Plastic Mold Parts

Plastic Mold Parts Explained

Plastic injection molding is an efficient method for producing complex shapes with smooth contours in large volumes. The actual Interesting Info about Plastic Mold Parts.

Sprues are the main conduits through which plastic first enters a mold, connecting directly with both its runner system and gate system.

Ejector Pins

Ejector pins act like the “bouncers” of plastic molds. By applying pressure, they eject parts from their respective molds with ease – sometimes leaving behind marks known as witness marks on finished products. A mold designer at Protolabs strives to position these pins so as to minimize cosmetic witness marks in final products.

Pins need a flat surface against which they can push, and this surface must be perpendicular to the direction of pin movement. Therefore, when designing a mold with pins that require pushing against them a surface should be installed which meets this criteria perpendicularly – this requires pads or bosses on problematic areas such as ribs and metal inserts which act as ejector plates when opening the mold. It may also help reduce risk by spreading out force among multiple pins so as not to increase their strength so dramatically, as one individual pin can break under pressure from exertion exertions that occur when one pin has increased strength due to increased force being spread among several pins rather than increasing strength in one pin alone.

Once again, lubricating the front half of each pin requires using an oil-free grease such as Heavy-Duty Mold Release and Pin Lube is highly recommended, as it contains no chlorinated solvents and operates up to 450degF without ever overheating or becoming problematic. Furthermore, its food-grade formula makes it safe for most injection molding applications.


Runners are channels used to transport molten plastic from an injection molding machine’s nozzle directly into each cavity in a mold. A well-designed runner system can significantly enhance the quality of final plastic products by reducing cosmetic defects such as sink marks and surface blemishes, helping ensure an even coating process across every mold cavity.

Designing a plastic injection mold requires careful consideration of its length, diameter, and shape for runners. They should be as short as possible to reduce material waste and cycle times while still being wide enough to allow an adequate flow of plastic into the cavity. Furthermore, the length and shape of gates are crucial, ensuring sufficient pressure builds up to enter into the cavity through them.

A runner balance analysis is an invaluable way to optimize runner layout and ensure that plastic flows evenly into mold cavities. An optimally designed runner system will help ensure parts form properly without warp, shrinkage, or residual stresses. Various kinds of runners, such as cold runners, hot runner systems, and multi-cavity sprue systems, are available on the market today.


Every injection molding process utilizes gates as conduits through which molten plastic flows to fill its mold cavity. Gates can be found along mold separation lines or strategically throughout the cavity to control flow; they may have tapered or constant diameters for easier control.

Gate design and placement can have an enormous impact on cycle times, lead times, and tooling costs. Furthermore, choosing an inappropriate gate can lead to cosmetic defects like sink marks and voids; an experienced contract manufacturer like Providence Enterprise understands this complex problem, which must balance rapid cycles with minimizing cosmetic and structural defects as effectively as possible.

Edge gates (also referred to as conventional gates or side gates) feature rectangular cross-sections and are typically placed along the parting line, making them popular on flat components. Although it is easier to trim manually after injection molding processes have concluded, edge gates leave visible marks on finished parts that must be manually trimmed away after the completion of manufacturing processes.

Submarine gates or tunnel gates are designed to direct plastic from its runner into a mold cavity at an angled position and provide for de-gating as well as increased production. While this type of gate allows de-gating, its size limits its functionality as it could create gas trapping in your mold.

A pinpoint gate (also called a needle gate) features a narrow profile for easy placement into hard-to-reach parts of the mold. After injection molding has taken place, this gate can easily be trimmed by shearing after shearing has been completed and can even help reduce short shots for thin-walled parts.

Injection Unit

The injection unit is one of the fundamental plastic mold parts. It serves to facilitate the melting of raw thermoplastic material before molding by way of its hopper, barrel, and reciprocating screw components, which convert plastic pellets to liquid material that can then be injected directly into mold cavities.

Injection molding is typically utilized for creating durable plastic products requiring high levels of strength and load-bearing characteristics. Depending on their application needs, manufacturers often turn to different polymers, such as plasticized rubber (TPU), polycarbonate (PC), or polypropylene (PP).

When performing injection molding, molten plastic is injected directly into a mold via an injector unit connected by a nozzle. Once enough material has been injected into the mold, it remains under pressure for up to an extended period that could range anywhere between milliseconds and minutes in order to compensate for thermoplastic shrinkage.

Once enough time has elapsed, the injection unit retracts, and the mold opens, allowing the plastic to cool off before it’s ejected from the injection machine. This cooling stage is essential to ensuring the final plastic part will retain its shape when released from the machine. During this period, changes in viscosity and pressure must be kept to a minimum to avoid issues caused by factors such as varying temperatures or uneven wall thicknesses.