Plastic part design for injection molding

plastic injection inner coverNowadays the plastic parts is as common as you can see it to be used for every products. Whatever in automotive, medical, industrial, and consumer electronics. Plastic parts always pays an important components as it should be. This is why we would like to discuss some design tips on the plastic part design.

As you know, the plastic parts is made from plastic mold with injection machine, we call this manufacturing process as injection molding. and injection molding process requires a system work condition and facility, like injection molding machine, raw material, injection mold, and workers.

Plastic parts design is a very comprehensive process and it requires rich experience in product design, mold design, and product manufacturing industry. As you have to consider many engineering factors.

Wall Thickness

A proper wall thickness is very important to affect both the part function and appearance, this is why we put the wall thickness top priority to discuss. Basically we follow a rule that design your plastic parts as thinner as possible, as long as the part can perform its function and stiffness. For a thicker wall thickness, we can also save lots of manufacturing cost.

On the other hand, a plastic part with varying wall thickness will experience differing cooling rates and different shrinkage. In such case achieving close tolerance becomes very difficult and many times impossible. Where wall thickness variation is essential, the transition between the two should be gradual.

CORNERS and radius

In theory , when two different surfaces meet, there must be a corner. And the corners could be outwards or inwards. When we design a plastic parts, we normally take the thickness of the corner 1.4 times thicker than normall wall thickness. If we do not follow this rule, the plastic part might encounter a shrinkage. Meanwhile we try to add some radius on the coners so as to enhance the strength of parts.



Rib is a protrusion feature in plastic part improve stiffness of wall, hole, boss. It can also enhance mold flowing as they hasten melt flow in the direction of the rib. So you must design some ribs on your plastic parts. And choose a proper thickness for the ribs.

According to our decade molding experience, we summarize some design data as below for your reference.

  • Rib thickness should be between 0.4 to 0.6 times nominal wall thickness to avoid shrinkage issue.
  • Rib height should be 2 to 4 times nominal wall thickness.
  • Rib base should be added radius 0.25 to 0.4 times nominal wall thickness.
  • Distance between two ribs should be 2.5 to 3 times nominal wall thickness.


MOULDABILITY(mold making and injection molding consideration

After we complete our plastic part design, we must take a look at the parts you designed, just think about it is possible and easier to make the plastic mold, and it’s cost-saving to make the injection molded parts? Otherwise you should always optimize it


Draft angles

It’s a common sense that draft angle is an important factor when designing plastic parts. You must design draft angle for your parts. Because of shrinkage of plastic material, injection molded parts have a tendency to shrink onto a core. This creates higher contact pressure on the core surface and increases friction between the core and the part, thus making ejection of the part from the mold difficult. Hence, draft angles should be designed properly to , assist in part ejection. This also reduces cycle time and improves productivity. Draft angles should be used on interior and exterior walls of the part along the pulling direction.

Normally we leave a 1degree per side for most of plastic parts, but for some big plastic parts, we may make the drawft angle between 2~ 5 degree per side would be preferable. It really depends on the part size and material.


After reading the material we shared above, we believe you must have some basic knowledge on how to design a proper plastic parts for your products. Hope this tips will be helpful for you. In case you still have other question or query, you welcome to to contact Inno Molding team, we will be always here in China for you.