The Process And Characteristics Of Injection Molded Parts

- Oct 26, 2017-

  • Utilizes screw-type plunger to force molten plastic material into a mold cavity

  • Produces a solid or open-ended shape (typically a cavity and core shape) which conforms to the contour of the injection molded part

  • Injection molds require the use of thermoplastic or thermoset materials

  • Parting line, sprue, and gate marks are present

  • Ejector pin marks are usually present on injection molds


How did the world of injection molded parts begin? In 1868, billiard ball maker Phelan and Collander, John Wesley Hyatt invented a way to make billiard balls by injecting celluloid into a mould. Hyatt improved the celluloid so that it could be processed 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 machines used by today's injection molding companies. It contained a basic plunger to inject the plastic into a mold through a heated cylinder. The industry progressed slowly over the years producing injection molded products such as plastic collar stays, buttons, and hair combs. In the 1940's the concept of injection molds grew in popularity. This is because World War II created a huge demand for inexpensive, mass-produced products.

In 1946, James Hendry built the first screw injection molding machine, revolutionizing the plastics industry with an auger design to replace Hyatt's plunger. The auger is placed inside the cylinder and mixes the injection molded material before pushing forward and injecting the material into the mould. This allowed colored plastic or recycled plastic to be added to the virgin material and mixed thoroughly before being injected. Today screw injection molding machines account for 95% of all injection machines companies. The industry of injection molds has evolved over the years, from producing combs and buttons to a diverse array of custom injection molded products for the following industries: medical, aerospace, consumer, toys, plumbing, packaging, automotive, and construction.

Applications of Injection Molds

Plastic injection molding is the preferred process for manufacturing plastic parts. Injection molds are used to create many things such as electronic housings, containers, bottle caps, automotive interiors, pocket combs, and most other plastic products available today. Plastic injection molds are ideal for producing high volumes of plastic parts, due to the ability of making multi-cavity injection molded parts, where multiple parts are made with one cycle. Some advantages of injection molding are high tolerances, repeatability, a wide range of material selection, low labor cost, minimal scrap losses, and little need to finish parts after molding. Some disadvantages of this process include an expensive tooling investment and the need to prototype, as some custom complex parts may encounter problems during the injection molding process such as warp or surface defects. Therefore, injection molded parts must be designed with careful molding consideration.

Examples of Polymers Best Suited for Injection Molds

Most polymers may be used for molds, including all thermoplastics, some thermosets, and some elastomers. There are tens of thousands of different materials available for injection molds and that number is increasing every year. The materials can mixed with alloys or blends of previously developed materials. This allows product designers to choose from a vast selection of materials so they can choose exactly the right properties for the injection molded part or parts they need. Mold materials are chosen based on the strength and function required for the final part and each material has different parameters for molding that must be considered. Common polymers like Epoxy and phenolic are examples of thermosetting plastics while nylon, polyethylene, and polystyrene are thermoplastic.