As one of the most popular manufacturing techniques for mass production, plastic injection molding can be used with many different thermoplastics to make parts with complicated designs that would be nearly impossible to create with any other method. If you’d like to know how it works and how it measures up to 3D printing, keep on reading.
Before we go into the ins and outs of the injection molding process, you’ll first need to know about the machine’s various components and what they do.
All the important parts of an injection molding machine are shown in the image below, with descriptions of each in the table further down.
Plastic injection molding uses thermoplastic pellets, which need to be melted first. These are fed into the hopper and make their way to the barrel, where the reciprocating screw pulls back to make room for them to get through. The screw then goes forward again to force the plastic through the nozzle. The pressure from the platen makes sure that no plastic can get away by bringing the nozzle and mold closely together tightly. Here, the melted plastic is pressurized, something that makes it go into all the mold cavity’s parts, taking up the space that the air previously occupied. As the plastic fills every single crevice (including the sprues and their buddies, runners, and gates), the air that was in that empty space now comes out of the mold vents.
The mold has to stay steadily at a temperature that aligns with the specific material’s melting point so that the part inside can cool and harden evenly. In addition, the holding pressure also has to be constant so that there’s no backflow of material into the barrel. This also keeps the shrinking under control. More pellets are then put in the hopper so there’s no downtime—once the part is cooled and taken out, it’ll be ready to go again. The platen opens when it’s time to eject your creation, and the screw does its job again—allowing room for material before pushing it through… it basically works like this on a loop.
Before you see all your wonderful products being made, you need to take care of two important things: your mold design and the actual mold.
First things first, your product needs to be designed. This is typically done as a CAD file or other transferable format, and you’ll follow the necessary design guidelines for the particular injection molding process you’re using. For the best chance of success with injection molding your plastic parts, try to include features like bosses for threaded inserts or fasteners, hollow cavities for thicker sections, rounded edges, ribbed supports for extra strength, snap-fit joints, or friction fits as joining features, living hinges where you need some flexibility, and draft angles on vertical walls. You’ll also want to make sure that wall thicknesses are consistent (or as much as possible) and avoid features that can lead to defects. These include overly thin/thick walls, sudden changes to the shape, i.e., sharp corners, randomly placed holes, badly designed ribbing, and undercuts or overhangs.
The tooling mold is undoubtedly the star of the show in the whole injection molding process. Making this, however, is no walk in the park—it’s the longest and most expensive part of the whole process and needs to be done by professional machinists who know exactly what they are doing. Based on your design, these experts craft the “tool” (that’s what those in the know call a tooling mold), then make their own blueprint that has all the necessities (cavity, sprues, gates, ejector systems, etc.). If you think that you’ll get your mold on next-day delivery, think again; the process of making the mold (including approvals) can take 20 weeks… sometimes more! So imagine how long you’ll have to wait (and how much it’ll cost) if you need to make changes to the mold… Suffice it to say, it’s important to get the design right before handing it over.
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