Design Guide: Top 5 Rules for Injection Molding

Injection molding seems like a very simple process given the mass amounts of plastics parts and products in our world today.  There are however a number of critical design issues that can impact part appearance and quality.  Luckily there a number of key design guides to
know about.  Implementing these in design will increase part quality and time to market.  Contract manufactures will also appreciate correctly designed parts as it will speed their production times as well.  Designing parts for manufacturability and assembly at the design stage will prevent costly rework and speed time to market.

1. Consistent Wall Thickness:  First start with consistent wall thickness.  Different wall thicknesses cause sink marks to appear on part surfaces.  As plastic cools thicker areas shrink and cause divots to appear on the part surface.  This can be seen commonly on inexpensive plastic trays where appearance is not critical.  But sink marks are unacceptable on the cover of a consumer electronics bezel.  Maintain consistent wall thickness throughout the entire part.  Changes in part thickness cause molding issues such as surface sink marks and possibly voids.  Where a wall change is necessary, add smooth transitions to ensure ease of material flow and prevent critical stress points.
2. Add Ribs and Bosses for Strength:  Ribs and bosses are added to bring strength to part surfaces and corners.  Ribs are added to increase part rigidity.  Consistent or near consistent wall thickness applies here as well and designers should avoid a combination of thin and thick ribs.  Rib thickness should be 60% to 80% of the nominal wall thickness and the maximum rib height should not exceed 3x the nominal wall thickness.  To increase product rigidity, it is better to increase the number of ribs rather than the rib height.  The minimum spacing between ribs should be 2x the nominal wall thickness.  Fillet radii applied to ribs should be no greater than 50% of the rib thickness and extra thick ribs (and bosses) should be cored out.  Cross ribbed patterns are preferred over unidirectional patterns as they ensure more uniform stress distribution.  Bosses offer strengthening properties and provide alignment during assembly.  Similar to rib design, it is important to consider the wall thickness when designing bosses.  The boss thickness should be 60% of the nominal wall thickness; if the part thickness is greater than 4mm, reduction of the boss thickness can be to 40% of the nominal wall thickness.  The boss height should not exceed 2.5x the diameter of the hole in the boss.  Corner bosses should be cored out to reduce excess material accumulation and part sink.  Ribs added to bosses help material flow. Gussets should also be added to distribute the load during screw/insert insertion.  The fillet radii should be applied at the base of a rib or boss to allow better stress distribution.  Stress concentrates if no fillets are applied which can lead to cracking and part failure.
3. Include Draft Angle:  In order for the injection molding machine to remove a part from the mold, the part must have a draft angle.  This is an often forgotten design rule.  Some injection molding tool suppliers will add an arbitrary draft angle if one does not exist and change the part.  To minimize surprises, add draft.  High pressures of injection molding and material contraction from cooling causes the part to grip the mold.  Thin-walled parts generally need more draft.  Smooth surface recommendation is 0.5° to 1° draft per side.  Textured surface should include 1.5° per 0.025mm depth of texture in addition to the normal draft amount (so about 2° and up).  Once a polymer is selected for the part it is a good idea to talk to the injection molding supplier.  They will have valuable feedback on the design and can offer suggestions on draft changes based on the part, geometry, size, and texture.  Mold designers can also add ejector pins to facility part removal.  If you can fit it in, use 1 degree of draft or more.  A rough rule of thumb is 1 degree of draft for every inch of depth, up to 3 degrees for 3 inches.  Light texture requires 3 degrees of draft minimum on vertical walls.  Medium texture requires 5 degrees.
4. Add Part Radii:  Sharp corners create localized stress concentrations.  This is where cracks form and cause part failure. Fillet radii are added to reduce stresses and improve plastic flow. At corners the inside radius should be 0.5x material thickness and the outside radius should be 1x material thickness plus the part thickness.
5. Tolerance as Needed:  Injection molding can accommodate ±.002" tolerance.  But, if a customer will only notice 0.02”, then tolerance to that.  Tolerance for function plus a tad to allow for tool wear.  Using narrow tolerance increases part cost and increases scrap.  Parts also shrink as they come off the mold.  Shrink tolerance depends mainly on part design and resin choice. It varies from 0.002" for stable resins like ABS and polycarbonate to 0.020" for unstable resins like Santoprene.  High tolerance is like adding undercuts.  They should only be added if the product absolutely needs it.  Both high tolerance and undercuts add complexity and cost.

Injection molding has been around the industry for decades. These tried design guides are used can help designers create functional parts that are also moldable.  These rules reduce surprises when moving from design to production.