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Injection molding is a high-precision manufacturing process that injects molten plastic into a carefully designed mold, where the plastic cools and hardens into the specified part or product. The piece is then ejected from the mold, either as the final product or as a near-final product that is sent on for secondary finishing.

The injection mold consists of two parts: the mold core and the mold cavity. The space that these two parts create when the mold is closed is called the part cavity (the void that receives the molten plastic). Depending on production needs, “multi-cavity” molds can be designed to create multiple identical parts

Designing your part for the Injection molding is critical to realize those advantages. You'll also find that designing your part correctly will yield a balance of optimum performance and cost-effectiveness. The following information is an overview of the most important design elements for injection molded parts and represents both general design best practices and decades of personal experience. For the full guide.

Material Choice

 Amorphous plastics have polymer chains with a random, entangled orientation. They are stronger and more suited for structural applications. Although they are strong, they are susceptible to stress fractures. They also do not work as well as Semi-Crystalline plastics for bearing surfaces. Here are some of the most common types:

>>ABS   >>Acrylic (PMMA)  >>Polycarbonate(PC) >>PVC

Semi-Crystalline plastics have random sections of crystalline structures. In other words, they are a hybrid of amorphous and a completely crystalline structure. They make great bearing surfaces, living hinges, and provide good chemical resistance. The downside is that they shrink and warp more than Amorphous plastics. Here are some of the most common types: 

>>Acetal >>Nylon >>PBT >>HDPE >>LDPE >>PET >>Polypropylene

Selecting a Parting Line

One of the first considerations when designing an injection molded part is selecting the parting line, or where the two halves of the mold come together. For some applications, there's an obvious choice; but in others, the best option may not be so clear.

The first step is deciding which direction the line of draw (the direction the mold opens) will be for the mold. In the image below, you can see how the mold is in two halves, the line of draw, and how the two halves would come together at the parting line.

​Adding Draft  

An injection molded part needs to come out of the mold without damage or too much resistance. To avoid these issues, you want to angle the walls of the part from the parting line (drafting). Generally speaking, there should not be any surface of the part that is exactly 90 degrees to the line of draw on the mold. Not drafting a part can cause defects like drag marks and create difficult ejection. In the image below, the A-side tooling is colored blue, the B-side is red, and the part is yellow. Notice the small amount of draft (referenced by the angled lines) that allows the part (yellow) to break free from the tooling block (red).

The amount of draft you should apply to the part depends on the application. The general rule is to have at least one degree for every inch of depth. Below is a list of different design considerations where you want to add to the amount of draft.

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Avoiding Thick Areas

The thickest area of the molded part will determine the amount of cooling time. Cooling time often represents the longest part of the injection molding process. Longer cooling time makes a longer cycle time, which increases costs. Excessive wall thickness can also create part defects like sink marks and voids.  

For most applications, excessive wall thickness is larger than .125 - .1875 inches. Thick wall sections are moldable, but they open up the potential for part defects and long cycle times. Although there are certain materials and additives to achieve thick wall sections, it's best to start thin. Doing so reduces cycle time and saves on material usage, both of which save on the ongoing piece price.

Design note: Having to "add plastic" to a molded part means removing steel from the mold. Machining away material in the mold is much cheaper and easier than welding and machining. We call this being "steel safe." It's common to stay steel safe on critical dimensions, test run the mold, check dimensions, and then remove steel to finalize that dimension.

Coring & Ribbing

To avoid thick sections of a part, you can add coring and ribbing. These features reduce cycle time, reduce part weight, and could make the part stronger. Designing these features into the B-side of the part is common and is the best practice. These features can help pull the part to the B-side or ejection side of the mold; they would then be on the non-show side of the part. However, it's possible to have them on the A-side of the tooling as well. 

The image above shows a perfect example of coring a part. Since this part required one large wall section, you can remove large sections of material without sacrificing strength. Think of this in terms of other common structural components like tubing and I-beams. Because of the way forces are applied to these components, removing the material doesn't change its structural integrity. The image on the left is cored out, and the image on the right is not. Using simulation software, we were able to predict that the part without coring required double the cooling time. The added time in the molding machine increases the cost of the parts, and over time, those numbers can be huge.  

From the images above, you can see that the coring is not just a large section of removed material. Instead, its more a webbing of wall sections (known as ribs). Adding ribs is a great way to add significant strength to a part without affecting cycle time. Ribs can also reduce the amount of material that is used in the part. The image below shows a boss before and after adding support ribs. Since these ribs are thin, they do not increase cooling time, but they do add strength. Notice that the ribbing towards the corner of the part is also creating coring (right). Its common to have bosses like this on the corner of a part.  When possible, its best to allow enough room to have coring completely surrounding the boss. This helps to avoid thick wall sections that could show through in the form of part defects (like sink marks from the material shrinking more in thick areas) on the outside show surface of a part. Don't forget draft!

Uniform Wall Thickness

Keeping all major wall sections throughout a part as consistent as possible is key to injection molding. Doing so keeps material flow in the cavity consistent and makes for even cooling. The previous images from the coring and ribbing section are good examples of this. Most injection molding materials do not react well to flowing through large differences in thickness. Inconsistent wall thickness can cause pressure spikes, material degradation, too high of differential pressure across the part, and poor process control. The plastic also cools at different rates when you have varying wall thickness.  

The mold surface is cooling the part from the outside surfaces inward. This means that the middle area of a wall section will cool last. Having areas of the part cooling at different rates can cause warping issues (covered later).


Adding ribbing to a part can generate a thicker wall section in a small area where the rib meets the base. As discussed above, thick wall sections can cause sink issues because the material is shrinking as it cools. To avoid sinking, design ribs slightly smaller to reduce the wall section at the connection point. Taking the same part from above, you can see the thick areas that could create sink marks on the show side of the part (shown on the left image). In the right image, you can see that the ribbing is thinner than the main wall section. The radii are also slightly smaller to reduce the potential for a section that is too thick.


Another exception to the rule of having uniform wall thickness is living hinges. Living hinges are used to create a base part with a closing lid in one piece. They are common in injection molding but create a significant difference in the wall thickness. Living hinges will be discussed in a later chapter. 

Adding Radii

Not only are radii more visually appealing, they also help plastic flow more smoothly and reduce the likelihood of fractures. When designing radii, keep in mind the uniform wall thickness best practice. The images below shows a simplified and sectioned cover. Adding radii helps make sure plastic flows through the part consistently, keeps the wall sections even, makes the part more visually appealing, and avoids potential

Surface Finish

Building injection molds requires several different pieces of equipment. To create molding surfaces, they may be CNC machined, ground, EDM'd, turned, and so on. Each of these manufacturing processes will generate different surface textures. These textures (tooling marks) may be acceptable for non-show surfaces, but in many cases, tooling marks need to be smoothed out or textured. Creating a texture not only makes the surface of the molded part consistent, but it also has implications on part design.

Basic Surface Finishes

The surface of the plastic part is a direct translation of the surface of the mold. For example, to mold a clear lens, the mold will have a highly polished or mirrored surface. In these cases, even a fingerprint or a hairline scratch can show through on the molded part. Below is a surface finish guide created by the Plastic Industry Association.

As you can imagine, going up the scale to the "A" surface finish gets more time consuming, and therefore, raises tooling cost. Starting at C-3, each stage on this scale builds on the one before it. That is, you would not start with diamond polish on a surface with tooling marks. You would work your way up the scale through the different finishes.

EDM Texture

Sinker EDMs create sandpaper like texture as a normal part of their operation. In other words, the finish resembles sandpaper, not the finish created by sandpaper. Since EDMs use electricity to remove material, the roughness of the texture is directly related to the intensity of the settings on the EDM.  Surface roughness, in general, can be a complicated subject with several different standards. For the sake of understanding an EDM finish, weve put together a basic guide referencing the equivalent sandpapermeaning the roughness of the sandpaper itself and not the finish the sandpaper would leave.Note that this is a general guideline and is for reference only. The point here is to help you understand the look and feel of an EDM finish, not to establish an accurate means of specifying a surface finish.

Other Textures

There are also more textures available through acid etching, laser engraving, or CNC machining designs right onto the surfaces. Some of the specialized surface finishes like acid etching are common in other industries; therefore, most molders outsource that aspect of the tool build. The mold builder will usually prepare the surface to a specified polish level before sending the components out for the texturing process.

Note that this is a general guideline and is for reference only. The point here is to help you understand the look and feel of an EDM finish, not to establish an accurate means of specifying a surface finish. 

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