Overview
Paper valve sack manufacturing converts flat kraft paper rolls into finished multi-layer cement bags through a two-machine process: tubing (on a tuber machine) and bottom closing (on a bottomer machine). This guide walks through every step of the production process.
The Production Flow
Kraft Paper Rolls → Tuber Machine → Flat Tubes → Bottomer Machine → Finished Valve SacksThe process has two main stages:
- Tubing — Converting paper rolls into flat tubes with a valve opening
- Bottom Closing — Folding and gluing the bottom of each tube into a sealed bag
Stage 1: Tube Formation (Tuber Machine)
Step 1: Paper Roll Loading
- Mount 2–5 kraft paper rolls (one per layer) onto the tuber’s reel stands
- Thread each paper web through the machine’s guide rollers
- Set paper tension controls for consistent feeding
- If PE film is used (4–5 layer bags), mount PE film roll on dedicated station
Step 2: Paper Web Alignment
- Guide rollers align all paper layers precisely
- Edge sensors ensure consistent width and positioning
- Any misalignment causes crooked tubes and waste
Step 3: Glue Application
- Hot melt adhesive is applied to the overlap area where the tube seam will bond
- Glue nozzles spray adhesive in precise patterns
- Temperature: typically 160–175°C for hot melt
- Glue must be applied consistently — too little causes weak seams, too much causes waste
Step 4: Tube Forming
- Paper layers pass over a forming mandrel (a shaped metal form)
- The flat paper sheets are wrapped around the mandrel into a cylindrical tube
- The glued edge overlaps and bonds to create the tube seam
- All layers are formed simultaneously into a multi-layer tube
Step 5: Valve Cutting
- A rotary cutter or punch creates the valve opening at one end of each tube
- The valve is where cement will be filled
- Valve dimensions must match the filling machine’s nozzle
- Typical valve size: 100mm × 35mm to 130mm × 45mm
Step 6: Tube Cutting
- The continuous tube is cut into individual bag-length sections
- A rotary knife cuts precisely at predetermined intervals
- Bag length accuracy: ±1mm
- Typical bag length: 480–580mm for 50kg cement bags
Step 7: Flat Tube Stacking
- Cut tubes are stacked neatly on a conveyor or pallet
- Automatic counting keeps track of production
- Stacks of 50–100 tubes are transferred to the bottomer machine
- Quality check: visual inspection of tube straightness, seam quality
Stage 2: Bottom Closing (Bottomer Machine)
Step 8: Tube Feeding
- Flat tubes are loaded into the bottomer’s feeding magazine
- Suction cups pick up one tube at a time
- Proper feeding alignment is critical for bottom fold quality
Step 9: Bottom Opening
- The tube end is mechanically opened from flat to rectangular shape
- Gripper arms pull the tube walls apart
- This creates the flaps that will form the bottom
Step 10: Bottom Folding
- Side flaps are folded inward first
- Then top and bottom flaps fold over
- The fold pattern creates an interlocking bottom that distributes weight evenly
- Precision folding is essential — uneven folds cause weak spots
Step 11: Glue Application (Bottom)
- Hot melt or chemical adhesive is applied to the folded bottom
- Adhesive must cover all fold overlaps completely
- Insufficient glue = bottom opens under cement weight
- Excess glue = waste and machine fouling
Step 12: Bottom Pressing
- Pressure rollers press the glued bottom firmly
- Heat may be applied to accelerate adhesive setting
- The bond must be strong enough to hold 50+ kg of cement
Step 13: Valve Reinforcement (Optional)
- For premium bags, reinforcement patches are applied around the valve
- Paper or cloth patches are glued over the valve area
- This prevents tears during the valve filling process
Step 14: Bottom Patch (Optional)
- An additional paper patch can be glued over the entire bottom
- This adds extra strength and dust-proofing
- Common for export-quality and heavy-duty bags
Step 15: Quality Inspection
- Finished bags pass through inspection stations
- Checks: bottom seal integrity, valve function, bag dimensions, print quality
- Defective bags are ejected automatically (on premium machines)
- Acceptable defect rate: <1% for quality operations
Step 16: Counting and Bundling
- Bags are counted automatically
- Stacked in bundles of 25–100 bags
- Bundled with strapping tape or shrink wrap
- Labeled with batch information
Production Parameters
| Parameter | Typical Value |
|---|---|
| Machine speed (tuber) | 60–350 bags/min |
| Machine speed (bottomer) | 60–300 bags/min |
| Paper tension | 80–200 N |
| Glue temperature | 155–180°C |
| Tube width | 380–600mm |
| Bag length | 450–900mm |
| Number of layers | 2–5 |
Common Quality Issues & Solutions
| Issue | Cause | Solution |
|---|---|---|
| Crooked tubes | Paper misalignment | Adjust guide rollers, check roll quality |
| Weak tube seam | Insufficient glue or low temperature | Increase glue volume or temperature |
| Poor bottom fold | Worn folding mechanisms | Replace worn parts, check alignment |
| Bottom opening under load | Weak adhesive bond | Check glue coverage, try stronger adhesive |
| Valve too tight/loose | Cutting tool wear | Replace cutter, adjust valve dimensions |
| Wrinkled paper | Incorrect tension | Calibrate tension settings |
Equipment Required
| Equipment | Purpose | Cost Range |
|---|---|---|
| Tuber machine | Forms paper tubes | $25,000–$1,200,000 |
| Bottomer machine | Closes bag bottom | $20,000–$900,000 |
| Flexo printer (optional) | Pre-prints paper | $15,000–$50,000 |
| Conveyor system | Connects machines | $5,000–$20,000 |
| Compressor | Powers pneumatics | $3,000–$15,000 |
| Glue system | Adhesive supply | Included with machines |
Frequently Asked Questions
How many bags per minute can one line produce? Budget Chinese lines: 40–80 bags/min. Standard Chinese/Taiwanese: 80–150 bags/min. Premium European: 200–400 bags/min.
How many workers does a paper bag line need? Typically 3–5 workers per shift: 1 tuber operator, 1 bottomer operator, 1–2 helpers for loading/stacking, 1 quality inspector.
Can I produce bags for different cement types on the same line? Yes! The same line can produce bags for OPC, PPC, and other cement types by changing the printed paper rolls and adjusting bag dimensions if needed.
Learn about tuber machines in our Tuber Guide or compare complete production lines in the Production Line Guide.