5 Cheapest Workhorse Plastics for Injection Molding (And Their Limits)
Your new product needs injection molded parts, but the budget is tight — and the part absolutely cannot fail. That’s where most engineers start sweating.
Here’s the reality I’ve seen across hundreds of projects: the cheapest material per kilogram isn’t always the cheapest per part. Resin cost is one variable. Cycle time, scrap rate, tool wear, and field failure risk all feed into the real cost. I’ve watched teams pick bargain-bin resin and lose ten times the savings in rejected parts and warranty claims.
This post covers the five plastics that offer the most performance per dollar — and the specific limits that’ll bite you if you ignore them.
PP (Polypropylene): $1.20–1.80/kg
$1.20–1.80/kg makes PP the default for high-volume, cost-sensitive applications.
The material’s defining feature is fatigue resistance. PP can be molded with an integral living hinge that flexes hundreds of thousands of cycles without cracking. Flip-top shampoo caps? That’s PP. No secondary assembly, no hinge pin, no extra cost.
PP also delivers excellent chemical resistance — it shrugs off most acids, bases, and organic solvents at room temperature. For containers holding cleaning chemicals or automotive fluids, PP is often the only commodity plastic that works.
The limits are real. Max continuous service temp is 100°C — above that, PP softens fast. UV exposure without stabilization causes embrittlement in months. And the surface finish is fair at best; PP won’t give you the glossy, Class-A surface that ABS or PS can produce.
Best applications: living hinges, chemical containers, battery cases, caps and closures.
HDPE (High-Density Polyethylene): $1.30–1.90/kg
HDPE comes in just above PP on cost, with a different tradeoff set.
Where HDPE excels is chemical resistance and moisture barrier. It’s the standard for bottles and containers — its moisture vapor transmission rate is among the lowest of any commodity thermoplastic. Contents stay dry.
The catch: aesthetics. HDPE looks and feels cheap. Its surface is waxy, difficult to paint or bond, and prone to visible sink marks. Semi-crystalline shrinkage (1.5–4%) makes tight-tolerance parts harder to hold. Warpage and creep are common failure modes under sustained load.
Max service temp is 90°C — slightly below PP. Above that, creep accelerates hard.
Best applications: containers, bottles, fuel tanks, chemical storage, cutting boards.
PS (Polystyrene): $1.10–1.50/kg
The cheapest on this list — but you get what you pay for.
PS comes in two forms. GPPS is crystal-clear, rigid, and brittle. HIPS is opaque, tougher, and less prone to shattering. Both share the same polymer backbone, but HIPS adds rubber particles to arrest crack propagation.
GPPS delivers optical clarity that competitors can’t touch at this price. Need see-through windows at $1.20/kg versus PMMA at $3.50/kg? GPPS works — as long as you can accept the brittleness.
HIPS fills the other need: tough, low-cost parts that don’t need chemical resistance or outdoor durability. Yogurt cups, takeout containers, disposable cutlery.
The failure modes are punishing. GPPS shatters under impact like glass. HIPS stress-cracks in contact with oils and solvents. Max service temp is 80°C — the lowest on this list. Neither grade survives outdoors.
Best applications: disposable packaging, clear display components, yogurt cups, CD cases, model kits.
ABS: $1.80–2.50/kg
The most expensive on this list — and the most versatile.
ABS delivers the best balance of mechanical properties, surface finish, and processability among commodity plastics. It’s the default for consumer electronics housings, automotive interior parts, power tool enclosures, and toys. Chances are the keyboard you’re typing on is ABS.
What makes ABS special is its toughness-stiffness balance. 40–50 MPa tensile strength, decent impact resistance (200–400 J/m Izod), and a surface that takes paint, texture, and EMI shielding beautifully.
The limits: poor UV resistance (yellowing within months outdoors), poor chemical resistance (stress-cracks in contact with hydrocarbons), and max service temp around 95°C. ABS also gives off a characteristic sweet-smelling styrene vapor during processing — good ventilation is non-negotiable.
The cost premium over PP (roughly 1.5–2x per kg) buys you vastly better surface quality, dimensional stability, and impact strength. For visible parts with aesthetic requirements, it’s usually worth it.
Best applications: enclosures, housings, automotive interior trim, consumer electronics, medical device shells.
SAN (Styrene-Acrylonitrile): $1.60–2.20/kg
SAN is the lesser-known cousin of ABS — same styrene-acrylonitrile backbone without the butadiene rubber.
The payoff is stiffness. SAN has a flexural modulus around 3.5 GPa versus ABS’s 2.3 GPa. For clear parts that need rigidity — refrigerator crisper drawers, cosmetic packaging, water filter housings — SAN beats GPPS on impact resistance and beats ABS on stiffness and clarity.
SAN also offers better chemical resistance than ABS. It handles mild acids, alkalis, and food oils without stress-cracking. That’s why it’s common in food-contact applications where ABS would craze.
The tradeoff: SAN is less impact-resistant than ABS (GPPS-level brittleness). Max service temp is 90°C. It yellows under UV, same as ABS. And while SAN flows well in the mold, it’s prone to molded-in stress that shows up as crazing when the part contacts certain chemicals.
I’ve seen designers pick SAN for clear cosmetic parts, then watch them crack around a threaded insert because they didn’t account for residual stress. The material demands careful gate design and annealed parts for critical assemblies.
Best applications: clear refrigerator components, cosmetic packaging, medical fluid handling, water filter housings, kitchen appliances.
Comparison Table
| Material | $/kg | Max Temp | Impact | Chemical | Aesthetics | Failure Mode |
|---|---|---|---|---|---|---|
| PP | $1.20–1.80 | 100°C | Low | Excellent | Fair | UV embrittlement |
| HDPE | $1.30–1.90 | 90°C | Medium | Good | Poor | Warpage, creep |
| PS (GPPS) | $1.10–1.50 | 80°C | Low | Poor | Excellent | Brittle fracture |
| ABS | $1.80–2.50 | 95°C | Medium | Poor | Excellent | UV yellowing, stress |
| SAN | $1.60–2.20 | 90°C | Low–Med | Good | Good | Crazing in chemicals |
How to Choose
Here’s a decision tree I’ve developed over years of quoting projects:
Need chemical resistance or living hinges? → PP
Need a container that keeps moisture out? → HDPE
Need optical clarity at the lowest possible cost? → GPPS
Need a tough, paintable surface for a visible housing? → ABS
Need a clear, rigid part that handles mild chemicals? → SAN
The surprising truth: ABS is often the cheapest option overall — not the cheapest resin — when you factor in surface finishing, secondary operations, and yield rate. I’ve seen projects where specifying PP to save $0.008 per part caused $0.05 per part in scrap and rework. Run the full cost model before you lock in your choice.
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