Recycled Bag Making Machine Technical Deep Dive: Melt Filtration and Contaminant Management
Recycled bag making machines often process post-consumer recycled (PCR) and post-industrial recycled (PIR) resins, which contain contaminants such as paper, adhesive, dirt, and degraded polymer. These contaminants can cause film defects (gels, black specks, weak spots) and damage machine components. To address this, the extrusion section (if integrated) or the film handling section (if using pre-made recycled film) must include robust filtration. For integrated extrusion, the melt filtration system is placed between the extruder and the die. It uses a screen changer – a device that holds a metal mesh filter (typically 40-200 mesh) to trap contaminants. The screen changer can be manual (requiring operator intervention) or automatic (continuous, with a sliding plate or a rotating disk). Automatic screen changers are preferred for high-volume production; they allow the filter screen to be replaced without stopping the line. The screen mesh is selected based on the contaminant size and the film thickness; finer mesh (200) removes smaller particles but causes higher pressure drop. The pressure drop across the screen is monitored; an increase indicates clogging, and the screen is indexed to a clean section. The filter is often a single-layer or multi-layer (with a coarse mesh for pre-filtration and a fine mesh for final filtration). For high-contaminant materials, a melt filter with a backflushing function can be used to clean the screen online.
Film handling for pre-made recycled film: If the machine uses pre-made recycled film rolls, the film may have contaminants on the surface, such as dirt or static-attracted particles. The machine includes a film cleaning station: a set of brushes (rotary or oscillating) that remove surface particles, followed by a vacuum extraction system. The brushes are made of nylon or carbon fiber, with adjustable pressure. The cleaning station is placed before the sealing section. The film may also have uneven thickness due to inconsistent melt flow; the machine's tension control must be adaptive to prevent film breaks. The registration system must handle films with variable print quality.
Plastic Bag Making Machine
Contaminant detection: The machine can include an inline vision system that detects gels, black specks, and other defects in the film. The system uses high-resolution cameras and machine vision algorithms to classify defects. Defective bags are rejected. The rejection data is used to assess the recycled resin quality and provide feedback to the supplier. The machine can also include a metal detector to prevent metal particles from damaging the sealing bars or cutter; if metal is detected, the machine stops.
Process optimization for recycled resins: Recycled resins have lower melt strength and narrower processing windows. The sealing temperature may need to be increased (5-10°C) to compensate for contaminants, but too high causes degradation. The machine's control system uses a conservative approach: start with lower speed and higher dwell, then gradually increase speed while monitoring seal quality. The film thickness may vary; the machine's thickness gauge adjusts the sealing temperature and pressure to maintain consistent seal strength. The cooling system must be efficient because recycled films may have higher heat retention.
Maintenance: The melt filter screen must be replaced or cleaned regularly; the frequency depends on the contaminant level. The film cleaning station's brushes require periodic replacement. The vision system's lighting and lenses need cleaning. By managing contaminants effectively,
recycled bag making machines produce bags with acceptable quality while using low-cost recycled materials, supporting circular economy initiatives.
