The unfortunate result of most case interior designs is improper internal cushioning for its contents. There are many great cases on the market, and choosing a sturdy case is much easier than designing a proper interior. Most people would think this problem stems from not having enough foam in the case. In actuality, most cases either have too much foam, or the wrong kind of foam in them for the weight loading and expected external stresses. Knowledge of foam types, and their shock absorption properties, is essential to keeping your valuable case contents safe and secure.
The two main types of foam used in cases are polyethylene (the stiff stuff) and polyurethane (the “spongy” stuff). The so called “Pluck Foam” kits sold by the case manufacturers are all made of the less dense polyurethane foam for the simple reason that it can be inexpensively die-cut in thicknesses up to several inches. Stiffer polyethylene cannot be die cut in deep thicknesses, and is seldom used as the factory supplied cushioning in travel-sized cases. Because polyurethane is the standard foam option sold with most travel cases, people are using low-density pluck foam to carry around their rare and fragile lead-weight collections. Pluck foam does not age well in the best of circumstances. With heavy loading, it ceases to provide proper cushioning, and stuff tends to get broken!
Polyethylene is manufactured in sheets, and it is therefore easy to cut up and make into layer kits. Conversely, polyurethane foam is cooked in large “buns” and must be sized down (think labor cost) to make into sheets. Not coincidentally, polyethylene is the foam of choice for the manufacturers standard layer kits for larger cases. Your friend with the large collection of rare and fragile paper dolls has them “protected” by the stiff polyethylene foam that came standard in the shipping case he purchased.
The deep, dark secret here is that case manufacturers are assuming that if you have an airline travel case for check through or carry on, you have relatively light-weight items being transported. They further assume that if you have a larger shipping case, you have very heavy, non-fragile items. Do not let their assumptions drive your cushioning choice.
One needs to consider the fact that, in order to cushion; the foam needs to provide enough compression but not too much. In simplistic terms, cushioning is a function of the total weight of the item, and the surface area of the item that sits on the foam in any given direction. There are, of course, several other complicating factors. The bottom line is; if it is fragile and valuable, consult a custom case professional to discuss proper cushioning.