Is Rubber a Plastic? What a Resin Fossil Project Taught Me About Silicone Flower Molds and JSP Mold Costs
Last March, a Customer Asked Me a Simple Question
Last March, a customer from a small craft company called our front desk. They were about to launch a “resin fossil” line—clear resin blocks with dried flowers, leaves, and maybe a plastic dinosaur embedded inside. Actually, it was a fake trilobite, not a dinosaur. But you get the idea. They needed custom silicone flower molds to make them. The second question out of their mouth was: “Is rubber a plastic?”
I get that question more than you might think. At JSP, we help customers with custom molding projects—rubber parts, plastic parts, and everything in between. I’m the person who watches our material costs, manages the procurement line, and keeps the vendor scorecard. Over the past six years, I’ve documented every custom molding order in our cost tracking system and compared at least a dozen mold suppliers. So when a customer asks a basic materials question, I answer it instead of rolling my eyes. Materials knowledge drives cost knowledge.
Is Rubber a Plastic? Short Answer: No
Rubber and plastic are both polymers, but an elastomer is not a plastic. Here’s the plain-language difference:
- Thermoplastics soften when heated and harden when cooled. They can be melted and reshaped multiple times. Polypropylene, ABS, and polycarbonate live here.
- Thermosets cure into a permanent shape. Once set, they don’t remelt. Epoxy resin, polyester resin, and polyurethane resin are thermosets.
- Elastomers stretch and return to their original shape. Natural rubber, synthetic rubber, and silicone belong here.
Silicone is a synthetic elastomer, so a silicone flower mold is a rubber product, not a plastic product. A finished resin fossil, though, is basically a cured thermoset plastic. That’s part of why people get confused: the mold feels soft, but the casting is hard. It’s not a matter of “plastic or rubber” in the store. It’s about material behavior under heat and stress.
I kept that explanation short because the customer’s real question was about money.
The Quote, the Sticker Shock, and the Cheap Mold Temptation
The customer wanted 500 silicone flower molds plus a small run of casting resin. We quoted $28 per JSP mold, including a 30-day warranty and design support. The total, including resin, was around $18,200.
The customer flinched. “We found the same-style mold online for $7.50,” she said.
“Did you add shipping? Customs? Tooling?” I asked.
No.
I have mixed feelings about our pricing. On one hand, I know our domestic, small-batch molds cost more than overseas craft molds. On the other hand, I’ve seen what hidden fees and short mold life do to a project budget. I’ve been that buyer. Back in 2023, a “free setup” offer cost us $450 in hidden charges after the vendor billed for revisions and proofing. That became the year I started putting every purchase into a TCO spreadsheet.
The most frustrating part of this project was watching a smart customer almost choose based on unit price alone. You’d think someone who sells on Kickstarter would understand that cost per good unit matters more than cost per item. But sticker shock blinds everyone.
The TCO Math That Changed the Project
I walked her through my spreadsheet. The numbers were plain.
Cheap mold: $7.50 unit price, plus $650 freight and insurance, plus 8% customs on the goods, so about $9.40 per mold. If I remember correctly, the freight quote was $700, not $650, but I don’t want to overstate the point. Let’s call it $9.40.
What she didn’t consider was lifespan. I don’t have hard data on how long craft-grade silicone flower molds survive in production, but based on what I see in vendor returns and warranty claims, a $9 mold typically lasts maybe 40 to 60 pulls before it loses detail or tears. I usually assume 50 for planning.
At 50 casts per mold, the cost per cast is $9.40 / 50 = $0.19.
Our JSP mold: $28, no freight if bundled with resin, conservatively 500+ pulls with normal care. That’s $28 / 500 = $0.056 per cast. The “expensive” mold was 70% cheaper on a cost-per-good-part basis.
I also added rejection rates. Cheap molds tended to leave visible seam lines and cloudy spots. If you’re throwing away 12% of your casts, your effective cost per good part jumps. Our molds had about a 2% reject rate in production. Maybe 3%. I can’t say exactly from memory. The point is, yield matters more than most people think.
And then there was her labor. She was casting fossils herself for the first 200 units. Every ruined cast meant wasted resin and wasted hours. The spreadsheet couldn’t capture that, but the calendar did.
The Three-Week Detour
She bought the cheap molds anyway. I get it. Upfront cost was half, and she needed inventory before her Kickstarter fulfillment date.
Three weeks later, she called back. The petal details had gone soft after about 45 pulls. One mold had torn near the edge. She said she’d thrown out 30% of the casts—I want to say 30, but I might be misremembering; it was enough to hurt.
“It’s a $9.40 mold,” she said, “that should have been a $28 mold.”
To be fair, the cheap mold wasn’t garbage. It was fine for a single prototype. It just wasn’t production-grade. It did exactly what a $9 mold does: it proved the design, then wore out.
By then, the project was two weeks behind schedule. She’d spent roughly $1,200 on wasted resin and $800 of her own time redoing casts. She placed the JSP order on the same call. (I didn’t say “I told you so,” though I thought it.)
What I’d Tell Any Buyer of JSP Molds
If you’re thinking about a JSP mold for a resin fossil, a craft product, or a custom plastic part, here’s the lesson I’d pass along: compare cost per good cast, not unit price.
Ask four questions:
- How many casts will this mold survive before the detail degrades?
- What’s your rejection rate on casts?
- Are there hidden costs like customs, freight, setup fees, or minimum order requirements?
- What is the value of your time when a mold fails mid-production?
One more thing. The customer wanted to put “recyclable” on the resin fossil packaging because the product was technically a plastic. I steered her away from that claim. Per FTC guidance (ftc.gov), if you say a product is recyclable, you need to show that a meaningful percentage of consumers actually have access to recycling for it. A cured resin fossil with embedded flowers doesn’t meet that standard—not in most municipal programs. That’s not me being conservative; that’s a 16 CFR Part 260 issue.
An informed customer asks better questions and makes faster decisions. That’s not a pitch. It’s the reason I still answer questions like “is rubber a plastic?” with a straight face.
My experience here is based on about 300 custom molding orders we’ve handled over six years, mostly for craft, industrial parts, and replacement components. If you’re using molds for food-contact, medical, or high-tolerance applications, ignore every number in this article. Compliance changes the cost model completely.
At the end of the project, the customer’s resin fossils looked good. The silicone flower molds worked. And the phrase I remember is what she said last week when she ordered a second batch: “I still don’t love the price. But I love the cost.”