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Polymer Engineering DISCUSSION
A polypropylene cover with a 3 mm wall has stiffening ribs that are also 3 mm thick. Every part shows a visible sink line on the outer face opposite each rib, and the flat cover bows by about 2 mm across its 250 mm length after it cools.
The moulder has raised the holding pressure and extended the cooling time, which helped a little but did not remove either problem. Is this something the process can solve, or does the part design have to change?
The sink marks are a design problem first. Where a 3 mm rib meets a 3 mm wall there is a local mass of material much thicker than the wall itself. The outer skin freezes quickly against the mould, the thick junction stays molten longer, and as it cools and shrinks it pulls the still-soft skin inward. Polypropylene is semi-crystalline with a high volumetric shrinkage, so it shows this more than ABS or polycarbonate would.
The usual rule is a rib thickness of 50 to 60 percent of the wall at its base, so 1.5 to 1.8 mm for your 3 mm wall, with a small root radius and 0.5 to 1 degree of draft per side. On a high-gloss visible face some designers go down to 40 percent.
What the process can do is limited by gate freeze. Holding pressure compensates for shrinkage only while the gate is still open; once it freezes, extra pressure or time does nothing. A gate-freeze study shows where that point is: mould parts at increasing hold times, weigh them, and the hold time at which the part weight stops rising is the gate-freeze time. Set the hold time just beyond it.
If the sink remains with the hold time past gate freeze, the gate or the flow path to the ribs is freezing before the thick junction does, and only a larger gate, a gate nearer the thick area or thinner ribs will fix it. Fill from thick sections toward thin ones, not the other way round.
Warpage comes from shrinkage that is not the same everywhere. For a flat cover with ribs on one side, the common causes are: the thick rib junctions shrink more and later than the wall, pulling that side shorter so the part bows toward the ribs; the two mould halves run at different temperatures, so the hotter side shrinks more; and flow orientation, because PP shrinks differently along and across the flow direction.
Polypropylene mould shrinkage is typically in the range of 1 to 2.5 percent. A difference of only 0.1 percent between the two faces of a 250 mm long, 3 mm thick plate is enough to bow it by a couple of millimetres, which is the size of what you are seeing.
Extending the cooling time treats the symptom and costs cycle time, which scales with the square of the wall thickness. A common estimate is t = (h² / (π² × α)) × ln((4/π) × (T_melt - T_mould) / (T_eject - T_mould)). With h = 3 mm, a thermal diffusivity of roughly 0.09 mm²/s for PP, melt at 230 °C, mould at 40 °C and ejection at 90 °C, this gives about 16 s. A 4 mm section under the same conditions needs about 28 s.
So the rib junctions are still hot inside when the 3 mm wall is ready to eject. Thinner ribs bring the whole part to ejection temperature together, which shortens the cycle and removes the cause of both the sink and much of the warp.
