Manufacturing



The former shapes the paper from the rolls into tubes.

Constructing The Paper Shipping Sack

The term "multiwall" refers to multiple plies of kraft paper and other materials used to construct paper shipping sacks. Multiwall sacks are flexible containers usually made of more than one ply (wall) of paper or other protective barriers.

The Tuber

The initial production of a paper sack occurs on a tuber, a machine which takes several rolls of paper and other materials and combines them over a rigid former into a flat or gusseted tube (a tube with creases or folds in the sides not found in flat tube sacks), bonded together with adhesives. Tuber speeds typically run from 150 to 250 feet per minute.

In addition to forming the paper plies into a multi-wall tube, other operations take place during the tubing stage which might be necessary for final production of a particular sack's specifications.

For example, a straight-cut tube might be suitable for sewn open mouth sacks, but if a stepped-end sack is required, the individual plies must be perforated – not cut – before being brought together. Instead of being cut from the tube, the tube is snapped apart at these perforations. Other examples are overall perforations for valve sacks and non-skid coatings applied to the outer ply of the tube during the tubing stage.



Scoring wheels crease the paper prior to folding.


Some sacks require perforations to permit air to escape during filling of powdery products.

The tubes are then taken to the bottom-forming operation as required by the particular specification of sack they will become. Some are sewn closed, making sewn open mouth or sewn valve sacks. The tubes are placed in bundles on automatic feeders which deliver them one at a time at speeds of 30 to 50 sacks per minute.

Other tubes are taken to a pasted bottomer and form either a pasted open mouth, stepped-end pasted valve or a pinch bottom open mouth sack.

For example, the valve sack is pasted closed at both ends during manufacture with only a corner opening or valve at one end, through which the sack is filled.

The pinch bottom sack is designed with overlapping plies so that the outer ply extends beyond the inner plies in a stepped fashion. Stepped plies are coated with a hot melt adhesive so that when the long side is folded over the short side and pinched closed after the sack is filled, a strong multi-ply bond is obtained.

 

 



A sack tube moving through a bottom-forming operation.

Multiwall Sack Types

Sewn Valve Gusseted Sack
This sack is completely fabricated in the plant with a small opening or valve in one corner through which the sack is filled. The valve is made with an inner flap which operates as a self-closing check valve after filling. The sack is available with a plain valve or a valve with an extended inner sleeve.

 

 

Sewn Valve Gusseted Sack with Tuck-in Sleeve

After this sack is filled, the sleeve is manually folded down and back into the pocket under the sleeve for a securely held, positive closure against sifting. On occasion, sewn valve sacks are made of flat tubes (non-gusseted).

 

 

Sewn Valve Gusseted Sack with Tuck-in Sleeve
After this sack is filled, the sleeve is manually folded down and back into the pocket under the sleeve for a securely held, positive closure against sifting. On occasion, sewn valve sacks are made of flat tubes (non-gusseted).

 

 

 

Sewn Open Mouth Flat Tube Sack
The bottom closure on this sack is made in the sack factory. After the sack has been filled, the top closure is made by sewing or other means. For added protection, sewn closures are sometimes dipped in a sealing compound which is absorbed into the tape, thread and filler cord, making the closure moisture-resistant, yet flexible. Various other moisture-resistant closures have been developed as well. Some multiwall sacks made with polyethylene liners or polyethylene-coated papers can be heat-sealed for better closure.

 

Sewn Bottom and Top Open Corner Sack
An "intermediate" variety of package somewhere between the basic valve and open mouth type sacks. These sacks are often used for commodities packed in a molten state which solidify upon cooling, taking the shape and size of the sack. These sacks are usually filled by a pipe inserted into the open corner. The walls forming the corner are then brought together and stapled. Tape is stapled down over the closure for further reinforcement. 

 

Pasted Valve Sack
This is completely fabricated in the sack factory with an opening or valve in one corner through which the sack is filled. The valve is made with an inner flap which functions as a self-closing check valve after filling. It is available with a plain valve or with an extended inner sleeve of paper or film..

Pasted Valve Sack with Tuck-in Sleeve
After this sack is filled, the sleeve is manually folded down and back into the pocket under the sleeve for positive closure against sifting. Sleeves that are thermoplastic-coated may be heat-sealed.

 

Pasted Open Mouth Satchel Bottom Sack
Made from a flat tube (non-gusseted), with a satchel-type pasted bottom. After filling, the top closure may be made by gluing, by tying with twine or wire twisted manually or by machine. Top closures may also be made by sewing, stapling or sealing with gummed or pressure-sensitive tape according to product requirements.

Pasted Open Mouth Automatic (S.O.S.) Sack
This sack is gusseted with a factory-made pasted square bottom. After filling, the top closure may be made by sewing, gluing, stapling, wire-tying or sealing with gummed or pressure- sensitive tape. For added strength and resistance to sifting, bound over tape or tape over sewing may be applied to the closure.  

Pasted Open Mouth Stepped-End Sack (Gusseted or Flat Tube)
A pasted bottom open mouth sack manufactured with stepped walls at the top. After the sack is filled, these walls are folded to form a multi-ply, sift resistant closure by applying a suitable adhesive or by reactivating factory applied adhesive.

Double Fold Sack
The bottom of this sack is folded over twice and then adhered to the sack wall. The top of the sack can be manufactured for stepped-end pasted open mouth or sewn open mouth closure.

Balers
Balers are sewn or pasted bottom open mouth sacks designed for the shipping or transporting of groups of small, filled paper bags also known as consumer bags. Small packages nest within the baler, providing a solid, compact unit during handling and shipping. Baler capacity is usually not more than 60 pounds.