There are thousands of injection molding designers in the United States but there are only a few select engineers within this group who design parts for injection molders. As there are multiple stages of injection mold design before the components are documented and released to the molder, the final step is the most critical since changes cannot be made without adding time or cost. Since every molder has its tooling techniques and preferences, it is imperative to design for consistency with these five considerations in mind:
One: Critical Tolerances
While injection mold design professionals usually provide generous tolerances, there are many times where tight tolerances are required to maintain appearance, fit, and function. Part of this is to ensure clearance to accommodate tolerance variation which depends on tool design, materials, and process control. Acceptable tolerances vary between molders, so it is essential to discuss critical tolerance specifications as early as possible.
Two: Gate Location
The gate location is agreed upon by the designer, toolmaker, and molder, and is related to all characteristics of an injection molded component because it impacts tolerances, wall thickness, appearance, physical properties, and surface finish. Designers should conduct a mold flow analysis as a tool to help determine the gate location and design.
Three: Material Options
Materials must be specified as early in the injection mold design process as possible and agreed upon by all parties. Many molders purchase considerable amounts of resins at a discounted price so knowing the material up-front saves money. There are many material options with a variety of performance attributes depending on what the customer needs. Resin can be selected for chemical resistance or or chemical-resistance properties but may not be able to hold specified tolerances or could be difficult to mold so it is important to collaborate on the resin and grade.
Four: Sink Marks
Experienced designers always try to avoid sink marks during the injection molding process which is unavoidable with certain materials. The recommended max wall thickness at the bottom of a rib should be less than 60-percent of the perpendicular wall to accommodate shrink marks but this is not always acceptable with the Quality Control Department. Sink marks also depend on tool quality, gate location, surface finish, viewing angle, color, nominal wall thickness, and additives.
Five: Shut-Off Angles
Shut-off angles are the minimum angle between the cavity and core which creates an opening that otherwise requires a cam. Every molder wants to maximize the angle between the cavity and core, but designers want a minimal angle. This compromise typically lands between 3- and 5-degrees. This is one consideration that truly needs communication between the designer, molder, and toolmaker to save countless hours in the long run.
Following these five considerations and having regular communication between your designer, toolmaker, and mold maker throughout the development process will save time and money in the long run. Continual analysis throughout the design process will result in a successful product launch where everyone can share the glory!