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Package design is always an interesting, challenging task. Let's break down your requirements into two separate answers:
1. How do I know how many inches the bag should be?
Generally this should be provided by someone to you. Most likely the person/company that will be manufacturing the product would provide this information. If you were to try and determine the size yourself, you could come up with a large range of numbers. The size will generally be determined by the largest chicken that could also weigh the set amount.
2. If I am designing for a plastic bag, what are some things I need to keep in count?
Let's start with the most general and work our way to the most specific:
- just like when you print on a flat, paper surface with special inks, plastic almost always require special inks to print correctly, and you may be responsible for determining where they should be applied (very similar to how you would define what should be spot glossed, by using a spot colour)
- the bag will stretch, therefore putting small, intricate details in some areas will not be effective. For example, the script writing and brand label have been printed in a place that will normally be "stretched" across the product inside, making it easy to read. If this text had been near a pinch in the plastic, it would have been illegible
- specific to your product, generally the center of the printing surface will represent the "resting" side of the product (the bottom) and the outside edges represent the top. In this way, the plastic is wrapped around the chicken and the outside edges are tied together to form a seal at the top. With this in mind, you can plan for where the graphics will go. Inner center = bottom of bag, outer edges = top of bag, middle ring = main graphical area