About Injection Molding

- Jul 16, 2019-

About Injection Molding


Injection molding is the most common modern method of manufacturing plastic parts. It is used to create a variety of parts with different shapes and sizes, and it is ideal for producing high volumes of the same plastic part. Injection molding is widely used for manufacturing a variety of parts, from the smallest medical device component to entire body panels of cars. A manufacturing process for producing plastic parts from both thermoplastic and thermosetting materials, injection molding can create parts with complex geometries that many other processes cannot.

he first step of getting a plastic part injection molded is to have a computer-aided design (CAD) model of the part produced by a design engineer. The three-dimensional (3D) CAD model then goes to an injection molding company where a mold maker (or toolmaker) will make the mold (tool) that will be fitted into an injection molding machine to make the parts.

Molds are precision-machined usually from steel or aluminum, and can become quite complex depending on the design of the part. Plastic materials shrink at different rates when they cool, so the mold has to be constructed with consideration for the shrinkage rate of the material being used for the parts. In other words, a formula is applied in the construction of the mold to slightly increase the size so that when the plastic shrinkage occurs, the part will be to the dimensional specifications of the CAD model.

The Process

Plastic injection molding is a manufacturing process where resin in a barrel is heated to a molten state, then shot into a mold to form a part in the shape of the mold. The resin begins as plastic pellets, which are gravity fed into the injection molding machine through a funnel-shaped hopper. The pellets are fed from the hopper into a heated chamber called the barrel where they are melted, compressed, and injected into the mold’s runner system by a reciprocating screw.

As the granules are slowly moved forward by a screw-type plunger, the melted plastic is forced through a nozzle that seats against the mold sprue bushing, allowing it to enter the mold cavity through a gate and runner system. The injection molded part remains at a set temperature so the plastic can solidify almost as soon as the mold is filled.

The part cools and hardens to the shape of the mold cavity. Then the two halves of the mold (cavity or “A” side and core or “B” side) open up and ejector pins push the part out of the mold where it falls into a bin. Then the mold halves close back together and the process begins again for the next part.

History of Injection Molding

The plastic injection molding process is generally dated back to 1868, when John Wesley Hyatt of billiard ball maker Phelan and Collander was searching for a suitable replacement material for the ivory in billiard balls. Hyatt invented a way to inject celluloid into a mould that processed it into a finished form. In 1872 John and his brother Isaiah patented the first injection molding machine. This machine was relatively simple compared to the complex machines used by today’s injection molding companies. It consisted of a basic plunger to inject the plastic into a mold through a heated cylinder. The industry was slow to adopt the injection molding process, eventually beginning to produce plastic items such as collar stays, buttons and hair combs. Not until the 1940s did the concept of injection molding really grow in popularity because World War II created a huge demand for inexpensive, mass-produced products.

The plastics industry was revolutionized in 1946, when James Hendry built the first screw injection molding machine with an auger design, replacing Hyatt’s plunger. The auger mixes the injection molded material in a cylinder and pushes the material forward, injecting it into the mould. This allowed colored plastic, or recycled plastic, to be mixed in with the virgin material before getting injected into the mould.

Today, screw-type injection molding machines account for 95% of all injection machines. The industry has evolved immensely over the years due to technological advancements and machine automation. It has come from producing combs and buttons, to a multitude of custom injection molded products for virtually every industry including automotive, medical, construction, consumer, packaging, aerospace and toys.

Plastic Injection Molds

Steel injection mold

Injection molds, or mold tooling, are the formed halves that come together in the injection molding machine to be filled with molten plastic and produce the plastic parts in their image. The cavity side, or “A” side, is typically the half which will form the “best” surface of the part, and the core side, or “B” side, will typically show the visual imperfections caused by ejector pins because the parts get ejected from this half.

Injection molds are manufactured by machining or by Electrical Discharge Machining (EDM). Standard machining was the traditional method of building injection molds with a knee mill. Technology advanced the process, and Computer Numerical Control (CNC) machining became the predominant method of making complex molds, with more accurate details, and in less time than the traditional method.

EDM is a process in which a shaped, copper or graphite electrode is slowly lowered onto the mold surface, which is immersed in paraffin oil. Electric voltage applied between the tool and the mold causes spark erosion of the mold surface in the inverse shape of the electrode. EDM has become widely used in mold making - many injection mold companies now have EDM in-house. The process allows the formation of molds which are difficult to machine, such as those with features such as ribs or square corners. It allows pre-hardened steel molds to be shaped without requiring heat treatment.

Compared to other plastic manufacturing processes such as CNC machining or 3D printing, injection molding has a high up-front investment because the tooling is expensive. However, for large production runs of thousands or even millions of identical parts, injection molding is typically less expensive in the long run, despite the high initial tooling investment, because of a lower piece price at high volumes. In addition, it is a much faster manufacturing process than the others mentioned.

Molds can be made of pre-hardened steel, steel that is hardened after the mold is produced, aluminum, and/or beryllium-copper alloy. The choice of mold material is determined in part by the number of parts to be produced.