Why this matters
Polypropylene is easy to mold but unforgiving to design around: it shrinks more than most resins, warps on large flat faces and sinks where ribs are too thick. Starting points for wall thickness, ribs, radii, living hinges and shrinkage.
Why polypropylene needs its own design rules
Polypropylene is a semi-crystalline resin. As it cools it crystallizes and shrinks more than amorphous resins such as ABS or polycarbonate, and it shrinks differently along and across the flow direction. That is why geometry that molds cleanly in ABS can warp, sink or end up out of tolerance in PP, and why the numbers below are starting points to confirm with the resin datasheet and the molder, not fixed limits.
Keep walls uniform and change thickness gradually
Published guides give a workable wall thickness for polypropylene of roughly 1 to 3 mm, with a wider window of about 0.6 to 3.8 mm also cited. Uneven walls cool at different rates and cause warpage and sink marks, so keep the nominal wall uniform and blend thick and thin sections gradually, with adjacent walls staying within about 40 to 60 percent of each other. Core out thick sections instead of leaving them solid.
Ribs add stiffness but also sink
Because PP shrinks so much, ribs should be thinner than they would be in other resins, about 40 to 50 percent of the wall they join. Above roughly 60 percent a rib tends to show as a sink mark on the opposite face, and far below 40 percent it becomes hard to fill. Keep rib height under about three times the rib thickness, add a fillet at the base of at least a quarter of the rib thickness, and prefer several low ribs to one tall one.
Use generous radii
Sharp internal corners concentrate stress and slow the flow of the melt. Internal and external radii of at least 25 percent of the wall thickness are a common minimum, and closer to 75 percent is better where space allows.
Living hinges depend on grade, thickness and gate position
Polypropylene is the standard choice for living hinges because a correctly molded hinge can flex many thousands of times without cracking. Published guidance puts the hinge at about 0.2 to 0.5 mm thick, with generous radii and a flat recess, and a homopolymer grade with high melt flow is usually recommended. The gate should sit so the melt crosses the hinge in one direction, and the hinge is often flexed soon after ejection. Prototype tools often cannot reproduce this, so confirm the hinge on production-intent samples.
Plan the shrinkage allowance from the grade, not from a rule of thumb
Typical mold shrinkage for polypropylene is quoted as about 1 to 2.5 percent, and some sources give up to 3 percent, depending on grade, wall thickness and flow direction. Filled grades shrink less than unfilled ones, and thicker walls shrink more. The effect adds up: an error of half a percent on a 200 mm dimension is about 1 mm. Use the datasheet value for the exact grade, expect the molder to adjust the cavity size after the first samples, and tolerance fit-critical dimensions with that in mind.
Control warpage in the design, not only in the process
Large flat faces are the usual warpage risk in PP. Ribs, a slight crown or curve, uniform wall thickness and a gate position that fills the part evenly all help, and uneven cooling across the mold makes it worse. If a flat part must stay flat, say so in the RFQ so the molder can plan cooling and gating from the start. Problems found at first samples are covered in the defects guide.
Choose homopolymer or copolymer by function
Homopolymer is stiffer and suits hinges and rigid caps, while impact copolymer is tougher at low temperatures and random copolymer is clearer and softer. Name the application, the temperature range and any food or medical rules so the supplier can suggest a grade instead of defaulting to the cheapest.
Design starting points for polypropylene
| Design feature | Typical PP starting point | What to check with the molder |
|---|---|---|
| Nominal wall | About 1-3 mm, kept uniform | Gradual transitions; adjacent walls within about 40-60 percent of each other |
| Rib thickness | About 40-50 percent of the wall | Above about 60 percent risks sink; far below 40 percent is hard to fill |
| Rib height and base | Height under about 3 times rib thickness; base fillet of at least a quarter of rib thickness | Tall thin ribs fill poorly and can warp |
| Corner radii | At least 25 percent of wall thickness | Sharp corners concentrate stress and slow the melt |
| Living hinge | About 0.2-0.5 mm thick, homopolymer high-flow grade | Gate position, radii and a production-intent sample |
| Shrinkage allowance | About 1-2.5 percent (up to 3 percent in some sources) | Exact grade, wall thickness and flow direction from the datasheet |
What to send for a PP molding quote
Practical takeaway
Treat these numbers as starting points: confirm shrinkage and hinge thickness against the datasheet of the exact grade, and let the molder adjust the cavity after first samples.
How to apply this guide
Use this guide as a preparation step before asking for mold price or production lead time. A stronger RFQ names the current project stage, the file format available, the expected first order, the annual volume, the target resin and the approval evidence needed after T1 samples.
If the project is still early, the guide can also show whether the next move is design cleanup, rapid tooling, production mold planning, material selection or quality-document scoping. That makes the inquiry more specific and helps the first engineering reply focus on decisions instead of basic clarification.