Thin wall polypropylene: high flow PP for cups, tubs, lids and IML containers

Which PP type for which thin-wall part

Every melt flow value on this page is MFR at 230°C / 446°F, 2.16 kg (ASTM D1238 or ISO 1133). A polyethylene melt index at 190°C / 374°F is not comparable. Caps and flip-top closures often run lower, about 10–35 g/10 min; see closure resin.

Typical published windows that match our PP grade finder. Confirm on the data sheet for the grade you run, and let a chemist check any equivalent before you switch.

Typical values from producer data sheets

These are values for specific high-flow grades. They show where each type sits on stiffness and impact. Compare grades only on the same test: ASTM and ISO results for modulus and Izod are different numbers.

Values from individual producer data sheets for specific grades (U.S., European and Asian sheets); grades vary, and some grades in the table are not sold in North America. The impact copolymer values fall inside the notched Izod window of about 80–130 J/m on our PP grade finder for freezer tubs.

Melt flow, flow length and cycle time

There is no standard definition of thin-wall. One producer's rule of thumb for packaging is a wall under about 0.6 mm (0.025 in) with a flow length more than about 200 times the wall thickness. The thinner the wall and the longer the flow, the higher the MFR you need to fill it at a pressure your press and mold can hold.

• Higher MFR fills further. It lowers injection pressure, fills thinner walls and longer flow paths, and lets the part cool sooner. Within a family, impact often drops as MFR rises.
• Cycle times are short. Thin-wall cups, tubs and lids often cycle in a few seconds in high-cavitation molds. Nucleation, mold cooling and the press decide how close you get.
• Run hot, but not too hot. High-flow grades often run toward the top of the PP melt range on our processing chart (190–274°C / 374–525°F for homopolymer). Overheating raises odor and taste, especially with clarified grades.
• MFR spread matters as much as MFR. A lot at the top of its window and the next at the bottom can change fill, short shots and part weight. Ask for the MFR range on the COA, and see melt flow index explained.
• Short shots and flash on a new lot often trace back to MFR. See short shots.

Stiffness, impact and the freezer

• Stiffness lets you take weight out. A stiffer grade holds top load and stacking strength with a thinner wall. Put a flexural modulus floor on the RFQ so a substitute does not make a lid or tub floppy.
• Homopolymer turns brittle near 0°C / 32°F. It suits chilled and room-temperature packs that are not dropped cold.
• Freezer packs need impact copolymer. Ask for notched Izod or Gardner impact at the coldest temperature the pack sees, by the same method, or drop-test filled packs at that temperature. A 23°C / 73°F value says little about a freezer.
• Clear and frozen is a hard pair. Clarified random copolymer is less tough in the cold than impact copolymer. If a clear pack goes in the freezer, ask the producer whether the grade is rated for it and test filled parts.
• Stress whitening. Impact copolymer can whiten where it is bent or struck, which shows on lids and hinges.
• Microwave and hot fill. Homopolymer has the most heat resistance. Random copolymer melts lower, about 140–150°C / 284–302°F against 160–165°C / 320–329°F. Check the FDA letter's conditions of use for the temperature.

Nucleated, clarified, antistatic and slip

• Nucleated. A nucleating agent raises the crystallization temperature, so parts set up sooner and the cycle shortens. It also raises stiffness and changes shrinkage. A swap between nucleated and non-nucleated grades can move dimensions and lid fit, so treat it as a material change.
• Clarified. A clarifying agent makes thin walls near-transparent. Haze rises with wall thickness, so ask for haze by ASTM D1003 at a stated thickness. A plain random copolymer is translucent, not clear. See clear PP random copolymer.
• Antistatic. An antistat blooms to the surface so containers collect less dust on the shelf. It works best some time after molding.
• Slip and release. Slip agents help lids and cups denest and release from the mold. They can also change how a label or print adheres, so tell us if you print or label.
• Color. Some pigments nucleate PP and change shrink and cycle. Approve each color on molded parts, and keep the masterbatch carrier and let-down ratio fixed. See masterbatch vs. precolored resin.

Controlled rheology vs. reactor grades

There are two ways to make high-flow PP. A reactor grade is made at high MFR in the polymerization reactor. A controlled-rheology (CR) or visbroken grade starts as a lower-MFR resin and is broken down with peroxide in an extruder to raise the flow and narrow the molecular weight distribution.

• CR grades fill evenly and tend to warp less, at a little less stiffness, because the peroxide cuts the longest chains and narrows the distribution. Peroxide by-products can add odor and taste, which matters for water, dairy and other sensitive foods.
• Reactor grades avoid the peroxide step. Producers often sell them for low odor and taste and for stiffness, and some high-flow reactor grades are nucleated for fast cycles.
• Why it matters when you switch. Two grades at the same MFR can fill, shrink and taste differently if one is reactor-made and the other is CR. Ask which route the grade uses, and test parts on your product, not just pellets.

IML containers

• What IML is. In-mold labeling places a printed label in the mold before the shot, so the label becomes part of the container wall. A PP film label on a PP container keeps the pack one material for recycling.
• Resin and label work together. The label holds heat on its side of the wall, so that side cools slower. That can add warp and cycle time. Nucleated, stiff grades help. Tell us the label film and supplier.
• Flow behind the label. IML walls are often the thinnest in packaging, so flow and MFR consistency matter even more.
• Cold impact still counts. IML ice cream and freezer tubs need impact copolymer, the same as unlabeled ones.
• Process on the RFQ. IML, stack molds and high cavitation change what grade fits. Say what you run.

Food contact and recycled content

• FDA letter for the exact grade. PP food-contact resins are covered by 21 CFR 177.1520 (olefin polymers). Ask the producer for its letter by grade name. It states compliance and the conditions of use, meaning food types and temperatures such as hot fill, microwave or freezer. A similar grade from another plant needs its own letter.
• Additives and colors. Nucleators, clarifiers, antistats and slip need their own clearances. The producer's letter covers the grade as sold. Colorants and additives you add at the press need their own documents. See FDA food-contact compliance.
• Canada and export. A Canadian customer may ask for a Health Canada letter of no objection, and EU sales need a Regulation (EU) 10/2011 declaration.
• Recycled PP in food packs. For post-consumer recycled PP in food contact, customers expect the recycler to hold an FDA letter of no objection for its process, covering PP and the conditions of use. Food-grade rPP supply is limited, and thin walls show specks, gels and odor first. Some producers offer certified circular grades allocated by mass balance from chemically recycled feed; ask your customer whether that counts.
• State laws. Several states now have packaging producer responsibility laws, and some set minimum post-consumer content for certain containers. Rules differ by state and product and step up over time, so check what applies to your pack. See packaging resin sourcing and adding recycled content.
• Molding with PCR. Start with a low blend and step up while the part passes. See molding with PCR resin.

What to put on the RFQ, and how it is priced

Thin-wall PP follows the propylene-based contract market, and the type and additives add steps on top: impact and random copolymer often cost a little more than homopolymer, and clarified grades more again. See polypropylene price per pound for the drivers. Large thin-wall molders often buy railcars on producer contracts. We can still help with trial lots, a qualified second source and cover during an outage or allocation. We do not post prices. We quote a specific grade, quantity, package and ship-to city through our supply partners, from one gaylord (about 1,000 to 1,600 lb) up, and every prime lot from our suppliers ships with a COA.

• The grade you run now, with its data sheet and three to six recent COAs if you have them.
• Type, in words. Homopolymer, impact copolymer (and impact level) or clarified random copolymer.
• MFR range, at 230°C / 446°F, 2.16 kg, and the range your mold can run.
• Additive package. Nucleated, clarified, antistatic, slip, and whether the grade is reactor-made or controlled rheology.
• Stiffness and impact. A flexural modulus floor, and impact at your coldest use temperature by a stated method.
• Clarity. Haze by ASTM D1003 at your wall thickness, for clear parts.
• Documents. The FDA letter with its conditions of use, low odor and taste needs, Health Canada or EU 10/2011 if needed, and any recycled-content claim.
• Process. IML or not, cavitation, stack mold and target cycle time.
• Quantity, package and ship-to. Gaylord, bulk truck (about 40,000–50,000 lb) or railcar (about 180,000–210,000 lb), yearly use and the city. See send an RFQ package.

Common questions

If your grade is not listed, tell us what you run. We source commodity, engineering and specialty resins.