When you develop a new product out of an idea all the way through market, there has always been a need for prototypes, which can be used for design evaluation, testing, proof-of-concept and as bridges to volume production. Injection Molding, CNC Machining and Vacuum Casting are the common manufacturing processes to create the desired parts, whether that is prototypes or end-use parts. However, there are quite a few differences of those processes that should be considered when choosing the right one in different stages of product life cycle.
The following article is the differences of Injection Molding, CNC Machining, and Urethane Casting in detail, including processes themselves, manufacturing lead times, relationships of part quantities and cost, materials, the right process in different development stages. These differences will help you to utilize the strengths and mitigate the limitations of each process in manufacturing your parts.
Injection Molding is a manufacturing process for producing parts by injecting material into a mold. Material for the part is fed into a heated barrel, mixed, and forced into a mold cavity, where it cools and hardens to the configuration of the cavity. It is widely used for manufacturing a variety of parts, from the smallest components to entire body panels of vehicles.
CNC Machining is a subtractive manufacturing technology. This means that a block of material, or a blank, is carved away bit by bit until your part is created. Drills and rotary cutters are some of the main tools used to achieve this removal of material. This process is mostly automated and requires very little supervision once the machine has been calibrated and the workpiece has been clamped.
Urethane Casting is a fast and cost-efficient process to produce low-volume parts from a silicone mold. A silicone mold is made with the help of a prototype which serves as the basis for the duplication process of the original master pattern. Thanks to the flexibility of the silicone, complex intermeshing geometries can be achieved without demolding.
1. Manufacturing Lead Times
Every manufacturing process requires time for production arrangement, machines setting up, surface treatment, packing solutions, delivery, etc. Those factors are predetermined. Factors that are unable to be confirmed, such as the company operational capacities and current workload should be taken into considerations when determining manufacturing process in terms of lead times. Different companies have different manufacturing lead times. However, the following lead times of three manufacturing process can be referred to in general.
3-5 weeks
1-2 weeks
1-2 weeks
Injection molding
CNC machining
Urethane casting
2. The Higher Quantities, the More Costly?
Injection Molding
The total cost of injection molded parts mainly consists of mold cost (aluminum tooling or steel tooling) and the cost for the molded parts. For example, an aluminum tooling usually costs more than a thousand dollars, depending on the complex geometries, materials and finishing options. Part quantities starting from 1,000 pieces for injection molding technology are deemed to be economically. The good thing is that the mold can be used repeatedly. Certainly, mold replacements are required if the part quantities are higher and higher. 
CNC Machining
There is no tooling in CNC machining. The single part cost is relatively higher than injection molding cost. The cost of machined parts includes Non-recurring engineering cost (NRE), the raw materials and the manufacturing time and more. NRE cost of CNC machining covers the cost of transforming CAD to CAM, which usually a one-time fee. Raw material is another factor affecting. The complex geometries or the hardened material dominates the manufacturing time, following by the fluctuating manufacturing cost in CNC machining. 
Urethane Casting
Urethane Casting is a manufacturing process for low-volume parts out of Urethane Resins. The costs consist of tooling, master pattern, the parts and more. The tooling is usually the silicone material, which can be used for one-time or repeated according to quantities and lead time. 3D printing is usually used to produce the master pattern. This is a relatively economical process for the lower-volume plastic parts, comparing with two other manufacturing processes.
Injection molding
CNC machining
Urethane casting
The figure illustrates the relationship between cost of per part and the part quantity. Part quantity is a key factor to consider when selecting a proper manufacturing process in terms of cost.
3. What Property of Material Do You Need?
There are over 100 different materials thermoforming plastics to choose from for your project. In addition, mechanical and chemical characteristics, such as mechanical strength, transparency, wear resistance, impact resistance, etc., can also be achieved.
Can you image, CNC Machining allows for numerous of plastics, woods and metal materials, including production grade plastics and metals? You can choose materials for CNC machining to see what material works best for your product, or test the performance like tensile strength, heat resistance, impact resistance, chemical resistance and more.
Hundreds of casting polymers are available to repeatedly produce parts with fine surface textures. It is also possible to make parts that are completely transparent or with solid color according to your applications.
4. What’s Your Final Goal?
Injection
Molding
This can be used for a bridge tooling, trial run and low- to mid-volume production before volume production. The proper quantity is ranging from hundreds of parts to thousands of parts. But you must notice that there are certain constraints for part design, including wall thickness, draft angles, undercuts and more. For some manufacturing suppliers, minor modifications on molds are allowed even in this stage in case your design requires iteration. RPWORLD is one of the few manufacturing suppliers that offers such service.
Do you want the high precision parts with properties of end-use part, part quantity is relatively lower, the lead time is around 1-2 weeks, and the design files are not final. CNC machining is right for your product.
CNC
Machining
Urethane
Casting
Product development is the process of design evolvement. This process is ideal for initial development, such as various testing, new product introduction, pre-sale, crowdfunding and on-demand production. The process is suitable for complex structure like undercut. The fee for modifying the mold is much lower than cost of Injection molding.
Urethane Casting is often used in the early stage of product development. It is economical, fast and modifiable, and able to simulate the performance of true materials. When you have higher requirements on quantity and quality, your product design is nearly frozen and ready for low- to mid-volume production, injection molding process is the right choice. CNC Machining is ideal to produce small batches and prototypes in which the production of expensive injection molding tools is not feasible. In addition, it machines the parts with high quality and accuracy, can be the end-use parts. A glance at the pros and cons of each manufacturing process:
Manufacturing Process
Injection Molding
CNC Machining
Urethane Casting
Pros
Wide materials choice
Economical for higher quantity
Wide materials choice
Design flexibility
Short lead time
High precision
Complex geometries available
Lower cost per part
Design flexibility
Short lead time
Cons
High total cost
Part defects
Long lead time
High cost per part
Lower quantities
Limited materials
The practical production of single product is way more complicated than the above. You may wonder: should I be using Urethane Casting, CNC Machining or going straight with Injection molding? But there is no simple answer to use which one, two, or all these processes. A manufacturing expert should be along with the development life cycle of your product. Feel free to contact us for a free design DFM analysis at [email protected].
