Industry News

PP woven square bottom valve bags, rice raffia cement packaging valve bags, PP woven valve cement bags and other valve bags play a key role in building materials, chemical, food and other industries. Its unique design not only improves the packaging efficiency, but also protects the quality of materials by moisture-proof and leakage-proof features. As a valve bag manufacturer, have you ever encountered the following problems: customers complain about valve mouth powder leakage; printed pattern peeling affecting the brand; blistering of the coating layer causing moisture-proof failure? This article will present the production process of valve bags in a concise manner, revealing the precise manufacturing logic and quality control points behind it from raw material preparation, flexo printing, forming, quality inspection and other aspects.

Central Impression Flexo Printing Machine for Powder Resin Packingvalve Bag

Raw Material Preparation

The choice of raw materials for valve bag directly determines the quality and performance of the product. At present, the mainstream materials of valve bag are PP, PE, kraft paper, paper-plastic composite materials and so on.

Polypropylene(PP)

PP has the advantages of high strength, good toughness, acid and alkali resistance, tear resistance, etc. It is strong and durable, very suitable for multiple transportation. However, it has limited temperature resistance and tends to become brittle in low temperature environments. It is not resistant to high temperature. Therefore, it is more suitable for chemical packaging valve bag such as chemical vitamin powder, PP bag chemical bulk bag with valve and other chemical raw materials packaging.

Polyethylene(PE)

PE has excellent moisture resistance, outstanding low temperature resistance, chemical stability and good flexibility. However, it has low strength, poor high temperature resistance and weak UV aging resistance. Therefore, it is more suitable for commonly used in tea packaging valve bag, coffee bag with valve flat, black matte coffee bean valve bag and other food grade packaging.

CI Type Flexo Press for Heavy Duty Fertilizer Block Bottom Valve Bag

Kraft Paper

Kraft paper is made from trees, which is a renewable resource, and is more environmentally friendly than petroleum-based plastics (e.g., PP, PE), which can be recycled and reused to reduce the overall packaging cost. It is tear-resistant, highly durable, and has extremely high moisture resistance and sealing properties, suitable for wall putty valve bag, cement valve bag and other building materials packaging.

Paper-Plastic Composite materials

Combining the strength of paper with the moisture resistance of plastic, paper-plastic composite materials are able to meet the needs of multiple scenarios. It combines high strength, moisture resistance, environmental friendliness and economy, and is favored by cement sugar packaging valve bag, fertilizer packing valve bag, nuts valve bag and other manufacturers.

Central Drum Flexographic Printer for Fertilizer Packing Valve Bag

Flexo Printing

In the production process of valve bag, flexographic printing is the most important link that can enhance the brand value and the added value of the product, but it is also the process that is most prone to problems. The following three dimensions from the selection of flexographic printing machine, ink option, material pretreatment and tension control, and optimization of printing plates and anilox rollers to dismantle the core points.

Selection of Flexography Machine

The production of valve bags such as block bottom PE valve bags and powder valve bags demands exceptionally high printing accuracy and stability. A central cylinder flexographic press can be equipped with automatic web guiding and tension control systems, offering a high level of automation.

Meanwhile, the CI drum flexo printer, with its concentric multi-color stations arranged around a central impression drum, achieves perfect register with zero gear gap. It supports a maximum printing speed of up to 160 m/min, making it especially suitable for high-volume continuous printing of valve bags.

Typical substrates for valve bags include kraft paper, PP/PE woven fabric, and paper-plastic laminated films. The central Impression flexo printing machinery offers excellent material versatility, enabling one machine to print on various substrates, making it a cost-effective choice for flexible packaging production.

CI Type Flexo Presses for Tea Packaging Valve Bag

Ink Option

Flexographic machine uses water-based ink that balances environmental friendliness with performance. It is non-toxic and safe, which makes it very suitable for printing on Packaging Plastic Bean Coffee Valve Bag, Juice Milk Powder Valve Bag and other food grade valve bags.

Central Drum Flexographic Printing Press for Recyclable Valve Coffee Bags

Material Pretreatment and Tension Control

The typical substrates for valve bags are kraft paper, paper-plastic laminated materials, or PP/PE woven fabric. To enhance ink adhesion, a corona treater of the flexo paper bag printing machine is used for surface pretreatment of the substrates.

A closed-loop tension control system, equipped with servo motors and tension sensors, ensures stable tension control throughout unwinding, printing, and rewinding processes—maintaining variations within ±5N. This prevents material stretching and deformation, thereby ensuring precise registration during multi-color printing.

Optimization of Printing Plates and Anilox Rollers

Using laser-engraved ceramic anilox rollers with high line screens (e.g., 400–600 LPI) in combination with a closed doctor blade chamber system ensures uniform transfer of water-based inks, enhancing both color saturation and fine detail reproduction.

Selecting the appropriate durometer of photopolymer printing plates based on the substrate is essential—for instance, a 35–40° Shore A plate is recommended for paper-plastic laminated materials, striking a balance between dot gain control and print durability.

Forming

Forming is mainly divided into 3 steps: bag making machine forming, valve mouth making, bottom forming and sealing.

Bag Making Machine of Windmöller & Hölscher

Bag Forming

First, the substrate is unwound through the unwinding unit of the bag making machine, with a web guiding system ensuring accurate alignment. Then, based on the required structure of the valve bag, the substrate undergoes micro-perforation and slitting. Finally, the material is folded into a tubular shape using a gusseting and folding mechanism.

Valve Construction

At one end of the bag body, a PE or PP valve film is heat sealed or ultrasonically bonded to form a one-way valve structure. An ultrasonic cutting knife (20kHz) is then used to precisely cut the valve opening, ensuring excellent sealing performance during unidirectional filling.

Bag Making Machine of NEWLONG

Bottom Forming and Sealing

To enhance the stability of products such as 100% Recyclable Valve Coffee Bags, Heavy Duty Fertilizer Block Bottom Valve Bags, and Coated Woven PP Valve Bags, a block bottom structure can be formed by folding and hot air welding.

For improved load-bearing capacity, reinforcement patches can be additionally applied to the bottom area.

Quality Inspection

Quality inspection is mainly divided into visual inspection and performance testing. Visual checks focus on verifying the alignment of the valve area, and ensuring that there are no printing defects such as missing prints or color misregistration.

Performance tests include drop tests and leakproof testing. For example, the bag must withstand three drops from a height of 1.2 meters without rupture, and remain leak-free after being inverted for 24 hours post-filling with powder.

CI Type Flexo Printing Machine for Cement Packaging Valve Bags

From the careful selection of raw materials to precise flexographic color registration, from tight valve sealing to rigorous final quality control, every step in the production of valve bags—such as cookie packaging valve bags, powder resin packing valve bags, black tea packaging valve bags, and cereal valve bags—reflects a commitment to moisture resistance, leak prevention, and brand presentation.