Injection molding has been one of the most important fabrication tools for the plastics industry since the reciprocating screw machine was patented in 1956. Today, it’s almost impossible to do anything without using injection molded parts. They are used in automotive interior parts, electronic housings, housewares, medical equipment, compact discs, and even doghouses. Injection molding is used to fabricate pallets, toys, crates, and pails, thin-wall food containers, promotional drink cups, lids, and milk bottle caps.

The injection molding process involves melting the plastic in an extruder and using the extruder screw to inject the plastic into a mold, where it is cooled. Speed and consistency are vital keys to running a successful injection molding operation, since profit margins are normally below 10 percent.


A molder will maximize output by minimizing cycle time which is the amount of time that is taken to melt the plastic, inject it into the mold, cool, and eject a finished part.

Using larger molds that produce more than one part each time the machine performs a cycle can also increase output. These molds are known as multiple cavity molds.


Consistency, or elimination of scrap and downtime, is just as important as output in a successful molding operation. The most consistent processing results from careful control of plastic temperature, plastic pressure as it fills the mold, the rate at which the plastic fills the mold, and the cooling conditions. These four primary molding variables are interdependent and can often be used to understand process changes and solve problems. While the variables apply to almost all injection molding processes, the process will be slightly different in each shop, depending on the application, the plastic being used, and the molder’s preferences.

Fill rate:

In thin wall applications, the material must be injected into the mold as quickly as possible to prevent the plastic from freezing before the part has been completely filled. The newest resin and machine technologies in this area almost always focus on faster, easier fills. In addition to minimizing cycle time through better filling ability, the molder could realize resin cost savings through the ability to fill thinner molds or achieve higher outputs by using larger, higher cavity molds.

Thin wall molding is accomplished using machines that inject the material in less than one second and are big enough to support large, multiple cavity molds. Thin wall lids and containers tend to be small, so molds may be used to fabricate over 100 small lids at a time.

Reference 19/11/08

An Introduction to the History of Plastics

Plastic Polymers

Plastics are polymers – long-chain carbon-based or “organic” molecules. These chains are made up of repeating fundamental molecular elements, or “monomers.”

The term plastics covers a range of mostly synthetic or semi-synthetic organic condensation or polymerization products that can be molded or extruded into objects or films or filaments. The name is derived from the fact the properties are in a semi-liquid state that is malleable, or has the property of plasticity. Plastics vary immensely in temperature tolerance, hardness, resiliency. Combined with this adaptability, the general uniformity of composition and lightness of plastics ensures their use in almost all industrial applications today.

Natural Polymers

People have been using artificial organic polymers for centuries in the form of waxes and shellacs. A plant polymer named “cellulose” provides the structural strength for natural fibers and ropes, and by the early 19th century natural rubber, tapped from rubber trees, was in widespread use.

Eventually, inventors learned to improve the properties of natural polymers. Natural rubber was sensitive to temperature, becoming sticky and smelly in hot weather and brittle in cold weather. In 1834, two inventors, Friedrich Ludersdorf of Germany and Nathaniel Hayward of the US, independently discovered that adding sulfur to raw rubber helped prevent the material from becoming sticky.

In 1839, the American inventor Charles Goodyear was experimenting with the sulfur treatment of natural rubber when, according to legend, he dropped a piece of sulfur-treated rubber on a stove. The rubber seemed to have improved properties, and Goodyear followed up with further experiments, and developed a process known as “vulcanization” that involved cooking the rubber with sulfur. Compared to untreated natural rubber, Goodyear’s vulcanized rubber was stronger, more resistant to abrasion, more elastic, much less sensitive to temperature, impermeable to gases, and highly resistant to chemicals and electric current.

Vulcanization remains an important industrial process for the manufacture of rubber in both natural and artificial forms. Natural rubber is composed of an organic polymer named “isoprene.” Vulcanization creates sulfur bonds that link separate isoprene polymers together, improving the material’s structural integrity and its other properties.

Polystyrene and PVC

After the First World War, improvements in chemical technology led to an explosion in new forms of plastics. Among the earliest examples in the wave of new plastics were “polystyrene” (PS) and “polyvinyl chloride” (PVC), developed by the I.G. Farben company of Germany.

Polystyrene is a rigid, brittle plastic that is now used to make plastic model kits, disposable eating utensils, and similar knickknacks. It would also be the basis for one of the most popular “foamed” plastics, under the name “styrene foam” or “styrofoam.” Foam plastics can be synthesized in an “open cell” form, in which the foam bubbles are interconnected, as in an absorbent sponge, and “closed cell,” in which all the bubbles are distinct, like tiny balloons, as in gas-filled foam insulation and floatation devices.

PVC has side chains incorporating chlorine atoms, which form strong bonds. PVC in its normal form is stiff, strong, heat and weather resistant, and is now used for making plumbing, gutters, house siding, enclosures for computers and other electronics gear, and compact-disk media. PVC can also be softened with chemical processing, and in this form it is now used for shrink-wrap, food packaging, and raingear.