An A-to-Z Guide for Injection Moulding Terminology

By Protolabs

Injection Moulding has become incredibly versatile as a manufacturing process. This is a direct result of advances in the technology we use to create product designs and produce moulding tools. You will hear various technical terms throughout the product design and manufacturing process. In this blog, we share an A to Z of some of the terminology you will hear when dealing with Injection Moulding specialists.

Core and Cavity

ABS (Acrylonitrile Butadiene Styrene): A common thermoplastic polymer known for its versatility, durability, and impact resistance. It’s widely used in various industries and applications due to its favourable mechanical properties and ease of fabrication.

Burn Marks: Refers to dark or discoloured spots or streaks on the surface of a moulded part. These marks can detract from the part’s appearance and may indicate issues during moulding. Burn marks occur due to localised overheating of the plastic material within the mould cavity. Several factors contribute to burn marks, including excessive heat, residence time, injection speed and pressure, inadequate venting, and material selection.

Cavity: The hollow space inside the mould where molten plastic material is injected into. This hollow space is the shape of the final part/product. The cavity is a crucial component in the injection moulding process as it defines the external shape and features of the part. Cavities are usually formed of two main halves: the cavity side and the core side.

Clamping Force: The force applied by the clamping unit of the injection moulding machine to keep the mould halves securely closed during the injection and cooling phases of the moulding process. The clamping unit usually consists of hydraulic or mechanical systems and must withstand and counteract the high pressure injected into the mould during injection. It’s a critical aspect that ensures proper mould closure and maintains the integrity of the mould during the process.

Cooling Time: The time taken for the molten plastic within the mould to solidify and cool sufficiently for the part to be ejected without distortion or damage. A crucial phase which directly impacts production cycle times. Various factors can influence cooling times, such as material type, part thickness, mould design and cooling medium (such as water or oil)

Injection Moulding Defects

Defect: imperfection, abnormality, or flaw in the final moulded part that deviates from the desired specifications or quality standards. These defects occur for various reasons and can include things such as:

  1. Sink marks - Depressions or surface irregularities caused by differential cooling rates, often due to varying thickness.
  2. Warping – Occurs when parts cool unevenly, causing them to bend or distort from their final shape.
  3. Flash – Excess material that extends beyond the intended part geometry, usually found along the parting line or mould edges.
  4. Short Shot – Mould doesn’t completely fill, resulting in a part that’s incomplete or missing sections.
  5. Jetting – When molten plastic shoots through the mould, leaving a visible mark due to high injection speed.
  6. Burning or Degradation – Overheating of the material can cause degradation or burning, resulting in discolouration or reduced mechanical properties.
  7. Knit Lines – Visible lines or marks where two or more flow fronts meet and join during moulding.

Draft Angle: a slight taper or angle applied to the vertical walls of a moulded part. This angle is incorporated into the mould design to facilitate the ejection of the finished part from the cavity. The purpose of adding a draft angle is to remove the part from the mould easily without causing damage or getting stuck.

ejector pins

Ejector Pins: An essential component of an injection moulding system. They play an important role in ejecting the finished part from the mould cavity after the plastic has solidified. These pins are part of the ejector system within the mould. They are often positioned on the moving half (ejector half) of the mould. When the mould opens after the cooling cycle, these pins are activated to push out the finished part from the mould. They are strategically placed around the mould cavity to apply controlled force, so the part can be ejected without causing damage. They come in various shapes, sizes and designs depending on the complexity of the part being produced, mould design and ejection requirements. Some pins may have special tips or coatings to minimise damage to the part surface. These pins must be regularly maintained and inspected to ensure they function correctly. Wear and tear can occur over time due to repeated use. Ejector pins are part of a broader ejector system that may include ejector plates, sleeves, or other components, which work together to remove the moulded part safely.

Extrusion: This is a distinct process, separate from injection moulding when molten plastic is pushed through a die (that shapes the material into the desired form) to create continuous shapes with a consistent cross-section. This process is commonly used to produce pipes, tubing, sheets, and profiles.

Fill Time: The duration for the molten plastic to fill the mould cavity completely once the injection process begins. Several factors influence fill time, such as material viscosity, part geometry, injection pressure and speed, mould design and venting. Optimising fill time is crucial for achieving consistent part quality and reducing production cycle times. If the fill time is too short, it may lead to incomplete filling of the part. On the other hand, excessively long fill times can increase cycle times and impact production efficiency.

Flash: as mentioned in the defect section, flash is a defect in a moulded part, referring to excess material that escapes the mould cavity during the injection process. It’s undesirable and appears as a thin layer or extra material around the edges of the part where the mould halves meet. Flash can occur for several reasons, including mould misalignment, excessive injection pressure, worn-out or damaged mould and poor venting.

Injection moulding gates

Gate: A specific point or opening in the mould, which the molten plastic is injected into the cavity. Gates come in various types, influencing the moulding process and the final part’s characteristics. These gates include the sprue gate, which is the primary channel for the molten plastic to enter the mould cavity from the runner system. Usually located in the thickest section and subsequently trimmed off after ejection. The direct or edge gate is positioned directly at the part’s edge. The submarine gate, concealed beneath the surface, minimises visible marks, though it requires additional effort to trim or remove excess material. Pinpoint gate (or hot tip gate) is a small-diameter gate that creates a tiny entry point for the molten plastic—commonly used for smaller, more delicate parts.

Gating System: The network of channels and passages that facilitate the flow of molten plastic from the injection moulding machine nozzle to the mould cavity. Components include

  1. Sprue – The initial channel through which molten plastic flows through from the machine nozzle into the mould. It directs material into the runner system.
  2. Runners – Channels that distribute the molten plastic from the sprue to the individual cavities within the mould. Runners serve as pathways for the material to reach the gates.
  3. Gates – As described above, specific entry points with the mould cavity.

Hot Runner System: a specialised injection moulding component used to deliver molten plastic from the nozzle into the cavities of the mould. Unlike traditional cold runner systems, which use solid runners to transport the molten plastic, a hot runner system maintains the material in a molten state within the runners. Key features include

  1. Manifold – This is the central distribution system that receives molten plastic from the machine’s nozzle and channels it to individual nozzles corresponding to each cavity in the mould.
  2. Nozzles – The components that deliver the molten plastic directly into the mould cavities. Each nozzle is connected to the manifold and controls the flow of plastic into the mould.
  3. Heating Elements – Hot runner systems employ heating elements to maintain the plastic with the runners and nozzles in a molten state, preventing it from solidifying during moulding.

Advantages of hot runner systems include reduced waste (hot runners eliminate the need for solidified runner material), enhanced quality (elimination of cold runners minimises potential defects associated with cold runner systems, such as vestiges or part inconsistency due to variations in material temperature) and finally increased efficiency (hot runner systems can offer faster cycle times as there’s no need to wait for solid runners to cool and eject before starting the next cycle).
Hot runner systems are often used in high-volume production runs where efficiency, reduced material waste, and consistent part quality are crucial. While they offer several advantages, they can be more complex and expensive to implement and maintain than cold runner systems.

Injection Moulding Diagram

Injection Moulding: involves the injection of molten plastic material into a mould cavity under high pressure and then allowing it to cool and solidify, forming the desired shape of the part. The main stages include feeding (the plastic pellets into the heated barrel), injection (of molten plastic), cooling (of the moulded part), and ejection (of the part from the mould).

Injection Unit: one of the primary components in the injection moulding process, responsible for melting and injection of the plastic material into the mould cavity. Key elements include the hopper (where the plastic pellets are stored and fed into the machine). Barrel (the hopper feeds the plastic material into a long, cylindrical barrel. Inside the barrel, a screw mechanism rotates and pushes the material forward while applying heat to melt it), screw or ram (responsible for melting and mixing the plastic as it moves along the barrel), nozzle and check ring (at the end of the barrel, there’s a nozzle that attaches to the mould. The check ring helps prevent backflow of molten plastic once injected into the cavity),

Mould: also known as the tool. A crucial component used to shape and form the molten plastic into the desired part or product. It consists of two parts, the core and the cavity, which, when together, form the enclosed space into which the molten plastic is injected.

Mould Release Agent: a substance applied to the surfaces of the mould cavity and core to help facilitate the release of the moulded part from the mould after the injection moulding process. It’s a critical component that prevents the part from sticking to the mould and ensures smooth and effortless injection.

Parting Line: Refers to the line or junction where the two halves of the mould meet and separate to create the final part. It is an essential consideration for mould assembly, flash prevention, surface finish and draft angles.

Platen: A large, flat plate or platform that provides a rigid and solid support structure for the moulding during the moulding process. The machine’s clamping unit contains two platens: stationery and moving platen. The platens functions include being responsible for exerting the clamping force required to keep the mould securely closed, being the housing for where the mould halves are mounted, supporting in the injection process by remaining closed and clamped to maintain part integrity and during the heating and cooling stage the platens are designed to support with maintaining the mould’s temperature (using heating elements or channels) and assisting with cooling systems (such as water channels).

Injection Moulding surface finish

Shrinkage: Reduction in the size and dimensions of a moulded part as it cools and solidifies after being ejected from the mould. Several factors contribute to shrinkage, including material characteristics, cooling rate, pard design and geometry, mould design and temperature control.

Sprue: Refers to the primary channel or passage through which the molten plastic flows from the machine's nozzle into the mould cavity. The initial entry point into the mould.

Surface Finish: Refers to the quality, texture, appearance, and smoothness of the outer surface of the moulded part. At Protolabs, we offer Mold-Tech finishes for our injection moulding services; learn more here.

Venting: The process of allowing trapped air, gasses, or other volatiles to escape from the mould cavity during moulding. Proper venting is crucial to ensure the production of high-quality parts by preventing defects such as air traps, burns, or incomplete filling.

Warpage: the deformation or distortion of a moulded part from its intended shape or dimensions. It happens when parts cool unevenly or experience internal stresses during the cooling and solidification phase of the process. Warpage occurs when cooling is uneven due to material properties and part design (such as sharp corners, varying thicknesses, inadequate draft angles, or complex geometries).

This covers just a few injection moulding terms. To read more, please visit our online glossary.