Molding, it’s not as scary as it seems.
Gener8 is a product development firm specializing in the engineering and design of complex instrumentation for medical and biotech systems. These systems often include injection molded parts which are commonly the preferred method for higher volume production and cosmetic instrument panels. This is because there are few manufacturing methods that can produce parts at such a low part cost with repeatedly high quality.
However, often we see hesitation to “cut tooling” for injection molding at opportune times during the design process. The two most common concerns about moving to injection molding is the seemingly high cost of tooling, and the perception that molding requires a design freeze, or is unchangeable. However, continuing low volume fabrication methods into production volumes may be costly and provide lower quality parts. So, when is the correct time to cut in molding? Turns out, mold tooling can be an attractive and financially viable option even before final production.
For the cost of tooling, the amortized cost for a couple hundred molded parts can underprice parts made from other processes like machining. While compared to 3D printed SLA parts, the cost of injection mold tooling and parts can be cheaper in less than 50 pieces. Machining and 3D printing prices increase even further if special finishing and painting for cosmetic panels must be included, compared to molding where texture and color are essentially included. In one case we saw large prototype and pilot instrument skins that were being machined in pieces, bonded and painted. The equivalent injection molded tooling and parts combined became cheaper at only 50 pieces, and greatly higher quality. Often molding is a financially viable option for parts where quantities are expected to be in the hundreds, even during the design phase prior to production.
The second common concern with committing to injection mold tooling is the idea that injection molded tooling requires a design freeze, or that it is exceedingly difficult to make changes once the tooling has been made. This is not usually as true as often thought. In fact, mold tooling is commonly modified after its initial creation.
In a typical process a mold maker machines the tool and the molder shoots sample parts for themselves, often called T0’s. They will measure these parts against the drawings and will rework the tool as needed to meet the requirements, sometimes more than once if tight tolerances are required. This is often internal to the molder and not charged to the customer. Once the molder believes the parts meet specification, they will send those parts (T1’s) to the customer for approval. If the customer believes features or dimensions are incorrect, they can request the tool be modified further to meet specification. And even if the customer desires to modify the dimensions of the part for design or fit reasons, this can also usually be done for a reasonable tool modification fee.
There are “metal safe” changes to the tool which mean the mold manufacture simply needs to remove more material to create the desired changes and is usually the lowest cost modification. However, even non-metal safe changes can be accomplished by milling out a pocket in the tool, bolting in a blank insert, and re-machining that section of the tool. Mold makers are particularly good at modifying these tools and only sometimes does a design or dimensions tweak require buying a whole new tool. Modifying tooling is common and can be as cheap as a couple of machined parts.
And finally, one of the most important reasons to seriously consider mold tooling earlier in a project than final production is that the cost of prolonging prototype and low volume manufacturing methods can be monumentally more expensive in development costs than the tooling cost for molding.
If a part is being machined during initial builds and then needs to change to injection molding for production the part must undergo geometry changes when going from a machined part design to a molded part design. That change creates risk. Even slight draft can affect holes fits and part clearances. The changes in materials can affect stiffness, thermal management, EMI shielding, or other characteristics sometimes overlooked during part material changes. That change risk can be costly when it requires engineering time to figure out new challenges brought on by late production method changes. And all the trial and error learning that occurred during use of the machined (or 3D printed) parts is as risk of needing to start over when the final change is made to molding. In general, we recommend using the method intended for long term manufacturing as soon as practical in order to minimize project risk and engineering costs by ensuring the learning that takes place early on does not need to be repeated later when the process changes.
Injection molding is a powerful method of manufacturing that results in quality parts at low part costs when compared to other manufacturing methods. And if using injection molding is the long-term production plan, utilizing injection molding early may reduce the risk design over-iteration from other prototyping methods as well as reducing risks from late stage manufacturing method changes. Like all methods of manufacturing, at Gener8 we weigh the pro’s and con’s for reasons specific to each project and application.