Last updated: 2026-09-05 · 5 min read

The short version: choose ABS when the part needs rigidity, painted or plated surfaces, dimensional stability and a solid feel; choose PP when the part needs chemical resistance, low weight, living hinges or the lowest material cost. They are not rivals competing for the same job — ABS is an amorphous engineering commodity for structure and appearance, PP is a semi-crystalline workhorse for containers, closures and flexible features. The right question is not “which is better” but which one matches the five inputs below — and the answer should be fixed before the mold is quoted, because the resin decides the shrinkage the cavity is cut against.

Key facts

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  • ABS: rigid, good impact, excellent paint and plating surface, amorphous — lower, more predictable shrinkage.
  • PP: light, chemically resistant, fatigue-tolerant (living hinges), semi-crystalline — higher, less predictable shrinkage.
  • Five selection inputs: mechanical role, chemical exposure, surface and finish plan, temperature, cost per part.
  • The resin choice changes the mold: shrinkage values, gate sizing, cooling and warpage strategy.
  • Specify family, grade and color system in the RFQ — “ABS” alone is not a resin specification.

    • MFI (Melt Flow Index) — MFI — grams of resin extruded through a standard orifice in 10 minutes at specified temperature and pressure. High MFI (20–40): flows easily, fills thin walls. Low MFI (5–10): for thick walls and large parts. MFI must match part geometry to avoid short shots or flash. The related guide covers adjacent topics.

    Why does this comparison come up so often?

    Because both resins sit at the affordable end of the spectrum and both mold beautifully, so early-stage designs often leave the choice open — and the decision lands on the mold quote as an assumption. The two materials are fundamentally different animals, and the difference is structural, not cosmetic:

    Amorphous vs semi-crystalline — ABS is amorphous: its molecules cool without forming ordered crystals, so it shrinks less and more predictably, and its surfaces paint and glue readily. PP is semi-crystalline: it crystallizes as it cools, which gives it chemical resistance and fatigue life but also higher, direction-sensitive shrinkage and more warpage risk. Every mold-side consequence in this article flows from that one difference.

    ABS and PP side by side

    Property ABS PP Verdict
    Rigidity and feel Stiff, solid, engineering feel. Softer, lighter, slightly waxy feel. ABS for structural parts that must feel solid in the hand.
    Impact behavior Good impact strength, holds up at room temperature service. Good with copolymer grades; homopolymer turns brittle in cold. Tie — grade-dependent; name the service temperature.
    Chemical resistance Attacked by many solvents, oils and fuels. Excellent against acids, bases, solvents and household chemicals. PP decisively, for containers and chemical contact.
    Surface finishing Paints, plates and glues well; good gloss range. Notoriously hard to paint or glue without surface treatment. ABS decisively, for painted or plated appearance.
    Heat tolerance Deflects lower; softens earlier under load. Holds shape to higher temperatures. PP for hot-fill, dishwasher-adjacent or under-hood-adjacent use.
    Shrinkage and warpage Lower, more predictable; friendlier to tight dimensions. Higher, crystallization-driven; needs mold design attention. ABS for dimensional stability; PP needs the tolerance conversation earlier.
    Flexibility features Rigid only. Living hinges, snap features, repeated flexure. PP — nothing else in this price class does living hinges.
    Material cost and density Higher price, higher density. Among the least expensive resins, and the lightest common one. PP — cost per part, not cost per kilogram, is the honest math.

    When does ABS win the job?

  • Painted or plated appearance parts. Housings, bezels, trim — the surface finishes that PP resists are ABS territory.
  • Rigid assemblies with tight fits. Predictable shrinkage makes snap fits, bosses and mating features behave as designed.
  • Parts that must feel solid. Consumer products where weight and stiffness read as quality in the hand.
  • Bonded assemblies. Where adhesive assembly is part of the plan, ABS glues with standard systems.

    • When does PP win the job?

  • Containers, caps and closures. Chemical resistance plus low weight is exactly the packaging formula.
  • Living hinges and flex features. Flip lids, integral hinges, repeated snap action — PP’s fatigue life is the reason it gets the job.
  • Chemical or wet service. Cleaning-product housings, fluid-contact components, outdoor weathering with the right grade.
  • Cost-driven volume parts where unpainted molded-in color is acceptable and every cent per part counts.

    • What the resin choice changes in the mold

    This is the part most selection guides skip: the resin is not just a line on the BOM, it is an input to the tooling itself. Pick a resin and you have quietly made four mold-design decisions at the same time. The consequences, side by side:

  • Shrinkage values. The cavity is cut against the resin’s shrinkage; ABS and PP use different design values, so switching resin after the mold is cut means re-cutting steel.
  • Warpage strategy. PP’s crystallization makes flat, thin parts more warp-prone — gate placement, packing and cooling balance matter more, and the DFM should show the plan.
  • Gate and runner sizing. Flow behavior differs; gate dimensions tuned for ABS are not automatically right for PP.
  • Ejection and finish. PP’s softer surface shows ejection marks differently; appearance faces need the finish and draft planned accordingly — the pairing rules are in the surface finish chart.

    • Practical consequence for sourcing: if the resin is genuinely undecided, say so in the RFQ and ask the factory to state the design shrinkage per option. Two quotes priced against two different assumed resins are not comparable, and the correction after steel cutting is the expensive kind.

    Five mistakes that make this choice expensive

    The resin decision looks trivial on paper — two short letters in a column — and that is exactly why it goes wrong quietly. The recurring failure patterns we see in incoming RFQs and transferred molds all trace back to the same handful of mistakes:

  • Leaving the resin as “TBD” after the deposit moves. The cavity gets cut against somebody’s assumed shrinkage, and the correction arrives as a change order — or worse, as parts that never hit dimension. Resin is decided before money, or it is decided twice.
  • Comparing kilogram prices instead of cost per part. PP’s lower density means each part consumes less material, so the gap between the two resins per finished part is larger than the resin datasheets suggest. The honest comparison is material cost per shot against your geometry.
  • Choosing ABS for appearance, then skipping the paint plan. ABS earns its place when surfaces get painted, plated or glued. If the part ships molded-in color and never sees finishing, part of ABS’s rationale is unused — and PP may do the same job for less.
  • Specifying PP homopolymer for cold service. Homopolymer PP turns brittle at low temperatures; impact copolymer grades exist precisely for this. The service temperature range belongs in the RFQ so the grade family follows from it.
  • Switching resin mid-program without telling the mold maker. Different shrinkage means the cavity no longer matches the material. If market conditions force a switch, state it openly and let the factory re-check dimensions against the new design shrinkage before production resumes.

    • What about the in-between options?

    Real programs often land between the two pure answers, and that is legitimate engineering rather than indecision. PP copolymer trades a little stiffness for cold-impact life. ABS blends — most famously the ABS/PC family — add heat and impact where plain ABS falls short. Filled grades of either family change shrinkage, stiffness and surface behavior together. The rule is the same as for the base choice: name the family, the grade characteristic and the requirement it answers, so the quote and the DFM respond to the real material instead of a generic label. When the part genuinely needs properties neither family offers, that is the moment to widen the search to engineering resins — and to say so before the deposit, following the DFM sequence.

    How to specify the choice in the RFQ

    1. State the family and the driving requirement. “PP copolymer for cold-impact” or “ABS, paintable grade” — the requirement lets the factory propose and justify the grade.
    2. Name the color system. Molded-in color versus painted surface changes both resin selection and finish spec; deciding it late costs a change order.
    3. Declare chemical and temperature service. What the part touches and the temperature range it lives in — this is where wrong assumptions become field failures.
    4. Ask for the design shrinkage in the reply. The factory should state the shrinkage value the cavity will be cut against, per option if the decision is still open.
    5. Keep the full document structure. Resin is one block of the nine in the RFQ checklist — it works together with finish, tolerance and volume, not alone.

    I never argue resin preference — I argue requirements. Ask a buyer what the part touches, what temperature it lives at, whether anyone will ever paint it, and whether anything flexes, and the ABS-versus-PP question usually answers itself before we finish. The problems start when the resin column says “TBD” and the deposit is already paid; shrinkage does not negotiate, and the cavity was cut against somebody’s guess.

    — Arthur, Tooling Engineer at aCatMould

    Frequently asked questions

    Which is stronger, ABS or PP?

    For stiffness and structural feel, ABS. For fatigue, repeated flexing and chemical exposure, PP. “Stronger” without the loading case is a question that produces wrong answers — define how the part is loaded first.

    Can PP be painted like ABS?

    Not with the same ease. PP’s surface resists adhesion; painting it needs surface treatment, special primers or molded-in color instead. If paint is the plan, that is itself an argument for ABS.

    Which is better for food-contact or liquid containers?

    PP dominates containers for chemical and moisture reasons, but food-contact suitability is not something any mold factory certifies — it comes from the resin grade’s documentation and your compliance pathway. We produce to the grade and file you specify; regulatory compliance for your market stays yours.

    Does switching between ABS and PP require a new mold?

    Usually a modified one at minimum. The design shrinkage differs, so cavity dimensions, gate sizing and cooling balance all shift. Late switches are exactly the change-order situation the DFM stage exists to prevent.

    Which one is cheaper per part?

    PP, in most cases — lower resin price and lower density both work in its favor, and the lower weight compounds the saving per thousand parts. Run the cost-per-part math against actual geometry rather than comparing kilogram prices.

    Can we quote both options at once?

    Yes — that is often the right move when the decision is genuinely open. Ask for the mold to be quoted with the design shrinkage stated per option, and compare cost per part rather than sticker price. The comparison only works if both options are priced against named grades.

    Can ABS and PP scrap be reground together?

    No — the two families are incompatible and a mixed regrind produces weak, unpredictable parts. If you plan to reuse runners and sprues, the factory must run each resin separately and keep its regrind stream clean, which is one more reason the resin choice should be fixed before tooling starts.

    Key takeaways

    1. ABS for rigidity, paintable surfaces and dimensional stability; PP for chemical resistance, light weight, living hinges and cost per part.
    2. The structural reason: ABS is amorphous with predictable shrinkage, PP is semi-crystalline with higher, crystallization-driven shrinkage.
    3. Five inputs decide: mechanical role, chemical exposure, surface plan, temperature, cost per part — not brand habit.
    4. The resin is a mold input: shrinkage values, gate sizing, warpage and finish strategy all follow from it, so switching after steel cutting is expensive.
    5. Specify family, grade characteristic and color system in the RFQ, and require the design shrinkage per option in the reply.

    If your part is sitting between these two resins, send us the geometry plus the service conditions — what it touches, what temperature it sees, what finish it needs. You will get a written DFM with the shrinkage values we would design to per option, and a line-item quotation priced against named grades, from a named engineer — evidence first, numbers second, in that order.

    Sources
    MatWeb — material property database for resin and steel datasheets
    ASTM D638 — Standard Test Method for Tensile Properties of Plastics, ASTM International
    ISO 20457:2018 — Plastics moulded parts: Tolerances and acceptance conditions

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet