Why a single measurement only partially reflects process capability in injection molding

One value that gets cited often in the plastics industry is the melt mass-flow rate, or MFI — or its counterpart, the melt volume-flow rate, MVR. Both belong to the standard toolkit for assessing how a material flows. And yes, they’re genuinely useful for a quick incoming goods inspection: the plastic is heated to a defined temperature, loaded with a fixed weight, and pushed through a small die. Whatever comes out in ten minutes is measured — and that gives you a comparative value that reliably picks up on batch-to-batch variation.

The trouble is, that’s exactly where the limits of the MFI value begin.

Plastics are, as a rule, shear-thinning when they flow. Put simply: the more shear, the thinner the melt becomes. How much shear actually occurs depends heavily on the processing method. Extrusion tends to operate in the lower shear range, while injection molding works at much higher shear rates. This shows up clearly in a typical viscosity curve — take PMMA at 260 °C, for example: at a low shear rate, viscosity sits above 250 Pa s, but at nearly 6,000 1/s it drops below 40 Pa s. Same material, two completely different flow states, depending on where you measure.

Limits of the MFI Value in the Injection Molding Process

And this is where the real problem lies: the test weight in the MFI test generates a fixed, but comparatively low, shear force. So the measurement only captures the lower shear range — precisely where viscosity is naturally high. For the real conditions in injection molding, where shear rates run far higher, this value simply doesn’t say much. You get a number, but no reliable indication of how the material actually behaves in the process.

Anyone who genuinely wants to assess a material’s process capability can hardly avoid producing a flow spiral. It’s made under real conditions, directly on the injection molding machine, and the flow length can be read off precisely on a scale. Because the actual process parameters are factored in here, the result is far more meaningful than a plain MFI value.

Just how big the difference can be is shown by one example that surprises a lot of people: even a 5% change in the MFI can translate into a flow-length improvement of up to 50% on the flow spiral. In the end, the MFI value is just a single point on an asymptotic curve. Nothing more — but nothing less, either.

Why does this topic matter to us? The bFI technology delivers a genuine flow boost — without compromising mechanical properties. Yet this benefit often isn’t fully captured by the MFI value alone, and the advantage for our customers, despite its significance, lies hidden within these physical characteristics. So please don’t hesitate to reach out to us at any time — let’s start the conversation!

In plastics and film processing, per- and polyfluoroalkyl substances (PFAS) and silicones have long been part of the standard toolkit of process aids. A closer look at both substance groups shows why that’s starting to change.

Proven tools with well-known drawbacks

As so-called Polymer Processing Aids (PPA), PFAS improve melt flow behaviour, prevent melt fractures such as the notorious “sharkskin” effect, and stabilise the extrusion process, for example in PE film production. Silicones, or siloxanes, are mainly used in injection molding as release agents. They allow for gentle demolding of complex geometries, shorter cycle times, and less wear on the tool.

All well established. The problem lies elsewhere.

Why both substances are increasingly viewed critically

PFAS are known for their extreme persistence, which is why they’re often referred to as “forever chemicals.” They accumulate in the environment and are suspected of being harmful to health.

For silicones goes nearly the same: degradation under certain conditions occurs so slowly that these substances could just as well be filed under the category of “forever chemicals”.

In injection molding, there’s an added complication: silicones migrate to the surface of the component and form a lubricating layer that’s difficult to remove. That’s a real obstacle for later process steps such as painting or coating, since it reduces adhesion. On top of that, mechanical machine parts can become permanently contaminated by penetrating silicone. It’s little wonder, then, that silicone-containing plastics are increasingly avoided in injection molding.

What’s notable is that both substances are usually added in only small amounts, typically 0.1 – 1 wt%. That, however, doesn’t change the risks described above.

So what’s the alternative?

Polytives’ additives show that there’s another way: they reduce sharkskin effects and improve flow properties in PE films at dosages as low as 0.5 wt%.

Would you like to learn more or try the product for yourself? Get in touch with us.

Recycling plastics in cars: Opportunities and Challenges of the EU End-of-Life Vehicles Regulation

Plastics in Cars: A Boon for Design, a Challenge for the EU End-of-Life Vehicles Directive

Plastics have been used in the automotive industry since the 1960s. While steel and aluminum remain important materials, plastics now account for an average of 15% of a vehicle’s total mass, which corresponds to approximately 150–200 kg per vehicle (Source: Plastics Europe). In particular, the weight reduction was made possible by the use of plastics such as polypropylene (PP). This material helps to lower fuel consumption and improve the environmental footprint.

With the EU End-of-Life Vehicles Regulation (ELV Directive), it becomes necessary to consider the efficient recycling of plastics in cars. The draft legislation targets various goals. However, the most significant change will be the increase in recycling rates to 15% for certain component groups and materials in a first step. Looking at the composition of a modern car, e.g., the VW ID.4, it is striking that steel dominates as a material with a share of 50–60% (and is still primarily used for the body), while plastic accounts for 10–20%. The proportions of plastics in the car, using the VW ID.4 as an example, are as follows (sources: VW):

• 63% thermoplastics
• 19% elastomers
• 9% thermosets
• 5% textiles
• 4% other Plastics

Why Plastics Remain a Sensible Material Alternative in Cars

The weight reduction mentioned at the outset, achieved by using lighter plastics, is crucial to the cost-effectiveness of vehicles. If one were to attempt to replace plastic components with aluminum or steel, the total weight of the vehicles would increase significantly—in extreme cases, by a factor of 1.5. Non-rigid elements would have to be replaced with leather, flax, or jute.

In addition to the burden of increased curb weight, important material functions would also be lost. Without a plastic layer, the windshield would lose its shatter-resistant properties. Headlights would have to be made of glass, which would lead to increased wear and tear and greater weight.

Modern automotive manufacturing is currently not feasible without plastics. They are not just a matter of weight, but also of safety, design freedom, and cost efficiency.

The Challenges of the EU ELV Directive 

Since automotive manufacturing involves an interplay of highly complex and meticulously regulated processes, any external legal intervention can throw the industry off balance. For example, the restriction remains in place that only PCR materials (post-consumer recyclates) count toward the targeted quotas, while PIR (post-industrial recyclates), i.e., waste from production, are not recognized.

This approach could lead to a supply crisis, as the necessary PCR materials—particularly the engineering plastics used in vehicles—are scarce on the market and can only be separated by type at great expense. To further enhance sustainability in line with the closed-loop approach, an additional 20% recycled content is to come from end-of-life vehicles. Here, too, significant challenges arise, as programs such as the so-called scrappage incentive have severely limited the availability of end-of-life vehicles in the EU.

How can sustainability and economic viability be reconciled?

It remains to be seen whether the targets set by the EU ELV Directive can be achieved without compromising the cost-effectiveness and quality of vehicles. Innovative solutions are needed that both ensure the availability of PCR materials and enable the efficient recovery of materials from end-of-life vehicles.

Rear lights are a clear example of plastic recycling in cars. They are typically made of PMMA (polymethyl methacrylate), a transparent plastic. PMMA casting compounds produced by extrusion or casting processes have very high viscosity, which normally makes them unsuitable for the standard injection molding process and thus for the manufacture of taillights.

Thanks to the innovative collaboration between Pekutherm Kunststoffe GmbH, Sitraplas GmbH, and Polytives GmbH, however, significant progress has been made in addressing this challenge: It is now possible to produce injection-moldable PMMA compounds from cast sheets without compromising the material’s valuable mechanical properties. This also enables the integration of recycled PMMA into taillights and opens up new opportunities for the circular economy.

The Future of Automotive Plastics

How will plastics in cars evolve in the coming years? Some trends can be predicted quite clearly:

• Due to the new legal framework, the recycling of plastics in cars will continue to advance.
• Research and development will focus on creating even lighter, more durable, and more sustainable plastics.
• The automotive industry will increasingly rely on circular economy concepts to return plastics to the production cycle after their useful life.

Thanks to TecPart e.V., we will continue to be kept closely informed about these and other important transformation processes in the industry. Closer cooperation between automakers, recycling companies, and policymakers is essential to address the challenges of the EU ELV Directive and ensure a sustainable future for the automotive industry.

In a previous article, we explained what xenon tests are, why they are indispensable for plastics, and which standards such as DIN EN ISO 4892-2 underpin them.

This post follows up on that with a concrete example of practical application from Polytives and Röhm. It shows how the processability of PLEXIGLAS® can be specifically improved without compromising its long-term optical stability, i.e., the weathering stability of PMMA.

This is often where the conflict of objectives lies: in many systems, increased flowability goes hand in hand with migration, yellowing, or mechanical losses. This is unacceptable for applications with high light-conducting and functional requirements – such as in lighting technology, design, or automotive engineering.

Process optimization meets material fidelity

In joint research, PMMA compounds based on PLEXIGLAS® 6N, 7N, and 8N were modified with a polymeric acrylate additive from Polytives (bFI A 3745).
What makes this special is that the additive itself is a polymer, chemically identical to PMMA. It is not low molecular weight, PFAS-based, or silicone-based. This allows it to integrate into the polymer matrix without changing the material properties.
Nevertheless, the effects on processing are clear:

• Reduction of melt-viscosity
• significant increase in MVR values of up to over 100%
• Cycle time reductions of over 20%
• Reduction in processing temperature, in some cases by up to 35 °C
• Pressure reduction of up to 50%

For processors, this means higher throughput, lower energy consumption, less wear and tear, and greater machine availability.

What happens during weathering?

To check whether these process-related advantages have a long-term effect on the material’s optical properties, PLEXIGLAS® samples were subjected for over 10,000 hours to the xenon weathering test in accordance with DIN EN ISO 4892-2 (method A).

A comparison of the zero sample with the modified variant shows

• no significant change in transmission in the visible range (400–800 nm),
• the Yellowness Index (ASTM E313) remains well below critical values even with 7% additive,
• no yellowing, clouding, or surface degradation occurs.

This shows that the weathering stability of PMMA with polymer additives is maintained even under extreme test conditions.

The relevance for developers and compounders

The combination of optimized rheology and stable optics opens up new possibilities:

• process windows become larger without material risks,
• process capability increases while flexibility also increases,
• design freedom increases (thin-walled, complex, long flow paths), with new and higher filling level concepts and new additive supplements (natural fibers, organic components, functional additives, e.g., blowing agents),
• energy consumption and CFP decrease,
• the material remains recyclable, as the solution is migration-stable and compliant with regulatory requirements.

Instead of having to choose between efficiency and durability, both goals can now be combined.

Conclusion

The example shown here proves that modern polymer additives not only accelerate processes, but also pass long-term tests under light, heat, and moisture.

Further information

We will be happy to provide you with the complete technical article with all diagrams and detailed values upon request.

If you would like to check the optimization potential of your process, we would be happy to discuss this with you directly.

Contact us – we would be happy to discuss your application personally.