Designing for Manufacturability (DFM) is an engineering design methodology that aligns production and engineering in the design phase to ensure cost and time efficiencies, superior quality, and regulatory compliance. It also helps avoid significant manufacturing defects that compromise product functionality and aesthetics.
Adding radii to corners and edges helps plastic flow consistently, eliminates stress concentrations that could lead to fractures or cracking during injection molding, and makes the part more visually appealing. Uniform wall thickness is also a best practice.
Choosing the Right Plastic
There are a lot of factors that go into choosing the suitable material for your plastic injection molds. First and foremost, you must choose a plastic that is fit for purpose, meaning that it will perform its intended function. Several plastics, including various polystyrene types, polyethylene (PE) with different densities, and polyethylene terephthalate (PET or PETE), fit this criteria.
Other material considerations include the desired color and texture and how the plastic will react to heat. For example, some plastics are unsuitable for injection molding because they will yellow or become hazy when exposed to heat. Finally, it would be best to consider how the plastic will be disposed of or recycled.
This may influence the choice of materials as some plastics are more accessible to recycle than others. The part’s design can also impact the selection of the appropriate plastic, including any living hinges or snap fits, as well as the location of any threads or undercuts.
Choosing the Right Tooling
Injection molding is an ideal manufacturing process for plastic components manufactured at scale. However, it can take some work to navigate. Thankfully, employing design for manufacturability best practices from the beginning of your project can reduce production and assembly costs and speed up turnaround times.
To start, work with an injection molder who understands the specifics of the material you plan on using for your product. They can provide you with an in-depth understanding of the limitations and capabilities of the plastic as well as give you suggestions on how to design your molds better.
For example, rounded corners are far preferable to sharp ones because the liquified plastic flows much more quickly on these surfaces. It also reduces stress concentration and prevents cracking. It’s also good to draft your edges for easier removal during the part ejection process. Ideally, the drafting angle will be between one and two degrees.
Choosing the Right Material
Injection molding is a manufacturing process that requires precise dimensions and multiple material options. There are more than 85,000 commercial options for plastic materials, so choosing the right one is an important decision that can impact the final molded product. It’s also essential that the chosen plastic is compatible with the injection molding process.
For example, a sharp inside or outside corner can cause stress concentration, leading to cracks in the finished part. Instead, designers use ample radii that dissipate stress throughout the surface area. This design feature also helps the resin flow during the injection process. Other considerations include the location of part lines.
These physical lines appear where the two halves of the mold meet when it opens up. These lines must be hidden or minimized, especially if visible on the finished product. This can be done by adding draft angles or using gussets. Sometimes, the injection molder may apply a texture that blends in with these lines, reducing their visibility.
Choosing the Right Finish
The finish that a plastic injection molded product is designed with can reduce production costs and maintenance requirements. For example, a matte finish that hides fingerprints is perfect for keyboard keys or a shop vac, while a smooth mirror-like surface works well for optical lenses. A uniform part wall thickness is essential to prevent many defects from occurring during the injection molding process. Thin plastic sections can warp or sink when they cool and shrink faster than the rest of the part.
The ejection of the molded parts from the injection mold is often complicated by undercuts (the portion of the part that extends past the sprue). It is possible to avoid undercuts by using a shut-off, which blocks the flow of melted plastic around an area of the mold. Using draft angles on vertical walls is a good idea to make ejection easier. Larger draft angles should be used for taller features.