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

Injection moulding 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 moulding does require a large upfront investment to manufacture the injection mould tool, it offers a significantly lower unit price which will drop even further the more parts you produce.

The heart of the injection moulding process

As the saying goes, at the heart of the whole injection moulding process is the mould. The term “mould” refers to the combination of the core and cavity inserts. These inserts are what gives the final product its shape. Well designed moulds 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 mould.

If built well, a single injection mould 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.

Modifying a mould versus building a new mould

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

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

A small and simple single cavity injection mould 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 mould?

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 mould. You may be able to modify an existing mould 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 mould, it is possible to incrementally remove material from the mould to increase your final part’s size, shape or wall thickness. The new design, should however, be followed by a mould 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.

Pro Tip: If you believe changes to your design may require the mould to be tweaked, try to make your design steel-safe, or in our case, “metal-safe.” Since metal can be removed from the mould 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 mould?

Due to the associated costs and time, it is not very appealing to have to make a new mould tool from scratch, but there are some situations where it isn’t possible to alter an existing mould and achieve your desired results. You may need to manufacture a new injection mould 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 mould, if you need to shrink the size of your part, you may need to manufacture a new set of mould inserts.
• Modify the parting lines: Modifications or additions to a part at the parting line will almost always require a new mould, as this change can affect your mould’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.
• 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 moulded part is out of your tolerance range. Different types of resins have their own shrinkage property; moulds are made to accommodate such shrinkage. If the new material has a lower shrinkage rate, you’ll need to build a smaller mould to accommodate its shrinkage.