What I've found is that the foam that comes with the Pelican case is a little flimsy, but that's not necessarily a bad thing. First, let's talk about foam.
Polyurethane foam (commonly called PU foam) is what is typically sent in a Pelican case. This is also what they use with the "pick & pluck" foam inserts. This is an open cell foam that will hold on to water, doesn't react well to solvents and has a squishy feel. Think of it like a sponge that you would use to wash a vehicle. This foam is also relatively inexpensive and easy to shape with an electric knife. This allows the end user to have several cutouts for one case for not too much money. I believe replacement foam for a Pelican 1700 is around $30 on Amazon for just the middle piece. PU foam usually looks like this.
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Polyethylene foam (commonly called PE foam) is what many aftermarket companies use to fill a Pelican (or similar) case. This foam is stiffer, closed cell, doesn't absorb water and reacts well with mild solvents and oils. It's also expensive to buy and if you don't know what you are doing, you can end up ruining that nice piece you just paid for. The alternitive is to pay someone like MyCaseBuilder to do it for you. Just keep in mind that YOU are the one responsible for the design and if you make a mistake and it doesn't fit...not their fault. Prices can be astronomical and exceed the price for the case it's going in depending on the options chosen. PE foam usually looks like this.
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I'm not saying that the PE foam is better or worse that PU, I'm saying it's a choice. Be informed and make a decision that best suits your needs and budget.
I'm currently struggling with how to accurately cut some PU foam for my 1700. I know from prior experience with my 1750 that it is quite flimsy without the firearms installed, but when they are in there...nothing is moving. My plan is to go slow and not make big cutouts. I might even try to use a friends scroll saw to do the job and keep the edges straight. Results to follow.