What You Should Know About Injection Molding

What You Should Know About Injection Molding

  1. What is Injection Molding?

    Injection molding is a manufacturing process in which parts are produced by injecting material in liquid form into a mold. It is most commonly performed with thermoplastic polymers, but can be used with a variety of other materials to include metals and glass. The process works by heating small pellets of material into a molten liquid and then forcing the hot liquid through a nozzle and into a mold whose internal cavity is the negative of whatever part is being created. The molten parts are cooled and then removed from the mold by ejector pins.

Injection molding follows a four-stroke cycle as shown in the diagram above. First the mold cavity is filled with molten material, then the material is rapidly cooled causing it to harden (turn solid). Next the mold cavity opens, exposing the part to the air and giving it a place to fall, which it does when at last it is ejected by pins.

  1. Where is injection molding done?

There are a number of injection molding companies in the United States. TMF has several strategic partnerships with injection molding firms in the United States  and across the globe- one if you are looking for 300-1000 injection molded parts, and another if you are looking for full manufacturing and assembly of parts at production volumes. We can get you from a novel prototype design to a fully engineered injection molding-ready tool and then through to completion of the manufacturing process.

  1. Why is injection molding done (i.e. what is injection molding used for?)

There are a number of significant advantages to injection molding. Here is a list of the most relevant:

  • Injection molding can be fully automated and thus permits very high rates of production (you can mass-produce parts very effectively through injection molding processes).

  • Injection molding allows for the production of highly detailed and intricately designed parts. You will need an engineering design firm or an in-house department to make credible designs, however, once the part is ready for and produced via injection molding, there is very little finishing work that needs to be done. Parts come out looking very finished.

  • You can use a variety of materials in injection molding as well as combine different materials to synthesize desirable material properties in your final product. The combination of more than one injection molded material (typically two different materials) is known as co-injection molding. Typically there is very little waste during injection molding as excess material can be easily repurposed for re-use.

  • You can utilize mold inserts to make new parts (this allows you to change the internal cavity without having to completely redesign and re-machine the aluminum or steel tooling via CNC). Interestingly enough, technology such as Polyjet from Stratasys is advancing to the point that you will be able to print mold tooling from a 3D printer for use in small scale (10-100 parts) injection molded manufacturing applications like prototype testing. Tool life will obviously depend on the material used in injection molding, and in particular on the part geometry and the barrel temperature of the injection molded material. High temperatures and delicate (thin) geometry will reduce tooling lifespan.

There are also some significant disadvantages to injection molding – namely the fact that high tooling costs keep it out of reach of the average independent designer. With that in mind, it’s useful to consider the alternatives.

  1. What are the alternatives to injection molding?

Several other technologies are available for manufacturing outside of injection molding. Most of these do not compare for large scale manufacturing but may be useful for small-scale projects where only a few hundred or a thousand units are required. Here are a few to consider:

  • 3D Printers (FDM et. al. machines): Advances in 3D printing have made it accessible for smaller scale users. For projects without a massive scale where materials aren’t necessarily “set in stone,” FDM, SLA, SLS, and SLM machines are a possible alternative.

  • Spin Casting: Spin casting uses a rubber mold and centrifugal force instead of the steel mold (typical for injection molding) and an injection molding machine to produce parts. Typically, spin casting is done using cured rubber molds that are generated by casting the molten rubber around the original object (prototype). The advantage to spin casting is that a rubber mold can be an order of magnitude cheaper to produce than a steel tool with an internal cavity. If you are only looking for a small number of parts, this is likely to be a better fit. Conversely, the advantage to steel tooling is that it holds up much better than rubber and is designed to be used in injection molding machines providing the ability to produce tens of thousands of parts from the same mold.

  1. What’s new in injection molding?

Technology is always advancing, and the trend in injection molding is keeping pace. It would be difficult to talk about all of the interesting things in production or soon to be incorporated in the world of injection molding, but here are a few interesting things we have read about ourselves:

  • Micro-molding (highly precise and very small part production)

  • Advances incorporating RFID tags into part production

  • Printed Injection Mold Tool (PIMT)

PIMT in particular is interesting as it would allow for a baseline steel tool to be retrofitted with 3D printed inserts that would be useful in certain injection molding scenarios. In particular, PIMT would be useful for prototype injection molding production where only a smaller number of parts are required (e.g. a few thousand) and the material they were created from was not critical. PIMT technology is limited by the material the printed insert is made from in that it likely will not sustain repeated high temperatures without deformation as is the case with steel tooling. Interested in PIMT?