Injection moulding is a manufacturing process that allows for parts to be produced in large volumes. It works by injecting molten materials into a mould (or ‘mold’ in the United States). It is typically used as a mass production process to manufacture thousands of identical items. Injection moulding materials include metals, glasses, elastomers and confections, although it is most commonly used with thermoplastic and thermosetting polymers.
Injection moulding is a method to obtain moulded products by injecting plastic materials molten by heat into a mould, and then cooling and solidifying them.
The method is suitable for the mass production of products with complicated shapes, and takes a large part in the area of plastic processing.
Discover the extensive process of injection moulding below.
Injection molding machine is divided into 2 units i.e. a clamping unit and an injection unit.
The functions of the clamping unit are opening and closing a die, and the ejection of products. There are 2 types of clamping methods, namely the toggle type shown in the figure below and the straight-hydraulic type in which a mold is directly opened and closed with a hydraulic cylinder.
The functions of the injection unit are to melt plastic by heat and then to inject molten plastic into a mold.
The screw is rotated to melt plastic introduced from the hopper and to accumulate molten plastic in front of the screw ( to be called metering ) . After the required amount of molten plastic is accumulated, injection process is stared.
While molten plastic is flowing in a mold, the machine controls the moving speed of the screw, or injection speed. On the other hand, it controls dwell pressure after molten plastic fills out cavities.
The position of change from speed control to pressure control is set at the point where either screw position or injection pressure reaches a certain fixed value.
The first stage of injection moulding is to create the mould itself. Most moulds are made from metal, usually aluminium or steel, and precision machined to match the features of the product they are to produce.
Once the mould has been created by the mould-maker, the material for the part is fed into a heated barrel and mixed using a helical shaped screw. Heating bands melt the material in the barrel and the molten metal or molten plastic material is then fed into the mould cavity where it cools and hardens, matching the shape of the mould. The cooling time can be reduced through the use of cooling lines that circulate water or oil from an external temperature controller. Mould tools are mounted on plate moulds (or ‘platens’), which open once the material has solidified so that ejector pins can eject the part from the mould.
Separate materials can be combined in one part in a type of injection moulding called a two-shot mould. This technique can be used to add a soft touch to plastic products, add colours to a part or produce items with different performance characteristics.
Moulds can be made of single or multiple cavities. Multiple cavity moulds can have identical parts in each cavity or can be unique to create parts of different geometries. Aluminium moulds are not best suited to high volume production or parts with narrow dimensional tolerances since they have inferior mechanical properties and can be prone to wear, deformation and damage due to the injection and clamping forces. While steel moulds are more durable they are also more expensive than aluminium moulds.
The injection moulding process requires careful design, including the shape and features of the part, the materials for the part and the mould and the properties of the moulding machine. As a result, there are various considerations that need to be taken into account when injection moulding.
A mould (or mold) is a hollow metal block into which molten plastic is injected to from a certain fixed shape. Although they are not illustrated in the figure shown below, actually there are many holes drilled in the block for temperature control by means of hot water, oil or heaters.
Molten plastic flows into a mould through a sprue and fills cavities by way of runners and gates. Then, the mould is opened after cooling process and the ejector rod of the injection moulding machine pushes the ejector plate of the mould to further eject mouldings.
A moulding consists of a sprue to introduce molten resin, a runner to lead it to cavities, and products. Since obtaining only one product by one shot is very inefficient, a mould is usually designed to have multiple cavities connected with a runner so that many products can be made by one shot.
If the length of the runner to each cavity is different in this case, the cavities may not be filled simultaneously, so that dimensions, appearances or properties of the mouldings are often different cavity by cavity. Therefore the runner is usually designed so as to have the same length from the sprue to each cavity.
There are a number of considerations to bear in mind before undertaking injection moulding:
The entry cost for injection moulding manufacture can be high – given the initial cost of machinery and the moulds themselves.
It is important to determine how many parts you wish to manufacture so as to decide whether injection moulding is the most cost-effective production method.
Minimising the number of parts and simplifying the geometry of your items will make injection moulding easier. In addition, the design of the mould tool is important to prevent defects during production.
Minimising the cycle time will aid production as will using machines with hot runner moulds and well thought-out tooling. There will also be cost savings from minimising assembly requirements, especially if you are producing many thousands of even millions of parts.
The “tonnage” is the maximum of clamping force that a injection molding machine can offer to keep the the injection tool tightly closed during the injection process.
This clamping force ensure the mold won`t move apart or getting loose and having products with sharps.The tonnage size is one of the most important factor when you want to select a injection molding machine. As it will determinate what kinds or size of plastic products you can produce with this molding machine.
We normally use the tonnage size as the main index of a injection moulding machine. You may heard some of these sentence:
I have a 200 ton injection machine, the mold will need a 300 ton molding machine,is the product can be produce with a 120 ton moulding machine?
Normally, most plastic injection engineers will rough calculate the machine`s tonnage with the product`s surface multiply by a index number.
If you are using square inch as the surface, then you can make it 2.5 times (higher flow materials like PS, PE and PP) to 4 times (lower flow materials like PA, PC and PSU) as the index number.
While your are using square cm as the surface, then you should make it 0.48 times (higher flow materials like PS, PE and PP) to 0.72 times (lower flow materials like PA, PC and PSU) as the index number.
There are many clamping force formula are used for different people for some different applications. Below we will try to explain you the popular clamping force calculation formula:
Empirical Formula 1
Clamping force (T) = Clamping Force Constant Kp * Product Projected Area S (CM * CM) * Safety Factor(1+10%)
To properly calculate necessary clamp tonnage, you must first know the projected area of the molded part/s plus any runners.
Area can be calculated by multiplying length times width, A = L x W or A=πr2.
Above clamping force empirical formula is most used for fast calulate with more easy to have a rough size. Most engineers will pick a bit larger size from the result even. As it still have so risk for the products which have complex construction or the thickness on different position have big changed, or there are many perpendicular runner/corners.
Injection moulding can be an expensive process, but there are several ways in which you can reduce mould costs, including:
Injection moulding is used to make a range of widely used products, including common plastic items like bottle tops as well as remote control casings, syringes and more. It is also commonly used for manufacturing larger items such as car body panels.
Injection moulding is mainly used where there is a need to manufacture many thousands or millions of identical parts from a mould.
The main advantage of injection moulding is being able to scale up production to produce a large number of parts. Once the initial costs of the design and the moulds have been covered, the price of manufacturing is very low. The cost of production drops as more parts are produced.
Injection moulding also produces minimal wastage when compared to traditional manufacturing processes like CNC machining, which cuts away excess materials. Despite this, injection moulding does produce some waste, mainly from the sprue, the runners, the gate locations, and any overflow material that leaks out of the part cavity (also called ‘flash’).
The final advantage of injection moulding is that it allows for the production of many identical parts, which allows for part reliability and consistency in high volume production.
With over 85,000 commercial plastic material option available and 45 polymer families, there is a wealth of different plastics that can be used for injection moulding. Of these, the polymers can be broadly placed into two groups; thermosets and thermoplastics.
The most common types of plastic used are high-density polyethylene (HDPE) and low-density polyethylene (LDPE). Polyethylene offers a number of advantages including high ductility levels, good tensile strength, strong impact resistance, resistance to moisture absorption, and recyclability.
Other commonly used injection moulded plastics include:
This tough, impact-resistant plastic is widely used across industry. With good resistance to acids and bases, ABS also offers low shrinkage rates and high dimensional stability.
This strong, impact resistant plastic has low shrinkage and good dimensional stability. A transparent plastic that is available in different optically clear grades, PC can provide a high cosmetic finish and good heat resistance.
There are many different types of PPA (or nylons), each of which has its own advantages. Generally-speaking, nylons offer high strength and temperature resistance as well as being chemically resistant, apart from against strong acids and bases. Some nylons are abrasion resistant and offer good hardness and stiffness with good impact strength.
Commonly known as acetal, this plastic has high hardness, stiffness, strength and toughness. It also has good lubricity and is resistant to hydrocarbons and organic solvents. Good elasticity and slipperiness also provide advantages for some applications.
PMMA, also known as acrylic, provides good optical properties, high gloss and scratch resistance. It also offers low shrinkage and less sink for geometries with thin and think sections.
This inexpensive resin material delivers high impact resistance in certain grades but can be brittle in cold temperatures (in the case of propylene homopolymer). Copolymers offer greater resistance to impact while PP is also wear-resistant, flexible and can provide very high elongation, as well as being resistant to acids and bases.
Good electrical properties makes PBT ideal for power components as well as automotive applications. The strength ranges from moderate to high depending on glass fill, with unfilled grades being tough and flexible. PBT also shows fuels, oils, fats and many solvents, and it also doesn’t absorb flavours.
A dimensionally stable material with high toughness, temperature and heat resistance, PPSU is also resistant to radiation sterilisation, alkalis and weak acids.
This high temperature, high-performance resin provides heat resistance and flame retardancy, excellent strength and dimensional stability, as well as good chemical resistance.
PEI (or Ultem) offers high temperature resistance and flame retardancy, along with excellent strength, dimensional stability and chemical resistance.
Sprues and runners among mouldings are not products. These portions are sometimes discarded, but in other cases they are finely reground and reused as materials for moulding. These materials are called reprocessed materials.
Reprocessed materials are not solely used as materials for moulding but usually used after blending with virgin pellets, since there is possibility of deterioration in various characteristics of the plastics because of the initial moulding process. The maximum allowable limit for the ratio of reprocessed materials is about 30 %, because too high ratio of reprocessed materials may spoil the original properties of the plastics used.
Moulding condition means cylinder temperature, injection speed, mould temperature etc. set in a moulding machine to obtain required mouldings, and the number of combinations of conditions is innumerable. Depending on the conditions selected, the appearances, dimensions, and mechanical properties of the moulded products change considerably.
Therefore, well-tried technology and experience are required to select the most suitable moulding conditions.
The standard moulding conditions for our materials are shown below. Please click the mouse at the following names of plastics.
At Roland Plastics, our injection moulding process combines advanced technology with expert craftsmanship to deliver high-quality plastic components.
From small, intricate parts to larger, more robust components, and whether you need low-volume prototypes or high-volume production runs, we can meet your needs.
Ready to elevate your project with our expert injection moulding services? Contact us today to turn your design into reality.
Plastic Injection Moulding & Bespoke Product Design in Suffolk, UK.
Copyright © 1985 - 2024 Roland Plastics Ltd
