Injection Molding vs. Casting - Which is Best?

Two incredibly versatile methods of manufacturing are molding and casting—but what makes them different from each other? While they may sound similar, both processes are unique in materials, process, results, and are specified for different applications. Both are also used throughout modern industry, which can make choosing one over the other confusing. While there is no “best” method per-se, this article aims to show which method—injection molding or casting—is the right manufacturing process for your existing capacity and which provides the quality of the necessary parts for your intended production line.

Die casting implements reusable metal molds known as “dies” and molten metal to create low-tolerance, high-accuracy, and complex metal parts. The dies are machined out of tool steel and last between 15,000-500,000 castings per die. A molten casting metal (aluminum, zinc, copper, and other non-ferrous metals) is forced into a pair of clamped lubricated die halves under extreme pressure, where the metal will conform to the die’s shape. Though highly expensive to produce, die machines produce the highest complexity casted parts available. Some dies contain water-cooled channels, runners, sprue holes, guide pins, cores, ejector pins, and other high-level accessories to ensure consistent quality castings. Die casting part quality is exceptional and is most useful for small to medium-sized parts with complex geometries for automotive parts, mechanical assemblies, and other industrial applications.

Sand Casting

One of the easiest, oldest, and most common casting methods, sand casting is both reasonable and highly effective. Sand casting uses molds made of (unsurprisingly) sand, where molten metal is then poured into the molds and solidifies into the mold’s shape. Sand is used because it retains heat, reduces cost, and allows for nearly infinite mold shapes if manufactured correctly. Sand is also more flexible than dies and investments, meaning it naturally allows for shrinkage of material while cooling without risk of cracking. The above image shows the entire process of sand casting but notice how there is no theoretical limit to sand mold sizes; one of the most beneficial aspects of sand casting is its use in casting massive, complex parts. Sand casting also allows for nearly any metal or alloy, if it melts and can be poured. Though best for short production runs, sand casting is a cheap, versatile, and highly adaptable casting method.

Investment Casting

Investment casting (also known as lost-wax casting) uses disposable wax patterns and ceramic shells to create molds and casting parts. The wax patterns are formed by a lower-temp casting process, where hot wax is poured into master molds and then removed and cleaned. This wax pattern is dipped in a refractory material (usually fine ceramic silica with binders) multiple times until a hard and thick shell forms. Once hardened, the mold is inverted and heated so that the wax melts out, leaves the hollow ceramic shell. Molten metal is poured to replace the wax, and then the shell is broken of once the metal is solidified. Investment casting leads to highly accurate, thin walled, and high-surface finish casts that require little cleaning. While generally more expensive than sand and plaster casting, investment casting works best in high-output, small scale applications where thin sections and low tolerances are specified.

As you can see above, solid plastic granules are conveyed and heated via the injection machine, where it is forced under pressure into the mold cavities. Injection molding tools are similar to the dies in die casting, but are generally smaller and restricted to thin-wall designs. Once solidified, the platen opens, releasing the two mold halves and the finished part, where the process is ready to mold another part with minimal/no cleanup necessary. The high volume production of injection molding machines is truly impressive—one machine can easily create 1000s to 10000s of parts in a workday depending upon complexity. Though initially expensive and limited to thin-walled parts, injection molding provides excellent surface finishes, high production rates, and requires little labor post-molding. Its throughput combined with the low material costs makes injection molding perfect for mass production applications like plastic packaging, production-grade plastic parts, urethane casting, and more. 

Types of Injection Molding

Just like casting there are also different types of injection molding techniques; however, they differ less on process and more on materials. Below is a brief list of other common injection molding techniques and how they differ from the traditional definition stated in the previous section:

Powder Injection Molding: metal, ceramic, or other powdered material is mixed with a binder and fed into injection molding machine, resulting in a part that can be debind and sintered into a final part. Powder injection molding allows for the high output production of metal or ceramic parts.

Co-injection Molding: two different materials are extruded through a coinjection nozzle, allowing for more versatility in manufacturing. A core of material can be surrounded by a second skin material, enhancing strength, durability, and parts quality.

Fusible core injection molding: a core is first molded/cast out of a low-melting-point metal in the shape of the hollow cavity for the part. It is then injected with plastic in an injection molding machine, and the part/core assembly is then submerged in a heated bath to melt the inside core leaving the finished part behind. This is most useful for more durable thermosets, and find applications in the automotive industry for intake manifold production.

These are not the only types of injection molding available but show how they mostly differ in material specification and not in the manufacturing process.

In general, injection molding is best for thermoplastics, thermosets, and other low viscosity materials while casting is best for metals, thermosets, and ceramic/glass materials. Die casting and investment casting are fairly expensive, but injection molding is typically the more expensive process due to the demands of the mold tooling. If looking to create high complexity, accurate, and aesthetic parts, casting is easily the winner, but injection molding will always win when it comes to volume. The choice will come down to material selection, production volume, parts quality, and everyday life considerations such as manufacturing space. Luckily there are many options in both casting and injection molding that will satisfy your requirements, providing high quality products in a variety of materials and shapes.