Plastics are polymeric organic materials manufactured by heating and pressurizing a resin matrix combined with fillers, additives, and other chemical materials. Plastics possess unique properties that surpass those of metallic materials;
they have demonstrated outstanding performance in numerous fields—such as chemical engineering, electrical and electronic machinery, and instrumentation—and are increasingly replacing metal parts.
Among them, engineering plastics are one of the fastest-growing and most widely used types of plastic materials. Engineering plastics are characterized by low density and resistance to acids and alkalis.
In addition, they possess excellent properties such as electrical insulation, wear resistance, vibration damping, and self-lubrication, making them suitable for most applications—and even indispensable in certain cases.
It is foreseeable that engineering plastics will account for an increasingly larger proportion of engineering materials; therefore, research into the machining of engineering plastics is necessary to select appropriate tool materials and parameters and improve machining efficiency.
Characteristics of Engineering Plastic Machining
Because the material properties of engineering plastics differ significantly from those of metals, the machining of engineering plastics differs considerably from that of metals;
furthermore, there are no standardized guidelines for engineering plastic machining as there are for metal machining.
During the machining process, attention must be paid to characteristics such as low cutting strength, poor thermal conductivity, plastic springback, material inconsistencies, and susceptibility to cracking.
Cutting Strength and Thermal Conductivity
Cutting engineering plastics requires much less cutting force than cutting metal materials.
While it is possible to increase the cutting depth of the tool, doing so presents a problem: engineering plastics have poor thermal conductivity.
When the cutting depth is increased, the active cutting area of the tool expands, and the friction area between the cutting tool and the engineering plastic also increases.
This heat cannot dissipate and accumulates rapidly over a short period of time.
Excessive heat can cause the cutting tool to overheat, accelerating edge dullness, while the engineering plastic may expand due to localized high temperatures, resulting in a decrease in the workpiece’s dimensional and shape stability, and precision will all deteriorate;
In extreme cases, overheating of the workpiece may even lead to charring or melting.
Therefore, during the machining process, cutting parameters are generally limited;
Cutting tools with high thermal conductivity are selected to dissipate the heat generated during cutting as much as possible;
High-speed cutting methods are employed; and compressed air is used as a coolant.
Plastics are brittle
After engineering plastics cool and cure, they become hard. When reheated, their behavior varies depending on their properties.
Generally, engineering plastics exhibit two types of behavior when heated: thermosetting and thermoplastic.
Thermosetting plastics lose their plasticity after heating and cooling, while thermoplastic plastics soften when heated and retain their plasticity even after repeated heating.
Therefore, machining thermosetting plastics is prone to cracking and chipping on the workpiece surface; in such cases, sharp cutting tools should be selected.
Thermoplastic materials, on the other hand, exhibit a certain degree of plasticity when heated.
During machining, the friction between the cutting tool and the plastic generates heat, causing the plastic to soften and form ribbon-like chips, making the machining process far less difficult than that of thermosetting plastics.
Elasticity and Abrasiveness
When cutting plastics, the material exhibits elasticity; after being cut or drilled by the tool, the workpiece will spring back, causing deformation and significantly affecting its dimensional and geometric accuracy.
Furthermore, some engineering plastics contain particles that act as abrasives—particularly certain fillers such as glass fibers, minerals, and other fibrous materials—which cause severe wear on cutting tools during machining.
This accelerates tool wear, dulls the cutting edge, increases machining difficulty, and raises the scrap rate.
Machining Accuracy and Surface Quality
Engineering plastics are inherently anisotropic, with complex compositions and non-uniform microstructures.
Therefore, during machining, attention must be paid to the material’s layering, elasticity, abrasive properties, and thermal conductivity;
otherwise, the machined parts will exhibit poor accuracy and rough surfaces.
Cutting Methods for Engineering Plastics
Today, engineering plastic parts used in various fields each have their own unique characteristics;
many do not come in standard sizes, and most plastic materials are anisotropic, exhibiting different strengths in different directions.
Depending on material differences, engineering plastics require different cutting directions and can be categorized into four cutting methods:
face cutting, longitudinal cutting, transverse cutting, and parallel cutting.
Selection of Cutting Parameters
All manufacturing processes focus on improving production efficiency and quality while reducing costs;
however, the relationship among these three factors is often contradictory. When balancing these factors, the parameters that yield the optimal production results should be selected.
The proper selection of cutting parameters can improve production efficiency, reduce production costs, ensure production quality, and minimize tool wear, making it a crucial aspect of optimizing machining processes.
Tool Parameters
The strength and hardness of engineering plastics are generally not very high, typically ranging from 30 to 100 MPa, whereas ordinary cast iron has a strength of 200 MPa.
Few plastics can match this level of strength; one such material is glass-filled nylon.
Therefore, the cutting forces involved in machining plastics are very small;
theoretically, large cutting parameters could be selected.
However, engineering plastics have poor thermal conductivity—their thermal conductivity coefficient is less than 1% that of steel—and their heat resistance is only around 100°C, or even as low as a few dozen degrees in some cases.
Such poor thermal conductivity, heat resistance, and inadequate heat dissipation conditions cause plastics to easily melt or char during the cutting process.
Furthermore, thermoplastics tend to stick to the tool, resulting not only in low yield rates but also in significant tool wear.
To address this situation and ensure productivity, necessary production quality, and reduced processing costs, cutting tools made of high-speed steel, diamond, or cemented carbide—which feature high hardness, low friction, and good thermal conductivity—should be selected.
General requirements for cutting tools:
The tools must be sharp to facilitate cutting and chip removal;
they must provide good heat dissipation and rapid cooling;
they must have a low coefficient of friction on the machined surface;
and they must be durable under friction with plastic while generating minimal heat.
Coolant Parameters
Coolant is sometimes added during the machining process to ensure workpiece quality. When machining engineering plastics, a large amount of chips is generated, and some plastics are hygroscopic (such as polyamide, polycarbonate, and polyimide).
Since liquid coolants are unsuitable for this machining environment, compressed air can be used for cooling; it is heat-resistant, does not expand easily, and plastics with low water absorption can be cooled using water or emulsion.
The selection of cutting parameters is influenced by various factors; careful consideration and analysis are necessary to select parameters appropriate for the specific situation.
In addition to using coolants, cutting methods such as intermittent cutting and whirl cutting can also achieve good heat dissipation results.
Cutting Examples
Engineering plastics come in a wide variety, and their material properties are complex. Both molded and laminated plastics exhibit anisotropy, and there are often significant differences between different types of plastics.
Some are brittle; common examples include thermosetting plastics such as fiberglass-reinforced plastic and phenolic resin. In contrast, nylon and polytetrafluoroethylene (PTFE) are thermoplastic plastics.
Therefore, to ensure machining quality, cutting parameters must be selected based on specific conditions.
The following discusses how to select parameters for machining under various circumstances.
Molded Plastics
A common product is phenolic resin filled with wood flour. The filler is powdery, brittle, and has an uneven structure.
When machining, it is essential to select wear-resistant cutting tools because the material itself is loose, brittle, and easy to cut;
however, it produces powdery chips that do not dissipate heat easily and have an abrasive effect on the tools.
Laminated Plastics
When machining laminated plastics, the direction of the plastic layers is a key consideration.
If cutting is not performed perpendicular to the layers, parallel cutting can easily result in a rough surface; drilling may cause cracking or delamination. Laminated plastics require a low feed rate;
if the feed rate is too high, it can lead to delamination, fuzzing, and surface roughness.
Fiberglass
Fiberglass is a difficult-to-cut plastic that is hard, wear-resistant, and has high mechanical strength.
It has poor machinability; its filler is glass fiber, and its main component is SiO₂.
It is generally based on phenolic resin, epoxy resin, or unsaturated resin; machinability varies depending on the resin matrix—for example, epoxy resin-based FRP is more difficult to machine than phenolic resin-based FRP.
When machining fiberglass-reinforced plastic, diamond and cubic boron nitride (CBN) cutting tools are the most effective and result in the lowest tool wear rate.
Acrylic
Acrylic is a thermoplastic that is easy to machine; however, the workpiece surface tends to become rough during machining.
Additionally, the material is not heat-resistant, so the cutting temperature must not exceed 40°C, and neither the cutting depth nor the feed rate should be too high to avoid an excessively high breakage rate.
To address the issue of surface roughness, use low cutting speeds, increase the rake angle, and select sharp cutting tools made of high-speed steel.
Nylon and Polytetrafluoroethylene (PTFE)
Nylon and PTFE are resistant to high temperatures and chemicals, possess high plasticity, and are highly machinable; chips do not break easily during machining.
It is important to note that although these materials are heat-resistant, once temperatures reach a certain level, they may adhere to the cutting tool, affecting machining quality and causing the workpiece surface to become rough.
Additionally, they have a high coefficient of linear expansion—7 to 8 times that of typical metals—resulting in significant dimensional changes due to heat during machining and a decline in precision; therefore, the use of coolant is required during machining.
Conclusion
Engineering plastics are now widely used in various new products, engineering structures, and equipment.
Aside from one-piece molded plastic parts, most applications require high-precision, non-standard geometric parts with specific dimensions that have been machined—and the demand for such parts is expected to continue growing.
Therefore, machining is an indispensable manufacturing method for engineering plastic products.
The surface quality and machining efficiency of plastic cutting primarily depend on the selection of machining processes, the appropriate choice of tool materials, and the adjustment of parameters such as the tool’s rake angle, cutting parameters, and cutting method.
Selecting cutting parameters tailored to specific applications is essential for improving quality and efficiency, meeting market demand, and ensuring corporate profitability.