CNC Machining vs Injection Molding

Before producing plastic parts, it is best to choose the right process in terms of cost and quality. Among all the plastic parts manufacturing processes, CNC machining and injection molding are popular for plastic prototyping and parts manufacturing. However, do you know what is the difference between CNC machining and injection molding? how do you choose between these two processes? Today, in this article, we will explore the advantages, disadvantages, and comparison of these two processes.

What is CNC machining?

CNC machining is a subtractive manufacturing process that removes part of a material block with instructions from a CAD model. The programmed computer software controls the movement of cutting tools and machinery. It is widely used in both plastic and metal prototypes and parts manufacturing.

Advantages of CNC machining

• High precision: Because of Computer control, CNC machining can create high-precision parts with Complex geometries. The minimum part dimension tolerance CNC machining can make is 0.02mm.
• Short lead time for small volume: Without the need for additional tooling, CNC machining can make one part quickly in less than 1 day. CNC machining will be a good choice for small volumes of less than 10 pieces.
• Flexible: CNC machining can create parts one by one, which means engineers can change the design after they find any issue from the first part. this can be realized by updating the program in the CNC machine without much cost.
• Material Versatility: CNC machining can work on a wide range of plastic and metal materials, covering almost all rigid materials. So whatever materials you need, CNC machining can make it.

Disadvantages of CNC machining

• Poor Consistency: CNC machining often requires manual post-finishing such as bonding, polishing, and painting, which can not accurately control the final quality of the product. the manual operations lead to poor consistency when CNC machining a small batch of parts.
• Not good for High-volume production: In terms of cost and speed, CNC machining is not good for high-volume production, because of the long per-part cycle time needed to make each part.
• High Labor cost: CNC machined parts need post-finishing processes which require much labor to finish the job. This will lead to a high labor cost and make CNC machining not cost-effective when producing large-volume products.

What is Injection molding?

Injection molding is a high-volume parts manufacturing process to make plastic products by injecting molten plastic material into the cavities of a mold that is made from steel. the molten plastic material rapidly cools down in the mold and forms the plastic parts.

Advantages of injection molding

• Low unit cost: Once the mold is manufactured, the unit cost per injection molded part is low because the Part is made automatically without labor cost.
• High production speed: The injection molding process only takes seconds to produce one part, which means it can produce a large number of parts in a short time and be perfect for high-volume parts manufacturing.
• Good consistency: Due to the high accuracy of tooling and injection machines, Injection molding can make mass-produced parts of consistent size and quality to meet quality standards.
• Material Versatility: Injection molding allows the use of a wide range of materials to meet specific performance and application requirements. Not only plastic materials But also soft materials like rubber and silicone materials to make the soft parts.

Disadvantages of injection molding

• High start-up cost: Due to the mold structure being complex, the cost of the mold for injection molding is high. Before any parts are made, this mold cost needs to be spent.
• Long lead time: it usually takes from 8 to 20 weeks to make the mold because of the complex process of mold making. this will result in a long lead time for injection molded parts.
• Difficult to make the design change: Once the mold is made, if any design changes, it will take a long time to modify the mold. and cause additional costs.
• Potential Scrap: Injection molded parts are sensitive to injection parameters, mold design, and operation environment, this could lead to a potentially high scrap rate.

Comparison of CNC Machining vs Injection Molding

To Understand clearly the difference between CNC machining and injection molding, let us explore the comparison from different aspects.

Part Geometry

In terms of Part Geometry, CNC machining offers better flexibility in part geometry than injection molding. for example, some undercut structures on the part may not be feasible in an injection molding process but usually can be made by CNC machining through several steps of machining to let the cutting tool access the undercut area.

Surface quality

For plastic parts, the quality of the surface matters, CNC cutting tools will leave milling marks on the surface of parts and require polish to remove the milling marks manually. the result will be not as good as injection molded parts because of the polishing process done on the steel mold and could get a better surface quality after injection. However, the injection molding process may cause defects on the surface such as sink marks, which means a high demand for the tooling design and injection process.

Part Strength

Injection-molded plastic parts usually have better part strength than CNC-machined parts. this is because plastic Injection Molding allows for the creation of parts all in one piece, and could use proper engineering materials, CNC machining works on plastic blocks which may not be the same materials as injection molding which leads to a weak strength.

Precision

In terms of precision, CNC-machined parts can have better precision than injection-molded parts. this is because due to the shrinkage character of the plastic materials, the injection molding process will have a larger tolerance. Generally, CNC machining can achieve a minimum 0.05mm tolerance while injection molding can achieve 0.1mm tolerance.

Production volume

Injection Molding is commonly for high-volume production because It can produce large numbers of parts quickly and consistently. In contrast, CNC Machining is commonly used for low-volume production because of the long per-part cycle time needed to make every part.

Lead time

injection molding needs to use a steel tooling which needs a long time to manufacture. usually from 8 weeks to 16 weeks for high-volume production. even if rapid tooling with soft steel for low-volume injection molding, still needs around 4 weeks. CNC machining can start with a simple setup, and only takes several days to finish the part. So, in the case of large-volume production, the injection molding process will be the right choice, but in the case of low-volume production, CNC machining will be the right choice.

Cost

Due to the high cost of upfront mold investment of injection molding, If only to produce a few or hundreds of parts, CNC can be cheaper because of no upfront investment. If a large amount of production is needed, injection molding will be cheaper for the piece price even including the mold cost.

Flexibility

CNC Machining offers better manufacturing flexibility. Changes can be made easily by only changing the program and settings of the CNC machine. However, changes for injection molding parts will take a longer time and higher cost of the tooling modification.

Materials

The injection molding process can work for soft materials like rubber which can not be used for CNC machining. However, CNC machining can work on metal materials and injection molding has limitations on metal materials. Generally, A broader range of materials can be used in CNC machining processes than in injection molding.

When To Choose CNC Machining vs Injection Molding

Based on the comparison above, we summarize a list of when to help you choose between CNC machining and injection molding.

CNC machining can be the first option in the following situations:

• When the volume is small, usually, less than 100 pieces.
• When you need a one-off fast prototype.
• When you have a complex geometry design that can not be feasible with a tooling.
• when your design is not fixed yet, you will have many modifications according to the parts made.

Injection molding can be the first option in the following situations:

• When the volume is high, it is over 1000 pieces.
• When you need exactly the same material as mass production, even for prototypes.

Alternatives of CNC machining and injection molding

Besides CNC machining and injection molding, other plastic manufacturing processes can also be considered to replace CNC machining or injection molding.

3D printing

3D printing can make more complex geometry parts than CNC machining, with a faster lead time and lower cost, however, the part strength is not as good as CNC machining and injection molding because it uses a layer-by-layer method to create the plastic part. Anyway, 3D printing prototyping is worth to be considered when you need to make plastic prototypes.

Urethane casting

Urethane casting is perfect when the volume ranges from 100 to 1000 because it uses silicone mold to duplicate the master pattern created by CNC machining, It solves the weakness of the long lead time of steel tooling for injection molding, replaced by silicone mold which is easy to make and the cost is cheap. Urethane casting prototypes are widely used in rapid prototyping industries to replace CNC machined prototypes.

Conclusion

Injection molding and CNC machining both have their advantages and disadvantages. In general, CNC machining is suitable for the small production run stage, where the parts demand is limited and the design needs to be validated quickly. Once the design has stabilized and entered the mass production stage, injection molding is usually a more economical and efficient option, especially suitable for mass production. Finally, The choice of plastic manufacturing process depends on the cost, volume, and design of the part. KUSLA specializes in both CNC prototyping and injection molding prototyping services, as well as part manufacturing services. we could help you choose the right process for your projects. welcome to contact us!

You may also interested in other manufacturing process comparisons

• Urethane casting vs injection molding
• CNC machining vs 3D Printing
• CNC Machining vs Urethane Casting
• Urethane Casting vs 3D Printing

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