Several considerations must be weighed during these conversions. Plastikos began its evaluation with prints of the part and two options: a single piece with molded threads, or a two-piece molded assembly.
Analysis revealed that the single-piece option with molded-in threads would need a tougher, stronger, more costly material to prevent the threads from tearing during the core-pull process. It would also mean a more complex, higher-risk tool design, which might cause difficulty during molding in filling out the threads with the stronger resin material. Instead, Plastikos recommended installing a separate threaded component after molding the part, for a relatively low cost and little complexity.
This option allowed the customer to use a more familiar material and a simpler mold design.
This example underlines the importance of engaging the mold making, molding, and material supply people, as well as the parts designer, up front, said Katen. “It’s best to get in with the customer early on, even when they’re at the paper napkin sketch stage. Several customers will say I have an idea -- is this feasible? What would you need to make it more feasible?”
For example, Micro Mold recommends part designers use consistent thicknesses wherever possible. A lot of thick-to-thin changes can cause problems in manufacturing, such as trouble filling out thicker walled areas.
The example also illustrates some of the controversy surrounding whether integrating multiple steps within a single molded part is always a good idea. There can be design engineering and design limitations to consider when incorporating more features into a tool, said Myers. “The industry has been seeing more multi-shot applications. But is the facility set up for it? Is the product’s application geared toward it?”
Reducing or eliminating post-processing handling does lead to reduced labor costs, said Clinton McDade, senior designer of plastic products for Schaefer Systems International. A single part from the machine that’s ready to package may look like the ideal solution from this perspective. “But to make this part in one piece, sometimes complexity must be added into the mold, such as moving slides to avoid die-lock,” he said. “This can lead to increased mold cost, longer part cycle times, and more downtime for mold maintenance, all directly affecting speed and cost. This one-part product may also require a higher tonnage machine, with its higher overhead rate.”
The two pieces of a two-part product can be made in less expensive, open/shut molds that run on smaller machines with lower overhead rates. On the other hand, manufacturing this final product requires two machines and two cycle times, in addition to the time required for handling and assembly.
“Regardless of how you juggle the manufacturing variables, the ultimate result must be a part that meets the customer’s functional needs,” said McDade. A two-piece product may make the most sense from the standpoint of manufacturing variables, “but if the product’s ultimate application subjects it to cyclic loading or temperatures, the fastening mechanisms could introduce a failure mode not present on a one-piece design.”