Quality assurance in drip irrigation pipe extrusion

Introduction

Industrial production lines for the manufacture of pipes, tapes and tubes for drip irrigation application have undergone extensive development over the past three years. New technologies were presented and indeed also implemented in the market place, both in the nature of the tubes, tapes and pipes as well as in the actual processes and manufacturing techniques. Typical production speeds of 60m/min for round and flat drippers were increased to some 300 m/min along with insertion rates of the drippers from 200 to 300/min to some 2000/min. The results were that new ways had to be adopted for example in sorting methods for this increase in dripper quantity and the respective down-stream drilling or punching for the accurate water outlet. Latest state-of-art technology platforms have been adopted quite unexpectedly and that hitherto would have been deemed unthinkable for such ‘simple products’. A significant challenge was the adherence to quality systems resolved with the use of the most modern, high frequency, speed, precision and high resolution visual camera detection systems along with the data processing capabilities necessitated for these demanding applications. New laser marking equipment was developed and implemented that enables the assured and ‘permanent’ printing or marking of products. All this allows for a very high rate of productivity and at the same time assures the highest levels of quality standards.

Line Configurations

For simplicity reasons not a full description is given for a typical production line for the extrusion and manufacture of drip irrigation tubes. The line layout (Fig.1) production and manufacturing steps for drip irrigation tubes describes in general how the individual quality assurance functions are interlinked between each of the main components in the line.

All main process steps (from left to right) are key components for full quality assurance starting with labyrinth tape or dripper sorting and insertion to tube or pipe extrusion and subsequent emitter adhesion to the tube through to tape or tube vacuum cooling, cooling and drying with subsequent high speed punching, drilling or slitting and finally high speed automated turret winding of the finished product on to card board spools or in form of coils.

Laser Marking

To date printing on to the pipes was achieved primarily by means of ink jet printing to a very limited degree. The problems and challenges with such solutions and equipment are well known.
One of the main challenges is the actual adhesion of the print to the tube surface. To obtain a reasonable adhesion, the surface of the substrate must be subjected to a mechanical, chemical or other pre-treatment typically a flame or corona pre-treatment. This however leads to additional technical challenges such as additional investment and utilities infra-structure necessitated for the corona treatment equipment itself. Furthermore, the use of conventional inkjet printers hardly assures acceptable results at line production speeds higher than 100 m/min.

Further negative factors are seen in the maintenance and consumables costs, such as inks and solvents, significant additional cost factors that must not be neglected. Thus to minimize for instance any potential equipment down-time a second (stand-by) unit must be acquired and be ready on-call and at a moment notice. All in all the use of ink-jet printers will necessitate and accrue additional on-costs calculated between EUR 40,000 and 50,000 (CHF 50,000 and 60,000).

In order to eliminate the above mentioned disadvantages, laser marking equipment was developed through which high speed permanent and abrasion free printing is achieved. There are no on-costs for additional maintenance or necessity for supplementary consumable items. The printing and marking is done without problem on all current products and at production speeds up to 300 m/min. There is no external or surrounding contamination with inks and no solvents are used.
A pre-requisite nevertheless is the use of a special laser technology. The use of CO2 lasers will initiate a thermal etching (engraving) of the printing into the pipe wall. This may well lead to issues especially with thin-walled products where the pre-scribed and specified minimum wall thickness can no longer be attained or maintained. In order to overcome this problem the overall tube wall thickness must be increased which leads to additional costs in the form of material usage on-costs amounting to some EUR 80,000 (CHF 100,000) and more annually.
The ‘special’ Laser Marking used, provides the advantage to print a product in such a way that the marking and identification is readable even after prolonged and extensive use of the product in the field. It should be added that many international standards now specific a permanent marking.
The ‘special’ Laser Marking as described, can be used safely and assuredly at speeds up to 300 m/min for the production of black polyethylene tapes, tubes and pipes without the use of any supplementary products and consumables. A typical Marking Unit is depicted in Fig. 2.

 

 

For the Laser Marking Unit Type MTL the following advantages can best be summarised:

• No consumables such as inks and solvents
• The surface pre-treatment of the PE pipes with flame or corona discharge is no longer needed
• The printing is fully abrasion resistant
• The marking does not penetrate in to the actual pipe wall, thus no wall thickness reduction
• Considerable cost savings in maintenance are achieved compared to ink jet printing solutions
• No health hazard or risk to operators
• No operational safety hazards or issues (e.g. fire, explosions, contamination…)

Quality Assurance

The following steps are envisaged for quality assurance:

• Spool weight control (spool, coil or reel)
• End product diameter control
• End product wall thickness control
• Water emitter rate per position
• CV Value
• Burst pressure

 

 

The data, detailed above, is captured and stored in the QC System of the DFC unit and are transferred or can be made available for call-off and retrieval by a central data management system. In addition the following line production data is captured and transmitted automatically to the QC system.

• Dripper nominal spacing deviation
• Correct positioning of water outlet opening
• Deviation of dripper quantity per defined length and/or spool

These values together with standard production process data are monitored through a PMS (Production Management System).

In detail the production data secured is processed as follows:

 

 

Data captured from the laser marking equipment for each spool, coil or reel produced is transferred along with the corresponding matching PMS production data can be transmitted to a bar code printer. After the reel or coil change over on the winding equipment a bar code label is printed and can be attached or adhered to the finished wound spool, coil or reel. This specific unique bar code will contain data as to whether the spool has been wound without any defect or whether defects or faults were logged and recorded during the winding process. The individual spool weight along with order and lot number is transmitted automatically to the Quality Control equipment DFC. A representative sample is taken from the individual spool to determine the physical properties which are also recorded within the QC equipment DFC.

 

 

Spools are transferred via conveyor belts to the down-stream packaging and overwrapping equipment. During this product transfer the spools pass a quality quarantine zone where faulty or non-conformance spools can be identified, segregated and withdrawn from the finishing process.

In the quality control system the following data is measured and captured, whereby the overall quality management system determines in which frequency and interval these control checks and measurements are carried out.

• Measurement of nominal flow rate at normal pressure (FIG3.) Here a relatively short test cycle is involved which is typically sufficient for NPC (No-Pressure Compensated) and labyrinth tape emitter type products. With this ‘rapid-test’ the overall process stability can be monitored.
• Flow at variable pressure, mainly for PC drippers (Pressure Compensated). This can also be used for regular NPC and labyrinth emitter tape type drippers. Here longer cycle times are involved during which the CV (Co-efficient Variation) value is determined. The results are depicted in FIG4.
• Burst strength of the tube, to determine the shelf life of the product, the weld adherence of the dripper to the inner wall surface and to determine and identify potential weak spots in the tube, tape or pipe.

Mechanical and hydraulic values are displayed in form of graphs by the system.

 

 

The above described process flow depicts how with relatively simple methods the overall line efficiency can be improved along with the overall product quality and consistency in quality thus enabling and leading to a production process capability of ‘just in time’ production. In addition it enables the actual end-user to trace, monitor and track over many years the performance capabilities of his product.

(article of Mr. Eberhard Kertscher of THE MACHINES YVONAND SA)

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