An Eaton hydraulic hose is not a slinky air hose. I know that sounds like a strange way to start a note about plastic sheet, but stay with me.
I manage purchasing for a 70-person packaging company. That means processing 60 to 80 purchase orders a year, roughly $180,000 in vendor spend, and a steady stream of requests that do not fit neatly on a purchase order. I am not a polymer chemist. My job is to ask enough questions to avoid ordering the wrong thing.
In March 2024, a client asked us to make a clear display tray for a set of handmade resin dice. The resin dice mold part of their process was already solved; they cast dice in silicone molds at their own workshop. What they needed from us was packaging. The client needed the first 500 sets before a tabletop convention, which left about six weeks from drawing to delivery. That order turned into a lesson in PET vs polypropylene, and it started with a price quote.
Search Terms Are Not Specs
The same week, our maintenance supervisor asked me to replace what he called the slinky air hose on a packing line. After two phone calls, I realized he meant a coiled retractable air hose. Slinky air hose is a nickname, not a part number.
Meanwhile, the mechanics needed an Eaton hydraulic hose for another machine. Eaton was a useful starting point, but it still came down to series, size, fitting, and pressure rating. The brand name alone could not keep a machine running.
Those two requests taught me to look past words. A request says what someone wants, but it does not tell you what will work. I started applying the same logic to plastic sheet. PET, PP, PVC, acetate—these names matter, but they are not complete instructions.
The Price Quote Looked Rational
When the drawing for the dice tray came back, our account manager asked what material to quote. I did the normal thing: asked three suppliers for a price on clear rollstock.
The lowest quote came from a supplier who said polypropylene was basically the same as PET, just cheaper. Their price per kilogram was maybe 15 percent lower, and they could ship in a week. I almost placed the order before seeing samples. That was my first mistake.
It is tempting to compare unit prices and stop there. But PET vs polypropylene is not a simple price comparison. The two materials do different jobs.
PET and PETG sheets are optically clear, stiff, and consistent for thermoformed trays and printed packaging. Polypropylene has a lower density, so a given weight of resin goes further. It resists repeated flexing, which is why you see PP in binding covers, document folders, and living-hinge packaging. But PP sheet is usually not as clear as PET, and at the same thickness it can feel less rigid. Untreated PP also gives inks and adhesives trouble because its surface energy is low. Honestly, I had to look that one up.
Polypropylene is not bad. It is just not automatically a substitute.
Samples Changed the Conversation
We requested a sample roll of clear PP and an APET sample from a different source. Under our work lights, the PET looked like glass. The PP looked transparent in the center, but the surface scuffed easily and the sheet felt floppier at the same gauge. To make a PP tray feel premium, we would have needed a heavier sheet, which ate into the price advantage.
For a clear tray, I cared about three things: clarity, rigidity, and ink adhesion. The PP sample lost on all three at the same nominal gauge. A PP sheet could have been made thicker or surface-treated, but that would have brought the quote closer to PET and added lead time.
Then came the printing test. The client logo was a specific red, referenced in Pantone. The art file was at 300 DPI, which is a typical commercial print minimum, so the designers did not expect a material problem.
Our production lead printed a test logo on both samples. The PET sample held the ink. On the PP sample, ink peeled off with standard packaging tape after curing. That is a classic untreated polypropylene problem. It can be fixed with corona treatment or another surface preparation, but the quote did not include it.
Even if the ink had stuck, color would still need a tolerance check. For brand-critical work, color difference is commonly measured in Delta E, and a target below 2 is often used for a match. The printed red on PP looked okay under the office light, but not beside the approved proof. For a small studio building a brand around handcrafted dice, close was not good enough.
The Supplier Who Knew What Not to Sell
The low-price supplier kept saying they could do PP, PET, PVC, or whatever we wanted. But when I asked what each sheet was designed for, the answer got vague. A generalist can be a problem when you need specific performance.
The manufacturer that earned the order asked questions before quoting. Their rep asked if the tray would be thermoformed, whether the logo would be printed before or after forming, how the dice would sit in the tray, and whether we needed anti-static film for shipping. Then they said something I still remember: for a clear tray like this, start with PETG. If you needed a binder cover or a folder with a living hinge, PP would make sense. But this was not that.
That answer built more trust than a discount. The rep basically told me that the cheaper material was wrong for my project. It would have been easy to sell me PP rollstock and let me fix the problem later. Instead they drew a boundary. I would rather work with a specialist who knows their limits than a generalist who promises everything.
What Happened in the End
We ordered PETG rollstock, not budget PP. The supplier was not the cheapest quote, and delivery slipped by two days, which made me nervous. But the trays formed, the logo stayed on, and the client approved the packaging.
The surprise was not that PET looked better. The surprise was that the so-called expensive material saved money. After I added up the PP samples, the printing rework, the extra shipping, and the lost time, the cheap quote no longer looked cheap. I do not remember the exact total, but I remember the lesson.
Since that project, I do not let an account manager write clear plastic on a purchase order. I want the specific resin, the nominal gauge, the tolerance, and the surface treatment if printing follows.
What I Would Do Differently
An Eaton hydraulic hose has a part number for a reason. A slinky air hose is a nickname, not a specification. A resin dice mold tells you that someone makes dice; it does not tell you what packaging material to use. PET vs polypropylene is the same idea in another form.
If someone asks me which is better, PET or polypropylene, my answer now is that it depends. What are you forming? Is there a living hinge? Does the logo need to survive a tape test? Is the sheet thick enough to feel rigid?
And if a supplier tells you material does not matter, treat that as a warning. Good suppliers know where their product fits and where it does not. Trust me on this one: the ones who define their boundaries are usually the ones who know their craft.