July 2013 Design Tip

Mould-Making with the Mod Squad

The purpose of prototyping is to either confirm that your design is ready for production or to spot problems so you can correct them. If there are problems, you’re probably going to fix them and order new parts. That may require a whole new mould or part of a new mould, or it might be done by simply modifying the existing mould, which will save both money and time.

“Steel Safe” is the common term for changes where material (in many cases steel) can be removed from the mould to tweak a design. Since Protomold does not necessarily use steel for our moulds, we use the term “metal safe.” If you believe changes to your design may require the mould to be tweaked, you’ll want to try to make your design metal-safe. Since metal can be removed from the mould but not easily 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. The basic rule is that we can usually add plastic in a mould mod, but removing plastic can be very difficult in a mould mod. Remember, not all changes can be accomplished by modifying an existing mould, as noted below.

Once you determine that you want to try a mould modification, there are a few essential points to keep in mind.


When milling moulds to change part geometry, there are minimum increments by which we can remove metal. For example, we can almost certainly remove 0.25mm of metal from an existing mould, but we may have a hard time removing less than that because of minimum cutting depths needed for milling.


Expanding a part at the parting line will almost always require a new mould, as this can affect mould features such as gating and venting. Gates are channels, often located in the parting line, where plastic enters the part cavity during injection. Vents are tiny channels cut into the parting line to release air trapped in the mould during plastic injection. Vents are located around the perimeter of the part and usually cannot be moved or altered once in place. If the parting line is non-planar, the parting line shutoff design may also preclude an outline expansion.


Protomold’s process is designed for speed and cost savings and does not permit extensive modification such as the addition of side-action cams to existing moulds. However, it is often possible to replace an entire side action if it can be done without changing the rest of the parting line.


Plastic resin shrinks as it cools, and moulds are made oversize so the part will shrink to the desired final size. Different types of resin shrink different amounts. If you are planning to produce prototypes in multiple resins, don’t plan to modify a mould to accommodate different shrink rates. It is difficult, if not impossible, to adjust a mould to accommodate all the dimensional changes that occur when shrink rates change. An easier way to test different resins in one mould is to design for the resin with the highest shrink rate and machine down the parts whose resins shrink less. Then, once you have made your final resin choice, we can cut a mould to match that resin’s shrink rate.


Finally, and this is essential, we cannot anticipate the need for future mould modification if you don’t tell us. As long as your design can be machined into a mould, i.e., there are no “required changes” raised by ProtoQuote®, we’ll build the mould according to your design. You need to plan for modification; we’ll be happy to help if you have questions. The good news is that, whether a change requires a new mould or modification of the old one, depending on the nature of the change we may only have to modify or remake one half of the mould, saving both time and money. That is why it is important to let us in on your design and make sure we are aware of the areas of possible change. Only then can we help you make special accommodations for producing a part that might need to be changed later.


Once you have a mould and you’re thinking of a change, you can see whether your new design can be produced by modifying the existing mould. The fastest and most reliable way to find out is to upload a revised model. In My Account, you can navigate to “Manage Quotes,” find the quote for the original mould, and click the “Upload a Revised Model” hyperlink. Or, you can find the original ProtoQuote email, open the ProtoQuote, and click the “Upload a Revised Model” link in Section 3. A slower and less reliable way is to just upload a new model and add a note to ask us to look for the corresponding mould. However you get us the model, if the modification is within our ability, we will provide a modification quote; otherwise we will provide a quote for a new mould.

As always, if you have any questions about undercuts, parting lines, or any of these modifications, please call our customer service engineers at +44 (0) 1952 683047.