The Insider’s Guide to Mold Selection

Select the Right Injection Mold for Your Project

You are about to finalize your part design and are ready to turn your attention toward your mold selection so you can start getting molded components.  Suddenly what you thought would be a straightforward “just-make-a-mold” task turns into a universe of decisions with tradeoffs. 

Is a mold just a chunk of metal with some holes in it, or is it a finely crafted work of master craftsmanship by skilled artisans with decades of experience? 

The rapid proto-places tell you how quick and cheap it is, but you heard through the grapevine that when the shoddy parts arrive, you’re stuck with what you get. 

Maybe your manager swears by overseas tools, while your engineer has vowed never again to suffer through overseas headaches. So who’s right?

Mold Rendering

Well, they could all be right.  You just need a way to organize the clutter of information during your mold selection process. 

1. Understand Mold Classifications

First, you want to understand that not all molds are created equally.  Different mold materials and mold construction methods serve different purposes.  Fortunately, the Plastics Industry Association, formerly SPI, developed a mold classification system that introduced a standard to the industry. This system helps mold buyers to make apples-to-apples comparisons and understand the tradeoffs of each option. 

For examples, an Aluminum mold (class 104) might be faster and cheaper to build, but it will not last as long.  A hardened steel mold (class 101 or 102) might last for 1,000,000 cycles, but the initial cost will be much higher.

The SPI categorized the molds that run in sub-400-ton presses into five separate classes ranging from a Class 105 prototype mold up to a more robust Class 101 mold. 

The standard details:

  • Specific hardness of various components of the mold
  • Construction approaches
  • Type of material typically processed
  • Number of cycles to be expected
  • General minimum cost 

These range from less than 500 cycles for a soft Class 105 mold up to over a million cycles for a hardened Class 101 mold. The Mold Selection Guide below helps to outline common situations and the best tool to match.

SPI Mold Class

The difference between SPI Class 105 and SPI Class 104 is vast.  The reality is that the options for something between 500 cycles and 100,000 cycles have expanded.  Further sub-categorization helps clarify the options.  This table is by no means an official standard but is useful in practice.

Mold Selection SubClass

Lead times should include both mold design and mold build.  Quoting a lead time from mold design is disingenuous, but I have had clients come to me and say they fell victim to this.  Assuming no mold design changes and assuming design lock is fair.  Mold ownership terms might require minimum quantity runs.

We have seen companies make the mistake of using softer tools for production on parts with fine features.  Once the tool is running, pins start breaking, and they must deal with time delays and additional costs associated with early tool wear.  On the other hand, we have seen companies skip past single-cavity molds and dive right into high-cavitation molds before they have proven out the part geometries and tested the market.  Changing the part design is then expensive.  Buying the appropriate grade of mold the first time saves time and money in both the short term and long term.