25kg External Valve Paper-Plastic Sacks for a South China Cement Group — Cutting Dust Leakage and Breakage on High-Speed Packing Lines

Our client operates a cement production base under one of South China's leading cement groups, with an annual capacity in the millions of tonnes and multiple automatic valve-bag packing lines. Their previous standard valve sacks had three recurring problems: an oversized gap between valve sleeve and filling spout caused cement dust to blow out during filling; weak bottom pasting led to high breakage rates in stacking and long-haul transport; and insufficient moisture barrier allowed the cement to cake during storage in the humid southern climate. They needed a 25kg external valve sack that would fit their existing bag placers precisely while solving dust, breakage and moisture at the same time.

Our technical team drew the specification from the client's bag placer and measured spout dimensions, issuing a controlled structural drawing (Document No. EN-VB-25K-BB-01) built on four modules.

① Formed Geometry & Nominal Dimensions (Figure 1): Bag length L = 580 mm ±5, bag width W = 400 mm ±3, bottom gusset G = 100 mm ±3, valve sleeve centred 150 mm from the bag edge. The top carries a sealed cover strip with vent holes — air escapes during filling, dust stays in afterwards. A 120 × 80 mm blank zone on the body is reserved for secondary coding and labelling.

② Valve Sealing Mechanism & Cutting Blank (Figure 2): A self-closing valve flap is pushed open by incoming material and then pressed shut by internal powder pressure once the spout is withdrawn — powder-pressure self-sealing that stops blow-back at the source. Sleeve OD φ100 mm, effective sleeve length 130 mm, valve patch 200 × 130 mm positioned on a 520 mm web so the formed sleeve lands exactly where the placer expects it.

③ Four-Ply Composite Structure (Figure 3): Outer extensible kraft paper 90 gsm (printable, scuff-resistant) + PE extrusion bonding layer 18 gsm + middle extensible kraft paper 80 gsm (strength and stiffness) + inner PE barrier film 25 μm (moisture vapour barrier) — total ≈210 gsm ±5%. Ply count (3–5) and PE gauge (20–35 μm) are selectable against strength, barrier and cost targets.

④ Technical Data Sheet (Table 1): Rated fill 25 kg, bag weight 210 g ±5%. Tensile strength ≥700 N/50 mm MD and ≥600 N/50 mm CD; interlayer peel ≥2.5 N/15 mm; drop test 1.2 m × 3 drops without rupture; stack load 8 layers × 72 h without deformation; water vapour transmission ≤5 g/m² per 24 h at 38 °C / 90 % RH; fill temperature ≤80 °C; valve tightness verified by inverting the filled bag. Manufactured to GB/T 9774 and tested to ISO 7965-2.

✔ After switching to our custom block bottom valve sacks, the client achieved:

✔ Dust leakage during filling essentially eliminated, cleaner workshop and lower material loss.

✔ Breakage rate reduced from about 1.5 % to below 0.3 % , with far fewer stacking and transport complaints.

✔ No caking after 6 months of storage in a humid climate .

✔ Valve sleeve matched to the bag placer, less jam-related downtime, line efficiency up roughly 15 % .