There are several different components often used during the process of plastic injection molding, and several of these involve transferring molten plastic that’s required for the mold into the proper area. One important component typically used for this purpose, and one that comes in a few different forms, is known as the sprue.

At EnviroTech Custom Injection Molders, we’re proud to offer a huge range of plastic injection molding needs, from standard and bulk injection molding to over-molding, slab stock molding and numerous other custom plastic molding services for industrial and commercial clients. We’re also happy to detail our processes to clients if needed, including the elements used to transfer hot plastic into the proper locations. What exactly is a sprue, why is it used within the plastic injection molding process, and what are some related alternatives? Here’s everything you need to know.

Sprue Basics in Plastic Injection Molding

As we referred to above, a sprue refers to a channel that allows molten plastic to flow within the molding process. There are a few different forms, though they all perform the same basic function of transferring plastic from the hopper to the desired location in a mold. In addition, there are several types used during this stage of production, and each have their own specific purpose and reason for being included within any given design.

Technically speaking, the sprue actually refers to the specific entry point of molten — the vertical passage that allows molten in before then splitting it off into various runners and gates. However, in many circles of plastic injection molding, “sprue” will be used as a broader term to describe the entire assembly used to transfer liquid plastic.

Essentially, the sprue is a connector for the passage of molten plastic between the machine and the mold. However, there are several important details to be aware of here, some of which we’ll go over below.

Sprue Options Available

While this is not an exhaustive list, here are a couple popular types of sprue used for plastic injection molding, plus how they work:

  • Cold runners (or cold sprue): The “conventional” sprue system for plastic injection molding, it involves a vertical passage that guides molten through the mold. This is not often used anymore due to its high number of failure points and need for high maintenance over time.
  • Hot runners: A modern alternative to the cold format is hot runners, which include plastic being heated in a way such that it never solidifies within the runners being used. This allows for fast production and high efficiency when it comes to molten the sprue.
  • Hot sprue bushings: Alternatively, the use of hot sprue bushings may be prudent in other cases. These are heated bushings that keep plastic molten hot when it would otherwise cool. Unlike hot runners, these bushings don’t keep runners or sub-runners in a molten state. This format reduces waste and cycle times, due to how long it takes the sprue to cool. In many cases, hot sprue bushings will be used as an intermediate step between cold runners and hot runners.

Excess Sprue Material

One common result of the use of a sprue for plastic injection molding is leftover plastic, which is common for each of the sprue types we listed above. Some view this as a downside of this format for injection molding due to waste — however, there are many settings where this extra material can be recycled and used for future molds, or even other objects.

Prior to recycling this plastic for reuse, it’s important that the excess material is first removed from the runner system. This can be safely done with a simple blowtorch or high-temperature flame — just be careful of any molten metal! A different option would be to use a chemical deglosser, which would dissolve the plastic while leaving any metal intact.

Not Always Needed

Molding with the sprue is a popular method of use for various items — however, it’s not always necessary to include one when working on injection molds. For example, there are some cases where runners and gates will be used instead. In this case, each runner would allow molten plastic to flow through it, until reaching the desired location.

For simple pieces with few sections, this format may work well in lieu of using a sprue. However, these cases are rare and not often seen in practice for most production items that require injection molding. When they do find their way into designs, they are generally used in spot-level injection molding, rather than large-scale applications.

Choosing the Right Sprue

If you’re confident you want a sprue used as part of your plastic injection molding process, choosing the proper material is crucial to the process. Using the right style of steel, for example, will make your sprue much more reliable in various ways. Some of these include increased tolerances when used with larger diameter runners, improved clamping force over time, and better feed characteristics that keep everything running smoothly.

Casting materials are not always necessary or recommended, but they are an option to consider in certain cases. A gate could also be used here, which is a much smaller opening that allows molten to flow through the mold. Larger cavities will require larger and more precise gates, though, so you must be sure that any manufacturer you’re working with is able to provide the size you need without issue.

Be sure to remain diligent in your search for a trusted supplier, as not all will have the ability to supply this material reliably. Using aluminum is quite common, but steel can be used just as well — choose whichever option is best suited for the specific needs of your product.

For more on the use of the sprue for plastic injection molding processes, or to learn about any of our custom injection molding or other products, speak to the staff at EnviroTech Custom Injection Molders today.