glue seam. Most glue suppliers offer detailed advice. They are familiar with the different paperboard types, the most common gluing and packaging machines and glue applicator systems.
Side-seam gluing involves applying glue to a side flap, pressing it to a side panel and maintaining the pressure until the glue seam has set. Side-seam gluing with hot melt glue requires precise machine settings. The glue seam must be fully developed after the application of pressure.
For end-fed cartons that are sealed with water-based glue, it is important that the glue seam sets quickly and the wet-strength of the paperboard is sufficient to hold the end flap in position until the glue seam has dried. Hot melt glue seams do not need this additional requirement because the bond strength is obtained by rapid cooling.
Top-fed cartons that are erected in the machine may be mechanically locked or glued. Heat sealing is predominant for plastic-coated paperboard in the erecting unit. The closure of the package is very often done with hot melt glue because the counter-pressure in the contact areas for the gluing is often insufficient for heat sealing.
The temperature of the product has major affect on the gluing operation.
The applicator applies the correct amount of glue with precision and is available in various types. Open time must not be too long and pressure time must be long enough. Important facts to consider when gluing are:
• The glue must wet and adhere to the substrate.
• There must be enough glue to form a bond but not so much that it squeezes out.
• The glue must be applied in the right place.
• The pressure must be maintained until the bond is strong enough.
Description of gluing methods
The glue is applied to the first paperboard surface with an applicator. The glue wets the surface and starts setting. The second paperboard surface is applied under pressure and a bond forms. If the glue is water based the paperboard first absorbs water, enabling the glue to set. Hot melt glues are cooled to solidify.
Open time is the time between the glue application and the application of the second surface.
The open time depends on the packaging machine and will affect the selection of glue and amount of glue required. It may even affect the choice of applicator and application pattern. Pressure time must be sufficient for the bond to develop before pressure is released.
The type of glue must suit the paperboard and the machine. The substrate is of key importance because it governs the type of glue. Most glue suppliers offer detailed advice.
Glue must usually meet the following requirements:
• The glue must be reliable with almost any available application technique.
• The glue must maintain a satisfactory bond on all paperboard products.
• The applicator must be easy to clean.
• The glue in combination with the technique employed must not add odorous or toxic substances that could impair the quality of food or other sensitive products.
• The glue must endure the environment of the product, for example in hot or deep freeze applications.
The task of the applicator is to apply the correct amount of glue in the right place. The glue may be applied in various patterns depending on the application and the applicator.
The most common methods of applying a suitable amount of glue are described below.
Examples of glue patterns.
I believe you have a Nordson's nozzle applicator (glue guns with solenoids) which pumps the glue from a storage tank to a nozzle provided with a high speed valve. The amount of glue can be set very accurately and the equipment can apply a predetermined pattern of glue in lines or dots. It should offer precise glue metering and glue pattern.
Key paperboard characteristics
The gluing or bond forming is usually the last operation performed on a package or graphical product, so here the function is critical. The paperboard must be suitable for the technique in question and the built-in properties must be predictable, both within an order, and between orders. This means that once the glue is selected and the machine is adjusted, the machine will only need routine supervision to ensure safe functioning.
The paperboard ply construction is of great importance because it governs the creasability. High elongation-to-break in the surface will permit the use of deep and narrow creases with low spring-back force after folding. This in turn facilitates good gluing because the carton’s side flaps will not place an excessive load on the newly formed glue seam, so it can dry and develop final strength with full surface contact in the glue seam.
Since the gluing operation is influenced by the surface ply, the surface strength properties are critical.
The importance of the design.
The crease resistance in the glue flap needs to be balanced as both a too high or a too low resistance may give a weak seam.
Every single procedure in the gluing process must be documented so that settings can be repeated. It may be hazardous to change glues without careful testing. The same applies for mixing glues. Minimise the number of glues used. The optimum is of course to use only one type of glue.
The glue is applied to the most difficult surface in order to first to obtain the best gluing result. Since the glue is in a tacky state during the open time it is important that the environment is constant with regard to temperature and ventilation so that the tack remains predictable.
The glue seam will become stronger if it is applied adjacent to an edge and far from a crease.
The following factors are essential for good gluing:
• the amount of glue
• the open time
• the pressure and pressure time.
The pressure and temperature are essential for good sealing.
Hot-melt application equipment performs three essential functions: melting the adhesive, pumping the fluid to the point of application, and dispensing the adhesive to the substrate in a desired pattern.
The tanks are made of highly thermal conductive material, such as aluminum, and they are heated by either a cast-in heating element or a strip or cartridge type heater. The side walls are usually tapered to provide good heat transfer and to reduce temperature drop. Adhesive melts first along the wall of the tank as a thin film. Internal circulation currents from the pumping action assist in transferring heat throughout the adhesive held in the tank. Even when the hot melt is entirely liquid, there will be temperature differences within the adhesive.
Extrusion guns are used on most packaging lines. This extrusion method entails applying beads of hot melt from and nozzle. The gun is usually fed from the melting unit through the hose or directly from the unit itself. Most high-speed applications use automatic guns that are triggered by timing devices or controllers on line with the parent machinery to place the adhesive on a moving substrate.
Automatic guns are actuated by pressure that forces a piston or plunger upward, lifting the attached ball or needle off the matched seat. Molten adhesive can then pass through the nozzle as long as the ball or needle is lifted off its seat by the applied pressure. The entire assembly can be enclosed in a cartridge insert or extrusion module.
The pressure to actuate automatic guns is either electro pneumatic by means of a solenoid or electromagnetic with a solenoid coil electrically signaled.
The melting device selected must be capable of handling the capacity required for the packaging operation. The unit must also have sufficient holding capacity to prevent the need for frequent refilling of the adhesive tank or hopper. Adhesive consumption is affected by line speed, bead size, and pattern. The adhesive consumption rate and maximum instantaneous delivery rate of the pump must be matched to the application requirements.
The pattern to be deposited will determine the dispensing device. Also, the pattern size and registration of the adhesive deposit must be matched with the cycling capabilities of that device.
The system must fit neatly into the entire operation with spacing considerations for mounting unit, gun, and hoses; location to point of application; and accessibility for maintenance.
Hope, this will help a bit in moving forward...