Manufacturing Process of Woven Sacks

The name ‘Raffia’ is derived from the natural fiber in leaves of Raffia Palm found in Africa, used for making mats, baskets, etc. The structure of woven sacks prepared from PP and HDPE tapes resembles those mats and thus the name “Raffia” was given to plastic woven sack manufacturing industry and allied activities.

Advantages for Woven Sacks Packaging:

1. Lower cost of packaging Vs alternatives like jute & paper.

2. Stronger and lighter bags (70 gm Plastic Woven Sack matches the load bearing capacity of 500 gm jute bag). 

3. Better aesthetics and printing possible for brand creation & product differentiation. 

4. Lower transportation and handling cost due to light weight.

5. Availability of quality raw material as well as downstream processing machines. 

6. Healthy growth in Cement, Fertilizer, Food grain & Chemicals sector expected to show sustained demand for sacking applications.

The Process of manufacturing PP Woven Sacks involves following six steps:

STAGE 1: Selection of raw material

STAGE 2: Tape Extrusion and winding – production of tape bobbins 

STAGE 3: Weaving (tubular fabric weaving on Circular Weaving looms)

STAGE 4: Extrusion Coating or Lamination – on woven fabric

STAGE 5: Printing – Flexographic

STAGE 6: Bag conversion, bailing and dispatch 

STAGE 1: Selection of raw material

The basic raw material – polymers used in manufacturing of raffia are generally polypropylene (PP) and high-density polyethylene (HDPE) available in granular form. Raffia Sector is highly technical demanding market and therefore need high performance polymers. With a strong focus on understanding the customer’s needs, there has to be a complete range for the segment with different grades for extrusion of raffia tapes, coating and differentiated grades with UV stabilization. 

The MFI (Melt Flow Index) is a critical parameter to choose appropriate grade of PP/HDPE for specific application. In PP, MFI ranges from 2 to 6 and in HDPE the MFI range is 0.9 to 1.0.

In PP generally MFI of 3 is very prevalent but new grades with MFI of 4.5 are available now to ensure an optimum balance of mechanical properties like tenacity and elongation at break and high productivity in terms of enhanced line speeds. Similarly, in HDPE less than 1 MFI is generally preferred.

Other than basic polymer (PP/HDPE) lot of other additives are used as below

  1. Calcium Carbonate Masterbatch/Filler – as an anti-fibrillation agent and cost reduction. Higher use of filler though results in deterioration of mechanical properties and diminishing UV Stabilization.
  2. UV Stabilizers – used where end product has an outdoor exposure. These includes Tarpaulin, Fertiliser bags, Jumbo Bags (FIBCs), food grain and sugar packaging.
  3. Color Masterbatch – to impart colors based on customer’s requirement.

STAGE 2: Tape Extrusion and winding – production of tape bobbins

In the Tape Extrusion/Stretching Lines, PP/HDPE granules, filler and other additives are blended, melted, homogenized and finally extruded through a T-die.  The melt is quenched in a water bath to form a solidified cast film of desired thickness.  The cast film is then slit into tapes of required width and stretched over godets rotating at different speeds under controlled temperature (inside a hot air oven for bringing tapes to softening point) before annealing to produce tapes with oriented molecules having desired level of tenacity and elongation.  Individual tapes are collected or wound on numerous winders at the end of the tapeline to form bobbins or cylindrical packages on steel/aluminium cores. Owing to the shape of the tape packages, they are also called cheese and the metal cores called cheese tubes.

The speed of extrusion lines available today are up to 600 meters per min and extruder melt capacity is up to 900 kg/hr. Due to market requirements the automatic changeover winders also are now widely popular and has been designed to meet the ever increasing demand for becoming more efficient and cost effective. 

STAGE 3: Weaving (on Circular Weaving Machine)

Warp and weft are the two basic elements used in any type of weaving to turn thread or yarn (in our case “tapes”) into woven fabric. The warp is the set of tapes which are mounted on the Creel of a circular weaving loom and enters from two opposite sides into the central portion of the loom where they are intertwined by Weft tapes mounted in shuttles which revolves in a circular motion inside the weaving sheds. 

Each model of loom is suitable for a range of tubular fabric widths (DFL).  Besides the DFL, the fabric is also defined in terms of mesh or number of tapes per 10cm in warp and weft direction, the denier of tapes (weight of 9000 meters of tapes in grams) in warp and weft directions and their corresponding widths in mm.  

The tubular fabric so produced may be wound into rolls or slit open before winding as flat fabric.

The high-performance circular looms are designed to weave light and heavy fabric (30 – 250 gms per sq. meter or GSM) for variety of end applications. For example, a 4-shuttle looms are suitable for weaving a ventilated fabric or Leno fabric up to almost 70 cm used for packaging of Horticultural products. Similarly looms with 6 shuttles are used for weaving of fabric used for fertilizer, cement, food grain and sugar packaging bags. Looms with Shuttles 6, 8, 10 and 12 are designated to manufacture bags for light and heavy GSM fabrics for FIBC or Jumbo bags, Tarpaulins and Geotextiles. Modern high shuttle circular looms have made it possible to make a fabric up to a working or double flat width of 295 cm. 

STAGE 4: Extrusion Coating or Lamination   

Depending upon the end requirement, the woven fabric may be coated with PP or LDPE on both side of fabric or on single side as per the end use requirement. The choice of raw material is very important and Copolymer grades of PP are available which can be used directly to eliminate the mixing of LDPE. Extruded PP/LDPE can also be used to paste another film on the surface being processed to produce laminated fabric.

STAGE 5: Printing

Roll-to-roll Printing

Most packaging requires to be printed, if not for aesthetic or marketing reasons, for reasons of identification and labelling. 

The fabric roll, coated or directly from loom, is fed to the printing machine which is usually capable of printing in multiple colours.  Graphics or texts may be printed as required.

High quality 4/6/8 colour flexographic machines with printing speeds of approximately 150 m/min are available. If fabric rolls are not printed before conversion, bottom stitched bags may be printed in a multi-cylinder bag-to-bag flexographic printing machine.  It is a much slower machine and less accurate than the roll-to-roll printing machine.

STAGE 6: Conversion, bailing and dispatch 

Tubular fabric may be manually cut to the desired length and stitched into bags of different designs. This is a very labour intensive process and has already replaced to a large extent with automatic bag conversion machines. The Bag conversion line combines the functions of measuring out, cutting, folding, stitching to deliver bottom stitched bags in neat stacks.  These bottom stitched bags are now ready to be filled in and top stitched. 

Finished printed sacks are then packed in lots of 500 nos.  (or a multiple of 500 nos.), or as desired by end user.  Stacks of bags are compressed with a bale press to expel the air trapped within and make compact pellets for transporting to end users. 

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