What Are Some of The Considerations For Injection Molding
Before you endeavor to produce a part via injection molding consider a few of the following things:
Entry Cost: Preparing a product for injection molded manufacturing requires a large initial investment. Make sure you understand this crucial point up front.
Determine the number of parts produced at which injection molding becomes the most cost effective method of manufacturing
Determine the number of parts produced at which you expect to break even on your investment (consider the costs of design, testing, production, assembly, marketing, and distribution as well as the expected price point for sales). Build in a conservative margin.
Part Design: You want to design the part from day one with injection molding in mind. Simplifying geometry and minimizing the number of parts early on will pay dividends down the road.
Tool Design: Make sure to design the mold tool to prevent defects during production. For a list of 10 common injection molding defects and how to fix or prevent them read here. Consider gate locations and run simulations using moldflow software like Solidworks Plastics.
Cycle Time: Minimize cycle time in as much as it is possible. Using machines with hot runner technology will help as will well-thought-out tooling. Small changes can make a big difference and cutting a few seconds from your cycle time can translate into big savings when you’re producing millions of parts.
Assembly: Design your part to minimize assembly. Much of the reason injection molding is done in southeast Asia is the cost of assembling simple parts during an injection molding run. To the extent that you can design assembly out of the process you will save significant money on the cost of labor.
An Example (Designing For Injection Molding)
Designing a part that’s suitable for injection molding versus one that’s suitable for machining, thermal forming, or 3D printing means taking into consideration some of the differences between the various fabrication techniques and recognizing when your project is better suited to one or the other. Typical parts you might want to injection mold include joints, brackets, or housings. For example, most consumer electronic tools are made with a plastic shell (housing) that’s injection molded and used for the body of the tool.
Consider the housing for an electric drill produced by Panasonic (see below)
One of the most obvious advantages to injection molding is that the housing serves multiple purposes. First, it serves as a handle for the end user to interact with. It also acts as a receptacle for the battery and motor as well the location of various screw bosses that will be used to fasten the device together once the internal parts are assembled. In other words, injection molding is extremely effective when you need to organize a lot of internal parts within a housing. As a consequence, it’s a fantastic way to reduce the number of total parts (“piece count”). Of note, this part is also an overmolded part. For more on this process read here.
Some of the other reasons that injection molding is a good fit for this example include the fact that the drill is being produced in large volume. That is, Panasonic is creating a large number of copies of the same drill handle. Injection molding is wonderful for this kind of high volume production because the high initial costs pay the manufacturer back over time with low per unit costs. For this same reason injection molding can be a poor choice for low volume production. Additionally of note, there are some design constraints if using injection molding. For example, the part has nearly uniform wall thickness (which is important in order to avoid defects), and the part is made with a thermoplastic material (allowing for solid plastic stock to be repeatedly melted for the procedure). If you were designing a part with a thermoset material then injection molding would be more nuanced. You can injection mold a thermoset material but you can only do it once. Trying to melt a thermoset plastic a second time will result in burning the material. Similarly, a part with varied wall thickness would require more attention in the mold tool design to ensure uniform cooling and to avoid defects during production.