Cracking is a common problem encountered during CNC machining of plastic. Minor issues include fine lines on edges and cracks in holes, affecting appearance and assembly; severe issues can lead to scrapped parts, increasing rework and production costs. Cracking is particularly noticeable in the machining of transparent parts, thin-walled parts, and high-precision structural parts. Sometimes, parts appear normal immediately after machining, but cracks suddenly appear after a period of rest, a typical phenomenon in plastic machining. Many people believe that plastic cracking is simply due to poor material quality, but this is not the case. Most cracking problems are related to the machining process, such as excessive cutting heat, insufficiently sharp cutting tools, excessive clamping pressure, and unreleased internal stress in the material. Therefore, to truly solve the problem of cracking in CNC plastic machining, it is not enough to simply repair cracks after they appear; rather, every step of the machining process must be carefully controlled to effectively reduce the risk of cracking and improve machining stability and finished product yield.
How exactly does plastic cracking occur?
Causes of Cracking
Cracking in CNC plastic machining is usually not due to a sudden material failure, but rather the result of a combination of factors including internal stress, cutting heat, clamping pressure, and tool impact. Plastics are inherently more fragile than metals and more sensitive to temperature and stress, making them more prone to developing fine cracks, edge chipping, hole cracks, or cracks that propagate along the grain during machining. Often, what appears to be a small nick on the surface will continue to expand if not addressed promptly, eventually rendering the entire part unusable.
Common Locations of Cracking
Cracks in plastic parts typically appear at hole edges, sharp corners, thin walls, screw locations, snap-fit locations, and stress-concentrated corners. These areas are either inherently thin or experience more concentrated cutting during machining, coupled with the release of internal material stress, making them prone to cracking. Transparent plastics, rigid plastics, and high-precision parts are particularly susceptible to developing micro-cracks after machining, requiring preventative measures rather than waiting until cracks appear before repair.
Why is plastic more prone to cracking than metal?
Plastics have poor thermal conductivity, making it difficult for heat generated during cutting to dissipate. High local temperatures cause the material to soften or become brittle. Furthermore, plastics vary greatly in elasticity and toughness; some are hard but brittle, while others are soft but heat-sensitive. In addition, CNC machining is a subtractive process; if the cutting tool cuts too aggressively, it can release internal stress, causing crack propagation. Therefore, the problem of plastic cracking is essentially a mismatch between material properties and processing methods.
What should be done after cracking occurs?
Immediately determine the stage at which the crack occurred
After discovering a crack, do not rush to continue processing or force assembly. Instead, first determine whether the crack appeared during processing, after processing and resting, or was caused by compression during assembly. Different stages correspond to different causes: cracking during processing is mostly related to the tool, cutting parameters, and clamping force; cracking after processing is mostly related to the release of internal stress; cracking during assembly may be related to structural design, hole accuracy, or stress concentration. 1. Locating the problem first is crucial to finding the root cause.
Immediately halt similar processes to avoid mass scrapping
If a batch of parts has already cracked, the most important thing is to immediately suspend similar processing and not continue mass production with the original parameters. Because plastic cracking is often recurring, especially when the material batch, fixture, and cutting tools are the same, the problem is likely to recur. Stopping promptly and checking the process is more important than continuing production and hoping for the best; this minimizes losses.
Review and verify the sample
After cracking occurs, review the sample: check if the material is dry, if the clamping is too tight, if the cutting parameters are too high, if the cutting tool is worn, and if there are any sharp turns or partial stops in the cutting path. If necessary, make a small sample and adjust one variable at a time for verification, such as reducing the spindle speed, decreasing the depth of cut, or using a sharper cutting tool, to see if the crack disappears. This method quickly pinpoints the source of the problem, rather than blindly modifying all parameters.
Categorize and process processed parts
Not all cracked parts can be reused. Minor cracks that don’t affect stress or assembly can be used as trial or verification parts; however, if the crack has extended to a critical location, it’s best to scrap it to avoid subsequent failures. For customer projects, integrity and stability are more important than “barely repairing” them. Categorizing and handling problematic parts saves costs and reduces potential problems.
How to avoid cracking?
Cutting parameters should not be too aggressive
Many cracking problems are actually caused by cutting parameters that are too “aggressive.” Excessive spindle speed, unreasonable feed rate, and excessive depth of cut all cause the tool to exert a stronger impact on the material, especially at hole edges and thin-walled areas, making them more prone to cracking. Plastic processing emphasizes “stability,” not just speed; it’s about minimizing impact while ensuring smooth cutting. For brittle materials, a small depth of cut and multiple passes should be used to gradually remove material.
The tool must be sharp and suitable for plastics
Dull tools are one of the major causes of plastic cracking. When a cutting tool is dull, it doesn’t cut the material but rather “squeezes” and “rubs” against its surface. This not only raises the temperature but also pulls out internal stress, making the edges prone to cracking. For plastic processing, it’s best to use specialized tools, especially single-edged or highly sharp tools, for smoother chip removal and a gentler cut. Replace worn tools promptly; don’t wait until obvious chipping occurs.
Clamping should be gentle and stable
If the clamp is too tight, the workpiece may appear fixed during processing, but once released, internal stress can be suddenly released, potentially causing cracks. This is especially true for thin-walled, transparent, and long parts, which are more susceptible to damage due to uneven clamping. The correct method is to ensure the clamping is “stable but not overly tight,” using large-area support, soft padding, vacuum suction, or distributed clamping to reduce concentrated local stress.
Heat control should address both cutting and environmental factors
Plastics are heat-sensitive; excessive heat can soften and deform the material, and make cracks more likely to propagate. During processing, minimize the tool’s stay in the same position to avoid repeated friction and heat buildup. If necessary, air blowing can be used to assist chip removal, allowing chips to quickly leave the machining area and reducing secondary friction. Simultaneously, the machining environment temperature should be kept as stable as possible; sudden changes in material temperature due to large differences will increase the risk of cracking.
Avoid sharp corners and abrupt stops in the machining path
In toolpath design, sharp corners, sharp turns, and sudden acceleration/deceleration all increase local stress. For easily cracked plastic parts, the path should be as smooth as possible, with rounded transitions at corners to reduce tool impact. Especially for hole edges, grooves, and thin-walled areas, it is best to machine using segmented cutting and a gradual feed method, rather than “breaking” the material apart in one go.
Crack Resistance of Different Plastics
POM is suitable for parts with both dimensional and toughness requirements
POM has good machinability and dimensional stability, and is generally less prone to large-area cracking like brittle materials. It is suitable for precision structural parts, gears, sliders, and functional components. If customers require precision but do not want the material to be too brittle, POM is usually a more reliable choice.
PMMA has a good appearance, but a high risk of cracking
PMMA, also known as acrylic, has high transparency and a beautiful appearance, but it is particularly sensitive to cutting heat and stress. If parameters are not properly controlled, edge cracks, whitening cracks, or microcracks can easily occur. If PMMA must be used, greater attention must be paid to tool sharpness, cutting parameters, and annealing treatment; otherwise, the probability of cracking will increase significantly.
PC has high strength, but is also susceptible to stress concentration
PC has better toughness than many transparent materials, but it is equally sensitive to processing and assembly stresses, especially at hole edges and thin-walled areas. Excessive drilling or overtightening of screws can also cause cracks. PC is suitable for parts requiring high strength, but only if process control is meticulous.
ABS is suitable for comprehensive applications
ABS has good machinability, a relatively moderate cost, and generally balanced crack resistance, making it suitable for many general-purpose parts. However, ABS can also crack at low temperatures or in structurally weak areas, so it is not “absolutely crack-free,” but rather easier to control. For mass production, ABS is usually a more practical choice.
Sélection des matériaux en fonction de l'application
If the product has a complex structure, thin walls, and concentrated stress, don’t blindly pursue the lowest-priced materials. If the product is merely a shell or a verification component, then materials with more stable processing and lower risk of scrap can be chosen. The truly suitable material is often one that is “not easily cracked during processing, can be used stably in the finished product, and has a reasonable overall cost.”
Problèmes courants
Why do samples not crack, but batches start to crack?
This is a very common problem. In the sample stage, the quantity is small, the processing time is short, the tools are new, and the operators are more careful, so problems are less likely to be exposed. However, in the batch stage, tools gradually wear out, continuous equipment operation generates heat changes, and there may be slight differences between material batches. These factors combined amplify the risk of cracks that were previously unseen. In other words, the absence of cracks in samples does not guarantee the safety of the batch; batch stability depends more on standardized processes and continuous monitoring.
Conclusion
Cracking during CNC machining of plastics is not uncommon. Often, it’s not due to “poor material quality,” but rather a mismatch between the processing method and the material properties. Plastics are more susceptible to heat and uneven stress than metals. Therefore, during machining, excessively high parameters, insufficiently sharp tools, overly tight clamps, or improper processing methods can all lead to cracks. Many cracks start small and are not noticeable, but if not addressed promptly, they tend to grow larger and larger, eventually rendering the entire part unusable.
Therefore, when encountering cracking, the most important thing is not to rush into repairs, but to find the cause. First, determine whether the crack occurred during machining, after machining, or during assembly. Then, systematically check the cutting parameters, tool condition, clamping method, and material selection. With a reasonable process, stable operation, and correct material selection, most cracking problems can be significantly reduced or even avoided. Ultimately, plastic machining emphasizes “stability,” not just cutting as fast as possible, but cutting more accurately, lightly, and smoothly. This reduces the likelihood of cracking and ensures more consistent quality.
