The broad range of properties that characterize polymers have made texturing a significant segment for the plastics industry. The value of a textured plastic part can be very high thanks to its added functionalities. Consumer products with surface textures are widespread, offering users a wide array of looks and feels. The texturing of plastic parts finds applications for both consumer and high-end engineering products. The functionalities introduced by surface texturing range from simple aesthetics (e.g., rigid plastic packaging) to advanced biomedical applications (e.g., scaffolds for tissue engineering). More recently, texturing has been used to functionalize injection molds to improve both the process’s filling and ejection phases.

Texturing plastic parts by replicating a micro- or nano-structured tool has been demonstrated as a cost-effective solution to generate different patterns on three-dimensional surfaces. Texture replication requires the consideration of design, material, and processing factors [1]. However, the extensive industrial polymer processing knowledge has constituted an excellent basis for developing the technology. The most common technologies used to replicate micro- and nano-structured technologies include injection molding ^[2][3][2,3], injection-compression molding [4], hot-embossing ^[5][6][5,6], roll-to-roll [7], and other variants of these technologies. The understanding of replication during processing requires the evaluation of polymer properties, processing parameters, and product functionality. Several researchers have studied extensively how different parameters affect replication for different processes.

2. Applications of Texturing in Injection Molding

Texturing finds different applications in injection molding technologies, ranging from plastic products to mold functionalization. Textures have been used to improve the appearance and functionality of plastic products, allowing significant opportunities for plastic manufacturers. More recently, the increased availability of texturing technologies has opened up opportunities for mold surface functionalization. The generation of micro- and nano-scale features on tool surfaces has improved the filling and ejection in plastic injection molding. Figure 1 6 summarizes the field of application and the functionalities of texturing for injection molding. The texturing technologies presented in this woresearchk have been used for molding applications with different levels of success. While some are well-established (e.g., chemical etching or micro EDM), others require more application-specific development in conjunction with polymer processing considerations.

2.1. Aesthetic Texturing

A plastic product’s aesthetic appearance can be modified by adding textures in the sub-millimeter on a micro-scale range. Smaller features, not visible to the human eye, are typically used for other applications. The texture design process is often based on creativity and marketing. However, the aesthetic surfaces often mimic natural textures like leather, wood, or animal skins. Other surfaces are textured to create grained, dull, soft, smooth, structured, or rough-shaded effects that are perceived as elegant cosmetic surface finishes. These textures change the customer’s perception of a product, thus creating market value. Examples range from the interior surfaces of different vehicles to PET soda bottles. However, these surfaces’ applications are limitless and have been successfully implemented in automotive, electronics, packaging, and more industries ^[8][61]. The features’ dimensions and design determine the selection of the most appropriate texturing technology for aesthetic applications. The most commonly used technology for aesthetic texturing is chemical etching due to its ability to work with large surfaces. However, more recently, laser-based technology has also been implemented thanks to its flexibility and smaller environmental footprint. For example, Brenner et al. used laser to texture a large and 3D mold used for an automotive application ^[9][62]. The successful replication of a textured injection mold requires particular processing consideration. Specifically, high mold temperatures may be required to replicate the texture geometry completely. These applications are supported by rapid heat cycle molding technologies, which allow heating of the mold above the polymer transition temperature during filling and rapid cooling before ejection ^[2][10][2,63].

2.2. Optics Functionalization

Surface texturing finds application in plastic products that require specific optical properties. Optics applications require replicating mold textures using transparent polymers, allowing the manufacturing of different optical components. Examples of micro- and nano-textured optical components include lenses, diffractive optical elements, diffraction gratings, blazed gratings, slanted gratings, diffractive diffusers, optical diffusers, and beam splitters. The industries implementing textured plastic lenses in their products are mainly automotive and electronics. In both industries, semiconductors for illumination and sensing systems led to the development of micro-lenses used as diffractive elements. The light beam generated by LED (Light Emitting Diode) or OLED (Organic Light Emitting Diode) is shaped by surface features when passing through the lens. The most common products are headlights for vehicles or camera lenses for smartphones. More advanced applications include components for photovoltaic systems, as manufactured by Fritz et al. using lithography for the insert texturing and hot embossing for replication ^[11][64]. The most common type of texture used for this application is micro-scale prisms (i.e., depth and width in the order of 10² μm and radius of 10¹ μm). The surface features must have a high surface finish and accurate dimensional tolerances. Hence, micro-milling, UV lithography, and laser writing are the most commonly used texturing technologies. Other technologies, such as μEDM, are more challenging because of their inherently high surface roughness ^[12][65]. The master tools’ surface structures are generally coated to increase their wear resistance and promote the release of the polymer at the end of the replication (Saha et al., 2016). The textured tools are replicated on a large scale using different manufacturing processes, such as injection molding, hot embossing, and roll-to-roll.

2.3. Information Scribing

Information scribing is an inexpensive, durable, and secure approach adopted to trace a product. Branding with appealing logos or artistic textures is widely used for both consumer and high-end products. By engraving a brand name, serial number, or other legal marks on a product, a company not only reinforces its brand image but also protects its name. Counterfeiting can be a challenge to a company’s ability to protect its name and brand identity. An engraved serial number or a complex geometry on a mold ensures a level of security for the company ^[13][66]. Moreover, engravings report information regarding the polymer grade, recyclability, and end-of-life strategies. The texturing technologies adopted for information scribing are mainly laser-based due to their ease of use and flexibility ^[14][67]. Information scribing range from letters or logos engraved using laser ablation to nano-scale structures used for surface coloring to micro-scale counterfeiting features.

2.4. Structural Colors

Texturing is an effective solution to create light diffraction effects on the surface of plastic products. Submicron scale patterns diffract the light when their pitch dimension is close to the visible light wavelength (i.e., 380–740 nm). Depending on the size and orientation of the structures and the angle of orientation of the incident light, different shiny shades of colors are visible throughout the entire visible spectrum ^[15][16][33,68]. The diffraction effect is similar to the one that can be seen on CDs and DVDs, but differently, as only one color is shown at a specific angle. Homogenous textures with long ripples are required to show structural colors, and smooth areas may alternate them to increase the shiny effect ^[13][66]. The diffraction effect is mainly obtained with laser-based texturing technologies, such as ultrafast femtosecond laser ablation.

2.5. Controlled Wetting

Textures with micro- and nano-scale features are used to modify and control a surface’s wetting properties. The wetting properties of a surface define its interaction with a liquid, and the contact angle is the most representative indication of the wetting behavior. The most famous example of the effect of texturing on surface wettability is found in nature on lotus leaves, which easily repel water. The presence of micro- and nano-scale hierarchical features modifies the interaction of the surface with liquids. Adding a texture to a product can lead to hydrophobic behavior ^[17][69]. Engineering applications that exploit this phenomenon include hydrophobic ^[18][70], hydrophilic, icephobic ^[19][71], anti-fogging, self-cleaning ^[20][21][72,73], anti-fouling ^[22][74], and more functional surfaces ^[23][75]. Oleophobicity (i.e., the ability of a surface to repel oils) and amphiphobicity (i.e., the surface’s ability to repel both water and oil) can also be controlled and enhanced using texturing. Hierarchical textures are commonly exploited to control wetting properties ^[24][76]. Two substantially different scale patterns define the two overlapped textures. The larger one is typically characterized by features of the order of 10¹ μm. The smaller textures, realized on top of the other, are typically smaller than 10² nanometers. For example, Wang et al. used a picosecond laser to generate different textures on mold steel with super-hydrophobic properties ^[25][34]. Similar surfaces were replicated using injection molding from inserts textured using EDM ^[26][77]. It should be noted that the polymer–texture interaction is also significant in these applications because the effect of patterning the surface enhances the polymer’s intrinsic surface properties. Typically, polymers show a low wetting with water that can be further improved, obtaining a more hydrophobic behavior (i.e., reducing its surface energy) ^[27][59]. Moreover, the polymer’s rheological and thermal properties influence the ability to achieve complete replication using a specific polymer processing technology ^[10][63]. Hierarchical textures have been manufactured using different texturing technologies. The most critical aspects of their manufacturing are related to the submicron scale texture generation, which needs to fit on the first texture accurately. The use of laser-based technologies allows excellent flexibility and control over small patterns ^[28][78]. High-energy texturing technologies have also been used, but they are typically carried out in clean-room environments.

2.6. Biomedical Functionalization

Textured surfaces find several applications in biomedical devices manufactured with polymers. Surface functionalities have been introduced to modify interactions with biological substances, such as cells, tissues, fluids, and more. The topography modification can significantly impact the biological response of the surface. Textured surfaces have been used to improve the interrelation between implants (e.g., pins, screws, rods, clips, etc.) and the surrounding tissue ^[29][30][79,80]. Other examples include tissue ^[31][81] and cell engineering scaffolds ^[32][82], microfluidic devices ^[33][83], organ-on-chip ^[34][84], and surgical tools. Texturing technologies are also being used to enable the large-scale manufacturing of microneedles ^[35][36][85,86]. The application of micro- and nano-structured plastic products for medical devices are expected to continuously grow, owing it to the wide range of properties that can be obtained using polymers. Biomedical applications exploit very different texturing technologies, and the most appropriate is typically selected based on the specific product.

2.7. Controlled Friction

The tribological properties of polymers are often poor, thus limiting their use for dry contact applications. In the industry, the problem is solved using lubricants (solid or liquid) or by changing the resin formulation. Surface texturing is an alternative to these techniques. The surface features reduce the contact area, entrap the wear debris, and improve surface/lubricant contact ^[37][87]. Depending on the lubrication condition and texture geometry, the tribological effect of a textured surface varies significantly. Thus, it is essential to design the right texture for a particular application ^[38][88]. Common textures used to modify the friction properties of a surface are micro dimples ^[39][40][89,90], micro stripes ^[41][91], micro grooves, and banded grooves ^[42][92]. The accuracy of feature definition, angle sharpness, edge angle, depth, and pitch all constitute factors that will influence the tribological properties. Surface wear needs to be considered for textured surfaces. However, it can be controlled through material selection and pattern design. For example, Aziz et al. used Fused Deposition Modeling (FDM) 3D printing to modify the tribological behavior of PLA surfaces ^[43][93].

2.8. Tool Surface Functionalization

The tool surface represents the boundary that controls the polymer/tool interactions. Tool topography can be modified to control polymer replication, achieving significant processing benefits. Different strategies have been used to achieve that, particularly surface modification using coatings and surface texturing ^{[44][45][46][47]}[94,95,96,97]. The most significant benefits of surface engineering were observed for the filling of the micro- or nano-scale features and the ejection of the replicated texture from the tool. This section focuses on the effects of texturing on polymer flow and part ejection from the tool.

2.8.1. Filling

Texturing has been demonstrated as an effective way to improve the filling of thin-wall injection molding cavities by reducing the required pressure drop ^[48][98]. Reducing the pressure requirement can lead to advantages for plastic product design, processing energy requirements, and resin selection. The effect of tool texturing on the polymer flow changes the rheological and thermal polymer/mold interface interactions. Submicron scale textures realized using ultrafast pulsed laser have shown an effect on the onset of the wall slip phenomena (Figure 217). When touching the mold surface, the polymer macromolecules cannot reach the depth of the texture valleys, thus the absorption points are less than a flat surface. The viscosity, the molecular mass distribution, and the polymer chains’ relaxation time play an essential role in the phenomena. A polymer with high viscosity, narrow molecular mass distribution, and long relaxation time is more likely to slip ^[49][99]. Micro- and nano-scale texturing can also affect the filling flow by changing the thermal boundary condition at the polymer/mold interface ^[10][50][63,100]. The air pockets trapped in the troughs in the texture have low thermal conductivity, thus they delay the heat transfer to the mold and keep the polymer warmer during filling. However, this effect is relevant only for thin-wall plastic parts, specifically an experimental threshold of 150 μm was identified ^[51][52][101,102].

2.8.2. Ejection

The topography highly affects the ejection of the polymer part from the textured tool. Several strategies have been adopted in the industry to decrease the force required to separate the polymer from the tool. In particular, the effect of surface roughness has been widely studied and exploited ^[53][103]. The force required to separate a plastic part from the mold generally increases with a rougher surface. However, highly polished tool surfaces also lead to high forces due to the vacuum generated at the interface. The optimization of mold topography through texturing has been demonstrated as an effective solution to solve demolding issues ^[54][104]. The understanding of ejection friction requires the analysis of the adhesion and deformation components ^[55][105]. The adhesion term is a surface effect related to chemical, electrostatic, and capillary adhesion. The deformation term is controlled by the mechanical interlocking generated between the polymer and the texture ^[56][106]. Surface engineering strategies control both components of ejection friction. In particular, mold surface generation using texturing controls the deformation component, while modification using coatings controls the adhesion ^[47][97].