Two Production Processes of Geomembrane

Blow Molding vs. Flat Extrusion: Unveiling the Performance Differences of HDPE Geomembranes

 

I- By production process, polyethylene geomembranes are produced using either blow molding or flat extrusion. Blow molding (tubular film) accounts for over 80% of total production, followed by flat extrusion (laminated film).

 

Blow molding: After the resin is melted and plasticized in an extruder, it is extruded through a circular die to form a thin-walled tube. This tube is then inflated, cooled, drawn, split, and flattened to create the geomembrane.

 

Flat extrusion: The resin is melted and plasticized in an extruder, extruded from a slit die at the die head as a molten sheet. The sheet is then calendered and cooled, drawn, trimmed, and wound up to form the geomembrane.

 

the blow molding machine : It's very tall ,up to 25m

II -The two key aspects of HDPE geomembrane manufacturing are raw material selection and formulation design, and process control.

a- The national standard "Geosynthetics - Polyethylene Geomembrane" GB/T17643-2011 stipulates the following regarding raw materials for HDPE geomembranes:

4. Requirements for Base Resins

4.1. Resin Density Requirements:

a) The resin used to manufacture high-density polyethylene geomembranes (high-density polyethylene HD or medium-density polyethylene MD) shall have a density of 0.932 g/cm³ or above.

4.2. Requirements for Recycled Materials: Up to 10% of clean recycled materials generated from the company's own production is permitted. The clean recycled materials used must be of the same formula as the geomembrane (or a similar formula where feasible).

4.3. Requirements for Fillers: Fillers are not permitted.

 

b- The performance of geomembranes is related to the resin's density, melt index, molecular weight, and molecular weight distribution.

1) HDPE geomembranes for environmental protection projects are produced using medium-density polyethylene (MDPE) resin with a melt index (MI) of 0.1-0.2 and a density (D) of 0.932-0.940. For example, HDPE Special geomembrane material TR131, melt index (MI) = 0.2, density (D) = 0.938.

2) Properties related to resin density include mechanical properties, environmental stress cracking resistance, stiffness, hardness, water vapor transmission coefficient, processing temperature, and oxidation induction time; properties related to resin melt index include melt strength, processability, and the correlation with additives; properties related to resin molecular weight and molecular weight distribution include mechanical properties, processability, aging resistance, and environmental stress cracking resistance.

3) Due to differences in equipment and processing techniques, high-density polyethylene (HDPE) geomembranes are more suitable for blow molding because they use raw resins with a melt index (MI) of 0.1-0.2 and a density (D) of 0.932-0.940. Geomembrane up to ASTM GRI GM13 standard is produced by blow molding process.

Flat extrusion is suitable for linear or low-density polyethylene geomembranes (LLDPE or LDPE) with a melt index (MI) of 1-2 and a density (D) of 0.915-0.926. LDPE).

 

Blown geomembranes have strict requirements for raw materials. Except for the national standard allowing the addition of up to 10% clean regrind (with the same formula as the geomembrane produced) from the company's own production, no regrind or fillers are added.

 

Otherwise, the melt strength will be too low, making film drawing difficult and resulting in waste. Adding regrind or fillers can cause minor defects on the film surface, affecting the appearance and tensile strength. Flat film extrusion (laminated film) offers a wide range of raw material options. Even if regrind is used or fillers are added, the film surface remains smooth after three-roll calendering, making it difficult to detect.

 

the flat extrusion machine: it's much shorter than blow molding machine 

III - Given the same raw material selection and formulation, geomembranes produced by flat film extrusion can have varying strengths in both the longitudinal and transverse directions due to differences in equipment and processing techniques. Because there is no transverse drafting, only longitudinal traction. There are certain differences. When producing geomembranes using the blow molding method, three-layer or four-layer co-extrusion blow molding machines are often used, and the top blow molding process is generally employed. The blow-up ratio and draw-down ratio not only directly determine the final width and thickness of the product, but also directly impact the performance of the geomembrane. A consistent blow-up ratio and draw-down ratio ensure balanced longitudinal and transverse strength in the HDPE geomembrane.