Ângela R. Rodrigues

Despite extensive research, predicting ejection forces during the injection moulding process remains challenging due to the complex calculation of the coefficient of friction. This coefficient comprises various components that are difficult to isolate, complicating independent predictions. Consequently, using literature-documented coefficient of friction for different material contacts often leads to discrepancies between the experimental data and the predictive models of ejection forces. The adhesion component is a particularly problematic aspect, and is frequently excluded from analyses due to the difficulties associated with measuring it directly. This study aims to present the development of a tool designed to measure both the friction and adhesion forces under conditions that replicate those of the injection moulding process. The measured values were then used to calculate the corresponding coefficients: the static coefficient of friction (COF) and the coefficient of adhesion (COA). The conducted measurements identified the most influential factors in both coefficient values using a Design of Experiments (DoE), revealing that replication temperature was the factor promoting the highest variation across all the tested polymers.

In this research, novel biocomposite filaments were developed by incorporating coffee silver skin (CSS) waste into polylactic acid (PLA) for use in Fused Filament Fabrication (FFF) technology. CSS was blended with PLA at concentrations of 0, 5, 10, and 15 wt.% to address the waste disposal challenge and produce environmentally friendly composite biofilaments for FFF, supporting circular economic efforts. These filaments have the potential to be used in sustainable prototyping, functional parts, and consumer products. A comprehensive analysis was conducted to examine the effect of printing temperature on dimensional accuracy, melt flow index (MFI), and mechanical properties. Higher printing temperatures and increased CSS content led to larger dimensions due to increased material fluidity, as confirmed by MFI results, which increased from 3.5 g/10 min (0% CSS) to 5.8 g/10 min (15% CSS) at 180 °C, reaching 26.3 g/10 min at 220 °C. Tensile tests on 3D-printed specimens indicated an improvement in elastic modulus with increasing CSS content at lower temperatures (180 °C), rising from 1622 MPa (0% CSS) to 1952 MPa (15% CSS), representing about a 20% increase. However, at higher temperatures, the elastic modulus decreased, possibly due to the poor dispersion and agglomeration of filler particles. Tensile strength generally decreased with CSS addition, especially at higher loadings, while yield elongation remained low (~1.4–1.7%), indicating a more brittle material. The findings also revealed no significant thermal changes with increasing CSS content, and good printability was achieved for all compositions, which was characterized by good layer adhesion, the absence of warping, and the ease of extrusion.

Ejection is the most critical phase in the injection moulding process and depends on several variables that can affect and difficult the demoulding process. Some phenomena in the mould, such as shrinkage, adhesion and friction, can lead to higher ejection forces. Friction, deformation and adhesion mechanisms are components of the coefficient of friction that occurs between the mould and the part surface. An increase in the coefficient of friction will result in higher ejection forces and consequently increase the energy consumption during the demoulding process. Mould and part material, mould surface roughness, coatings and ejection temperature are some of the variables that can affect the coefficient of friction in the injection moulding process. This review paper presents some considerations on the mechanisms, the process and material variables that can affect the coefficient of friction.