what is injection molding
what is injection molding
So you want to know what is injection moulding? Basically, it is just as the name suggests the injections of a liquid into a mould. The liquid is usually a plastic of some sort. Now the liquid can also be a molten metal in which case the process is called die-casting; however, it’s not as widely used as plastic injection moulding. Even though I have given you a straightforward overview of what injection moulding is, I don’t want to mislead you. It is actually a very technical with quite a lot of science behind the process. However, don’t worry I am not going to get into that just yet. Injection moulding is used to make everything from a plastic bottle cap to a plastic medical device and is the most widely used process to manufacture things.
Now I don’t know exactly how many injection moulding companies there are, however, if I do some back of the fag box maths, in the Republic of Ireland where I am from we have approximately 122 moulders in a population of 4.9 million. With a world population of 7.8 billion on the planet, according to my maths, there should be 1591.8 x 122 = 194,204 injection moulding factories around the world—that about one factory for every 40,000 people. The injection moulding machine market was valued at 16.72 billion USD in 2017 and is estimated to reach 19.68 billion USD in 2023, which is growing at a 2.9% rate. Anyhow you can see this is a huge industry.There really are there three main things you need for injection moulding, 1. an injection moulding machine, 2 a mould and 3. materials. I will go into a little bit of detail about each of these.
Injection moulding machine.
The injection moulding machine basically injections the liquid material into a mould, then opens the mould and ejects the part from the mould, then closes the mould. The machine is operated by a hydraulic system or via electric servo motors, that have very precise control over the speed and pressure of each of the movements. The injection moulding machine has a central control unit with an interface that can be used to easily control the sequence, speed, pressure and time of each of the moving parts of the machine.
So getting into a little bit of the details I will just introduce each of the movements most injection moulding machines will perform. Lets starting with the injection sequence. The machine pushes a screw forward at a set speed usually 40 – 200 mm/s, at a certain position or pressure the machine switches over to a holding pressure or as it is also referred to as a second stage injection. This really is just where the screw is moved very slowly forward at a certain pressure. So material has been injected into the mould where it is allowed to cool for a short time depending on the thickness, but it can be anything from 5 sec to 45 seconds usually.
During this cooling time, the screw within the barrel of the machine is rotated causing the material, which is usually in small pellets stored in a hopper over a small opening in the barrel where the screw is. So the material falls down onto the screw, and as the screw is rotating the material is drawn up to the top of the screw. The barrel usually has heater band around it that heat the barrel to a certain temperature. The rotation speed and the pressure at which the screw moves back at is fully controlled by the machine; this sequence is usually referred to as the plasticising or charging stage.
The machines then open its platens, which are just a big block of cast iron that the mould is attached. Again the opening of the mould is done at a controlled speed, and depending on the size of the machine, this mould opening distance can be anything from 200 mm to 5000mm. During the mould opening sequence, parts within the mould may need to be moved either in or out or rotate. This is usually done via a hydraulic piston or a pneumatic piston, and again the machine will control the sequence and speed of these movements, usually called core movements.
When the mould has fully opened the machines operates the ejectors, which is usually done via a hydraulic piston or an electric motor. Again the speed and pressure at which the ejectors are moved forward and backwards are all controlled. Delay and pause times can be set so that the part can be successfully ejected from the mould. The part(s) can be simply ejected into a bin, or a robot can come into the mould and take the parts out. Once the part is ejected from the mould, the mould is closed at a controlled speed and pressure. So that is a basic overview of a moulding machine. However, there are many variations in machine size and configuration. For example, the injection unit can be vertical or a horizontal configuration; likewise, the clamping unit can be vertical or a horizontal configuration. However no matter how big or small the machine is or what configuration set-up it is, they are all essentially opening and closing the mould, injection material and charging and melting material in a semi or the fully automatic way. The is a lot of detail within injection moulding machines; however let us move on to the next important piece for injection moulding, the mould.
The mould also spelt mold in America, has been referred to as the heart of the moulding process, and in lots of ways, it is. The mould is usually made of plates of steel; however, aluminium and softer metal have been used to make moulds.
Within the mould, there are a few main parts, namely the feed system or otherwise known as the runner system where the plastic is introduced into the mould. The plastic is injected into the mould usually via a sprue bushing, and down into a runner system. The runner system is a channel that the plastic can easily flow along. The plastic can then be divided into sub runners that feeds multiple parts within the mould. Each part within the mould will have a gate which allows the plastic from the runner system to flow into the part. A part may have multiple gates in it depending on the size of the part and how far the plastic has to flow. Once the plastic flows into the part air within the cavity is pushed out through vents. The molten plastic within the mould heats up the tool; this heat must be removed from the mould so that the part can cool down and solidify. Moulds are usually cooled via water channels, that are drilled into the mould plates and around the core cavity, these cooling channels can range in size from 4 mm to 8 mm in diameter.
There are usually two sides to the mould the fixed side and the moving sides; this can also be referred to as the cavity and core sides of the mould. In America, it is also referred to as the A and B side of the mould. The mould may be split into two or more places so this allows the mould to be opened and the parts to be ejected. If there are holes or undercuts in the plastic part, a metal core may need to be moved in the mould to allow the part to be freely ejected. Parts are usually ejected from the moving half of the mould; however, with hydraulic/pneumatic pistons the part can be ejected from the fixed half of the mould.
Due to the large injection forces, anything from 40 – 80 Mpa or 400 – 800 bar would be a typical injection pressure range within injection moulding. That may not mean a lot to you but consider your car wheel takes approximal 2 bar or air pressure, and I am not sure about you but whenever I am topping my car tyres up I am away afraid it will burst or blowout and smack me in the head. So in relative terms, there are between 20 to 40 times the injection pressure in an injection mould than is inside your car tyre.
This may give you some appreciation as to the forces exerted within the mould, so as to stop the mould from being pushed open due to the high injection forces a clamping force must be exerted on the mould to keep it closed. As a general rule of thumb, this clamping force can be anything from 1.5 to 5 metric tonnes per square inch of the projected part surface area. So for a part that is 1 inch by 1 inch, we are going to need 1.5 to 5 tons of force to keep the mould closed. Therefore, the mould needs to be designed and constructed in such a way that it is able to withstand these large forces.
There is also the need for alignment keys in the mould half such as pillars and bushings, to allow the mould to open and close while maintaining alignment of the tool halves. It should also be noted that the ejector system within the mould must be robust enough to be able to transfer force onto ejector pin to remove the part from the mould. The parts shrink onto the core very tightly, so quite a lot of energy is needed to ejector the part and runner system from the tool. So as you can see, there is quite a bit of engineering and design work that goes into building and making tools for injection moulding.
Materials for injection moulding.
There are many materials that can be used in injection moulding, but a large percentage of them are what is know as thermoplastics which are plastics that can be remelted over and over again. There are many different types of thermoplastics however the most common types of thermoplastics which would account for the largest percentage of moulded part would be made from the commercial resins such as, HDPE, LDPE, PP, PS, ABS and PC. There are other plastic materials know as engineering resins such as nylon, POM, PMMA and PBT but these are more specialised materials; however, they can all be injection moulded. Die casting which use a very similar process to injection moulding however metal is injected into a mould. This process is usually used for application where higher strength parts are needed; however, this is a somewhat less clean process compared to injection moulding. Also, with the improvement in thermoplastic material mechanical properties and the addition of fibre reinforcement, plastic parts strength is improving and in many cases just as strong as metal parts. Another category of material suitable for injection moulding are thermoset materials; these materials can not be reheated and melted over and over. For example, liquid silicone comes as a part A and part B material, when it is mixed within the screw and barrel of the moulding machine, it is injected as a liquid into a hot mould. The heat in the mould cures or crosslinks the material to form a solid silicon part. The Silicon can not be remelted to form a new part.
So, in essence, this is injection moulding, I hope this has given you some insight into what is injection moulding. I will be running a course on the fundamentals of injection moulding soon when I do I will put alike here to it. I simply provide training and consultation in the area of injection moulding, so if you have any question, please get in touch. Many thanks, www.pro-plastics.com