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In PP injection molding, selecting the right MFI is essential because it directly affects mold filling, surface quality, and production stability. However, a higher MFI is not always better, and there is no single value suitable for every product. Each application requires a different MFI range depending on wall thickness, mold design, and mechanical performance requirements.

So, what MFI is suitable for PP injection molding, and how can manufacturers make the right selection from the beginning? This article explains what MFI means, provides reference ranges for different applications, and outlines the key technical factors that influence material selection.

1. What is MFI?

MFI, short for Melt Flow Index, indicates the mass of molten plastic that passes through a standard die within 10 minutes under specified temperature and load conditions. It is commonly expressed in g/10 min. In technical data sheets, this value may also be listed as MFR, or Melt Mass-Flow Rate. When measured under the same conditions, a higher MFI generally indicates that the material flows more easily, while a lower MFI usually corresponds to higher melt viscosity.

Read more: Significance of Melt Flow Rate - Melt Flow Index in plastics processing

For PP, MFI is commonly measured at 230°C under a load of 2.16 kg, following ASTM D1238 or ISO 1133. However, MFI only reflects the material’s flow behavior under standardized test conditions. It cannot fully describe how molten PP behaves inside an actual injection molding system, including the screw, nozzle, runner, gate, and mold cavity.

Therefore, MFI should be used as a tool for material screening and quality control rather than the sole criterion for determining whether a PP grade is suitable for an injection-molded product.

International standards for MFI measurement

2. What MFI is suitable for PP injection molding?

The MFI range of PP used in injection molding is relatively broad. Thick-walled products with short flow paths generally use materials with low to medium MFI. In contrast, thin-wall containers, multi-cavity molds, and parts with long flow paths often require PP with a higher MFI so that the melt can fill the cavity before solidifying.

The following table can be used as an initial material selection guide:

These ranges are intended only for preliminary screening. They are not mandatory limits specified by ASTM D1238 or ISO 1133. The final MFI should be selected based on the material’s Technical Data Sheet, mechanical performance requirements, and actual mold trial results.

2.1. MFI of 5–15 g/10 min for thick-walled parts

PP with an MFI of approximately 5–15 g/10 min is often considered for products with relatively thick walls, large dimensions, or requirements for toughness and impact resistance. Because the melt flows through a larger cross-sectional area, it does not need to travel through an extremely narrow section before solidifying. Therefore, the material does not necessarily require very high flowability.

However, manufacturers should not assume that every thick-walled product requires a low MFI. A large part with long flow paths or multiple changes in wall thickness may still require a medium-flow PP grade to keep injection pressure within an acceptable range.

2.2. MFI of 8–25 g/10 min for general injection molding

An MFI range of 8–25 g/10 min is a suitable starting point for many common PP products, including containers, trays, cups, household goods, and medium-thickness components.

This MFI range often provides a practical balance between mold-filling performance and mechanical properties. The material can flow consistently without requiring excessively high temperatures or injection pressures. At the same time, it is generally less sensitive to flashing than some very-high-flow grades.

During testing, manufacturers should begin with a medium-MFI PP grade and then monitor filling pressure, cycle time, and finished-product quality. This approach is more reliable than immediately selecting a very-high-MFI grade simply to reduce injection pressure.

2.3. MFI of 20–40 g/10 min for complex parts

When a product has multiple flow-direction changes or areas located far from the gate, the material must maintain its flow for a longer period. An MFI of 20–40 g/10 min can improve mold filling and reduce the risk of short shots at the end of the flow path. This range is often considered for structurally complex caps, small technical parts, heavily ribbed products, or molds requiring relatively fast cycle times.

However, short shots are not always caused by low MFI. Before changing materials, technicians should inspect the gate size, runner balance, nozzle temperature, injection speed, and mold-venting system.

Switching to a higher-MFI PP grade may temporarily improve cavity filling, but it will not correct the underlying problem if the gate is too small, the runner system is unbalanced, or air cannot escape from the mold efficiently.

2.4. MFI of 30–100 g/10 min for thin-wall injection molding

Thin-wall products lose heat rapidly when the molten plastic comes into contact with the mold surface. If a solidified layer forms too early, the available flow channel becomes narrower, increasing the pressure required to fill the cavity.

For this reason, thin-wall containers and packaging commonly use PP with an MFI of 30–100 g/10 min. The exact value depends on several factors, including:

• Minimum wall thickness
• Flow length
• Number of mold cavities
• Gate size
• Injection speed
• Target production cycle

An MFI above 50 g/10 min is often considered for products with extremely thin walls, long flow paths, or multi-cavity mold systems. Nevertheless, high flowability does not automatically guarantee a high-quality product. The selected PP grade must also meet requirements for stiffness, impact resistance, sealing performance, hinge durability, and dimensional stability.

3. Reference MFI ranges for common PP injection-molded products

Based on the typical characteristics of commercially available PP grades, the following table can help narrow the initial material selection range:

These values are intended as starting points rather than fixed specifications. Even products within the same category may require different MFI ranges because of differences in wall thickness, gate position, mold size, cycle-time targets, or performance requirements.

4. Conclusion: Which MFI should be selected for PP injection molding?

There is no single MFI suitable for every PP product. An MFI range of 8–25 g/10 min is a practical starting point for many general injection molding applications. Thick parts or products requiring impact resistance may favor a lower range of approximately 5–15 g/10 min. Thin-wall products and parts with long flow paths often require a higher MFI of approximately 30–100 g/10 min.

More importantly, MFI must be evaluated together with the PP type, mechanical properties, mold design, machine capability, masterbatch dosage, and recycled-material content.

A Technical Data Sheet can help narrow down the available options, but actual mold trials provide the most reliable basis for approving a material for mass production. The best PP grade is not necessarily the one with the highest MFI, but the one that delivers stable processing, consistent product quality, and the required mechanical performance.

5. EuroPlas – Material solutions for PP injection molding

EuroPlas supplies a wide range of plastic material solutions for different processing technologies, including injection molding. Its product portfolio includes filler masterbatch, color masterbatch, additive masterbatch, engineering plastic compounds, and bioplastics. Selecting a compatible carrier resin and the appropriate masterbatch dosage is important for maintaining dispersion, flowability, color consistency, and production stability.

EuroPlas PP filler masterbatch is manufactured from CaCO₃, PP carrier resin, and a suitable additive system. The formulation can also be adjusted to meet the technical requirements of different injection-molded products.

Contact EuroPlas for support in selecting the right masterbatch, dosage rate, and material formulation for your PP injection molding applications.