Here's the thing about picking plastics — it's not just about finding something that "works." We've been running Eastmaster for over 25 years now, machining and molding parts for clients across industries, and we've seen the same pattern play out over and over: someone picks a material based on a spec sheet or a supplier's recommendation, runs production, and then discovers the hard way that it wasn't the right fit.

One client lost $80K in recalls because they swapped POM for PP in a gear assembly to save on material cost — the gears deformed under load within weeks. Another spent $15K more than necessary on tooling because they specified pure PC for a thin-walled housing when PC/ABS would have done the job perfectly at lower injection pressure.

So this guide isn't a textbook overview. It's what we've actually learned from making parts — the trade-offs that matter, the mistakes we see repeatedly, and the practical factors that spec sheets don't always make clear.

The Quick Comparison

If you just need a fast reference, here's the big picture. Detailed write-ups on each material follow below.

Material	Tensile MPa	HDT °C	Cost $/kg	Best Known For	Watch Out For
PP	31–41	100	1–2	Living hinges, chemical resistance	Can't paint or glue easily
PE	26–33	80	1–2	Moisture barrier, food safe	Warpage on large flat parts
PS	20–35	80	1–2	Crystal clear (GPPS), cheap	Brittle, poor chemical resistance
ABS	40–50	100	2–3	Tough + good finish + easy to mold	Yellows outdoors
PC	60–70	135	3–5	Unbreakable + transparent	Must dry thoroughly, expensive
PC/ABS	50–60	118	3–5	Best of PC + ABS, laptop housings	Still needs drying, not clear
POM	60–70	160	3–5	Precision gears, low friction	Hard to bond, formaldehyde when hot
PA (Nylon)	70–85*	220*	5–8	Strong, wear resistant	Absorbs moisture, dims change
PMMA	65–72	95	2–4	92% light transmission	Brittle, cracks on impact
PEI	100–110	205	25–40	Flame resistant (no additives)	Amber color, pricey
PEEK	90–100	260	80–150	Extreme heat, chem, wear	Virtually everything about the price
TPE/TPU	30–50	75	4–12	Soft touch, overmolding	Creeps under load, shrinkage varies
LSR	5–10	200+	8–20	Medical/skin-safe, wide temp	Special equipment, not recyclable

* Glass-filled (GF30) values. Unfilled PA is lower. Always check your supplier's datasheet for the specific grade.

Commodity Plastics — The Budget Options

PP (Polypropylene)

PP is the cheapest plastic that still does real work. It's what shampoo bottle caps are made of — those little hinges that flex open and closed a thousand times without breaking? That's PP's signature move. The material can handle millions of flex cycles because the polymer chains align naturally in the direction of flow during molding, and if you design the hinge right (0.4–0.6mm thick, with flow direction across the hinge), it'll outlast the rest of the product.

It's also chemically resistant enough that you can store aggressive cleaning products in it, and it's FDA-compliant for food contact in most grades. At $1–2/kg, it's hard to beat on price.

The catch: PP has very low surface energy, which is a fancy way of saying you can't paint it, glue it, or print on it without plasma treatment. It also shrinks 1.5–2.5% during cooling, so thick parts can warp. And below 0°C, it gets brittle — not the best choice for anything that lives in a freezer.

PE (Polyethylene — HDPE / LDPE)

PE is the world's most-produced plastic. HDPE is the rigid version (milk jugs, cutting boards); LDPE is the flexible one (squeeze bottles, plastic bags). Both are virtually waterproof and dirt cheap. If your part needs to hold liquids or sit in a wet environment, PE is a solid baseline choice.

Like PP though, it's hard to bond and paint. HDPE can warp badly on large flat surfaces, and neither version is going to win any structural strength awards.

PS (Polystyrene — GPPS / HIPS)

Crystal clear and cheap (GPPS), or tougher and opaque (HIPS). Polystyrene is easy to mold and very affordable, which is why it shows up in disposable cutlery, CD cases, and refrigerator liners. But it's brittle — GPPS cracks if you look at it wrong — and it's one of the worst plastics for chemical resistance. Many common solvents dissolve it on contact. Use it for low-cost, low-stress indoor applications and nothing more.

Engineering Plastics — When Commodity Isn't Enough

ABS

ABS is the default for a reason. Good impact resistance, nice surface finish (takes paint and plating beautifully), easy to mold, and affordable at $2–3/kg. It's what most consumer electronics housings are made of, and it's the material LEGO uses for their bricks — which tells you something about both its durability and its precision.

The big weakness: UV resistance. Put ABS outside and it'll yellow within a few months and get brittle within a year. If you need outdoor durability, look at ASA instead — it's basically ABS with UV resistance built in, at similar cost.

Shop tip: If your ABS part needs chrome plating, make sure you specify an "electroplating grade." Standard ABS has butadiene particles that are the wrong size for good plating adhesion, and you'll end up with peel-off issues. We learned this the expensive way early on.

PC (Polycarbonate)

PC is the unbreakable one. Its impact strength (600–850 J/m notched Izod) is in a completely different league from other transparent plastics. It's what bullet-resistant glass and safety helmets are made from. It also handles heat well — continuous service up to about 130°C.

But PC demands respect in processing. It must be dried to below 0.02% moisture before molding, or you get splay marks and degraded molecular weight. That means 3–4 hours in a dehumidifying dryer at 120°C. Skip this step and your "unbreakable" part becomes surprisingly breakable.

PC also scratches easily (most optical applications need a hard coat), and it's attacked by gasoline and strong alkalis. At $3–5/kg, it's significantly more expensive than ABS.

PC/ABS Blends

This is what most laptops and phone cases are actually made from — not pure PC, not pure ABS, but a blend (usually 50/50 to 70/30). You get PC's heat resistance and impact strength with ABS's processability and lower-temperature toughness. It's also easier to achieve UL 94 V-0 flame retardancy with PC/ABS than with either material alone.

Think of it as the practical choice: when PC is overkill and ABS isn't quite enough, PC/ABS sits right in the sweet spot.

POM (Acetal / Delrin®)

If you need precision, POM is your material. It holds tight tolerances because it barely absorbs moisture (less than 0.25%), so unlike nylon, your parts don't swell in humid environments. Its coefficient of friction against steel is 0.1–0.3 — low enough for gears and bearings to run dry.

One thing people get confused about: "Acetal" and "Delrin" aren't exactly the same. Delrin is the homopolymer version — slightly stronger and stiffer. Acetal copolymer has better chemical resistance and handles hot/wet environments better. Pick copolymer for wet conditions; homopolymer for maximum stiffness.

Downsides: almost impossible to bond with adhesives, poor UV resistance, and it releases formaldehyde if overheated during processing (ventilate your shop). Also fairly dense at 1.41 g/cm³.

PA (Nylon — PA6, PA66)

Nylon is strong, tough, and handles wear well. Glass-filled grades (PA6-GF30 or PA66-GF30) are beasts — tensile strength jumps to 70–85 MPa, heat deflection temperature to over 200°C, and shrinkage drops from ~1.5% down to 0.3–0.5%. For under-hood automotive parts and structural brackets, glass-filled nylon is often the answer.

But here's the thing about nylon that catches people off guard: it absorbs 1.5–2.5% moisture from the air. This means parts can grow 0.5–1% in size after they leave the dry-as-molded state. If you've designed a tight-tolerance assembly based on dry dimensions, it'll be out of spec after a couple weeks in a humid warehouse. Either design in tolerance for the growth, switch to glass-filled (less absorption), or use POM if the environment is wet.

Also: glass-filled nylon is abrasive. Expect 20–30% shorter mold life compared to unfilled grades. Budget accordingly.

PMMA (Acrylic)

Acrylic transmits 92% of visible light — better than polycarbonate and better than most glass. It's also UV-stable and scratch-resistant, which is why it's the go-to for outdoor signage and light fixtures. But it's brittle. Really brittle. Impact strength of 15–25 J/m, versus PC's 600+. If you're choosing between acrylic and polycarbonate for a transparent part, the question is simple: does it need to survive impact? If yes, PC. If not, PMMA.

High-Performance Plastics — When Money Isn't the Problem

PEEK

PEEK is what you use when failure is not an option. Jet engine components, spinal implants, semiconductor wafer carriers, oil well downhole tools — these are applications where the part has to work at 250°C, in aggressive chemicals, under load, for years, without excuses. PEEK does all of that.

It also costs $80–150/kg, requires melt temperatures of 370–400°C, and needs specialized molding equipment. For 95% of projects, PEEK is massive overkill. But for that 5%, there's simply no alternative.

PEI (Ultem®)

Think of PEI as PEEK's more affordable cousin. You get about 80% of PEEK's thermal performance at roughly 30% of the cost ($25–40/kg). The standout feature is inherent flame resistance — PEI achieves UL 94 V-0 without any flame-retardant additives, and it produces very little smoke when exposed to flame. That's why it's the standard for aircraft interior components.

It's naturally amber-colored, which limits optical applications. And like PEEK, it needs high processing temperatures (340–380°C). But if you need high-temperature performance plus fire safety and PEEK is too expensive, PEI is usually the answer.

Flexible Materials

TPE / TPU

When you need a soft-touch grip, a flexible seal, or a phone case that absorbs drops, TPEs and TPUs are the answer. They process on standard injection molding equipment (unlike thermoset rubber), and they're recyclable. TPU is tougher and more abrasion-resistant; TPE is cheaper and easier to work with.

The main thing to know: overmolding TPE/TPU onto a rigid substrate (ABS, PC, PA) requires careful material pairing for adhesion. Not all combinations stick. If you're planning a two-shot overmold, talk to your molder about compatible pairings before you commit to a material.

LSR (Liquid Silicone Rubber)

LSR is for when you need something that feels like rubber but is biocompatible and works from -50°C to +250°C. Baby bottle nipples, medical seals, baking molds — that's LSR territory. It's processed by liquid injection molding (LIM) at heated molds, which means you need dedicated equipment. It's also a thermoset, so it can't be remelted or recycled.

A Practical Way to Choose

Instead of starting with a material, start with your requirements. Here's the process we walk through with every client:

1. Write down what the part actually needs to survive. Not "it needs to be strong" — be specific. "Must withstand 80°C continuous, resist isopropyl alcohol, survive a 1.5m drop onto concrete." Quantified requirements eliminate 80% of candidates immediately.

2. Check regulatory requirements early. If your part touches food, you need FDA-compliant resin. If it's a medical device housing, you need ISO 10993. If it's an electronics enclosure, you probably need UL 94 V-0. These constraints will narrow your list fast.

3. Think about the manufacturing process. For injection molding, your material's Melt Flow Index (MFI) needs to match your part geometry — thin walls need MFI above 15, thick parts work better with MFI 5–10. For CNC machining, POM and PA are excellent; PE and TPU machine poorly.

4. Don't over-engineer. If your part only needs 60% of PC's impact resistance, why pay for PC? Find the material that hits your requirements at the lowest total cost — material plus processing plus tooling wear plus secondary operations. A $2/kg material with easy processing often beats a $1/kg material that fights you every step of the way.

5. Validate before you commit. Before cutting steel for production tooling, have your manufacturing partner run a DFM review and mold a few sample shots. We regularly catch material issues at this stage that would have been expensive to fix later.

Common Mistakes We Keep Seeing

Using ABS outdoors. We see this all the time and it always ends the same way — yellowed, cracked parts and warranty claims. Use ASA instead. Same cost, same processability, years of UV stability. There's no good reason to use ABS in outdoor applications anymore.

Ignoring moisture absorption in nylon. This one sneaks up on people. Parts pass QC in the dry-as-molded state, ship fine, and then grow 0.5–1% after sitting in a humid warehouse for two weeks. If your assembly has tight tolerances, either use glass-filled nylon or switch to POM.

Choosing by price per kilogram alone. A cheap material that's hard to process, wears out your molds, requires secondary operations, and fails in the field is more expensive than a pricier material that just works. Look at total cost, not just resin price.

Saying "clear plastic" without specifying performance. "Clear" could be PC, PMMA, PS, PETG, or several others — with vastly different impact strength, temperature capability, and cost. A clear safety shield needs PC. A display stand can use PMMA. Be specific about what "clear" has to do, not just how it looks.

Regulatory Quick Reference

Standard	Covers	Usual Suspects	Note
FDA Food Contact	Food/beverage safety	PP, PE, PET, BPA-free PC, PEEK	Verify specific grade — not all qualify
UL 94	Flammability (V-0/V-1/V-2/HB)	PC/ABS (V-0 w/FR), PEI (V-0 inherent)	Electronics housings often need V-0
ISO 10993	Medical biocompatibility	Medical-grade PC, PEEK, LSR, PP	Medical grades cost 3–5x more
RoHS / REACH	Restricted substances	Most modern resins comply	Old FR grades may contain restricted stuff

The Bottom Line

There's no "best" plastic — only the right one for your specific job. A material that's perfect for a car dashboard could be a disaster in a medical device. The key is being honest about what your part actually needs to survive, checking regulatory boxes early, and validating your choice with your manufacturing partner before you invest in tooling.

If you're not sure which direction to go, that's literally what we do — help people figure this out before they spend money on the wrong material. Drop us a line with your part design and we'll point you in the right direction. Usually takes about a day.

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Written by the Eastmaster Engineering Team. We've been machining and molding plastic parts in Shenzhen, China since 2010. Property values are typical ranges for unfilled grades — always verify with your resin supplier's datasheet.

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