Plastic Injection Molding Design Tips

6 Do's and Dont's from the Experts

August 7, 2018

When designing a product, it’s easy to create something that fits all requirements for the product's intent without any thought about the manufacturing process. Many times, it might not be clear what manufacturing process will be used to produce the final product.

At Cadrex, we are experts in working with plastic injection molding manufacturers. We understand the complications that come with adjusting a design to complement the injection molding manufacturing process. Our Design for Manufacturability (DFM) engineers have decades of experience helping customers make modifications to their designs to ensure their product is manufacturable, cost-conscious, can meet geometric tolerances, and works with the chosen material without compromising the integrity and functionality of the product.

*engineering tip: .step files are the best choice when sharing design files

We have put together 6 design tips to help engineers with designing for injection molding.

Tip 1: Do not design a part with non-uniform nominal wall thickness

  • Varying wall thickness creates uneven fill throughout the cavity
  • Thinner sections will cool faster than thicker areas causing highly stressed areas where distortion and warping will occur
  • It is recommended to core out the thick areas to create uniform walls, which will reduce cycle time and plastic usage and improve part cosmetics
  • Depending on the type of material being used, nominal wall thickness should not vary more than 15%

Tip 2: Do identify possible sink areas and design them out

  • Sink areas are likely to occur at intersections of a rib and nominal wall where proper design guidelines are not followed
  • Rule of thumb: Ribs should be a maximum of 70% of nominal wall, but 60% is preferred
Tip 2 | ideal scenario, corner radii are small, at 0.010"; rib thickness is 60% of nominal wall thickness. This is considered standard procedure. Non-ideal scenario, Corner radii are too large, and sphere pulls farther away from top surface, causing sink. Non-ideal scenario, Rib thickness is 100% of nominal thickness, which is too large; sphere pulls farther away from top surface, causing sink.

Tip 3: Do create radii on all corners

*note: too large of radii on ribs cause sink

  • Sharp inside corners become stressed and outside corners are difficult to fill and trap air
  • Avoid radii on parting line – causes excess expense on tool build
  • Adding radii improves fill of corners and distributes stress across a larger area
  • .030" radius minimum if possible and if radii are less than .010" they are not worth adding to design
  • Place radii at the end of design tree to make collaboration with DFM engineers easier

Tip 4: Do allow at least 1° draft for part release

  • For textured surface, more draft is required – follow texture guidelines
  • If possible, more than 1° draft is better
  • 3° draft or more is needed when parting line transitions from one level to another
    • Less than 3° draft causes the steel to squeeze together when mold closes making the mold harder to pull apart and creating drag marks on the part

Tip 3&4 | Poor part design 90 degree angle, sharp corners. Add draft. Add radii on corners.

Tip 5: Do not design bad steel conditions

  • When envisioning steel required to create part shape, avoid very sharp, tall, and fragile steel conditions
    • Steel will get hot and be brittle
  • Steel condition height should not be more than 4x the width whenever possible
  • When in a corner, one rib is better than two to allow the steel to cool better

Tip 5 | heat gets trapped between two ribs and creates brittle steel conditions. One rib creates better material flow and steel conditions.

Tip 6: Do not design deep ribs with square corners

  • It will trap gas and cause short shots
  • It will be more difficult to eject part