Comparison, Advantages and Disadvantages of CNC Machining and Injection Molding, How to Choose Between Parts Machining?
Choosing the right manufacturing method for a new product is often easier said than done. CNC machining and plastic injection molding are both common and cost-effective methods of producing parts. While both methods are similar in this regard, they are completely different manufacturing methods. It is a subtractive manufacturing process that uses a metal tool to remove material from a workpiece to form the final part shape. Plastic injection molding forces liquid plastic to conform to the shape of a mold to make a finished part.
1.CNC Machining Definition vs. Plastic Injection Molding
CNC Machining
A subtractive manufacturing process that uses machines such as mills, lathes, drills, and saws to create high-precision parts. Computer-aided design (CAD) data is developed during the product design phase. This CAD data is then used as the basis for programming and optimizing machine sequences and paths. The material is then machined using tools (such as end mills and drills) to create the part. Auxiliary machinery, including honing machines, hobbing machines, or grinders, may also be required to finish the part to customer specifications.
Plastic Injection Molding
A method of manufacturing using injection molding machines (IMMs). These machines contain components such as injection screws, injection barrels, and prefabricated molds. Plastic pellets are fed into the barrel and melted by the rotation of the screw and heating band. The molten plastic is injected into the mold and forms the geometry of the part.The IMM applies pressure to the mold until the product is properly formed and cooled. The finished part is ejected and the cycle begins again.
2. Advantages and Disadvantages of CNC Machining
Advantages of CNC Machining
Here is a list of some of the advantages of CNC machining over injection molding:
Beautiful surface finishes and tight tolerances can be easily obtained with it because small diameter tools can be used and each process parameter can be optimized.
A wide range of materials can be molded using it because both plastic and metal materials can be cut.
Modifying part designs is easy because programs containing CNC machine instructions and fixtures can be quickly changed to match successive design iterations.
CNC machining has faster tooling lead times than injection molding because it does not require the creation of complex molds.
Disadvantages of CNC machining
CNC machined products are more expensive to produce in large quantities and on a per part basis due to the longer cycle time per part required to manufacture the part.
3.Advantages and Disadvantages of Plastic Injection Molding
Advantages of Plastic Injection Molding:
Because multi-cavity molds can produce many parts in a single injection cycle, it is easy to produce large quantities of parts in a fast and repeatable manner.
Injection molding has the lowest price per part of any manufacturing process because of the long life of the molds and because multi-cavity molds can increase the capacity of each machine to support high volume orders.
Minimal post-processing is required because, depending on the type of gate used (the point of entry from the runner into the cavity), the part is usually ready to ship immediately after ejection.
Disadvantages of Injection Molding:
It is difficult to adapt to changes in product design because reworking complex molds with multiple plates, cams, and complex geometries can be cumbersome.
Defects inside the mold can lead to surface defects in the finished part.
Mold manufacturing requires a high upfront investment, as multi-cavity molds can cost anywhere from a few thousand dollars to hundreds of thousands of dollars.
Labor-intensive because workpieces, finished parts, tools, and tool holders must be fastened to or removed from the machine depending on the job.
4. Comparison of CNC Machining and Molding Key Dimensions
Cost Comparison:
CNC machining tooling costs are significantly lower than injection molding. Manufacturing jigs, fixtures, and sourcing raw materials and tooling drive its costs. Single- and low-cavity injection molds typically cost thousands of dollars, while multi-cavity molds can reach hundreds of thousands. Over a high enough production volume, that upfront mold investment quickly pays for itself.
Assuming you have the machines needed to make the part and only need to arrange for tooling such as molds, fixtures and jigs, CNC machining has a lower upfront cost.
Speed Comparison:
Both CNC machining and injection molding are known for their ability to produce parts quickly. For small production runs, CNC machining is usually preferred. For high volume production, injection molding is preferred because multi-cavity molds can produce parts quickly and reduce cost per part. Considering the lead time for mold production, it takes longer to start volume production with injection molding than with CNC machining. Consider using CNC machining for low volume production and injection molding for high volume production.
Volume Comparison:
Compared to CNC machining, injection molding can produce both more parts per basic machine and more parts per dollar invested. The molds used in injection molding can have multiple cavities – from one cavity to hundreds of cavities. As a result, injection molding can produce large quantities of parts quickly. In CNC machining, parts are usually manufactured one at a time. While CNC machining produces parts quickly, injection molding is even faster.
Material Comparison:
The CNC machining process can be used with a wider range of materials than injection molding.CNC machining is able to produce parts from certain plastics, as well as metals such as aluminum and steel. Many thermoplastics, thermosets and elastomers are too soft to be CNC machined. However, they can be easily made into parts using injection molding machines.