More often a small tolerances in pipe extrusions is required. This put the producers in a difficult position, as they can only match this requirements by use of automation control systems. Otherwise staff costs and wastage will rise. Here is an installation result report, where such high accuracy demands were requested.
A Netherlands producer has manufactured PE irrigation pipes with 3.2 mm outer dia and 0.9 mm inner dia. For matching a constant water throughput, an inner diameter of +/- 0.01 mm has to be guaranteed.
The solution for such a demand is an Ultrasonic System. This system measures all pipe dimensions and controls the extruder due to 2 feedbacks and holds these constant within the set point values.
The Ultrasonic System
The main unit of the system is a multi channel ultrasonic electronic equipment. In conjunction with a measurement chamber (gif 14k), in which are placed four, six or eight transducers, the wall thickness is measured, both the outer and inner diameters.
A comfortable operator interface displays the results by means of VGA colour screen and in addition contains a lot of other functions. These being, for example, tolerance monitoring, trend display, printer record and a regulation with 2 control feedbacks, for this particular request.
Each control feedback can be flexible and individually adjusted , especially as to what dimension it should operate. For this example the first unit controls the inner diameter by vacuum vent; the second unit the outer diameter by extruder screw speed.
Solving the temperature problem
The cooling water temperature deviates between morning and noontime or summer and winter by app. 5 degree Celsius. This deviation is transferred on the PE pipe and there arises a wallthickness error by 0.3% per degree Celsius.
It's well known that the ultrasonic wall thickness measurement is dependent upon calibration of the sound velocity, thus temperature is reflected. It is first essential that the conditions by an once done calibration, stay constant. Influences on the temperature can rise by process changes such as, haul off speed, cooling temperature, mass temperature and raw material. For the present case, besides the cooling of the water, all other influences stayed constant or were too small to raise problems.
A wall thickness error by 2/100 or an inner diameter error by 4/100 mm, was expected and could be prevented. Compensation for this problem was achieved by a new procedure, that considered the water temperature and readjusted automatically, the sound velocity.
A water temperature measurement was already integrated in the system because of the diameter measurement. Due to this, and an easy software application, a readjustment of the sound velocity of the material was achieved and by this the requested accuracy could be matched.
The Result
During the Extrusion's start up period, the system directs the dimensions within the first 1 - 2 minutes onto the set points of pipe dimensions. Further it keeps these during many hours within the tolerances of 1/100 mm inner diameter constant. This result could only be gained by long term stability of calibration.
It is also important, that the system has the advantages of a total dimension measurement, recording and control. All this proves the system to have excellent accuracy, high functionality and yet is still simple to operate.
Rolf Diederichs 18. Dec 1995, dieder@teuto.de
