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Strengths and Weaknesses of Injection Molding

Plastic injection molding is a pretty interesting process! You start by feeding plastic resin in pellet form into a machine. Then, the material is rotated forward by a screw and makes contact with a heated barrel.

This friction melts the plastic, which accumulates in front of the screw. Finally, the molten plastic material is injected into a metal tool which then cools and ejects the plastic part from the machine.

The machine has a program that turns off the screw motor when there is enough material. The screw is used to push the material out of the nozzle and into the mold. With the assistance of cooling channels, the material cools and hardens in the mold. After a set amount of time, the mold opens, the plastic part is ejected, and then the process starts over again.

Types of Molds

Injection molding production tools are typically made out of steel in order to ensure that the mold can handle the pressure of the injection molding process. Steel molds tend to last a while and can produce a lot of parts.

Molds that are not as tough can be made from aluminum, but they don’t last as long and aren’t as easy to take care of. They might be a good choice for short prototype runs with only a few hundred parts. MUD frames come in standard sizes that inserts are loaded into. They’re a good option for small parts and quick mold changeovers.

Single cavity tools produce one part at a time; perfect for low volume programs or large parts. Multi-cavity tools produce many of the same molded part in one cycle and are ideal for high-volume production, reducing manufacturing time and labor.

Family tools produce multiple different parts in one cycle. However, because of the varying sizes and features of the parts, this mold should be reserved for low quality, non-critical, non cosmetic parts.

Side Actions

Depending on the complexity of your part, the tool designer may need to add a side action. Side actions are special features that are designed into a mold to help achieve tricky part features. These add cost and increase mold complexity and maintenance.

Lifters help to release internal undercuts and once the part has cooled, the lifter can be pushed up at an angle to release the undercut. Slides are used to release external undercuts and the part is molded around the features cut into the slide. Once cooled, the slide is pulled away from the part by an angled pin attached to the core side of the mold.

Collapsible cores are really handy for creating threaded features. The part is molded around the core, and then once it’s cooled, the core collapses so that you can remove the part from the mold.

Unscrewing actions also work well for threaded features. Again, the material is molded around the unscrewing action, and then once it’s cooled, the action turns and unscrews itself from the part.

Strengths of Injection Molding

Structural foam molding is a process where you mix the part material with a foaming agent which then turns into gas and enters the mold. You inject less material into the mold than is needed to fill the cavity and the gas bubbles expand until it fills the cavity. 

This creates a cellular structure within the part. Because of the unpredictable nature of gas, structural foam parts may not look exactly how you want them too and they might have an uneven surface color. Structural foam molding is ideal for larger, thicker projects that need to be light and sturdy. It also uses less material than other processes, and requires a lower tonnage press.


Overmolding is the process of molding plastic onto plastic or metal components. A part is engineered to fit into a steel mold to allow plastic to be formed around the part, creating a strong bond between the part and the overmolded plastic and eliminating the need for assembly. Overmolding can be used to create a variety of different textures and colors, and it’s a great way to add an extra layer of protection to your components.

Two-Shot Molding

Two-shot molding is a process to achieve plastic on plastic overmolding results. As its name suggests, the completed molded part is created with two shots from the injection molding machine.

The first shot creates the base, typically rigid, plastic component. The second shot overmolds the commonly flexible plastic onto the first shot plastic. This process can be used to create a variety of different products, from cell phone cases to automotive parts.

Insert Molding

Insert molding is loaded into the mold either by hand or by machine. The mold then closes and plastic material surrounds the inserts. The mold must be designed to fit the required inserts, but this process eliminates the need for time-consuming assembly and creates a stronger bond between the part and its inserts.

Weakness of Injection Molding

Flow lines are wavy patterns that can be slightly different in color than the surrounding area. They often show up on narrower sections of a molded component or as ring-shaped bands near the gates where molten material flows through.

While flow marks don’t usually affect a component’s strength or performance, they can be considered unsightly by some people. This is especially true for consumer products where appearance is important, like high-end sunglasses.

When you’re injection molding, sometimes you’ll see a squiggly line on the surface of the finished product. This is called jetting, and it happens when the molten material doesn’t have time to fill the mold before it starts to solidify. Jetting often happens near the gate where the material is injected, and it can make your part weaker.

When a molded part is a different color than intended, that’s called “discoloration” or “color streaking.” Usually the discoloration is only in one area or there are just a few streaks of abnormal color. This usually just affects how the part looks, but it doesn’t make the part weaker.

Key Takeaway

Getting to know the ins and outs of injection molding is key when you’re ready to release a new product. However, before you get too deep into tooling and production, it’s important to chat with a Design for Manufacturability (DFM) engineer. They are experts when it comes to part and tool design, material selection, and cost avoidance.


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