Injection molding can be accomplished using a range of materials which include metal, glass, elastomers and confections. Injection molding is the technique used on most of the plastic pieces that you come into contact with everyday. Some of those common plastic materials are Polypropylene (PP), Acrylonitrile butadiene styrene (ABS), Polyvinyl Chloride (PVC), Thermoplastic Polyurethane (TPU) and many others.
The injection process is accomplished using a heated barrel where the material is first melted down and mixed. Thermoplastics will often begin life as pellets and will be fed into the heating bowl with a reciprocating screw. From here the material is then forced through a check valve to collect at the front of the screw in the exact volume needed for the mold – known as a shot. This is actually slightly greater than the volume of the mold cavity as it includes extra to compensate for shrinkage (which is particularly high for thermoplastics), to fill the ‘sprue’ and ‘runner’ of the mold, and to provide a ‘cushion’ that remains in the screw and prevents it from bottoming out. The material is transferred to the mold at high pressure and velocity, both of which are kept steady (spikes in pressure are not wanted). The injection process can take under a second in total – blink and you’ll miss it!
Once inside the mold, the material will conform to the contours inside the mold (in some cases a single mold will have multiple cavities to create separate part) where it will begin to cool and harden (this process is sped up and stabilized by passing a coolant through the mold). The mold itself is usually made from tool steels (carbon and alloy steels), though stainless steel and aluminium can also be used; aluminium being cheap, though less suited to parts with narrow dimensions and more prone to wear and damage. A single steel mold conversely may be used to process millions of parts, but can be expensive to fabricate (costing hundreds of thousands of dollars in some case).
Actually a mold consists of two parts – the injection mold and the ejector mold (or A plate and B plate respectively). When the material first enters the mold it will pass through a ‘sprue’ or ‘gate’, and will then flow into the runners via ‘runners’. Vents are also ground into the parting line of the mold thus preventing trapped air which can cause imperfections.
When the mold is opened (using ‘ejector pins’ in most cases), the cast should remain in the ejector side and should draw the material out from the sprue and runners with it. An interesting quirk is that you can’t have parts overhanging one another across the opening of the mold unless you utilize components called ‘lifters’ to compensate. In other words, molds are a lot more complicated than you might imagine!
Design Tips and Limitations
Injection molding is a quick and efficient method for creating a vast range of items and can be used for creating everything from storage containers and mechanical parts, to small furniture and toys. It is ideal for delivering large quantities of the same part and is the most common method in part manufacturing today as a result.
Despite the considerable benefits of injection molding however, the process is of course not suitable for all forms of manufacturing and does have its limitations. For one, most parts creating using injection molding will bear certain ‘birth marks’ where the parting line, sprue, gate marks and ejector pins were located. Skillful design however can hide these imperfections to the point that they often aren’t noticeable at all. Some deformation is also usually expected due to slight fluctuations in pressure during injection and other factors – but accounting for all of these can be very expensive.
At the same time there are certain things that of course can’t be achieved with injection molds or are more difficult. There are size limitations for example (parts larger than a small chair would typically be too costly and impractical) and complex parts require clever design and forward thinking when it comes to the creation of the molds. Elements such as hollow portions can be accomplished using techniques such as injection blow molding (commonly used for creating bottles and containers). The high cost of tooling can also be limiting for smaller production runs.
Really though, for large runs of smaller objects, injection molding is an ideal solution and the only real limitation is your imagination and ingenuity. With smart mold design you’d be amazed what can be accomplished.