4 Step Injection Molding Design Guide

Injection molding is a manufacturing process used widely across many industries to create plastic consumer products.  It is used for a wide variety of different products such as bottles, combs, packaging, children’s toys, gardening implements, and much more.  This three part injection molding design guide will help to provide a firm understanding of plastic injection molding basics.

The process of injection molding is fascinating, first plastic granules are loaded in a hopper.  They are heated, mixed and fed through a screw conveyor at a high temperature and pressure into your mold (tool) cavity through a runner system.  The runner system distributes the molten plastic into the mold so that it evenly fills the mold cavity.  The plastic cools and shrinks.  Once the product is cool, ejection pins push your part out of the mold.

In this injection molding design guide, we will discuss how part geometry plays a large role in the quality of the end product.  There are many possible defects that can happen during this process.  To minimize these defects, we have compiled a 3 part article to serve as a plastic part design guide.  In this first section of the article we will discuss plastic part geometry design guidelines.

1. Wall Thickness

It is ideal to have a uniform wall thickness. Thick sections of a part can shrink unevenly during the cooling process creating defects such as sink marks, voids, warping, and stresses. The maximum recommended thickness varies for different materials but generally should be in the 2 to 4 mm range. Having a consistent wall thickness can help to avoid these common injection molding defects.

If there is no way to make you part with a uniform wall thickness, then as gradual as possible transition should be used.

injection molding design guide draft for plastic part design

2. Draft

Draft is the angle off of parallel from the part’s wall to the direction the mold opens. This is required for injection molded parts so that the part can be removed or released from the mold. Typically a draft angle of 1 to 2 degrees will work with most situations, while a heavily textured surface could require as much as a 5 degree draft.


3. Radius

Radii should be added to all corners of parts. Avoiding sharp corners reduces stress points in your part and allows the molten plastic to fill the mold more evenly.

  1. The radius on the inner side of the part should be at least half the of the wall thickness. Ri= ½*T
  2. The radius on the outside of the part should be the part thickness plus the inside radius. Ro=3/2*T
  3. The radius on rib or boss should be a quarter of the part thickness.


4. Ribs

Ribs are used to create a more rigid part and increase loading capacity. If you desire to increase your parts rigidity and strength, the quantity of ribs should be increased rather than increasing ribs thickness or height.

  1. Thickness of ribs should be 40-60% the wall thickness.
  2. Rib spacing should be at least two times the wall thickness apart.
  3. Rib height should be limited to three times the wall thickness


Read part 2 of our injection molding design guide.