Injection molding design: 10 critical considerations for designing high-quality molded parts (2)

It should be noted that designing a quality injection molded part requires a designer to be knowledgeable about all the fundamental design parameters associated with custom plastic injection molding and to be highly skilled in the art. The molder/designer partnership is not intended to be an internship program—it is supposed to optimize handoff of the final design to production with few or no changes. If completed successfully, final production parts typically are cost effectively molded precisely to specifications for the following reasons.
1. Material options and consequences
Materials are often specified early in the design process and should be mutually agreed upon by both parties. Sometimes molders may purchase large quantities of specific resins at major discounts. These discounts can be passed on to customers. For example, if a designer can specify an ABS grade that matches the properties of one purchased in large quantities by a molder, many tens of thousands of dollars can be saved. A designer may discover certain high-performance resins may not be ideally suited for a molder due to viscosity, high glass content or crystallinity. A resin may be chosen for specific physical or chemical-resistance properties but may be very difficult to mold or maintain specified tolerances. Molders should be in agreement with specified resins and overall part requirements, since they will be required to actually mold the parts.
2. Critical tolerances
One of the greatest challenges for any designer faced with designing an injection molded part is providing enough clearance in the design for tolerance variation. Tolerance variation depends upon several variables, including materials, process control and tool design. Acceptable tolerance ranges in a design will vary greatly from one molder to another. It’s imperative that designers discuss reasonable critical tolerance specifications with a molder and consider options for possible mold revisions, if required. This may require certain design features to be intentionally designed with extra clearance, which will later be tightened by removing steel from the mold. No one wants to add steel with welding to remedy interference problems. Molders may offer a number of suggestions for maintaining tight tolerance control, including post machining, fixturing and gate locations.

(source:https://www.hthc-tech.com/injection-molding-design-10-critical-considerations-for-designing-high-quality-molded-parts-2/)