Polypropylene molds on a standard screw injection machine, with no drying for unfilled grades, at a melt temperature that producers put between about 200 °C and 260 °C, into a cool mold, with a fast fill and enough pack and cooling time to control a mold shrinkage of about 1–2 %.
PP crystallizes as it cools, so it shrinks more than amorphous resins, and its low melt viscosity fills thin walls and also flows into any gap at a parting line or vent. Where a drawing calls for tight tolerances or routine painting, weigh PP against ABS before the tool is cut.
Molded PP resists solvents, so printing, painting or bonding needs a surface treatment first.
Does Polypropylene Need Drying
Unfilled polypropylene does not need drying before molding. INEOS’s polypropylene processing guide states that pre-drying is not necessary under normal conditions, and Borealis’s data sheets for its unfilled grades ask only that the resin be stored dry and below 50 °C.
Filled grades are the exception: Borealis’s glass-fiber and mineral-filled polypropylene compounds call for pre-drying of about 2 hours at 80 °C.
Splay is the usual sign of wet resin, but INEOS’s troubleshooting table also traces it on PP to a sweating mold, a disorderly flow front and short packing. Check the mold face for condensation before adding a drying step.
Process Settings for Polypropylene
Barrel temperature and injection pressure matter most when molding polypropylene; INEOS calls them the two most important variables a molder has.
Melt Temperature
Mold polypropylene at the melt temperature its producer lists for the grade: Formosa’s PE/PP injection guide gives a melt temperature of 400–470 °F (200–250 °C), and Borealis’s unfilled injection grades typically fall between 210 °C and 260 °C.
Formosa’s window is the melt itself. Take an air shot, read it with a needle probe, and adjust the barrel zones, screw speed and back pressure until the melt lands in range.
INEOS quotes cylinder set points instead, usually 400–525 °F (204–274 °C), so a barrel display and a probe reading cannot be checked against the same number. INEOS’s rule for setting the cylinder starts from the part: run it 25–50 °F (14–28 °C) above the minimum temperature that fills the part, never above 570 °F (299 °C), with the feed zone 30–50 °F (17–28 °C) cooler than the nozzle.
At Borealis, copolymer does not need a hotter melt: at the same MFR, its impact and random copolymer windows start 10–20 °C lower than its homopolymers’ and share their upper limit.
Mold Temperature
Polypropylene runs in a cool mold: for unfilled grades the INEOS, Formosa and Borealis windows together span 10 °C to 66 °C, and each Borealis grade’s own window is narrower.
Within the window, a warmer mold gives better surfaces and fewer flow marks, weld lines and brittle parts; a cooler one reduces shrinkage, warpage, sinks and cavity sticking. Keep both mold halves at the same temperature, and keep the mold above the dew point so it does not sweat.
Injection Pressure and Speed
Fill polypropylene fast, at whatever pressure the part needs to fill and pack out: Formosa sets injection speed as fast as possible without cosmetic defects, and Borealis marks injection speed high on its unfilled grades.
INEOS gives the usual polypropylene injection pressure as 1,000–1,500 psi (6.9–10.3 MPa), with best results at up to about 75 % of press capacity, but its guide does not say whether that is hydraulic or plastic pressure.
A hydraulic press multiplies its gauge pressure by its intensification ratio to reach the plastic pressure at the nozzle; an electric press reads plastic pressure directly. When a PP mold moves between presses, carry over the plastic pressure.
Holding Pressure and Time
Hold polypropylene until the gate freezes, at a holding pressure Formosa puts at 50–75 % of injection pressure. Borealis lists 200–500 bar (20–50 MPa) of holding pressure on its unfilled grades as the minimum to avoid sink marks, without saying whether that is hydraulic or plastic pressure.
Pack controls part size on PP. INEOS’s fix for parts that come out small is more pack pressure or pack time, and for parts that come out large, less.
Back Pressure and Screw Speed
Run polypropylene at low back pressure and moderate screw speed: INEOS recommends a back pressure of 50–100 psi on the gauge (about 3.4–6.9 bar), and Formosa pairs 50–200 psi of back pressure with a screw speed of 60–125 rpm.
Raise back pressure only when the melt needs more shear for melting or pigment mixing. If recovery runs long, raise screw speed, lower back pressure or warm the rear zones.
Cooling Time
A polypropylene part needs cooling time in proportion to the square of its thickest wall, so doubling the wall quadruples the time in the mold.
INEOS recommends cooling a polypropylene part to about 130 °F (54 °C) before ejection.
Machine and Screw Setup
Polypropylene runs on standard injection machines: INEOS says no alterations are needed, and Formosa specifies a single-stage, general-purpose screw. The producers’ starting figures for sizing the press:
| Setting | Starting figure | Given by |
|---|---|---|
| Clamp force | 2 tons per in² of projected cavity area; at least 4–6 tons per in² for parts with integral hinges | INEOS guide |
| Shot size | 40–60 % of barrel capacity | Formosa PE/PP guide |
| Cushion | 0.25 in (6.35 mm) maximum | Formosa PE/PP guide |
| Screw L/D | 20:1 to 24:1 | Formosa PE/PP guide |
| Screw compression ratio | 2.5:1 to 3:1 | Formosa PE/PP guide |
How Much Polypropylene Shrinks
Unfilled polypropylene shrinks about 1–2 % in the mold. Borealis prints 1–2 % on its unfilled homopolymer and copolymer data sheets, depending on wall thickness and molding conditions, and INEOS’s property table puts copolymer slightly below homopolymer.
Wall thickness has the largest effect: in a study of eight PP grades, shrinkage rose by 0.001 mm/mm for each 0.4 mm of added wall and fell by 0.001 mm/mm for each 6.9 MPa of added cavity pressure.
Shrinkage also continues after ejection: in a 2021 study of 4 mm polypropylene bars from a 20 °C mold, shrinkage rose from 1.38 % one hour after molding to 1.56 % after three weeks, so measure parts once they have aged.
For tolerances, one PP design reference gives ±0.007 in on a 1-in dimension of a 0.125-in-thick part as commercial and ±0.0043 in as fine; the current standard for molded-part tolerances is DIN ISO 20457.
Uniform walls, and ribs and bosses thinner than the wall they join, keep shrinkage even.
Living Hinges
A polypropylene living hinge is molded thin, filled by a single melt front that crosses the hinge at right angles to its length, and flexed while still warm from the mold.
Trade guidance puts living-hinge thickness at about 0.23–0.38 mm (0.009–0.015 in).
Gate so the melt reaches the whole length of the hinge at the same moment. When one gate cannot fill evenly, place gates on both sides of the hinge so the fronts meet away from it.
Hinged parts need INEOS’s higher clamp factor and a stiff mold: mold plates that flex by only 0.001–0.002 in have a drastic effect on hinge performance.
Give the hinge its own temperature-control channel. INEOS’s guide notes that the hinge channel is often run hot, at 160–180 °F (71–82 °C), while hinge-design guidance in the trade press cools the hinge fast; decide between them by bend-testing the first shots. Bending each hinge a few times as it leaves the mold adds orientation and flex life.
Both homopolymer and copolymer make durable hinges: in a published comparison, both gave a longer bending life than an elastomer-modified compound. Copolymer’s better low-temperature impact makes it the safer choice for a hinge that will see cold service.
Molding Filled Polypropylene
Glass- and talc-filled polypropylene runs at much the same melt temperature as unfilled grades, but it needs pre-drying, a warmer mold and gentler screw settings, and it shrinks less. In Borealis’s data, most filled grades keep the 220–260 °C melt window of its unfilled homopolymers; what changes is:
- Mold temperature: 30–50 °C on most filled sheets and 40–80 °C for long glass, where several unfilled grades run at 10–30 °C.
- Screw and fill: back pressure and screw speed low to medium, and a flow-front speed of 100–200 mm/s on the glass grades.
Run long-glass grades to keep the fibers long: melt the pellets quickly with little work in the screw, and keep the flow smooth.
Fillers cut shrinkage, glass most of all, but glass cuts it unevenly: in a 2019 study of 1.8 mm plaques at 20 wt % filler, glass-filled PP’s difference between flow and transverse shrinkage was about four and a half times talc-filled PP’s.
How Much Regrind and Recycled Polypropylene to Add
There is no fixed share of regrind or recycled polypropylene: plants run anything from none, the usual answer for medical parts, to 100 %, and INEOS’s guide refers to 30–40 % fed back at a constant rate.
Three habits keep a regrind blend predictable:
- Constant share: a fixed ratio keeps the blend’s MFR steady from shot to shot.
- Fines: screen them out, because they show up as specks.
- Melt-flow check: test the blend; a 2016 study of PP regrind treated a melt volume rate more than 1.5 times the virgin value as a sign of molecular breakdown.
To set a plant’s own ceiling, mold parts from virgin resin and again after five regrind passes; if the fifth-pass parts still meet spec, allow three passes for margin.
Post-consumer recycled PP is a different material from in-plant regrind, with its own MFR and contamination, and is qualified as a new grade.
Additives Added at the Press
Color masterbatch and nucleating or clarifying masterbatch are the additives most often added to polypropylene at the press, with UV stabilizer joining them for outdoor parts. For an outdoor part the stabilizer can come in the same pellet as the color, as in a weather-resistant color masterbatch.
Antistats often come compounded into the grade and need nothing at the press.
Color Masterbatch
Polypropylene is colored at the press by blending a color masterbatch into natural resin at a fixed let-down ratio; our custom color masterbatch is made for let-downs of 1:20, 1:25 or 1:50 (5 % to 2 %), confirmed against the batch COA. Check the carrier as well as the ratio: a concentrate on a carrier incompatible with PP shows up as brittle parts.
Some pigments also change part size. Quinacridone and DPP pigments, and phthalocyanine blues and greens, nucleate PP and make it shrink more, so on a tight dimension a color change can need its own first-article check. A nucleated base resin keeps shrinkage consistent across colors.
Molding Nucleated and Clarified Polypropylene
Nucleating and clarifying agents make polypropylene crystallize at a higher temperature, so the part sets sooner and holding and cooling time can come down; clarified grades also need a melt hot enough to dissolve the clarifier.
In a 2025 study, 0.2 wt % of the sorbitol clarifier DMDBS raised polypropylene’s onset crystallization temperature, but at 0.7 wt % the crystallization temperature fell back slightly, so a higher dose does not shorten the cycle further.
Our MZT103 transparency masterbatch, added at 1–3 %, needs a melt above 230 °C; molders who run a standard 200–220 °C profile and get incomplete clarity usually have a dispersion problem rather than a dosage problem. Going hotter than needed costs crystallinity, stiffness and impact strength.
Clarified grades do not share one temperature window, so take the temperature from the grade’s own sheet. Clarifier powder such as CA-61S, at 0.15–0.25 %, is compounded in on an extruder, not dosed at the press.
Common Defects and Fixes
INEOS’s and Formosa’s troubleshooting tables for polypropylene center on short shots, flash, sticking, warpage, sinks, splay and brittle parts. On the tool side, INEOS’s guide accepts any gate type as long as the gate is at least half the maximum wall thickness and no less than 0.030 in (0.76 mm) across, and cuts vents 1 mil (0.025 mm) deep at the land, flaring to 5 mils (0.127 mm).
Warpage
Stop a polypropylene part warping by cooling it evenly and fully before ejection and by cutting molded-in stress, which INEOS does with a hotter mold, lower pressure and a moderate fill rate.
- Uneven cooling: keep the A and B halves at the same temperature and remove hot spots in the mold.
- Ejection too early: lengthen cooling.
- Gating: a center gate on a rectangular part warps opposite corners in opposite directions, so gate for flow in one direction, preferably from the end.
- Fill speed: INEOS and Formosa both slow the fill when warpage is the problem, though INEOS ranks fill time as a minor factor in warpage next to cooling and gating.
Sink Marks and Voids
Sinks and voids on polypropylene mean a thick section shrank without enough melt packed in behind it, so pack it harder and longer: raise holding pressure and time and keep a steady cushion. Lowering melt and mold temperature also helps.
If packing does not clear them, the fix is in the tool: open the gates, gate into the heavy section, and thin ribs and bosses. Formosa flags walls thicker than 0.25 in (6.4 mm) as prone to voids.
Flash
Stop flash on polypropylene by checking clamp force first, then reducing injection pressure, injection time and barrel temperature. On the tool, clean the faces and check the shutoffs and parting line; vents cut deeper than PP needs get welded up.
A resin with too high a flow for the setup also flashes, so check the lot’s MFR when flash starts after a resin change.
Short Shots
Fix a polypropylene short shot by adding shot size, injection pressure and time, and barrel temperature, then by opening gates, runners and vents when the press settings run out. When a job starts shorting with no setting changed, suspect the resin: Formosa lists melt-flow variation in the resin as a cause, and INEOS lists a flow too low for the setup.
Sticking in the Mold
Polypropylene sticks in the cavity when it is overpacked or undercooled, so cut pack pressure or time and lengthen cooling; it sticks on the core when it has shrunk on, so shorten the cycle and eject sooner. Polish, more draft and removing undercuts fix what settings cannot.
Mold release spray should not be needed: INEOS’s guide finds it cheaper to correct the tool than to lose decorating ability and cycle time to release agent. An antistat in the grade also acts as an internal release.
INEOS lists shrinking onto the core, and sticking when parts are not ejected fast enough, among the drawbacks of nucleated PP, so a switch to a nucleated grade can start core sticking on a tool that ran clean. Some nucleated grades carry a demolding additive to offset it.
Brittleness
A molded polypropylene part comes out brittle when the resin has degraded, the part carries molded-in stress, the color concentrate is wrong for PP, or the grade’s flow is too high for the part.
- Degradation: too hot a melt or too long in the barrel; lower the barrel temperature or the residence time.
- Molded-in stress: from overpacking or too cold a melt; reduce pack pressure or time, use a moderate fill rate and raise the barrel temperature.
- Color concentrate: an incompatible carrier, poor dispersion, or a nucleating pigment.
- Resin flow: higher-flow grades have lower molecular weight, so INEOS advises the lowest flow that fills the part.
- Part design: sharp corners, and knit lines in load-bearing areas.
Black Specks and Yellowing
Black specks and yellowing on polypropylene both come from degraded resin, so the fix is less heat and less time in the barrel: lower melt temperature, back pressure and screw speed, and clear the places where material hangs up, such as the screw flights, check ring and hot runner. Contamination and fines in regrind also show up as specks.
For yellowing, also lower the regrind share, because regrind has already lost part of its antioxidant on earlier passes.
Reading the Resin Datasheet Before Setup
The lines on a polypropylene grade’s datasheet that set the molding window are the processing table (melt and mold temperature, holding pressure, injection speed, any drying note) and the MFR with its test condition, which ranks the grade’s flow without setting a temperature by itself.
Polypropylene’s MFR is measured at 230 °C under a 2.16 kg load, by ISO 1133 or ASTM D1238. A melt-flow figure measured under another load or temperature cannot share a column with it.
INEOS’s guide uses MFR to choose a grade for a setup: take the lowest flow that fills the part at a melt of 400–450 °F (204–232 °C) and 1,000–1,500 psi.
Which family and MFR to buy follows from wall thickness, flow length and the impact the part must take, covered for the three main PP grades, thin-wall parts and thick-wall automotive parts.
Food-contact clearance is set per grade: polypropylene for food contact in the US falls under 21 CFR 177.1520, and the grade’s own regulatory statement says whether it is registered for food contact or medical devices.
Changing Resin Lot or Supplier on a Running Mold
When the polypropylene lot or supplier changes on a running mold, re-check the minimum fill temperature, part weight, critical dimensions and release from the core, because a matching MFR on the certificate does not mean the new resin fills the same way.
Start from the certificate of analysis, but read its MFR against the test’s own scatter. In an interlaboratory comparison, MFR results on one polypropylene differed by up to 7 % within a laboratory and up to 21 % between laboratories at 95 % confidence, so a COA value and an incoming test can differ that much with no change in the resin.
Producers set the MFR band for each grade and print it on the COA; there is no common industry tolerance to assume across producers.
On the press, re-find the minimum fill temperature and reset the barrel margin above it, then weigh parts, measure the critical dimensions and correct size through pack pressure and time.
Watch the pressure at transfer as well: with velocity control holding fill time constant, resin with shorter chains, such as a blend carrying more regrind, shows up as lower transfer pressure.
A change between a nucleated and a non-nucleated grade shifts shrinkage, cycle and release, so treat that switch as a new grade with its own first-article check.