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Injection Molding: Modify an Existing Mold or Build a New Mold?

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Injection molding is known as the most popular and most efficient production method for producing large volumes of high quality plastic parts. The process itself is extremely fast compared to other methods, and the high production output rate makes it even more efficient and cost-effective. While injection molding does require a large upfront investment to manufacture the injection mold tool, it offers a significantly lower unit price which will drop even further the more parts you produce.

The heart of the process

As the saying goes, at the heart of the whole injection molding process is the mold. The term “mold” refers to the combination of the core and cavity inserts. These inserts are what gives the final product its shape. Well designed molds can typically last between a hundred thousand to one million shots or cycles, depending on the material, operating conditions and other factors that affect the longevity of your injection mold.

If built well, a single injection mold can be used for multiple production runs, which is the beauty of the process. However, if you need to alter your design, change resins, or resize your part, you may not be able to continue using your current setup. Luckily, that doesn’t necessarily mean you’ll need to make a new injection mold from scratch. In some cases modifying your existing mold to better suit your needs makes more sense.

Here’s what you need to know to about modifying a mold versus building a new mold:

Cost of making a new mold vs cost of modifying an existing mold

A small and simple single cavity injection mold for low-volume production runs can cost anywhere from $2,000 to $6,000. Depending on the tooling material, size, complexity, and number of cavities, tooling for full-scale production of large, complex parts can cost tens of thousands of dollars — or even over $100,000. In comparison, you can expect a modification of an existing mold tool to cost much less than the cost of producing the full tool.

When is it recommended to modify an existing injection mold?

Creating brand new tooling means having to go through the entire manufacturing process, which consumes a lot of time and money. The good news is that — in some cases — you can avoid repeating the entire process and simply modify the existing mold. You may be able to modify an existing mold if you need to:


  • Increase the size, shape, or wall thickness of the part: while it’s costly and difficult (for all practical purposes, near impossible) to add metal to the inside of an existing mold, it is possible to incrementally remove material from the mold to increase your final part’s size, shape or wall thickness. The new design, should however, be followed by a mold flow analysis to determine any risk for potential sinks before proceeding with any modifications.
  • Add a small part or feature: in cases where there is enough room in the tool, a small part or feature with a similar style can be installed with a shut-off separating it from the initial cavity.


Tip: If you believe changes to your design may require the mold to be tweaked, try to make your design steel-safe, or in our case, “metal-safe.” Since metal can be removed from the mold but not added, voids in your first version that may be changed should start out larger, and masses like ribs, walls, and gussets should start out smaller than they might be in a modified version.

When is it recommended to make a new injection mold?

Due to the associated costs and time, it is not very appealing to have to make a new mold tool from scratch, but there are some situations where it isn’t possible to alter an existing mold and achieve your desired results. You may need to manufacture a new injection mold if you need to:


  • Reduce the size or shape of your product: Since it’s potentially costly and very difficult to reliably add metal to an existing mold, if you need to shrink the size of your part, you may need to manufacture a new set of mold inserts.
  • Modify the parting lines: Modifications or additions to a part at the parting line will almost always require a new mold, as this change can affect your mold’s vents and gates. Gates are channels, often located in the parting line, where the molten plastic enters the part cavity during injection. Vents are tiny channels cut into the parting line that allows air to escape from the
  • Change materials: If you need to use a different resin due to supply shortages, or you realize your selected material isn’t suitable for its end-use or environment, you may need to create a new cavity and core set, assuming the molded part is out of your tolerance range. Different types of resins have their own shrinkage property; molds are made to accommodate such shrinkage. If the new material has a lower shrinkage rate, you’ll need to build a smaller mold to accommodate its shrinkage.

Plastic injection molding with HLH Rapid

While the points listed in this article can help you understand whether to lean towards modifying an existing mold or building a new mold, each project is treated on a case by case basis. As such, it is always advisable to speak with an expert.

HLH’s design and engineering experts can help you through the design, prototyping and production phases, and advise on whether to modify or build a new mold depending on your designs and project needs. Ready to start your next injection molding project? Upload your parts here or contact us today.

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