Production process of Woven polypropylene bags contains knowlege about plastic industry, textile weaving, and even chemistry. This article is going to introduce the complete production process of how the polypropylene resin becomes a finished woven polypropylene sack step by step.
Sumarize of Woven polypropylene bags production process.
Overall, though the production process of woven pp sacks requires multiple equipments, it’s still an industry with simple process flow, easy control of process parameters, easy equipments operation, high production efficiency. Actually it’s mostly benefited from its stable and reliable equipments with safe operation and low maintenance rates. Specific manufacturing process steps of woven polypropylene (PP) bags will be as follows,
- Polypropylene tapes drawing.
- Circular knitting( weaving ).
- Coating and lamination.
- Cutting and sewing.
Production process of Woven polypropylene bags specific steps.
Step 1 , Polypropylene Tapes Drawing
1. Raw material
Raw material of woven polypropylene ( PP ) bags are divided to powder polypropylene and particle polypropylene. For different pp woven bag applications, manufacturers are taking different types of polypropylene materials and added with different additives. In the early stage of this industry, almost all the manufacturers were using 100% particle polypropylene material. With development of technology, more and more manufacturers awared that to add a certain percentage of modifying material ( mainly Caco3 ) could not only increase the stability of materials but also could greatly reduce material costs, the percentage is commonly controlled within the range of 20%-40%.
2. Manufacturing process of polypropylene tapes drawing
After raw material of Polypropylene and additives were put into the extruder, it is heated at 190-250 ℃ and sheared by the screw and barrel. After plasticization process, the material is extruded at constant pressure and velocity. After formed by the tooling die, materials enter into the cooling water in shape of melted film. After cooling, the film is slit into narrow embryo yarns by the blades. Then the embryo yarn is being stretched in the oven with high temperature until it comes out in shape of flat tapes. Then the flat tapes are heat set on the hot rollers, and shrink at low traction speed. Finally, the flat tapes are rewinded by the disk differential tension winding system. It becomes the base structure of pp woven fabric and woven polypropylene sacks.
3.Technological indexes of polypropylene tapes drawing.
The technological indexes of polypropylene tapes manufacturing are mainly divided into four categories:
- Physical and chemical modification index. It mainly includes blending modification, blending ratio, adding ratio of functional additives and blending ratio of recycled materials.
- Physical property and rheological index. It mainly includes draft ratio, inflation ratio and retraction ratio.
- Mechanical performance index. It mainly includes breaking force, relative breaking force, and elongation at break, linear velocity and linear density deviation;
- Dimension tolerance index. It mainly includes the thickness and width of flat tapes (yarn).
Step 2. Circular knitting ( Weaving )
1.Circular knitting process
First of all, the warp tape is drawn out from the bottom of each row of spindles on the warp frame. Through the porcelain hole on the warp frame, to the first vertical plate, to the yarn pressing roller, to the yarn feeding roller, to the yarn guiding roller, to the second vertical plate, to the yarn guiding roller,to the magnetic hole, to the tension rod, to the round steel buckle, to the warp fixing ring and reach the reserved cloth base. Meanwhile, the weft tapes are loaded into the shed, when the circular looms starts to run, shuttles move in circling running. With close cooperation of the warp tape supplying system and the shuttle pushing device, warp tapes and weft tapes are knnited as tubular woven polypropylene fabric. Traction device was equiped on the top of the loom which would guides tubular fabric to a press roller and then goes to rewinding device behind the warp frame. Common type 4 shuttles circular looms could knit around 2300meters of Woven polypropylene fabric rolls every 24hours.
2. Main manufacturing index of polypropylene circular knitting process
Weaving density refers to the number of warp and weft tapes in a 100 mm × 100 mm area. Different applications of woven polypropylene bags require different weaving density. A case in point, bags for flour packing will requires more tighter weaving to avoid leakage of flours. The density of commonly used woven fabric is 36 × 36 pieces / 10cm, 40 × 40 pieces/10 cm, 48 × 48 / 10 cm. (Some countries are using mesh density per square inch, like 7×7, 10×10, 14×14 etc.)
Tensile strength of woven fabric.
Tensile strength is also called tensile capacity, tensile force. For pp woven fabric, it bears tensile strength in warp and weft directions, so it is called warp tensile strength or weft tensile strength also. It’s the most important index of woven fabric which decides how heavy or capacity could a polybag carries. Unit of Tensile strength Newton/5cm.
Unit Weight per square meter of pp woven fabric.
The weight per unit area of woven fabric is expressed by gram weight per square meter, which is an important technical index of woven fabric. Gram weight per square meter mainly depends on the density of warp and weft and the thickness of flat yarn. Gram weight per square meter affects the tensile strength and load capacity of woven fabric. PP woven bags manufacturers control their costs of material by adjusting the g/m2 of the pp woven fabric what they are using. In some countries, Denier is also used to express the thickness of woven polypropylene fabric, as to how to cover denier to gsm please visit our article How to Convert Denier of PP Woven Fabric to GSM.
The width of woven fabric directly affects the bag making process. For tubular fabric, the width is expressed by half of the circumference. Width also involves the shrinkage rate. After cutting, printing and sewing, the width of the bag will be slightly smaller than that of the cloth width which just been woven, which we call it width retraction.
Step 3. Coating and lamination.
1.Manufacturing process of coating and lamination.
Working principle of coating and lamination are exactly same which is to compound the melted polypropylene and Polyethylene onto pp woven fabric. The difference is when we saying lamination, we are using BOPP film or kraft paper to compound with woven polypropylene fabric while doing coating. Coating and lamination could be made on one side or both sides of woven pp fabric. For different materials lamination, differnent ratio of resins are taken as lamination material. For instance, for Bopp to pp woven fabric lamination, perfect ratio is 3PP:1PE, but for Kraft paper to PP woven fabric lamination, the perfect ratio will goes to 3PE:1PP. Temperature setting on extrusion dies needs to be adjusted accordingly as well.
2.Main manufacturing index of coating and lamination.
Main manufacturing index of coating and lamination is the controlling skill on temperature setting, coating pressure setting, and coated material peeling off strength.
Step 4. Printing
There are two typical types of printing method for pp woven sacks.
- Flexographic Printing. Please visit our article”Can PP Woven Bags Be Printed” for detailed information about flexographic printing.
- Rotogravure Printing.Please visit our article”Gravure Printing for Woven Bags” for detailed information about Rotogravure printing.
Step 5. Cutting and Sewing
In the initial development stage of woven bag production industry, bag cutting and sewing are divided into two separate steps. With the development and updating of technology and production equipment, the cutting and sewing processes of woven bags have gradually merged into two different units on a machine.
The woven bag cutting is divided into heat cutting and cold cutting.
- Heat cutting. For those woven bags that are not been laminated, a high-temperature blade should be used to cut the bag while ironing the cut tapes to avoid scattering.
- Cold cutting. that is to cut bags with a blade at room temperature, is only for those woven bags that have been coated. The cold cutting process does not require heating blades, so the power consumption is less.
The sewing thread is polyester threads now which was made by cotton. Different thickness of the sewing thread, needle pitch would affects the strength of the sealing. Customer could choose to use single stitch or double stitch on the bottom of the bags.
The sewing and cutting machine is a type of all-in-one machine, it may intergrated with functions of sewing,cutting,gusseting,liner inserting, handle punching or even top hemming.