Plastic film squeezers remove free water and compact washed film into a form that is easier to convey and meter into an extruder. Two figures are often used to describe the result: output bulk density and compaction ratio. They are useful only when the material, moisture basis and measurement method are stated. A value copied from a brochure cannot predict performance on every LDPE, LLDPE, HDPE or PP stream.
Material density and bulk density are different
Material density is the mass of the polymer divided by the volume occupied by the solid polymer itself. Schüttdichte is the mass of loose flakes or compacted pieces divided by the total container volume, including the air spaces between pieces. A squeezer changes particle shape, packing and moisture; it does not normally change the basic polymer density.
ISO 1183-1 covers methods for determining the density of non-cellular plastics, while ASTM D1895 addresses apparent density, bulk factor and pourability of plastic materials. These properties answer different questions. For hopper sizing and extruder feeding, the plant normally needs a consistently measured bulk density, not a handbook value for virgin polymer density.
How to measure film bulk density
Use the same container, filling procedure and material-conditioning rule for every comparison. A simple plant method is:
- Select a container with a known internal volume.
- Weigh the empty, dry container.
- Fill it using a defined procedure without hand-packing the film unless the method specifically calls for compaction.
- Level the surface consistently and weigh the filled container.
- Subtract the container mass and divide the material mass by the container volume.
Bulk density = material mass ÷ filled volume. Report the result in kg/m³ and identify whether it is loose, tapped or otherwise compacted. Also record the resin mix, particle form, temperature and moisture. Wet input film can appear heavier than dry output because the input mass includes water, so an undefined wet-basis comparison can distort the result.
How to calculate a defensible compaction ratio
If the input and output are measured on a comparable mass basis, the volumetric compaction factor can be estimated as:
Compaction factor = output bulk density ÷ input bulk density.
For example, if a representative dry-basis sample rises from 30 kg/m³ before squeezing to 240 kg/m³ after squeezing, the measured factor for that sample and method is 8:1. This is an illustration, not a universal machine rating. A different film thickness, contamination level, cutter setting or filling method can produce a different result.
When the machine removes water, a direct ratio based on wet input mass and dry output mass is not a true volume-reduction comparison. Either determine dry solids for both samples or report moisture and bulk density as separate measurements. The test report should explain the basis so that a supplier and buyer can reproduce it.
Why published ratios vary so widely
Initial film form
Loose shopping-bag film, shredded agricultural film and washed film flakes start with very different void volumes. Extremely fluffy feed can show a large apparent compaction factor even when the final output density is ordinary. A ratio without the starting bulk density is therefore incomplete.
Resin and film construction
LDPE, LLDPE, HDPE and PP differ in stiffness and thermal response. Laminated film, labels, paper, aluminum and residual soil change how the material dewaters and packs. Mixed streams should be identified by composition rather than described only as “film.”
Flake size and washing condition
Particle size affects both drainage and packing. Water trapped in folds or fine material may remain after mechanical squeezing. Wash-water temperature, surfactant, rinsing and the time between washing and sampling can also change a moisture result.
Machine settings
Screw speed, counter-pressure, die or discharge restriction, cutter condition and heating all affect the output form. Increasing restriction can raise density but may also increase current, heat or the risk of a blockage. The useful setting is the one that meets the downstream feed and moisture requirements without unstable operation.
Measure moisture separately
Residual moisture is often more important to pelletizing stability than the largest possible compaction ratio. Agree on a sampling and drying method, then report moisture as a percentage on a defined wet or dry basis. The discharge should be sampled over time because a single handful may miss changes caused by feed interruptions or wash-line carryover.
A supplier may demonstrate a low moisture result with a clean, thin film under controlled conditions. That result should not be treated as a guarantee for heavily printed, multilayer or soil-contaminated feed. Ask for a trial with representative material and record throughput, power, water removed, discharge temperature and moisture distribution.
How density affects downstream pelletizing
Consistent bulk density helps an extruder feeder deliver a steadier mass flow. It can reduce bridging and the large feed fluctuations associated with loose film. This may support more stable screw loading and melt pressure, but the outcome also depends on feeder design, extruder venting, contamination and temperature control.
Very dense or partially fused pieces are not automatically better. Oversized, irregular chunks may bridge in a hopper or feed unevenly. The acceptance specification should include particle form and size distribution in addition to average bulk density.
What to put in a squeezer acceptance test
- Feed resin, source, film thickness range and contamination description
- Input and output moisture using the same declared method
- Input and output bulk density using the same container and filling method
- Dry-solids throughput over a stated test duration
- Average and peak motor current, discharge temperature and stoppages
- Output particle-size range and the proportion of fines or oversized pieces
- Time required for safe opening, cleaning and a normal wear inspection
The result can then be used to size storage, conveyors and the extruder feeder. It also provides a realistic baseline for later maintenance checks.
Choosing the right film dewatering route
A Filmquetschtrockner is one option after a PP/PE-Film-Waschanlage. Depending on the feed and final product, the plant may also compare an agglomerator, compactor or screw-press densifier. The detailed plastic film squeezer guide covers the broader process, while the densification technology comparison separates the main operating principles.
For pelletizing, review the required feed form with the equipment supplier and, where possible, test the squeezer and PP/PE-Filmpelletieranlage as a connected process rather than as separate catalogue items.
Häufig gestellte Fragen
Ist die Polymerdichte gleich der Pressausgangsdichte?
Nr. Die Dichte des Polymers oder Materials schließt Luftgaps zwischen den Partikeln aus. Der Ausgang des Pressgeräts wird in der Regel durch die Bruttodichte bewertet, die diese Lücken einschließt und von der Partikelform und der Messmethode abhängt.
Welcher Kompressionsverhältnis ist für Kunststofffolie gut?
Es gibt keine universelle Zielsetzung. Ein nützliches Ergebnis erfüllt die Anforderungen an die Nachlaufverarbeitung, Feuchtigkeit und Partikelgeometrie. Berichten Sie sowohl die spezifische Gewichtsdichte von Ein- und Ausgangsprodukten als auch die Prüfgrundlage anstatt sich nur auf ein Verhältnis zu verlassen.
Can bulk density be compared when the input film is wet?
Only with care. Water changes the measured mass. Determine dry solids or report moisture separately and use the same container and filling procedure for input and output samples.
