What Are Processing Aids for Plastics? // Broadway Blog

Plastics processing runs at speed and to tight tolerances, so minor issues can cause major costs. A part that sticks in the tool slows the cycle. A film with a rough surface fails inspection. A warped moulding misses its dimensional spec. In this blog, Broadway discusses how each of these problems adds scrap, downtime and cost to a production run.

Processing aids address these issues at source. They change how a polymer behaves during the manufacturing process, so lines run faster and reject rates fall. Broadway formulates processing aids as additive masterbatches, tailored to the polymer and the production conditions they need to work in.

What Does a Processing Aid Do?

A processing aid is an additive that improves how a polymer flows, releases, cools or handles during manufacture. Suppliers normally deliver processing aids as an additive masterbatch, which processors dose straight into the machine, alongside their base resin and colour.

The additive does its work during the production run. Once the part leaves the line, the benefit has already been delivered.

Several active ingredients sit behind effective processing aid formulations. These include metallic stearates (such as zinc stearate and calcium stearate), slip-agent fatty acid amides (notably erucamide and oleamide), specialised polyethylene (PE) waxes, micronised waxes, fluoropolymer waxes and functional anti-blocking silicas.

Processing Aids Compared with Performance Additives

These two types of additives solve different problems.

Performance additives change what the finished part does in service. UV stabilisers extend outdoor life. Antimicrobial additives limit bacterial growth on the part’s surface. Laser marking additives enable permanent coding and marking.

Processing aids change what happens on the production line. They reduce friction, control static, improve dispersion and stabilise dimensions during cooling.

Some additives do both. A slip additive lowers friction in the extruder and also helps the finished film run through a customer’s packing machine. However, the distinction remains useful when specifying a formulation.

How Processing Aids Work

Processing aids use three broad mechanisms.

Surface migration – The additive moves gradually from the bulk of the polymer to the surface of the part. Slip agents, antistatic agents and mould release additives all work this way. Migration rate depends on the polymer, the additive and the temperature, so timing matters as much as dosage.

Interface coating – The additive forms a thin layer on the tool or die surface. This lowers shear stress at the wall and allows the melt to pass more smoothly. Melt fracture additives use this mechanism.

Bulk modification – The additive acts within the melt itself. Nucleating agents change how the polymer crystallises. Moisture scavengers bind water molecules. Dispersion aids reduce surface tension so pigment clusters break down evenly.

Common Processing Problems and Solutions

Friction and Mould Release

Tool adhesion slows ejection and increases cycle times. Surface friction causes films and profiles to bind, scuff or tear as they pass over rollers.

Slip additives lower the coefficient of friction by forming a microscopic lubricating layer at the surface. Mould release additives prevent vacuum locking in complex cavities, which allows clean ejection every cycle and reduces mechanical wear on ejector pins.

Static, Nesting and Handling Problems

Static build-up causes parts to cling to conveyors and attract dust. Contamination then appears on parts that were perfectly clean when moulded.

Antistatic additives create a conductive surface layer that bleeds away electrical charge safely. Anti-block and de-nesting additives introduce a subtle micro-texture to thin-walled products such as cups, lids and trays. This breaks the vacuum effect between stacked parts, so automated lines separate them reliably.

Warpage, Shrinkage and Dimensional Instability

Uneven cooling and differential shrinkage create internal tension. Parts then bow, warp or drift outside tolerance.

Nucleating agents promote rapid, uniform crystallisation. Contraction becomes consistent across all axes, which improves dimensional stability and often shortens cycle times as well.

Trapped moisture creates a related problem. Water in the melt causes internal voids and structural bubbling. Moisture scavenging additives bind water molecules chemically within the melt, which helps considerably with recycled feedstocks and polyolefin film. Hygroscopic polymers such as PET and polyamide still require correct pre-drying, so treat moisture scavengers as support rather than replacement.

Surface Defects and Poor Dispersion

High shear at the die produces melt fracture, often described as shark skin. Volatile gases produce splay and silver streaking. Poor pigment dispersion produces streaks, specks and inconsistent colour.

Melt fracture additives coat internal die surfaces to reduce shear, which allows higher line speeds at lower energy. Dispersion additives reduce surface tension so pigment agglomerates break down and distribute evenly through the part.

Benefits of Using Processing Aids

Processing aids deliver measurable operational gains.

• Lower reject rates – Fewer parts fail on dimensions, surface finish or colour consistency.
• Faster output – Shorter cycle times in injection moulding and higher line speeds in extrusion processes.
• Reduced energy use – Lower shear and lower processing torque cut energy demand per part.
• Better recycled content performance – Recycled feedstock varies in viscosity, moisture and contamination. Processing aids help stabilise the run, which makes higher recycled loadings commercially viable.
• Fewer secondary operations – Less need for manual separation, rework or post-mould handling.

What to Consider When Specifying a Processing Aid

Base polymer and carrier resin. The carrier must be compatible with your base resin. Standardised formulations for PP, PE and PET cover most requirements, but the match matters for dispersion and clarity.

Dose rate – Under-dosing provides no measurable processing benefit, while over-dosing inflates raw material costs without adding value. Excess additive can cause plate-out – where the additive separates from the plastic and deposits onto machine metal, causing equipment build-up and costly downtime. It can also lead to surface bloom, pushing extra additive to the surface to form a hazy, oily film that prevents inks, adhesives, and welds from sticking properly. Establish the correct let-down ratio early to protect both product quality and profit margins.

Interaction with colour and other additives – Processing aids may often sit alongside other components such as colourants, UV stabilisers and performance additives in the same formulation. Where practicable, combining them into a single multifunctional masterbatch or plastic compound simplifies handling and dosing.

Regulatory requirements – Food contact, medical and toy applications carry specific compliance obligations. Confirm the processing aid formulation meets the relevant standards before specifying it.

Trial conditions – Processing aids respond to shear, temperature and residence time. Trial under production conditions rather than laboratory conditions, because behaviour changes with scale.

Discuss Your Requirements with Broadway

Processing aids solve production problems that resin selection alone cannot fix. As processors take on more recycled content and work to tighter tolerances, the case for using them strengthens.

Broadway has decades of experience in formulating custom masterbatches to suit your polymer, process and production conditions. They now offer a range of standardised processing aid masterbatches. Their technical team can identify the right processing aid for the problem you are seeing on the line. They can also engineer customised formulations and combine processing aids with your colourants and other additives in a single formulation.

Get in touch with Broadway to discuss your processing requirements.

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