PP vs. ABS — How Do You Decide Which Resin to Use for Your Injection Molded Housing, and What Are the Key Differences in Processability and End-Use Performance?
Last updated: 2026-09-03 · Reading time: about 5 min
PP and ABS are the two default housing resins, and they fail in opposite ways. Choose ABS when the part must survive a drop and be painted; choose PP when it must resist chemicals, flex as a living hinge, or stay light. Cost alone should not decide it.
See also related article.
Key facts
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Parting line — The line where the two mold halves (A-side and B-side) meet. Parting line location affects: gate placement, ejector pin marks, witness lines, and cosmetic appearance. A well-designed parting line minimizes visible witness marks. See also related guide.
What are the key property differences — and how do they affect part performance?
The two resins sit at opposite ends on impact, chemical resistance, weight and surface energy. The table is the selection baseline.
| Property | PP (homopolymer) | ABS (general purpose) |
|---|---|---|
| Tensile strength | 32–38 MPa | 40–45 MPa |
| Flexural modulus | 1,400–1,800 MPa | 2,000–2,500 MPa |
| Impact (Izod, notched) | 25–50 J/m | 180–250 J/m |
| Heat deflection (0.45 MPa) | 90–105°C | 90–100°C |
| Density | 0.90 g/cm³ | 1.05 g/cm³ |
| Shrinkage | 1.0–2.5% high, anisotropic | 0.4–0.7% low, isotropic |
| Chemical resistance | Excellent (acids, bases, solvents) | Poor (ketones, esters) |
| Surface energy | Low 29–31 dyne/cm — hard to paint | High 38–42 dyne/cm — easy to paint |
| UV resistance | Poor (needs stabilizer) | Poor (needs stabilizer) |
Four differences drive the decision. Impact: ABS is 5–8× tougher, so a housing that can be dropped goes to ABS. Chemicals: PP resists the cleaners and oils that attack ABS. Finish: ABS accepts paint and adhesive as-molded; PP needs corona or flame treatment first. Weight: PP is 15% lighter, which matters for handheld and portable parts.
How do the processing requirements differ when you switch between them?
Processing is where the two resins behave most differently — and where a mold cut for one cannot simply run the other.
| Parameter | PP | ABS |
|---|---|---|
| Melt temperature | 200–280°C | 210–270°C |
| Mold temperature | 30–60°C | 50–80°C |
| Drying | Not required (does not absorb moisture) | Required (80°C, 2–4 h, max 0.1% moisture) |
| Shrinkage | 1.0–2.5% — larger mold dimensions | 0.4–0.7% — tighter control |
| Warpage tendency | High (anisotropic crystallization) | Moderate (isotropic shrinkage) |
The mold must be cut for the resin you name, not the one you might use later. PP shrinks 2–4× more than ABS. Run PP in an ABS-sized cavity and the part comes out undersized; run ABS in a PP-sized cavity and it comes out oversized. Lock the resin before the steel is cut.
Drying is the other split. Wet ABS produces splay and loses impact strength, so it must be dried; PP absorbs no moisture and skips drying entirely — an advantage in humid shops without drying equipment.
“We ask which resin the housing will run before we quote, because PP and ABS shrink differently and cannot share a cavity. If you are undecided, we mold the flow study on both and show you where each one warps — then you choose against evidence, not a datasheet.”
— Project Engineer, aCatMould
Frequently asked questions
Can you use a PP/ABS blend instead of either resin alone?
Yes.
A 70/30 to 50/50 PP/ABS blend pairs the chemical resistance and low cost of PP with the impact and finish of ABS — roughly 2–3× the impact of PP alone at 10–20% below the cost of ABS.
The catch: PP and ABS are immiscible, so the blend needs a compatibilizer (usually maleic-anhydride-grafted PP). Without it the two phases separate under impact and the blend is brittle. It is common in automotive trim, appliance housings and power-tool bodies.
Which is more cost-effective per part — PP or ABS?
PP runs 20–30% less per kilogram ($1.20–1.80 vs $1.80–2.50/kg). On a 100 g housing that is about $0.06–0.07 less material per part.
But total cost includes processing. ABS needs drying yet paints as-molded; PP skips drying but needs corona treatment before painting ($0.02–0.05/part). Net, PP saves about $0.03–0.05/part — and if the part is left natural, the gap narrows to $0.01–0.02.
Which resin should you use for a living hinge — can ABS do it?
PP only. A PP living hinge withstands 100,000+ flex cycles; ABS cracks after 1,000–5,000. The hinge must mold in a single shot so the molecules orient along the flow — thickness 0.25–0.5 mm, length 3–5 mm.
If the housing needs ABS impact and a hinge, use a mechanical hinge — two parts joined by a pin or snap-fit — rather than a molded living hinge.
Key takeaways
- ABS wins on impact, rigidity and paintability; PP wins on chemical resistance, weight and living-hinge fatigue.
- Processing splits on drying and shrinkage — PP needs no drying but shrinks 2–4× more, so the cavity must be cut for the named resin.
- PP saves about $0.03–0.05/part, but a PP/ABS blend or a split-material design covers cases that need both property sets.
Decide PP versus ABS on the application, not the price per kilo. Impact, paint and tight dimensions point to ABS; chemical contact, low weight and a living hinge point to PP. When you need both, a compatibilized blend or a two-material design separates the functions.
Sources
MatWeb — material property database, reference for PP and ABS mechanical and thermal data
Plastics Technology — resin selection and drying practice for injection molding
LyondellBasell — polypropylene grade selection and property data
SpecialChem — polymer property database for PP/ABS blends and compatibilizers
DuPont — injection molding part design guidelines
Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet