Injection mold tooling can challenge even experienced plastics manufacturers. Your goal is to optimize the mold for the type of plastic, part geometry and finish, desired cycle time, production volume and, of course, cost effectiveness. But where and how do you begin with so many variables? Even after you’ve selected the ideal plastic for the project, a finished part is only as good as the mold used to make it.
In our experience, it’ll be worth your effort to consider the following before beginning your project.
1. Which Metal Is Best?
Injection molds are made from many types of metal: Tool steel (e.g., P-20 or S-7 hardened steel), copper alloys (e.g., copper beryllium), aluminum and other metals can be used alone or in combination. Choosing the right material comes down to balancing thermal conductivity with hardness.
- Metal in the mold transfers heat away from the injected plastic material, which cools and solidifies it into the final part. Faster heat transfer means faster cooling and higher cycle time.
- Hardness, as measured on the Rockwell C scale, affects the mold’s durability. Harder metals have less corrosion, rust, pitting and general wear and tear, so they’re good for high-volume production. Care must be taken in the design of the tooling, making certain areas of the tooling too hard can leave surfaces brittle, leading to early tool failure (especially on shutoff [parting line] and mating surfaces).
Thermal conductivity and hardness vary inversely, so while a fast-cooling mold increases cycle time, it will show wear and need maintenance sooner. Conversely, harder metal is more durable over time, but takes longer to cool and produce each part, lowering cycle time. Tooling experts can match the mold materials to your needs for cycle time and part volume (i.e., thousands of low-temperature ABS plastic parts quickly or a small quantity of parts made from rigid PVC, which is abrasive to the mold).
A knowledgeable mold maker will also know how to combine properties of different metals to boost efficiency and save scrap. As John C. Kuli, Jr. noted in MoldMaking Technology, “with proper design, inserts can be added to trouble spots in a steel mold after initial fabrication and first article operation of the mold so that hot spots can be reduced and dimensional stability from shot to shot can be improved.” A tooling expert can also determine which metals are appropriate for molds used to make child-safe or food- or medical-grade parts.
2. Increasing Mold Durability
Obviously, the longer the useful life of the mold, the more parts you can produce in less time, and with fewer repair costs. Some plastics are more abrasive than others, and some release corrosive chemicals as by-products of melting and cooling, all of which degrade the mold. Even the water used for cooling the mold contributes to rust and pitting from chlorine and other impurities. Applying coatings and plating (e.g., chrome, nickel, diamond-like carbon and hard anodizing) is a way to harden the surface of mold cavities, increasing durability.
3. Scheduled Maintenance
Cleaning and maintenance keep a mold in good working order and catch damage early. Smart manufacturers keep a regular schedule for in-press inspection and degreasing and also periodically remove the mold for disassembly and thorough cleaning. But, cleaning shouldn’t inadvertently damage the sections of the mold from friction or abrasion. One safe cleaning option is dry ice blasting, where particles of dry ice are blown into the mold cavities to remove debris and built-up plastic. Another is ultrasonic cleaning by cavitation. “The rapid forming and collapsing of millions of very small bubbles in a bath of water and suitable biodegradable cleaner . . . does not invade the surface of the steel the way conventional friction-based methods do,” wrote John Berg in MoldMaking Technology.
4. Prototyping Adds Value
Producing a custom mold is expensive no matter how you look at it, but some upfront costs can save you money in the long run. Mold making is an iterative process, which means the finished part may not be perfect the first time. Look for an experienced mold maker who can juggle mold design challenges like wall thickness and water channel placement as well as complex part geometry and finish requirements. Time spent reviewing the design, drawing on the mold maker’s experience and computer simulated molding might result in fewer iterations during prototyping. The bottom line is that your finished parts will be in production sooner and you’ll save on raw material and machining costs.
5. Look at Long-Term Costs
Even though it’s easy to focus on the price of raw materials, always look at the bigger picture. Advances in rough and finish machining tools, such as circle-segment cutting and welding technology, can get your mold to the press and running sooner. Also think about the long-term cost benefits of different metals for your mold, even if they’re more expensive to produce. For example, “if you need a steel that is wear-resistant, you are looking at a hardened tool steel. This will increase your tooling costs up front, but will reduce your maintenance costs in the long term,” wrote Randy Kerkstra in Plastics Technology.
Mold making is a complex process, but it doesn’t have to be overwhelming. If you need a new mold for your project, the experts at UGS can help. Contact us with questions about your project or to get a quote.