A steady drip

Drip irrigation pipes, as used in agriculture, look simple but the reality is more complex. Producers buy raw materials by weight and sell end-products by length, which means that three factors are of core concern: pipe dimensions, including diameter tolerances and wall thickness (concentricity); scrap minimisation; and line utilisation. Well-engineered manufacturing plants are more effective and cost-efficient than supposed low-cost solutions. The following study considers equipment for mass (high volume) production. Similar equipment is available for manufacturers looking for short production runs, involving less automation and simpler machinery.

Figure 1 is a schematic presentation of a typical high performance line, producing thin wall drip irrigation pipes at 250m/min and with 2,000 drippers/min. The key points for optimisation include: drippers adapted to the process; sorting, accumulation and feeding of the drippers; high-speed pipe extrusion; and fixed-centre extrusion head. Water management; in-pipe dripper detection; ink-free printing and ID ‘on the fly’; spool or coil handling; and inline quality management are also influential.

Fig 1: Line LIR 106 HP

Drippers adapted to the process

Injection-moulded drippers involve highly technical design and extremely tight tolerances. They are designed for high output rate in the injection process, low mass volume, and high output rate and reliability in the extrusion process. Symmetric drippers offer higher feeding capacity, larger flow path (less clogging), they are suitable for thin and thick walls (0.125-1.2mm), for tube diameters Ø16mm and above and for both low and high water flow. Asymmetric drippers offer higher injection capacities, meaning less cycle time, low volume advantages, they are suitable for thin walls (0.125-0.65mm) and tube diameter Ø12mm and above, offer reduced tube pressure loss, less packaging volume and lower raw material cost.

Fig 2: Symmetric dripper (left), assymetric dripper (right)

Mould architecture is based on modular construction with sections of four cavities, whose inserts can be easily exchanged for others with different flow rates. The injection system itself can be delivered as full hot runner “shut valve” or “flat drip”. Among the distinct characteristics of hot-tip hot runner systems are low maintenance and higher injection pressure. Shut valve hot runners offer shorter cycle times; low gate clog sensitivity; and lower injection pressure. However, both costs and maintenance requirements are higher.

Fig 3: Injection moulding machines with full electrical operation offer the highest capacities for high-performance moulds.

The main considerations for dripper design are: physical dimensions, raw material and injected dripper quality.

Sorting, accumulation and feeding

Sorting and feeding rates of 2,000 drippers/min can be achieved with well-specified symmetric drippers. The same machine can achieve approximately 60% of the symmetric sorting capacity with asymmetric drippers. Length and weight are also important; attempting a linear sorting speed of more than 60-70m/min leads to system instability. Precision injection moulds and suitable design are essential for ensuring the dripper’s shape complies with tight design tolerances.

Figure four shows a sorting and feeding machine, which enables damaged drippers to be extracted and flow maintained. An accumulator with a 1000-unit capacity is located between the centrifuge and the inserting unit. In the event of any blockage or feed interruption the accumulator will continue to deliver drippers to the inserting unit while the centrifuge is repairing itself and production continues. The accumulator preheats the drippers to facilitate perfect welding against the tube wall.

Fig 4: Bowl and inserting

Feeding distance can be set digitally and the machine automatically corrects pitch to specification. Each dripper is counted and memorised, with the result compared with the number of laser-punched holes. Mismatch means the product is out of tolerance.

High speed pipe extrusion

It has been possible to achieve extrusion rates of 250m/min and over with specially-designed extruders. Considerations include screw design, plastic material feeding, raw material compositions, head and tool design and calibration. While 36:1 extruders are generally used, older designs with 30:1 can be considered with modifications to the screw design.

Fig 5: Extruder for high-speed production

Extrusion head design

High-speed machines with heads designed for cable sheathing are not ideal for drip irrigation. Flow is not optimal for thin wall pipe extrusion; stabilisation time is lengthy, centring is difficult and both stripe marking and skin extrusion have been found to be problematic.

Heads have to be compact. Excess length creates problems with the insertion of drippers and can lead to unstable welds. Micro porosities can also generate substantial amounts of scrap, with rates getting worse as walls get smaller and thinner. Correct material selection is essential and changes in formulation generally require a new recipe. Not all materials are suitable for these types of products, including regrind.

Strip material used in conventional stripe marking can penetrate too deeply into the wall and the use of non-modified masterbatch can lead to porosity along the stripe lines. To avoid this, the stripe is applied on the surface only, penetrating 10% at most into the pipe wall. The method is the same as for any skin layer application (fig 6).

Fig 6: Surface striping

Conventional sheathing heads divert material coming from the extruder by 90° and split it into two flows, which are subsequently brought together to form a circular section. The two flows generate welding lines, which can become weak points. Problems in even distribution have also been identified. As head pressures can be very high (over 400bar) in thin wall production, centring can take over an hour to get properly set. Accuracy within 0.02mm is required for wall thicknesses of 0.15mm and less. A well-adapted cross-head can be centred while the line runs at full speed, saving time and minimising scrap. Modern fixed-centre heads require no adjustment after mounting but they require very precise head parts and well-adapted, precise tools. Options for manual centre heads are a motorised centring device or automatic centring with an eccentricity monitor, which can be also used for quality management (see fig 7).

Fig 7: Eccentricity gauge (top) and monitor (bottom)

Water management

Modern extrusion lines are equipped with a decalcification circuit, reverse osmosis and micro filtering systems. Water temperature is controlled in a closed circuit and consumption is reduced to a few litres/hour.

Fig 8: Water treatment

In-pipe dripper detection

Figure 9 shows an in-pipe dripper detection solution that uses a specially developed electrode, which is placed in the same position as the mechanical sensor in conventional systems. The electrode measures the real time difference for absorbed energy in the pipe and determines both the location and length of the dripper.

Figure 9: Contactless dripper detection

High speed laser printing ‘on the fly’

Inkjet printers are generally limited to speeds of 100m/min, ink is expensive and it tends to contaminate the production environment and machinery. ‘Permanent’ laser marking does not require any extra costs per metre produced. It can mark at any wall thickness at high speeds, while maintaining a ‘clean’ environment. It is abrasion resistant and cannot be erased. The laser printer does not need ink or solvents and is maintenance-free. The environment is unpolluted and there is no health hazard.

Fig 10: Laser printing sample

Handling spools and coils

Irrigation drip pipe is wound on cardboard spools, in the case of thinner walls (generally up to 0.6mm) and on coils for larger thicknesses. Winders are generally fully automated. Frequent changes require a measurement protocol in order to check, pack and label the spools or coils.

Figure 11 shows spools being packed and labelled in a stretch film applicator. Several extrusion lines can be served from a single packaging unit.

Fig 11: Spool packaging

Thick wall pipe coils are usually strapped but the strapping operation is a weak point; it requires a lot of maintenance and post-production shrinkage leads to loosening, allowing movement during handling and loss of shape.

Fig 12: Strapped coil

High-stretch film strapping is almost maintenance free, can be handled easily and can incorporate UV resistant films. Figure 13 shows spools produced on a two-minute cycle; a very high degree of automation can be achieved in the winding cells, which also helps to improve quality.

Fig 13: Stretch foil packing

Fig 14: Automatic spool handling

Quality assurance

All measurable parameters are monitored and controlled during production but some can only be measured in-line with high additional investment. For this reason a quality monitoring unit provides offline measurement to several production lines. Samples from each spool or coil are checked for pipe wall thickness and concentricity; inner wall diameter; flow per hole; flow pressure; burst pressure; and spool weight. Parameters are reported per spool or per shift. They are presented as graphs and can be transmitted to a central quality assurance department; together with inline quality management, nearly all output can be documented and the packed products can go directly to dispatch.

Fig 15: DFC 1001quality monitoring unit

Inline quality and plant management system

The Plant Management System (PMS) is designed to provide information and data to drive operational improvements for reducing production costs, increasing productivity and cutting scrap rates. An initial KPI template generates a summary report with basic information and rejection percentage over a period of time. The alarm and event monitoring functions are designed to provide direct connection to the servers.

Security functions ensure the use of the line at four levels of access: designer, manager, supervisor and operator. Exchanges between operators and managers are efficient and fully involve staff in production improvement.

Summary

The drip irrigation pipe production plants went through a highly dynamic evolution, from very simple extrusion to completely automatic production lines. Despite the high investments for these lines the production cost/km is now considerably lower than with so-called low cost lines.

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