It’s not always as easy as it seems to form the perfect bags on your vertical form fill seal bagging machines.
There are many variables and settings which all have to be spot on to achieve the bag size and look that you are going for.
This blog post aims to address some of the main issues around setting up your bagging machine to get it to form and seal perfect bags.
Packaging Film Choice:
Laminate film is by far the easiest type of packaging film to use. It is strong, versatile, quick to seal and looks very good. It is also much easier to print on to for pre-printed film and it is ideal for gas flushing and freshness as it does not leak like some polythene types.
Laminate films also allow for a euro-slot or hole punch in the top seal of the bags to help with hanging or product displays. This is possible to achieve on polythene film, but the process is more complicated and the outcome is not as aesthetically pleasing.
Sealing laminate film does generate a larger sealed area in comparison to polythene film.
For the purposes of this post, I will assume that you are working with laminate film rather than polythene or other alternatives.
How big should my bags be – Flat bag widths worked out…
Before you order your packaging film, you need to think about how wide you want your bags to be. Bag lengths are irrelevant, unless you are using printed film, but the width of your bag (or flat bag width) is a very important factor.
A single forming set (tube and shoulder) is manufactured to a specific outside diameter which directly corresponds to the flat bag width. For instance, to create a bag that is 200mm wide, a forming set with an outside diameter of approximatley 130mm will be required. The flat bag width is half of the forming set’s circumference so therefore the diameter required can be calculated from the following equations…
Forming Set Equations:
Diameter = Circumference divided by Pi (3.14)
Circumference = Flat Bag Width multiplied by 2
So diameter = (Flat Bag Width multiplied by 2 ) divided by Pi(3.14)
So for our Flat bag width of 200mm the forming tube’s outside diameter = (200mm x 2)/3.14 = 127.4mm
As forming sets are rounded to the nearest 5mm, a 130mm diameter forming set will be required.
As much as this tells us what diameter forming set we require for our desired bag widths, we need to then take a moment to work out if our product will actually fit down the forming tube and fall freely into the bags. This is not always an issue for loose products such as granules or flakes, but for solid products like fruit, vegetables and meat etc. this is always work thinking about.
If the outside diameter of the forming set is 130mm, then the inside will be about 5-10mm smaller than this depending on the thickness of the metal that is used to manufacture the forming set.
Assuming its a relatively thin metal (3mm stainless steel), this leaves us an inside tube diameter of 124mm. If your product pieces can be larger than this then you may need to rethink the flat bag width you are going to require. Otherwise, if your product pieces are consistently smaller, then you’re good to go to the next stage.
What about bag lengths…
So you’ve decided what width you want your bags, and you know that your product will fall freely down the forming set and into the bag. Now you need to think about how long you want your bags to be.
The good news is that your bag length can be any length you like, within the working parameters of the machine. The size of the forming set makes no difference!
What does make a difference is the volume of product that you are putting into your bag. You need to note that there will be an amount of empty space needed in the bag to allow for sealing without damaging your product.
All products have different weights, densities and therefore different volumes. Most packaging is done by weight, however some is done by volume. Rather than getting overly technical with volume, density and area equations, the best way to decide your bag length is to physically test it.
Get an empty bag, (the same width as your proposed flat bag width) or ask for some bag samples from your packaging film supplier if you don’t have anything suitable to hand. Use some scales to measure out your required target weight (or volume) and empty the product into the bag. Do this a few times if your product pieces vary and if they aren’t consistent. Don’t shake the bag to settle the product as it is unlikely that your bagging machine will be able to replicate this.
Then measure from the bottom of the bag to the heights point where your product has come to rest. Once you have noted this height, it is always a good idea to add another 30% to this so that there is plenty of room for the bagging machine to seal the bag without catching or damaging the product. On most machines, as the sealing jaws come together, the bag is lifted slightly to bring both sides together to make the seal. The seal itself will also contribute to the height of the bag which will be dependant on the sealing jaws that you have on your machine.
So, if your product takes up 200mm within the bag, add another 60mm for empty space. Then add the height of your sealing jaws (probably around 40mm). This should give you 300mm which is your overall bag height.
This value isn’t too important unless you are using printed film as the height can be adjusted easily on the bagging machine to suit.
If you are going to use printed film, it is a good idea to run some tests on plain film to get your bag sizes absolutely spot on otherwise you could end up wasting a lot of money on printed film that doesn’t quite fit.
Hopefully that’s enough to get you going for now, I will be back shortly with part 2 of this post covering back seal types, overall film width and eye-mark detection.
More posts will following covering machine temperature settings, speed setup, testing and tweaking to ensure you get perfect bags every time.