Injection Molding Design Guideline

Jack Lie CNC machining expert

Specialize in CNC Milling, CNC Turning, 3D Printing, Urethane Casting, and Sheet Metal Fabrication Services.

In order to guarantee final product quality and repeatability in injection molding process, we need to consider several factors in injection molding designing. There are certain design guidelines to yield full benefit of injection molding.

In this blog, we outline common injection molding defects, basic and advanced guidelines in design, minimize cost recommendations.

Common Injection Molding Defects

Melted material flow and non-uniform cooling rate cause most defects in injection molding, we list of main defect as following to notice while designing injection molding parts

  1. Warping

Once certain section cool faster than other sections, the internal stress will result in permanent bend of molded part. Special for parts with non-constant wall thickness.

  1. Sink marks

Sink marks will appear once parts interior solidify before its surface, it show as a small recess in flat surface. Thick wall parts or poorly designed ribs parts are inclined to sinking.

  1. Drag marks

In ejection process, mold will slide and scrape parts walls, result to drag marks. Vertical wall parts without draft angle are more prone to drag marks.

  1. Knit lines

Knit lines always occur once 2 melted materials meet with hair-like discolorations. Which affect parts aesthetics and decrease parts strength. Parts with holes or abrupt geometry changes are most prone to knit lines.

  1. Short shots

Trapped air in mold will inhibit material flow and result to incomplete parts. Parts with thin walls or  inferior designed ribs are more inclined to short shots.

Injection Molding Design Rules

Injection molding can form complex geometries easily, apply single part with multiple functions. Complex parts can be reproduced with low cost once mold is manufactured. But any adjustment in mold design will be very expensive in later development stages. Our guidelines will assist you to avoid most common injection molding defects.

Constant wall thickness

Uniform wall thickness and avoid thick sections through parts in possibility. Non-uniform wall can lead to warping in cooling process. For different thickness section requirement, use smooth transition as chamfer or fillet, in order to ensure material flow evenly inside cavity and fill fully whole mold.

Wall thickness from 1.2mm to 3mm is suitable for most plastic materials, we summary recommended wall thickness for common injection molding materials:

Material Recommended wall thickness [mm]

Polypropylene (PP) 0.8 – 3.8 mm

ABS 1.2 – 3.5 mm

Polyethylene (PE) 0.8 – 3.0 mm

Polystyrene (PS) 1.0 – 4.0 mm

Polyurethane (PUR) 2.0 – 20.0 mm

Nylon (PA 6) 0.8 – 3.0 mm

Polycarbonate (PC) 1.0 – 4.0 mm

PC/ABS 1.2 – 3.5 mm

POM (Delrin) 0.8 – 3.0 mm

PEEK 1.0 – 3.0 mm Silicone 1.0 – 10.0 mm

Design Notice:

  1. Uniform wall thickness in recommended values.
  2. Smooth transition chamfer or fillet are required for section, which length is 3 time than thickness.

Hollow out thick sections

Thick sections will lead to warping and sinking, limit your design thickness under maximum recommended values in any section, hollowing is the most essential way. In addition, use ribs to improve hollow section strength and stiffness while reducing wall thickness, ribs should also apply wall thickness limitations to avoid sink marks.

Design Notice:

  1. Hollow thick sections and improve sections strength and stiffness by ribs.
  2. Maximum ribs thickness is 0.5 time of wall thickness.
  3. Maximum ribs height is 3 times of wall thickness.

Round all edges

Vertical features as ribs, bosses and snap-fits should also be rounded for smooth materials flow. For internal edges, radius of at least 0.5 time wall thickness must be used. For exterior edges, use radius as interior radius plus wall thickness. This will make wall thickness is constant at corners, avoid sharp corners with concentration stress result to weak parts.

Design Notice:

  1. Add 0.5 time wall thickness at internal corners.
  2. Add 1.5 time wall thickness at external corners.

Add draft angles

Add draft angle to vertical walls will make part ejection process easier. Otherwise, drag marks will be created on parts surface in reason of high friction between mold and no draft angle walls. Our minimum draft angle is 2°degree, maximum of 50°degree will be used for tall features. We recommend increase one degree draft angle for every 25mm. All these rules are also necessary for ribs.

Design Notice:

  1. Add at least 2°degree draft angle to all vertical walls.
  2. Increase draft angle by one degree every 25mm for all features taller than 50mm.
  3. Increase draft angle by extra 1-2°degree for parts with textured surface finish.

Undercuts solutions

As undercuts like thread teeth or snap-fit joint hook, cannot be manufactured by simple straight-pull mold. In reason of materials will be in the opening or ejection ways. So we need some ideas to deal with undercuts.


Shutoffs are useful to deal with undercut on internal regions for snap-fits or on sides for holes and handles. Essentially, remove materials in area below undercut and eliminate this issue.

Parting line

Moving parting line to intersect with undercuts features is the simplest way. It is suitable for undercuts on external surface, in addition, we need to adjust draft angles accordingly.

Stripping undercuts (Bumpoffs)

Stripping undercuts is used for flexible parts deform over molds in ejection process. Such as bottlecaps with threads. It has strict usage conditions:

  1. Stripping undercut must be kept way from stiffening feature like corners or ribs.
  2. Molded part must be flexible to deform and have space to expand.
  3. Striping undercut need 30°to 45°lead angle.
  4. Avoid fiber reinforced plastic.

Thread fasteners

We normally add thread fasteners in 2 ways, add boss on parts and add threader insert.


Bosses are common features for attachment or assemble points in injection molding process. They combine of cylindrical cylindrical projections with holes, which are used to receive screws, thread insert, or other fastening hardware.

When use bosses as fastening points, boss outer diameter should be 2 times of screw or insert nominal diameter, and inner diameter equal to screw or insert diameter. Boss hole depth should extend to base-wall level for wall thickness uniformity. Chamfer will be added fro screw or insert easy insertion.

Design Notice:

  1. Avoid bosses merge into main walls.
  2. Support bosses with ribs, or connect bossed to main walls.
  3. Bosses outer diameter is 2 time of insert nominal size.


Metal thread inserts can be added to plastic injection molding parts, which provide durable holes for machine screws. Insert are installed through thermal, ultrasonic or in-mold insertion ways with many cycle of assemble and disassemble.

Design Notice

  1. Avoid threads add directly on parts.
  2. Add 0.8mm relief on thread edges.
  3. Use thread with pitch bigger than 0.8mm.
  4. Use trapezoidal or buttress thread.
  5. Place external threads along with parting line.


Ribs are always used to improve molded parts stiffness once maximum wall thickness in recommendation cannot satisfy functional requirement.

Design Notice:

  1. Ribs thickness is 0.5 time of main wall thickness.
  2. Ribs height is smaller than 3 time of ribs thickness.
  3. Base fillet radius should be greater than 1/4 of rib thickness.
  4. Add at least 0.25°-0.5°raft angle.
  5. Distance between ribs and walls should be at least 4 time rib thickness.

Snap-fit joints

Snap-fit joint is a simple and economical way of parts connection without tools or fasteners. There are various possible design of snap-fit joint. Its deflection depends on its length and width permissible force.

Design Notice:

  1. Add a draft angle one vertical wall in snap-fit joints.
  2. Snap-fit joints thickness should be at least 0.5 time of wall thickness.
  3. Control deflection and permissible force by width and length adjustment.

Letters and Symbols

Text can be used for labels, logos, warnings, instructions and diagrams. Because CNC machining for mold manufacturing, it is economical to add embossed text than engraved text. Raising text 0.5 mm above parts surface will make letters easy to read. We recommend style of bold, rounded font,uniform lien thickness, 20 points size or bigger.

Design Notice:

  1. Embossed text instead of engraved text.
  2. Uniform thickness and minimum font size of 20 point.
  3. Align text perpendicular to parting line.
  4. Text height or depth greater than 0.5mm.