How an Injection Molding Machine Works


An injection molding machine acts like a large hypodermic needle injecting molten plastic into a mold in order to produce its final product, and it is one of the most cost-effective and dependable ways of producing high volumes of similar parts. Often, the remarkable fact about horizontal injection molding machines is that.

Raw thermoplastic material is fed into a heated barrel equipped with a rotating screw, where it melts and uniformly distributes thermal energy among its polymer chains.

The Process

Injection molding is the process by which thermoplastics are injected into a mold to produce components with precise shapes and sizes, such as machine widgets, automotive components, plumbing fittings, or toys. Molds made of strong tooling materials like aluminum and steel can withstand high injection pressure; however, they must first be designed and constructed before production begins – increasing upfront project costs considerably.

Raw thermoplastic material is fed into a barrel by means of a reciprocating screw, which delivers it to a heated nozzle, or sprue, through which molten polymer flows freely into a mold cavity and through an internal channel called the sprue. As it advances, shearing of individual chains occurs before melting; viscous fluid forms inside of it that runs down through it into mold cavities while being cooled by the machine.

As part of the injection and clamping process, two halves of the mold must be brought together tightly enough to prevent plastic from escaping through its interface with mold cavities—known as “flash.” Flash can significantly reduce strength in finished parts or lead to total scrappage of molding operations; several factors, including excessive shot size, injection pressure, or using an outdated mold, could contribute to flash.

Once both the sprue and injection cavity have been filled, packing pressure must be applied until all molten plastic solidifies around its gate (cavity entrance). When filling 95% of an injection cavity’s capacity is achieved, speed control typically changes over to packing pressure control on an injection machine’s control system.

Thin-walled injection-molded components tend to cool and solidify more rapidly, making production quicker than thicker parts. However, it is essential to balance this against reduced stiffness caused by having less material compared with its thicker counterpart. A good solution would be adding ribs to key design elements—adding these will significantly increase stiffness without increasing production costs or times.

The Materials

Injection molding can be used for an assortment of engineered thermoplastics. A wide variety of additives can alter the mechanical properties of plastic, such as glass fibers that increase strength or colorants that offer more than one hue.

Injection molding is a highly repeatable manufacturing technology, making it ideal for producing mid- to high-volume parts. This is due to relatively constant tooling costs, which are determined by production volume. Therefore, it may be possible to reduce costs by designing parts that minimize production time and costs.

To achieve this goal, several design elements must be considered when planning plastic parts. These include wall thickness, rib and boss designs, weld line transitions, corner transitions, and gate placement. Understanding how a part will be used will allow designers to optimize its structure and performance by including additional ribs or gussets as needed.

At the core of injection molding lies plastic melt, which is introduced via a nozzle into a mold through runners and distributed across individual cavities that determine the dimensions of a finished plastic part. An optional gate may then be added to one or more runners to focus the flow of melt directly to a particular location in each cavity.

Once the gate has been placed in, a clamping unit presses together the two halves of the mold under high pressure before using a cooling channel to allow any melt to cool and solidify before being released from its mold.

Injection-molded parts are typically composed of thermoplastic materials such as acrylonitrile butadiene styrene (ABS), polystyrene (PS), and polypropylene (PP), with glass fiber reinforcement available to improve strength and stiffness, fillers, plasticizers, and flame retardants injected in injection molding for enhanced performance as fillers and plasticizers to boost its capabilities for manufacturing mid to large volumes of plastic and liquid silicone rubber components; CNC or 3D printing could offer alternatives without incurring tooling costs associated with injection molding production techniques for smaller production runs;

The Equipment

The injection molding process employs high-pressure injection of thermoplastics into a mold to produce plastic parts. The mold is an instrument for shaping polymers into their desired forms and is typically made of hardened materials like stainless steel or aluminum for maximum durability.

Injection molding machines (IMMs) are typically utilized to carry out this process. Engineered for precision and energy efficiency, IMMs can produce small, medium, or large parts with various degrees of complexity ranging from simple designs to highly complex ones requiring cleanroom environments.

Injection molding machines use a reciprocating screw and heaters to transform plastic pellets into liquid material for injection into molds. As the screw turns and heats the pellets to melt them down, a heated nozzle dispensing this plastic into the injection mold is activated; typically, it’s located within an injection mold bushing containing a cavity-shaped depression for this nozzle that seals tightly with its injection barrel to avoid air entry during melt-injection, potentially leading to flash.

Once molten plastic enters a mold, it travels along runner channels to reach its destination in each cavity. Each runner features a gate at its end that controls how much plastic flows into each cavity—this gate must be placed exactly where needed so each cavity receives just the correct amount of molten plastic.

Too much plastic results in parts that are unreasonably heavy and lack sufficient strength, while too little leaves voids or excess material around its edges, creating weak points that compromise structural integrity. Both scenarios may be mitigated through reduced shot size or lower injection pressure, but these steps come with their own set of challenges.

Specialized auxiliary equipment, such as granulators, may be needed to convert sprue and runners into consistent-sized granules that can then be conveyed and blended with virgin pellets or other feedstocks to maintain consistent material quality. This is especially critical for hygroscopic resins, which absorb moisture from their environment and lead to internal defects like voids, discoloration, and holes. These defects result in increased processing costs and voids in finished products.

The Parts

Injection molding involves multiple components and pieces, including the mold, injection machine, and barrel. The barrel feeds thermoplastic material into the machine before being melted down and injected into the mold via its main parts, called the A and B sides. The A side contains visible features, while the B side may contain hidden features like bosses, ribs, and snap-fits that won’t show in finished products. The B side often bears signs of marks from ejector pins used to push formed plastic out from within once it has set.

The A and B sides are connected by a channel through which molten plastic flows, known as the runner system. A sprue bushing seals tightly against the nozzle of an injection machine to direct this plastic to cavity images on either A or B sides; additionally, this bushing connects it with channels machined into each side of the mold (called gates), making these materials the only waste products generated during injection molding process – up to 30% can be recycled or reused! Sprue and gate systems represent 15-30% of total waste produced during injection molding processes.

One major component in the cost of injection molded plastic parts is their initial investment cost; this has an impactful ripple-through to production costs even after just 1000 units have been produced. However, most injection molding costs come from raw materials and labor expenses.

Therefore, it is vitally important to select the appropriate plastic material for an injection molding application. Various polymers are available, and each has different properties that affect how the final product performs.

More rigid plastics will increase the stiffness of finished parts. Wall thickness must also be optimized in order to minimize cooling cycles; thinner parts tend to cool and solidify more rapidly, which could lead to warping issues. Ribs may be added for additional stiffness if necessary.