Plastic Sheet Buyer's FAQ: 3M, Resin Molds, Garage Mats & PET vs Polycarbonate

Plastic Sheet Buyer's FAQ: What You Need to Know

I'm an office administrator for a mid-sized manufacturing company. I manage all our non-production purchasing—roughly $200k annually across 8 vendors. Plastic sheets and rolls are something I order regularly, and I've picked up a few lessons the hard way. Below are the questions I wish someone had answered when I started.

FAQ

1. What's the difference between 3M marine filler and 3M premium filler?

To be fair, I was confused about this too when I first started. Both are fillers, but they're designed for different jobs. 3M marine filler is formulated for high-moisture environments—think boat repairs, outdoor equipment, or anything that'll be exposed to water. It's more resistant to water absorption and cracking. 3M premium filler, on the other hand, is a general-purpose body filler for automotive and industrial use. It sands easier and is better for smooth finishes on dry surfaces. In my experience, using premium filler in a marine application leads to failure—I learned that after a repair job I did in Q3 2024 delaminated within months. The extra $12 for marine-grade was absolutely worth it.

2. Can I use garage rubber mats under resin molds?

Short answer: yes, but it depends on the resin. I went back and forth on this one for about a week when we were setting up a new workbench. Garage rubber mats are great for protecting floors from spills and providing a non-slip surface—they're durable and fairly chemical-resistant. But if you're working with epoxy or polyester resin, the uncured resin can eat through some rubber compounds. I'd recommend testing a small area first. In our workshop, we've found that thicker mats (about 3/8 inch) hold up better for occasional use. For heavy-duty resin work, a dedicated silicone mat is a safer bet. That said, for quick demos or light projects, garage mats work fine—just don't leave uncured resin on them overnight.

3. What's better for vacuum forming: PET or polycarbonate?

This is probably the most common question I get from our engineering team. It really comes down to what you're making. PET (polyethylene terephthalate) is the go-to for thermoforming and vacuum forming. It's easy to heat, forms cleanly, and has good clarity. It's also more affordable—about 20-30% less per sheet than polycarbonate. Polycarbonate is tougher—it's impact-resistant and can handle higher temperatures, but it's harder to form because it needs more heat and is more prone to warping. In my experience, if you're making packaging or display trays, PET is a no-brainer. If you need structural strength or the piece will see rough handling, polycarbonate is worth the extra cost. I've had projects where using PET saved us $400 in material costs, and the final parts worked perfectly.

4. How do I choose between PET and polycarbonate for my project?

Bottom line: consider the total cost of ownership, not just the per-sheet price. Here's a quick breakdown based on what I've learned:

• Clarity: PET has better optical clarity (around 90% light transmission); polycarbonate is slightly lower (around 88-89%).
• Impact resistance: Polycarbonate is 10-15x more impact-resistant than PET. If it needs to survive drops or impacts, go with polycarbonate.
• Formability: PET is easier to form—less heat, faster cycles, fewer rejects. This can save you time and labor costs.
• Chemical resistance: PET is better against oils and acids; polycarbonate is more sensitive to chemicals (it can craze with some solvents).
• Cost: As of January 2025, a 0.060" thick PET sheet is roughly $8-12 per square foot; polycarbonate is $12-18 per square foot. Verify current pricing with your supplier.

If your project needs clarity and cost-efficiency, go PET. If it needs toughness and can handle a higher budget, go polycarbonate. I usually end up choosing PET for about 70% of our applications.

5. What's the best way to clean and maintain garage rubber mats?

This seems simple, but I've seen people ruin good mats by using the wrong cleaners. Garage rubber mats are pretty low-maintenance. For regular cleaning, sweep or vacuum them to remove dirt and debris. For deeper cleaning, use a mild soap and water solution—don't use harsh chemicals like bleach or solvent-based cleaners, as they can break down the rubber over time. A pressure washer works great if you have one, just keep the nozzle at least 12 inches away to avoid damaging the surface. In our facility, we clean our mats once a month, and they've lasted over three years. I've had one mat that we cleaned with a citrus-based degreaser once—big mistake. It got sticky and started cracking within weeks. Stick to gentle soaps, and you'll double their lifespan.

6. Is 3M premium filler worth the extra cost over standard body fillers?

I get why people go for cheaper body fillers—budgets are real. But the hidden costs add up. 3M premium filler is more expensive upfront—about $35-45 per gallon vs. $25-30 for generic brands. But here's what I've learned: the cheaper stuff tends to have more pinholes, requires more sanding, and takes longer to cure. In a production setting, that extra time is money. Our team found that using premium filler reduced finishing time by about 25% on average. Plus, the final finish is smoother, which means less rework. I had a project in Q2 2024 where using a generic filler saved $60 upfront—but we spent an extra $200 on labor and sandpaper. So the 'budget' choice ended up costing us more. Now I calculate total cost before comparing any vendor quotes.

7. Can I use PET plastic for resin molds?

Yes, but with a caveat. PET sheets are often used as a mold surface for resin casting because they're smooth, non-stick (with a release agent), and fairly durable. I've used them for small epoxy projects with great results. The key is to apply a proper release agent (like a mold release spray or even PVA) before pouring resin. Without it, the resin can bond to the PET—and that's a nightmare to clean. I learned this the hard way when I skipped the release agent on a mold for a custom tabletop. The resin fused to the PET sheet, and I had to scrap both. That mistake cost me about $80 in materials and 4 hours of work. So, yes, PET works for resin molds—just don't get lazy with the release agent. For larger or more complex molds, you might want polycarbonate for its higher heat resistance, but PET handles most home and small shop jobs fine.

8. How do I ensure I'm getting the best price on plastic sheets and rolls?

When I took over purchasing in 2020, I used to just go with the cheapest quote. But I've learned that process matters. Here's what works for me now:

• Request quotes from at least 3 suppliers for the same spec—thickness, size, quantity, and delivery time.
• Don't just compare unit price. Ask about shipping, minimum order quantities, lead times, and return policies. A low unit price can vanish when you add rush shipping or pay for rejected parts.
• Build relationships with 2-3 vendors. Our main supplier gives us a 5% discount just because we've ordered consistently for 3 years. That loyalty discount saved us $1,200 last year alone.
• Buy in volume if possible. For commonly used sheets (like PET for packaging), we buy a quarter's worth at once. That cut our per-sheet cost by about 12%.
• Check pricing regularly. I do a quick market check every quarter. Prices change—knowing when to lock in a rate matters.

In Q4 2024, I tested 4 vendors for a 500-sheet order of 0.060" PET. Pricing varied by nearly 35% for identical specs. Total cost, including shipping, ranged from $2,800 to $3,650. The cheapest vendor wasn't the one with the lowest unit price—it was the one with the best total package.