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Injection Mold (Tool) Modification Process

Before you start the mold injection process, you need to consider potential modifications. Due to the subtractive nature of mold production, you can’t add material back to the mold; you can only carve it out. If you don’t get the design right, you might have to replace the molding tool altogether.

Other than the above scenario, there are times when you might have to change the molding tool. Doing so can be costly and time-consuming, so you should do everything possible to avoid it.

This article will give you some tips for planning with mold modification in mind. But first, let’s discuss the situations where changes are even possible.

Injection Molding multicavity mold

Modify or Rebuild the Mold?

A solid set of molds involves a significant investment. Luckily, it’s usually more than worth it, especially if you produce high volumes of plastic parts or products. Your per-unit costs will stay low as long as your products don’t require changes.

As your business grows, you might have to change your production. Perhaps you’ll have to resize your product, add or remove features, or change the material. You can make some changes without having to replace your mold. These include:

  • Expanding part geometry – You can make incremental metal reductions in your molds. This way, you can slightly increase the product’s size or change its geometry. Wall thickness can be increased and through-hole diameter can be decreased.

  • Adding features – If you use well-built inserts that fit into your mold’s cavity and core, you should be able to add small features to it – like holes, slots. However, you should discuss such changes with your engineering team first.

  • Adding or extending ribs, bosses, gussets – If the wall or other geometry is not sufficiently strong it is possible to add ribs, bosses and gussets by machining some portions of the mold.

  • Making Snap fit tighter – when the fit of two mating parts is not sufficiently tight it is always possible to make the fit stronger but not vice versa, thus it is good to start with looser fits.

  • Adding back the material to the mold (doing the impossible) – at some cases it is still possible to increase the diameter of the hole just by substituting pins that shape the hole. Also, at some extreme cases it is possible to solder, weld back some mold material so that it could be again machined and fixed.

  • Changing surface finish – What is also good about injection molding that mold texture and surface roughness can be changed. For example, if sand blasted surface finish appeared too soft it is possible to make EDM machined texture. The only thing is that the overall dimension of the part can slightly increase.

advanced materials molding

Basically, whenever you need to make small changes to your product that involve adding to it, you should be able to modify the mold accordingly. On the other hand, here are some common changes that involve a replacement:

  • Reducing a part’s size – While you can remove some metal from the mold, you can’t add it in. That’s why shrinking a product requires a new mold. In some cases, you might be able to change the core, though this often doesn’t do the trick.

  • Changing the parting line – Any parting line changes will impact the mold’s gates and vents. Because of this, parting line modifications call for new molds.

  • Changing the materials (if tolerances are strict) – Molds are generally built with specific resin shrinking in mind. If you change the resin type, it will likely have different shrinking properties, so you’ll need a new core and cavity set to accommodate this.

Other than product changes, you might need to replace the molds due to wear and tear. Metal-on-metal contact is bound to amortize your molds after a few years, and no modification will help.

Tips for Mold Modification

variety of plastic parts molded

If your circumstances allow for mold modification, here are some tips for ensuring it goes smoothly.

1. Prepare in Advance

You should plan for all changes to your product or molds in the design phase. The design should be “steel-safe,” meaning that it allows for the metal to be removed from the mold to account for any changes.

In addition, you should plan all product features and parameters ahead. If you don’t have your mind set on them, make sure there’s room for changes that won’t involve new molds. For instance, if you are unsure about wall thickness, start with thinner product walls. You can always increase thickness by machining the mold, but not vice versa. The same goes for holes and pins; you can only reduce the diameter of the hole.

Make sure your design team knows how to account for potential mold modifications, and you’ll save yourself many headaches down the line.

2. Be Wary of Geometry Changes

Will your product or part need undercuts? If so, this is another thing you need to consider before production starts. While adding side-action cams to the mold is possible, it’s an extensive change that won’t always make financial sense.

Eliminating undercuts is generally considered an excellent way to reduce production costs. But if this isn’t possible, plan such complex features in advance.

3. Consider the Minimum Cutting Depth

As mentioned, you can remove some metal from the mold to increase the product’s size or slightly change its geometry. However, you should keep in mind that you can only remove the metal in fixed increments.

To remove metal from the mold, you’ll have to mill it, so you need to factor in the minimum cutting depths. For example, you may be able to carve out 0.250 mm of metal, but removing less than that might not be possible. So, if you plan to expand your product, ensure the expansion matches the cutting depth.

4. Choose Your Resin Wisely

2 shot molding

How much will the plastic you use shrink during the molding process? Will your product need different materials with varying shrink rates? Will your material have glass fiber additive (which is abrasive for molds)?

These are some of the most critical questions you need to answer before you start the molding process. Depending on the extent of the shrink rate change, it may be difficult to impossible to modify the mold so that it accounts for the variations.

You can find many resources online that offer detailed shrink rates for each resin. Use them as guidelines for building a mold that will fit your materials. When testing different resins, it’s always best to design for those higher shrink rates. You can machine down prototypes that shrink less and configure the final mold appropriately.

Aluminum mold will not work well with abrasive GF filler; thus steel molds have to be used. However, for low-volume production and several cycles aluminum could help, and thus manufacturing volume must be also taken into account prior tooling process.

Be Proactive

They say that a good plan is half the job done. When it comes to injection molding, it’s much more than that. The initial stages are often the most complicated but worth your time and effort. This is especially true if you consider that many mold modifications aren’t a result of planned changes, but design errors.

To avoid costly mistakes and ensure that production goes according to plan, take your time to scrutinize every detail of your product’s design. Only then can you build a mold that will let you reap the cost benefits of mass production.

Mold changes are likely to happen anyway due to amortization, but there’s no reason to expose yourself to additional costs due to design mishaps. Think a few steps ahead, and you’ll save a significant amount of money in the long run.


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