Citation: Borysiuk, P.; Auriga, R.; Wilkowski, J.; Auriga, A.; Troci´ nski, A.; Seng Hua, L. A Study on the Susceptibility of PLA Biocomposites to Drilling. Forests 2022, 13, 1950. https://doi.org/10.3390/f13111950 Academic Editor: Zeki Candan Received: 26 October 2022 Accepted: 16 November 2022 Published: 18 November 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Article A Study on the Susceptibility of PLA Biocomposites to Drilling Piotr Borysiuk 1, * , Radoslaw Auriga 1 , Jacek Wilkowski 1 , Alicja Auriga 2 , Adrian Troci´ nski 3 and Lee Seng Hua 4 1 Institute of Wood Sciences and Furniture, Warsaw University of Life Sciences (WULS-SGGW), 159C Nowoursynowska St., 02-776 Warszawa, Poland 2 Faculty of Biotechnology and Animal Husbandry, West Pomeranian University in Szczecin, Janickiego 33 St., 71-270 Szczecin, Poland 3 Department of Mechanical Wood Technology, Pozna´ n University of Life Sciences, Wojska Polskiego 28 St., 60-637 Pozna ´ n, Poland 4 Laboratory of Biopolymer and Derivatives, Institute of Tropical Forestry and Forest Product, Universiti Putra Malaysia—UPM, Serdang 43400, Malaysia * Correspondence: piotr_borysiuk@sggw.edu.pl; Tel.: +48-225938547 Abstract: Wood–plastic composites (WPCs) increase the range of applications of materials by creating new material solutions. As part of this research, PLA (polylactic acid)- and HDPE (high-density polyethylene)-based composites were manufactured. Softwood sawdust or conifer bark with different sizes (large and small) were used as filler. In selected cases, the addition of 3% additives, such as calcium oxide in the case of PLA or polyethylene-graft-maleic anhydride in the case of HDPE, were tested. The manufactured composites were examined for their density profile and their susceptibility to drilling, defined by the value of the axial force occurring during drilling. The obtained results re- vealed that the type of matrix had the greatest influence on the axial forces during drilling. Regardless of the composite formulation, composites based on PLA had 25% to 56% higher axial forces during drilling than those based on HDPE. Furthermore, increasing the proportion of lignocellulosic fillers resulted in a decrease in the value of axial forces during drilling, with PLA composites experiencing a greater decrease than HDPE composites. The type and size of the filler had a minor impact on the axial force values during drilling. The statistical analysis indicated that the additives had a greater influence on HDPE than on PLA. Keywords: PLA; HDPE; lignocellulosic filler; biocomposites; drilling; machinability; density 1. Introduction Owing to their properties, wood–plastic composites (WPCs) are widely used in a variety of industries. A significant portion of WPCs is made from polymers derived from fossil fuels. For example, polyethylene (PE) is used for decking, construction, and consumer goods; polypropene (PP) for automotive parts, construction, and consumer goods; and polyvinyl chloride (PVC) for decking and construction [1]. Biodegradable thermoplastics, such as poly (lactic acid) (PLA) [2,3], are gaining in- dustry attention as an alternative to WPC production. PLA is easily compostable and has no negative effects on the natural environment [4]. The composite materials within PLA are also ecological due to the fact that the filling material is made of wood fibers. Furthermore, apart from solid wood, WPC fillers can be made of shredded postconsumer wood materials [5–7], recycled fibers [8,9], bark [3], bamboo fibers [10], or particles of annual plants [2]. WPCs are typically manufactured using extrusion or injection methods [2,11], result- ing in the production of generally finished products that do not require any additional mechanical processing. However, as these materials become more widely used in a va- riety of products, the demand for machining grows. Drilling mounting holes, profile Forests 2022, 13, 1950. https://doi.org/10.3390/f13111950 https://www.mdpi.com/journal/forests