Plastic Pipe Defects Minimmization


M. Narasimha
R. Rejikumar


This study present a systematic approach to find the root causes for the occurrence of defects and wastes in plastic extrusion process in the case company. Particularly defects such as surface roughness and scratch, bulging, sink marks, uneven wall thickness, dimensional variation, centering problem, tears and marks were identified defects.

To identify the root causes of these defects, cause-and -effect diagram were implemented. From the diagram, the major root causes of each defect were the extrusion process parameters such as, the vacuum pressure, temperature, take-off speed, screw speed of the extrusion process and raw material properties. According to the data of the company about 76.758% these root causes are caused by operators. This is due to inappropriate setting of operational parameters. The most frequently occurring defects were identified through Pareto chart. After identifying these frequently occurring defects, by applying Taguchi's method of quality engineering for optimum setting of process parameters for the plastic product so that those defects were minimized by reducing variation of performance (quality) from target value.

Bruh Tesfa Irrigation and Water technology PLC (BTIWTPLC) company, used as a case in order to conduct Taguchi's method (design of experiment) together with quality loss function for the selected products. 

Particularly, the quality loss for the current performance variation was calculated using Taguchi's principle of loss function and requirement for improvement was verified. In this case design of experiment was applied to optimize the process parameters of the four selected products and those products were selected from pipe HDPE1 í˜ 50mm and Plain pipe ∅ 25mm. Four independent process parameters were investigate, namely Vacuum pressure, Take-off speed ,screw speed and Temperature were considered for DOE. .Commercial Minitab 15 software was used to analyze the result of the experiment based on the result of analysis optimum process parameters were selected.

Here loss function was calculated and compared with the quality loss before applying of design of experiment. From this it was understood that, using Taguchi's method of design of experiment the quality loss because of performance deviation (scrap inclusive) improves by about 85.201% for the selected products, Here also the Structural tests of main pipe mechanical properties (tensile and hydrostatic pressure property) have been tested using the universal testing machine. From the test conducted that, the plastic product characteristic were within the target application without failure.

In addition, to minimize defects caused by non-linear properties (i.e. sharkskin, melt fracture and extrudate swell) mathematical modeling and analytical calculation were made. Accordingly, the best operating parameters including; maximum pressure, residence time, the required operating torque and speed of the extruder screw were determined.