The best time to consider Package Design depends on your type of manufacturing process and your product.
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If your products are larger dimensionally and/or have varying assembly times, the best time to address your packaging should be prior to any manufacturing.
Treat the packaging as part of the product rather than an afterthought.
If your product is moved, handled, or staged during production, it may be cost effective to incorporate the packaging into your process as a function of material handling.
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If your products are manufactured in large quantities that roll off an assembly line, the time to address your packaging is during the production phase.
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If you develop customized products with varying designs to meet customer requirements, packaging designs are best addressed once the product is completed as the product may undergo last minute modifications.
Open communication with your packaging vendor can help you avoid shipping delays due to package size errors. It may be cost effective to ship your product assembled in a single shipment or dis-assembled with few ancillary packages.