Injection molding tooling represents a substantial investment in the total cost of developing a product, and while many product developers are aware molds are expensive, they are still shocked when they get quotes.
Often the reason for high tooling cost is a design that doesn’t take advantage of the injection molding process while minimizing difficult to create features. While almost any part can be tooled if you have the right vendor and enough capital, knowing the following tips can help you achieve your design intent without breaking the bank.
Before getting into why one feature might cost more than another to tool we first have to understand what contributes to a tool’s cost. A basic injection mold consists of two parts; a core and cavity, the negative space between these is what forms your part. Any feature that can’t be directly pulled out of the cavity or slid off the core requires what’s known as “action”. Action in a tool is when, either by nature of the tool opening or a secondary actuator, a piece of the tool is moved to allow the finished part to be ejected off the core. Not all side actions are created equal; small undercuts can be formed for a nominal increase in price using angled ejectors, whereas large undercuts or side features may need a slide which can be a significant portion of the tool’s cost. Because adding action to a tool increases the tooling complexity the cost naturally goes up. As a rule of thumb, we estimate about $1500 for each angled ejector and about $4000 for a side pull.
Let’s start with an example project, a simple electronics enclosure. To package the PCB shown below there may be holes for a couple lights and a place for the power jack to come in.
Figure 1: PCB to be packaged
Don’t get hung up on holes
Making a quick enclosure with holes for the LED’s and mounting bosses is fairly straightforward, but a common issue arises when accounting for the power jack. Many people see a circular connector and assume they should place a circular hole to fit the jack. The problem is that to create a hole in the side of a molded part requires action in the tool, which adds cost.
A low cost option to achieve the same result is to add a “mouse hole” which is formed entirely by manipulating the parting line, maintaining a simple core-cavity tool. If the hole is close enough to the parting line the opening at the bottom of a mouse hole can generally be tolerated, but if unacceptable an enclosed hole can be formed by having a separate piece of the assembly fill in the in the lower area as shown. It should be noted that the sides of this mouse hole are created by the “shut off” between the two halves of the mold, so at least 7° of draft is required to seal the joint against the flow of plastic.
Figure 3: Mouse hole
Figure 4: Mouse hole filled in with a mating piece
Knowing Which Button to Push
Buttons are an important ergonomic and aesthetic item for many products, so often an increase in cost is acceptable to create the look and feel that is required. However if the buttons are in a non-visual area there’s a way to create buttons without adding extra parts or cost. By creating a “diving board” that is formed by the core and cavity coming together we can provide enough length of material that it will flex and actuate a switch. Imagine our PCB has a reset button on the bottom, as it’s not a visual surface we can use a large diving board to create an easy to use button as shown below. Forming this within the part is much cheaper than adding another part and assembling.
The Design Hinges On It
Another feature many people don’t realize adds to the tool cost is hinges. Often people imagine a solid plastic hinge with a metal pin to pivot on, similar to what they see every day in a door hinge. However unless the pin hole is in the opening or closing direction of the mold it will require action or a secondary operation to create. There are two great options to avoid this; the first is to use a living hinge if the material allows it. A living hinge uses material such as polypropylene that when flowed through a thin section allows the parts to be flexed thousands of times without breaking. If a living hinge can’t be used for geometric or material reasons, the hole for a hinge pin can created by alternating mouse holes to create a hinge as shown below.
Figure 6: Expensive molded hinge
Making Hooks is a Snap
If you’re designing a hinge, chances are pretty good you will also want some mechanism to hold the hinged component in place when closed, and this is a typical application for a snap. Snaps by their very nature have undercuts and while these can be achieved with only a moderate increase in cost by using angled ejector, if a hole can be tolerated in the part then snaps or hooks can be created without adding any tool cost.
Figure 8: Snap requiring a lifter/ angled ejector
These features by themselves do not make for the most aesthetic product, however keeping them in mind early in the design process will lead a product that achieves the visual, functional, and cost goals. Working early on with toolers or experienced design firms will allow you to get to market on budget while keeping all the features you need for your product to function.
