Why PP and PA6 MFR Numbers Cannot Be Compared Directly

A buyer sends one line: "What is the MFR of this grade?"

We usually do not answer that straight away. Not because we are holding anything back, but because the question is missing half of what it needs.

The same MFR number can describe two completely different things.

Three mistakes come up repeatedly in real conversations.

Mistake 1: comparing numbers across materials

MFR (melt flow rate) is measured to ASTM D1238. The principle is simple: fixed temperature, fixed load, extrude the material through a standard die and weigh what comes out in ten minutes.

But the temperature is not the same for every material. Each resin family has its own test temperature.

From our own specification sheets:

PP at 230 °C, PA6 at 235 °C, PC/ABS at 265 °C, PA66 at 275 °C, PC at 300 °C.

Raise the temperature and the polymer chains move more easily. The same load pushes more material through in the same ten minutes, so the number comes out higher.

Comparing the MFR of PA6 against the MFR of PP is not a comparison of flow. It is a comparison of test temperature.

To compare flow across materials properly, convert to a common temperature, or use a different indicator altogether.

Mistake 2: quoting MFR without wall thickness

MFR describes thin-wall behaviour.

The standard die is thin, roughly 0.87 to 1.03 mm. Flow resistance in a 3 mm wall part is a completely different proposition.

The same batch fills a thin small part and fails to fill a thick large one. Short shot or pressure that will not come down.

Quote MFR together with wall thickness. A 1 mm part and a 3 mm part are not asking for the same thing.

Shear is the related issue. Thicker walls run at lower shear rate, and the polymer chain stays in the tool longer, so degradation is higher. High-MFR grades tend to cope better at high shear, while lower-MFR grades discolour and embrittle more readily in thick sections.

Mistake 3: assuming higher MFR is always better

High MFR means good flow. That is useful, and it has a cost.

Too much flow means lower melt strength. Lower melt strength shows up as visible weld lines, poorer dimensional stability and shrinkage that drifts from the design intent.

Three properties have to be balanced together:

MFR high enough to fill a complex tool
Melt strength high enough for clean weld lines and stable dimensions
Shrinkage inside a controllable range so assembly gaps hold

All three have to be read together. A grade selected on MFR alone usually moulds fine and performs poorly.

What to do instead

Step one: confirm the test condition. Ask for the temperature and the load together with the number. Only figures measured at the same temperature are comparable.

Step two: state wall thickness and geometry. How thick is the part, how long is the most demanding section, are there thin runners or abrupt thickness changes.

Step three: read MFR, density and shrinkage together.

Two grades from our own RF-1331 series show why:

Going from 20% to 45% glass fibre lifts density from 1.26 to 1.46. Flow under the same test condition is comparable, but both numbers have to be read together: as density changes, the mould shrinkage compensation and the part weight both shift.

One more point on those two specification sheets: the MFR value field itself is left blank. Only the test condition is stated.

That is deliberate. MFR is sensitive to batch, fibre orientation and processing history. For some grades we lock the test condition but not the number, so that an unreliable figure is never used to judge a material.

The numbers worth pressing on are the firm ones. Density, shrinkage, ash content and tensile strength are all supplied as ranges.

Qingdao Yunsu Polymer Material Technology Co., Ltd.