Injection molding is a method to obtain molded products by injecting plastic materials molten by heat into a mold, and then cooling and solidifying them.
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The method is suitable for the mass production of products with complicated shapes, and takes a large part in the area of plastic processing.
The process of injection molding is divided into 6 major steps as shown 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.
A 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 mold through a sprue and fills cavities by way of runners and gates. Then, the mold is opened after cooling process and the ejector rod of the injection molding machine pushes the ejector plate of the mold to further eject moldings.
A molding 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 mold 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 moldings 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.
Sprues and runners among moldings are not products. These portions are sometimes discarded, but in other cases they are finely reground and reused as materials for molding. These materials are called reprocessed materials.
Reprocessed materials are not solely used as materials for molding but usually used after blending with virgin pellets, since there is possibility of deterioration in various characteristics of the plastics because of the initial molding 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.
For the properties when reprocessed materials are used, please refer to "reprocessing capability" in the plastic data base.
Molding condition means cylinder temperature, injection speed, mold temperature etc. set in a molding machine to obtain required moldings, and the number of combinations of conditions is innumerable. Depending on the conditions selected, the appearances, dimensions, and mechanical properties of the molded products change considerably.
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Therefore, well-tried technology and experience are required to select the most suitable molding conditions.
The standard molding conditions for our materials are shown below. Please click the mouse at the following names of plastics.
Once you have developed the molds, the process is extremely fast with cycle times as short as 10 seconds. It is excellent for medium and high-volume production runs for anything from 10,000 parts to well over 100,000 depending on what molds you use.
Sometimes you can increase production by using a multi cavity or family mold, where several parts are produced from one press to further increase the manufacturing rate.
If you are outsourcing your molding then it's also important to consider an efficient front-end design and order process ' take a look at our on demand manufacturing.
There is a huge range of plastic materials that you can select from depending on what properties you need from your final part. And you are not limited to plastic, Protolabs also offers liquid silicone rubber molding.
You can even use fillers in the molding material, which adds greater strength to the completed part, and you have a huge range of colors to choose from as well. Talk to us about what you need your part to achieve ' there are generally a number of different options.
If you are committing to steel mold tools for high production volumes (100,000+) then it can take a great deal of time and machining to produce and this can be a significant capital cost. If you need lower volumes of parts then this will affect your cost per part price and it may be worth exploring other options such as using aluminum molds instead.
It can take up to 12 weeks to produce steel tooling. If you need to get production running before this then take a look at our on-demand production; by using aluminum molds and digitizing our front-end process we can ship anything from 25 to 10,000+ parts in 15 working days or less from the point that you first upload your CAD. Sometimes parts shipped in as little as one day.
For very low production runs there are other technologies such as 3D printing or CNC machining which could provide a more cost-effective answer more quickly.
You will need to consider certain design elements, such as:
You also need to consider where to place your gates, ejectors and cooling lines if aesthetics are important. And remember it's also hard to change the design of a mold ' you can remove part of the mold, or add plastic to your final part, but not the other way round.
For smaller production runs, 3D printing allows you to design virtually any shape or geometry that you require. As a rapidly developing technology there is an increasing number of plastic and other materials, even metal, that we can produce.
Most people think that they should only turn to injection molding for orders of 100,000+ parts; but there are other options when you can use this process for smaller production runs.
Using more affordable aluminium molds and quick turnaround times (as fast as a day) our on-demand service helps keep your per part cost down. It is ideal for anything between 10,000 and 25,000 parts, but can help control costs for production runs even smaller than this.
For low production runs measured in the hundreds or less then it is also worth exploring other manufacturing technologies such as 3D printing and CNC machining. With these technologies the cost per part will be lower and you will not face the initial set up costs or lead times involved in producing molds.
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