How Does Plastic Injection Molds Work?

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.

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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.

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.

As one of the most popular manufacturing techniques for mass production, plastic injection molding can be used with many different thermoplastics to make parts with complicated designs that would be nearly impossible to create with any other method. If you’d like to know how it works and how it measures up to 3D printing, keep on reading. 

How it Works

Before we go into the ins and outs of the injection molding process, you’ll first need to know about the machine’s various components and what they do. 

Components

All the important parts of an injection molding machine are shown in the image below, with descriptions of each in the table further down.

Process

Plastic injection molding uses thermoplastic pellets, which need to be melted first. These are fed into the hopper and make their way to the barrel, where the reciprocating screw pulls back to make room for them to get through. The screw then goes forward again to force the plastic through the nozzle. The pressure from the platen makes sure that no plastic can get away by bringing the nozzle and mold closely together tightly. Here, the melted plastic is pressurized, something that makes it go into all the mold cavity’s parts, taking up the space that the air previously occupied. As the plastic fills every single crevice (including the sprues and their buddies, runners, and gates), the air that was in that empty space now comes out of the mold vents.

The mold has to stay steadily at a temperature that aligns with the specific material’s melting point so that the part inside can cool and harden evenly. In addition, the holding pressure also has to be constant so that there’s no backflow of material into the barrel. This also keeps the shrinking under control. More pellets are then put in the hopper so there’s no downtime—once the part is cooled and taken out, it’ll be ready to go again. The platen opens when it’s time to eject your creation, and the screw does its job again—allowing room for material before pushing it through… it basically works like this on a loop.

Getting Started

Before you see all your wonderful products being made, you need to take care of two important things: your mold design and the actual mold. 

Design

First things first, your product needs to be designed. This is typically done as a CAD file or other transferable format, and you’ll follow the necessary design guidelines for the particular injection molding process you’re using. For the best chance of success with injection molding your plastic parts, try to include features like bosses for threaded inserts or fasteners, hollow cavities for thicker sections, rounded edges, ribbed supports for extra strength, snap-fit joints, or friction fits as joining features, living hinges where you need some flexibility, and draft angles on vertical walls. You’ll also want to make sure that wall thicknesses are consistent (or as much as possible) and avoid features that can lead to defects. These include overly thin/thick walls, sudden changes to the shape, i.e., sharp corners, randomly placed holes, badly designed ribbing, and undercuts or overhangs.

Tooling mold

The tooling mold is undoubtedly the star of the show in the whole injection molding process. Making this, however, is no walk in the park—it’s the longest and most expensive part of the whole process and needs to be done by professional machinists who know exactly what they are doing. Based on your design, these experts craft the “tool” (that’s what those in the know call a tooling mold), then make their own blueprint that has all the necessities (cavity, sprues, gates, ejector systems, etc.). If you think that you’ll get your mold on next-day delivery, think again; the process of making the mold (including approvals) can take 20 weeks… sometimes more! So imagine how long you’ll have to wait (and how much it’ll cost) if you need to make changes to the mold… Suffice it to say, it’s important to get the design right before handing it over.

Compatible Materials

The materials listed in the below table are all suitable for plastic injection molding.

Disclaimer

The content appearing on this webpage is for informational purposes only. Xometry makes no representation or warranty of any kind, be it expressed or implied, as to the accuracy, completeness, or validity of the information. Any performance parameters, geometric tolerances, specific design features, quality and types of materials, or processes should not be inferred to represent what will be delivered by third-party suppliers or manufacturers through Xometry’s network. Buyers seeking quotes for parts are responsible for defining the specific requirements for those parts. Please refer to our terms and conditions for more information. 

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