How to Assess a Recyclate Batch Before Serial Injection Moulding of Technical Parts

One batch of recycled polymer may produce parts with consistent dimensions and a low rejection rate, while another delivery of nominally the same material can require process adjustments or lead to a higher number of defects.

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Such differences result from the natural variability of secondary raw materials. Their properties depend on factors such as feedstock origin, contamination level, previous processing history, additives and the consistency of the recycling process.

For this reason, process stability depends to a large extent on the qualification of each incoming material batch before full-scale production begins.

Working with recycled polymers can require more extensive incoming-material control than processing a tightly specified virgin grade. Appropriate quality-control procedures should therefore begin before the material reaches the injection moulding machine.

Which Properties of a Recyclate Batch Should Be Checked Before Production?

Before serial moulding starts, one of the important parameters to assess is the consistency of melt flow behaviour.

For many thermoplastics, the Melt Flow Index or Melt Flow Rate — MFI/MFR — provides a practical indication of how easily the polymer flows under defined test conditions.

Differences between batches can indicate changes in:

• molecular structure,

• degradation history,

• polymer composition,

• filler content,

• contamination,

• blending proportions.

A significant change in melt flow may alter the behaviour of the material during cavity filling.

If viscosity is higher than expected, the risk of short shots may increase. If the melt flows much more easily than the validated material, flash or other filling-related defects may occur depending on the mould and process settings.

For technically demanding applications, MFI/MFR should therefore be treated as one screening parameter rather than as a complete description of rheological behaviour.

Contamination and Material Purity

Another important step is assessing the cleanliness and homogeneity of the recycled material.

Depending on the polymer and application, incoming inspection may include:

• visual contamination,

• foreign polymer content,

• metallic or mineral particles,

• density,

• ash content where relevant,

• filtration behaviour,

• colour variation.

Ash content can be useful when mineral fillers or inorganic contamination are important, but it is not a universal measure of recyclate quality.

Foreign particles can interfere with melt flow, affect the surface of the moulded part and, in some applications, increase wear or create weak points in the finished component.

In hot-runner systems with narrow flow passages, poorly controlled contamination may also increase the risk of process instability or blockage.

Why Is Moisture Control Important?

Moisture content can be critical, particularly for hygroscopic and hydrolysis-sensitive polymers such as PA, PET, PBT or certain grades of PC.

If these materials are processed with excessive moisture, water can contribute to polymer-chain degradation at elevated processing temperatures.

Possible effects include:

• reduced molecular weight,

• deterioration of mechanical properties,

• surface streaks,

• gas formation,

• increased process variability.

The acceptable residual moisture level depends on the specific polymer grade and supplier specification.

For materials such as PP or PE, moisture behaves differently and should not be treated in the same way as for highly hygroscopic engineering polymers.

Moisture analysis should therefore be selected according to the material being processed and the level of accuracy required.

How Does Feedstock Origin Affect the Properties of the Final Part?

The origin and previous history of a recycled polymer influence its behaviour during subsequent processing.

Post-consumer material can contain a broader range of polymer histories, additives and degradation levels than controlled internal regrind.

This may affect:

• melt flow,

• shrinkage,

• impact resistance,

• tensile properties,

• colour,

• dimensional stability.

For semi-crystalline polymers, variations in composition and cooling behaviour can also influence shrinkage and warpage.

The presence of fillers, pigments or traces of other polymers can further change the way the material behaves during cooling.

For this reason, the same moulding parameters should not automatically be assumed to be optimal for every new recyclate batch.

Why Can Colour and Appearance Vary Between Batches?

Recycled materials may show greater colour variability than virgin polymers.

This can result from:

• the original colour of the waste stream,

• thermal history,

• oxidation or ageing,

• pigments from previous applications,

• minor contamination with other materials.

Colour correction may therefore require masterbatch or another controlled colouring strategy.

However, the amount of colourant should be selected carefully, because additional additives can influence material properties and processing behaviour.

Where consistent appearance is important, colour should be treated as a measurable quality parameter, not only as a visual judgement.

Does a Higher Recycled Content Always Reduce Mechanical Performance?

No. The effect of recycled content on mechanical performance depends on:

• polymer type,

• number of previous processing cycles,

• contamination,

• degradation level,

• stabilisation,

• additives and fillers,

• blending with virgin material,

• recycling technology.

In some cases, impact resistance, elongation or fatigue behaviour can decrease as recycled content increases.

In other applications, a properly qualified recyclate can meet the required specification without a significant loss of functional performance.

For this reason, there is no universal percentage reduction in strength that can be assigned to recycled plastics as a group.

The decision should be based on testing against the requirements of the finished component.

Why Can a New Batch Require Different Injection Settings?

A new batch may have a different viscosity, composition or thermal behaviour even when it belongs to the same general material category.

This can affect:

• filling speed,

• injection pressure,

• holding pressure,

• melt temperature,

• cooling time,

• shrinkage.

A sudden change in melt-flow behaviour can shift the validated process window and increase the risk of:

• short shots,

• flash,

• sink marks,

• warpage,

• dimensional deviation.

For this reason, trial runs and parameter verification are particularly useful when material consistency is critical.

The objective is not to continuously “chase” the material with process settings, but to establish an acceptable incoming-material window and a validated process window.

Which Products Are More Tolerant of Recyclate Variability?

Products with broad dimensional tolerances and moderate mechanical requirements may generally tolerate more material variability than high-precision or safety-critical technical components.

Examples can include selected:

• transport containers,

• storage products,

• advertising components,

• POS elements.

This does not mean that only basic checks are required.

Even relatively simple components should be evaluated against the requirements relevant to their application, including:

• mechanical strength,

• dimensions,

• appearance,

• chemical resistance,

• long-term behaviour.

Precision housings, automotive parts or other technically demanding components usually require tighter control of raw-material properties and more rigorous process validation.

How Should Recycled Plastics Be Introduced Into Serial Production?

Before recycled plastic products are introduced into high-volume production, the material and process should be validated under conditions representative of actual manufacturing.

This may include:

• trial moulding,

• dimensional inspection,

• mechanical testing,

• evaluation of visual defects,

• confirmation of the process window,

• comparison of successive material batches.

NOEX states that its ECO-LINE includes products manufactured using recycled raw materials and that its wider production process includes quality testing of raw materials, production materials, components and finished products.

Noex also processes a wide range of polymers, including materials made from recycled waste, and manufactures injection-moulded parts for different industrial applications.

A structured qualification process helps determine whether a recyclate is suitable for the intended component and whether its variability can be controlled within the validated production window.

FAQ

Which Recyclate Parameters Should Be Checked Before Serial Production?

Common parameters include MFI/MFR, moisture where relevant, contamination, density, particle or pellet consistency, colour and mechanical properties.

The exact test plan should be adapted to the polymer and the requirements of the finished product.

Why Is Recyclate Moisture Important?

Excessive moisture is particularly important for hygroscopic and hydrolysis-sensitive polymers.

If not properly dried, such materials can degrade during processing, leading to poorer mechanical properties, surface defects and unstable production.

Does Recyclate Always Require Different Injection-Moulding Settings?

Not always.

If the new batch remains within the validated material specification, existing process parameters may remain suitable.

However, significant changes in melt flow, moisture or composition may require additional verification or adjustment.

What Problems Can Contamination in Recyclate Cause?

Depending on its type and quantity, contamination can contribute to:

• surface defects,

• unstable flow,

• blocked narrow passages,

• mechanical weak points,

• increased wear,

• dimensional inconsistency.

Can Recyclate Be Used for Precision Technical Components?

Yes, provided that the material has sufficiently stable properties and the production process is validated for the required tolerances and performance.

Supplier qualification, incoming-material control and technological trials become especially important in such applications.