Lots of things in life have confusing names for example, why do we park on driveways and drive on parkways? Luckily for us, Injection Molding isn’t one of those. At its root, injection molding is the process of injecting a material that has been heated to melting into a mold. This process is very common in manufacturing and used on all kinds of materials including metals, glasses, and (most commonly of all) plastics.
Below we’ll talk about the parts and the process for thermoplastic injection molding, discuss the key benefits, and discuss some of the disadvantages of this process.
The Anatomy of a Press
Without getting too technical, this article is going to focus on the steps happening in two major components of an injection molding press, the barrel and the mold. The barrel is where plastic pellets are heated to the point of melting and then put under pressure and injected into the mold. The mold is where the molten plastic cools and the actual form of the part being produced is set.
The Barrel
The barrel can either be oriented horizontally or vertically on the press, but consists of the following parts:
Hopper
Where the plastic pellets are stored before being melted.
Heater Bands
These bands of ceramic or mica heaters are wrapped around the outside of the barrel and are used to help heat the barrel to bring the plastic pellets inside to their melting temperature.
Reciprocating Screw
Located inside the barrel, the screw is what drives the plastic pellets and eventually melted plastic forward as it melts. It also adds heat to the process through friction and pressure.
Nozzle
This is the tip of the barrel and comes in contact with the mold and directs the flow of molten plastic into the mold.
The Injection Mold
The mold is what forms the molten plastic into a solid plastic piece and normally consist of two halves. For every piece produced using injection molding, a unique mold must be made. Molds consist of many different steel plates and become increasingly more complex as the complexity of the part increases. However, all Molds consist of the following parts:
Cavity
A void cut into the steel that is the same shape as the part being produced.
Runners
Channels cut into the mold to allow the flow of plastic from the sprue into the cavity.
Sprue
The main channel allowing the flow of plastic from the nozzle into the mold.
Parting Line
This is where the mold parts in half allowing for fully formed and cooled parts to be ejected.
The Process
Now that we’re familiar with the parts, let’s talk through what happens when we injection mold something.
Melting The Plastic
1
First plastic pellets fall from the hopper into the barrel and are heated. The combination of the turning heated screw and the heating bands on the outside of the barrel melt the plastic.
2
As the plastic melts, it is pushed forward into the nose of the barrel just behind the nozzle. The screw draws back allowing a reservoir of melted plastic to build up.
Injecting The Plastic
3
When enough plastic is built up. The screw plunges forward, forcing the melted plastic into the mold. The plastic flows along the sprue, into the runners, and finally into the cavities of the mold.
Cooling & Ejecting
4
The plastic is then allowed to cool in the mold for some time. This can be as quick as a couple of seconds or as long as a couple of minutes depending on the type of plastic being used, the size of the part being molded, and the design of the mold itself.
5
The mold is pulled apart at the parting line, and pins in the mold, called ejector pins, eject the cooled plastic from the mold. The mold is then clamped back together making it ready for the next shot of plastic from the barrel.
This is a very basic sketch of what happens in an injection molding machine to explain the concept. With technology today injection molding machines are extremely complex, programmable manufacturing centers that have a lot more moving parts then the ones described above, but they all accomplish the same goal of melting material and forming it using a mold.
Key Benefits
So why choose to injection mold something?
Complex Geometeries
Plastic injection molded parts can hold very tight tolerances on extremely small components that aren’t possible with other manufacturing methods at a cost-effective price. This allows injection molded products to be more complex or better fitting for certain applications.
Speed & Scale
Plastic Injection Molding operates best when it is quickly making a large number of parts. Molds can contain multiple cavities all producing identical parts in a single shot. Normally thermoplastic cooling times are short too. Even large parts can be produced in a minute or two. This means that every minute you can produce hundreds of parts from a single machine and a single operator.
Lightweight Strength
Plastic as a material is extremely strong for its weight especially when molded. That’s why it’s often used for carrying cases, outer housings, and coverings. Add to that the fact that you can mix the plastic used with resins to resist UV corrosion, increase its anti-microbial properties, make it anti-static, and other applications makes it a very durable material choice for parts.
Cosmetic Control
It is a lot easier to control the look and feel of plastic injection molded parts then other components. Colors are melted directly into the plastic allowing for consistent coloration without the need to run a second operation like powder coating or painting. Also, molds can be finished to provide part finishes ranging from slick and smooth to matte and textured.
Additionally, labels and logos can be engraved right into the part by the mold itself, so it comes out clearly legible and on every piece.
Main Disadvantages
Injection molding comes with some disadvantages when compared to other processes.
Design Limitations
Due to the fact that the mold has to open and eject the part, there are some designs that can’t be injection molded or will be extremely difficult to mold.
High Upfront Costs
Injection molding involves investing in an injection mold that can cost anywhere from $10,000 to $50,000 depending on the complexity of the part and size of the mold. This mold can normally only make the part as it is designed and will need to be modified if the part is modified.
Longer Design Period
Before production can being fully on a part, the mold has to be designed and built which might increase project timelines for getting parts into production.
How to Mitigate the Risk
If you’re reading this article, then you probably aren’t an expert in plastic injection molding. If that is the case, then our advice is to either hire someone who is an expert in plastic injection molding to help start up your operation or to partner with a company, like Whitman, that is capable of bringing a staff of capable experts to the table to help you produce the parts you need.
Partnering with a manufacturer like Whitman can significantly reduce the disadvantages listed above. Our design staff has over 40 years of experience designing plastic injection molded parts for production. We know what traps to avoid and how to modify parts to achieve the same results without making them impossible to produce.
Additionally, our in-house tool shop can produce molds quickly on site at a low cost which significantly reduces the investment required to get started, so you can start reaping the benefits faster.
We also have ways of creating soft tooling or bridge tooling that is a lot quicker to into production, so we can get parts in your hands, while we build harder tooling that is capable of cutting costs.
We would love to talk to you more about your next plastic injection molded part or any parts that you’d like to switch to plastic. Just contact us below.