3. Sink marks
Experienced designers are always faced with the challenge of avoiding sink marks in injection molded parts. Although the recommended maximum wall thickness at the base of a rib or boss should be less than 60% that of the perpendicular face wall, some molders prefer 50% or less. It should be noted that this is a guideline and not a guarantee that the part will be acceptable to the QC department.
Cosmetic surface imperfections are dependent on gate location, tool quality, nominal wall thickness, material, additives, surface finish, color and viewing angle. Production problems can be avoided by clearly establishing acceptable surface quality with the molder well before any of these decisions are made. Reputable molders will provide honest expectations and backup plans well before production starts. Molders may suggest eliminating all features on the inside of a part, while others may suggest special coring techniques.
4. Steel safe areas
When we are designing injection molded parts, we’re often faced with details requiring tight tolerances such as snap fits, alignment features or interlocking parts. It’s easy to perfectly align and match these features in CAD, but it’s not that easy to repeatedly produce them during production. Details that cannot be confidently reproduced by a molder are often designed “steel safe.” For the benefit of those not familiar with the term, steel safe means the design feature is detailed with enough clearance to allow a tool maker to easily machine away steel in the mold to tighten up the clearances after initial test shots are molded. Most molders prefer these precautionary measures to avoid welding material back into the mold, which is then later machined.
Welding always compromises tooling quality, is expensive and delays production startup. Close collaboration with a molder or tool maker early in the design process will minimize revisions in your design, enabling both of you to agree on critical dimensions that should be made steel safe and on the amount of clearance to include in the design. Typically, these cooperative, well-planned decisions add little or nothing to the tooling budget and have a minimal effect on production launch. Conversely, some molders want custom plastic parts designed exactly as expected and don’t want added clearance. That’s why close communication with your selected molder is important.
Suzhou HuiTong HuiChuang Technology 