การกรอง PET หลอมเหลว: การปกป้องค่า IV ในกระบวนการรีไซเคิลแบบขวดสู่ขวด

What Is PET Melt Filtration?

PET melt filtration is a process in which molten polyethylene terephthalate (PET) is forced through stainless steel screen packs or filter discs — typically rated between 20 µm and 120 µm absolute — to remove solid contaminants before the melt reaches downstream forming equipment. In a recycling line the filtration step operates at melt temperatures of roughly 260–290 °C, where PET is fluid enough to pass through the media yet viscous enough that every retained particle concentrates stress in the melt and degrades polymer quality if left uncontrolled. By capturing dirt, paper, polyolefin fragments, aluminum fines, gels, and carbonized black specks, melt filtration does more than protect the die and downstream tooling: it is the single most influential mechanical step for preserving intrinsic viscosity (IV), keeping product color and clarity acceptable, and holding line uptime high in bottle-to-bottle and flake recycling operations. For an overview of how the same principles apply across extrusion processes, KAIFIL's plastic extrusion filtration application page explains media selection in detail.

Why PET Recycling Needs Dedicated Melt Filtration

PET is a condensation polymer whose molecular weight is expressed as intrinsic viscosity (IV). Virgin bottle-grade resin typically enters processing at an IV of 0.80–0.84 dl/g. Every thermal and mechanical history — drying, extrusion, solid-state polymerization, re-melting — tends to pull IV down, and re-melting scrap or flakes accelerates that decline because the polymer has already been through a heat cycle. Reprocessing can reduce IV by roughly 0.02–0.10 dl/g per pass depending on moisture control, temperature, and residence time.

Melt filtration protects IV indirectly but decisively. Contaminants such as polyolefin fragments (PP/PE), paper, adhesives, and degraded polymer act as stress concentrators and as sources of moisture and oxidation. A particle trapped against a screen creates localized high shear; the resulting viscous heating can raise local melt temperature well above the 260–290 °C setpoint, driving hydrolysis and chain scission that lower IV. Remove the particles before they can cause these local hot spots, and you reduce the real-world IV loss.

The economics push in the same direction. In bottle-to-bottle recycling, a drop of 0.05 dl/g in IV can be the difference between a resin that meets food-contact specifications and one that must be blended or downgraded. Because upgrading IV costs energy — solid-state polymerization at around 220 °C for many hours — it is far cheaper to preserve the IV you already have with good melt filtration than to restore it downstream.

Flake Quality and Contamination in PET Recycling

The contamination load entering a PET melt filter depends heavily on flake quality. Well-washed post-consumer flakes still carry measurable impurities. In practice, the solids content of washed rPET flake typically ranges from a few hundred to over 1,000 ppm before hot washing and drying; after a good wash line, residual contamination can be reduced to below 100–200 ppm. For food-contact applications, the filtration step works together with decontamination processes such as solid-state polymerization to keep the final resin compliant, but even in those lines the melt filter is what stops visible particles and gels from reaching the preform.

Typical contaminants in PET flake:

• Polyolefin fragments (PP bottle caps, PE labels) — lower melting point than PET, they survive washing and must be screened out, aided by a melt filter that removes solid particles.
• Paper, glue, and label adhesive residues — partially burnt during extrusion, they form dark specks.
• Aluminum and metallic debris from caps, aerosol cans, and processing equipment.
• PET dust and fines generated during grinding.
• Degraded, crosslinked, and oxidized PET — the direct source of gels and black specks.

Because these contaminants vary in size, a single fine mesh is rarely the answer. Lines commonly use a coarser upstream filter (120–300 µm) to protect the extruder and a finer downstream filter (20–40 µm absolute) close to the die. This two-stage approach is standard practice in PET recycling and keeps the fine filter from blinding prematurely.

Typical Mesh and Micron Ratings for PET Melt Filtration

Screen packs for PET recycling are described two ways: by wire mesh count and by absolute retention rating. Mesh count refers to the number of openings per linear inch — a 40-mesh screen has roughly 425 µm nominal openings, 60 mesh about 250 µm, 80 mesh about 180 µm, and 100 mesh about 150 µm. Because a pack stacks multiple layers, and because particles are retained by the tortuous path through the weave, the practical "absolute" rating of a well-designed pack is finer than the top mesh number alone suggests. For demanding PET work, multilayer packs and sintered media reach absolute ratings of 20–40 µm.

The mesh numbers above are nominal opening sizes for plain square-weave screens. In practice, filter mesh for PET recycling is engineered as a stack — a coarse support layer (often 20–40 mesh) to hold the fine layer, intermediate layers to distribute pressure, and a fine retention layer on the melt side. The way these layers interact determines both filtration sharpness and service life, and getting the combination right is where most of the practical expertise lies. KAIFIL's guide to extruder screen pack mesh combinations walks through layer-by-layer design in detail, and our plain wire mesh filter discs are available in the weave patterns used in these packs.

Screen Changer Types for PET Recycling Lines

The screen pack must be changed as it loads with contaminants, and the mechanism that performs this swap is the screen changer. Choosing the wrong type has a bigger effect on PET line profitability than almost any other filter decision, because the melt must not stop and must not be over-exposed to heat while the swap happens.

For bottle-to-bottle lines, a continuous or dual slide-plate changer is common because any interruption in melt flow translates into a pressure pulse that can shift the pack, alter residence time, and momentarily raise melt temperature — all of which threaten IV and gel formation. Rotary designs go further: a 12-cavity rotary changer can operate for weeks before the full disc needs service, which is why high-throughput plants favor them. The rotary disc is typically loaded with fine filter discs; KAIFIL's extruder screen product range covers the media for these changers, from single-layer screens to sintered composites.

Filter Change Intervals and Pressure Drop

A screen pack behaves like a filter cake that grows with throughput. Operators track two numbers: initial differential pressure across the fresh pack, and the rise as contaminants accumulate. A typical fresh-pack differential at startup is 5–20 bar depending on media fineness; as the pack loads, differential climbs, and most PET lines trigger a change when differential pressure reaches 50–100 bar — or when the pressure ratio (current divided by initial) passes roughly 3–4×. Pushing beyond this window has three consequences: the pack can collapse under pressure, the retained cake can compact into a solid mass that is difficult to purge, and the elevated back pressure raises melt temperature through viscous dissipation, accelerating IV loss and gel formation.

Several factors determine how quickly a pack loads:

• Flake cleanliness — 100–200 ppm residual contamination loads a fine pack far faster than a 50 ppm feed.
• Filtration fineness — a 20 µm absolute pack catches far more particles by count than a 60 µm pack and may need changing proportionally more often.
• Throughput — screening area and melt flow set the cake growth rate.
• Media design — more open area per unit area (multilayer Dutch weaves, sintered mesh) extends the interval.

A practical trick: log the change interval per pack design. If a 20 µm pack lasts four hours but a 25 µm pack lasts six, the small downgrade in fineness often more than pays for itself in uptime — provided the product still meets specification. The same pressure-drop logic applies to the two-stage approach: the coarse upstream filter protects the fine filter, so the fine filter's change interval reflects upstream performance.

Impact on IV and Product Quality: Gels and Black Specks

The two quality defects that melt filtration most directly controls in PET recycling are gels and black specks.

Gels are localized regions of crosslinked or degraded PET with a different melt behavior from the surrounding polymer. In extruded sheet and preforms they show up as translucent lumps or "fish-eyes"; in fiber spinning they cause breakages and filament defects. Most gels form from oxidized polymer and moisture-induced hydrolysis inside the extruder, and once formed they are solid particles that can be caught by the screen — but a fresh gel formed after the filter will not be. This is why gel control depends on both filtration and process discipline (dryer dew point, residence time, temperature).

Black specks are carbonized material: overheated, charred polymer, aluminum fines, or burnt paper that has degraded at melt temperatures of 260–290 °C. They are visible in the finished product at very low concentrations, and food-contact and cosmetic packaging have essentially zero tolerance. A fine retention layer, typically 20–40 µm absolute, is what captures them before they reach the die.

The effect on IV is more subtle but arguably more valuable. Because every retained particle converts to a local high-shear, high-temperature zone, fine filtration reduces the cumulative thermal insult the melt experiences. PET recyclers routinely report that moving from a 100 µm filter to a 30–40 µm absolute pack improves measured IV retention by several hundredths of a dl/g on the same extruder — the equivalent of hours of solid-state polymerization time saved. For the same reason, filtration fineness and IV retention should be discussed together when a line is being specified.

Common Mistakes in PET Melt Filtration

1. Assuming finer is always better. A 10 µm pack on a flake line with 300 ppm contamination will blind in under an hour, forcing constant changes and raising melt temperature. Match media fineness to the product target and upstream flake quality.
2. Ignoring the pressure-drop change window. Running packs until the differential hits 150+ bar saves a change but costs IV and risks pack collapse. Set a change trigger and honor it.
3. Single-stage filtration on dirty flake. One fine pack cannot both protect the extruder and produce food-grade melt. Use a coarse stage early and a fine stage near the die.
4. Overheating during screen changes. A melt that sits at 280 °C for the duration of a manual change degrades visibly. Choose a changer that minimizes residence time, and keep temperature control tight during swaps.
5. Guessing the pack design. Layer order, support mesh, and weave type all change performance. A systematic approach to screen selection is worth far more than intuition — see KAIFIL's guide on selecting extruder screens for plastic recycling lines for a practical method.
6. Treating filtration as a substitute for drying. A well-dried PET flake feed (dew point below −40 °C) is a prerequisite. No filter recovers IV that moisture has already destroyed.

FAQ

What is PET melt filtration? PET melt filtration is the process of passing molten PET through stainless steel screen packs or filter discs — typically 20–120 µm absolute — to remove solid contaminants such as polyolefin fragments, paper, gels, and black specks before the melt reaches the die or forming tooling.

Why does melt filtration matter for intrinsic viscosity (IV)? Contaminants trapped in the melt create localized high-shear, high-temperature zones that drive hydrolysis and chain scission, lowering IV. Fine filtration removes these particles before they can cause thermal damage, helping recyclers retain several hundredths of a dl/g that would otherwise be lost.

What mesh or micron rating should I use for PET bottle-to-bottle recycling? For food-grade bottle-to-bottle lines, 20–40 µm absolute filtration is typical, achieved with multilayer screen packs or sintered wire mesh media. Coarser 60–120 µm ratings are common for fiber, strapping, and sheet applications.

How often should I change the screen pack in a PET recycling line? Change when differential pressure across the pack reaches about 50–100 bar, or when the pressure ratio exceeds roughly 3–4× the fresh-pack baseline. The actual interval depends on flake cleanliness, media fineness, and throughput — log it per pack design.

Do I need a continuous rotary screen changer for PET recycling? Not always, but for high-throughput food-grade lines it is the usual choice. Continuous rotary changers index a fresh cavity on a pressure signal and can run for weeks unattended, avoiding the pressure pulses and residence-time spikes of manual or single slide-plate changers.

Get Custom Screen Packs for Your PET Recycling Line

Melt filtration performance is decided long before the first ton of flake enters the extruder — it is decided by media fineness, pack design, and the changer you install. KAIFIL manufactures stainless steel custom wire mesh filter discs, multilayer extruder screen packs, and sintered wire mesh filter cartridges in Shijiazhuang, China, and ships to recyclers worldwide. Whether you need a 20 µm absolute pack for a bottle-to-bottle line or a two-stage setup for a flake line, our engineers can match media, layer order, and disc geometry to your extruder and screen changer. Contact KAIFIL for a quote and application support, and let us help you protect IV, cut gels and black specks, and keep your line running.