Multi-response optimization of drilling performance in chemically treated kenaf/epoxy hybrid composites reinforced with Citrus Limetta peel fillers

Drilling-induced damage can compromise the hole quality and structural integrity of natural fiber-reinforced polymer composites. This study investigates the drilling performance of sodium bicarbonate-treated kenaf/epoxy hybrid composites containing 10 wt.% citrus limetta peel (CLP) filler. A full-factorial experimental design comprising 81 combinations was employed to evaluate the effects of feed rate (0.05–0.30 mm/rev), spindle speed (700–2800 rpm), drill diameter (4–8 mm), and drill point angle (90°–135°). Thrust force, surface roughness, and entry and exit delamination factors were considered as performance responses. Grey Relational Analysis (GRA) was used for multi-response optimization, while analysis of variance (ANOVA) quantified the contributions of the machining parameters and their two-factor interactions. The optimum combination was identified as 0.05 mm/rev feed rate, 1400 rpm spindle speed, 4 mm drill diameter, and 135° drill point angle. Under these conditions, the measured thrust force was 14.65 N, surface roughness was 1.2042 µm, and entry and exit delamination factors were 1.011 and 1.028, respectively. Drill diameter was the most influential main parameter, contributing 26.01% to the variation in Grey Relational Grade (GRG), followed by feed rate (21.68%), drill point angle (15.64%), and spindle speed (10.02%). Independent confirmation tests yielded a mean GRG of 0.84118 ± 0.02151, compared with the prediction from the additive model of 0.78275, corresponding to a prediction error of 7.47%. The findings demonstrate the applicability of full-factorial experimental design and GRA for optimizing drilling conditions in CLP-filled kenaf/epoxy composites.