Every industry has its own language. If you don’t speak it fluently, it can be problematic to understand the nuances and details of a new quote for business. But don’t fret. Learning the key terms for plastic injection molding isn’t like learning a foreign language. If you’re interested in a new project involving plastic injection molding, these are the key terms you should know.
There are four major categories involving injection molding that you should know about: raw materials, molding machines, injection molds, and process defects.
Raw Materials
Resin – Resin is the basic raw material for plastic injection molding. There are thousands of resins available, from simple to highly engineered resins formulated for unique characteristics. Choosing a resin for a specific project can be difficult given there are so many resins available, so a discussion with an expert molding partner like SEA-LECT Plastics is a wise decision.
Colorant or Pigment – Resin in its raw form has no color, and pigment or colorant is typically added to resin to achieve a desired color. Coloring can be added as the resin is formed into small pellets, or the coloring can be added as a separate pellet and mixed at a specific ratio for the desired color.
Additives or Fillers – Injection molded parts can have many different characteristics, and these can be honed and adjusted via additives or fillers. Specific compounds can be added to base resin to improve the overall performance and appearance.
Hardness – This describes the resistance of a plastic material to indentation, scratching, or compression. The harder a material is rated, the less likely it will be affected.
Shrinkage or Shrink Rate – As the resin cools after molding, it shrinks or reduces volume by a determined amount. It is generally indicated in a percentage.
Thermoplastic – A Thermoplastic is a material that can be heated and cooled repetitively without changing the material structure from the effects of melting.
Thermoset – A Thermoset is a material that is heated and formed into a specific shape. The heating process changes the material structure, so they can’t be reformed once set into its permanent shape.
Molding Machines
Tonnage – Tonnage indicates the clamping force or pressure a machine can exert on an injection mold. Most injection molding machines are rated from 5 tons to beyond 4,000 tons. Higher tonnage requires larger machines, so keep in mind the material being used for the molded part, the overall size of the part.
Hopper – The hopper stores the plastic resin pellets prior to being melted for the injection molding process. Some resin materials may require a dryer to remove moisture from the resin in the hopper. You can also find small magnets around r inside the hopper to trap any loose metal shavings from entering the molding machine.
Barrel – The hopper feeds pelletized resin into the barrel where the resin is melted into a molten state.
Cure or Curing – This is the process of allowing a heated plastic to cool to a hardened state.
Cycle or Shot – The time (beginning to end) it takes for the plastic injection molding process to complete a finished part.
Flow rate – The volume of material passing a particular point per unit time.
Machine Shot Capacity – The maximum volume of resin that can be injected in a single stroke of the injection machine.
Multi-Shot Molding – A molding process that uses two or more plastic materials to form a single part.
Nozzle – The nozzle is at the end of the barrel and contacts the injection mold. The nozzle forms a tight seal to the mold while the molten resin is injected through the nozzle into the mold.
Purging – Purging is the process of cleaning the injection machine of resin prior to changing materials or colors. It is typically done with a clear (or non-colored) resin to indicate a clean point between the old and new materials.
Ram – The Ram pushes the melted resin into the injection mold.
Injection Molds
Injection Mold – The injection mold contains the shape that the plastic resin will take when cooled. They can be made from steel or aluminum based on the part volume, temperature, and other factors that determine the best material for durability.
Mold Cavity – The cavity side of the mold could be considered the female half of the mold. It contains the finalized shape of the molded part and has no protrusions outside of the shape.
Mold Core – The core side of the injection mold that could be considered the male side. It contains any protrusions inside the mold, which help form the inner surface of the molded plastic part.
Runner System – This can also be called simply “the runners” as they allow resin to flow through the injection mold to fill the part cavities. They are tailored to suit the mold and part shape. There are two main types: hot runner and cold runner systems.
Wall Thickness – The thickness of the final part. The goal is to create a uniform thickness and at the smallest (thinnest) level required. Thicker parts require more cooling time and can lead to air becoming trapped in the mold and weld / knit lines to form.
Degassing – Degassing is the simple operation of opening and closing an injection mold to allow any trapped air (or gas) to escape. Trapped air can cause multiple defects as blistering and bubbles.
Dimensional Stability – The goal with any formed part is to keep its formed shape without deviation or warping.
Draft or Draft Angle – The angle (labeled in degrees) of taper in a side wall to help with removal of formed parts from the injection mold. Draft angles are always positive.
EDM or Electric Discharge Machining – EDM is a manufacturing process used to create molds. It uses electrical discharge to create the shapes within the mold cavity by removing metal material.
Ejection or Ejector Pin – Each injection mold can contain metal rods or pins that push or eject the molded parts from the injection mold.
Gate – When molten plastic enters the mold, it come through the gate. There are multiple types of gates available that can be tailored based on the need of the parts involved. A pinpoint gate can be used in hot-runner molds to control the flow of the material. A Fan gate has a wider width to reduce warping of the finished part. A Flash gate helps to distribute molten resin at thinner sections to create a linear melt flow into the injection mold cavity.
Sprue – The Sprue comes after the gate and distributes molten material from the nozzle to runner system in the mold.
Family Mold – An injection mold that contains multiple cavities to create multiple parts.
Surface Finish – The surface finish is the texture on a finished molded part.
Mold Release – Mold release is a surface treatment to help with removal of cooled parts from the injection mold. It can be sprayed into the mold or included as an additive in the resin.
Over Molding – Over molding is a two-shot molding process where two plastic materials are injected into a mold sequentially to form a part. Most processes usually inject a harder base material first with a coating of softer material overtop.
Prototype or Soft Tool – This is an injection tool that is made to produce a limited number of initial parts as samples. They are typically used to make the final adjustments or changes before creating the final production tool.
Undercut – This can be a mistake made during the design that results in an indentation or protrusion inhibiting the removal of the formed part from the injection mold. Undercuts can also be used as a feature in a mold to ensure a formed part holds onto the correct side of the mold.
Vent – A feature that allows trapped air to escape from the injection mold as molten material is injected into the cavity.
Process Defects
Flashing and Burrs – Flashing on a finished part is a small amount of excess resin that wasn’t contained within the mold. It most commonly appears at parting lines or ejector pin locations in the mold.
Parting Line – The parting line is where the two halves of the injection mold come together.
Blisters or Bubbles – Blisters and bubbles appear on the surface of a finished part. They are generally created by gas or air bubble voids that have formed inside the plastic part.
Crazing – Crazing causes small cracks to be exhibited in the finished part often caused by over-stressing the material.
Delamination – This often appears as a flaky surface layer on the finished part and is caused by contamination or excess moisture left in the resin pellets prior to melting in the barrel.
Flow Marks – These look like a wavy pattern, off-color, or mild discolorations caused by too slow of an injection speed during the molding process. The slow speed allows the molten resin to cool too quickly.
Orange Peel – Peeling appears like a rough surface on the finished plastic part. It is often caused by excessive moisture left in the mold cavity during the cycle or an incomplete fill of the material.
Short Shot – Short shots are due to an incomplete fill of material in the mold cavity.
Weld Line or Knit Line – This is where two or more material flows inside the mold meet and cause a line due to cooling material. They typically occur around obstructions, voids, holes, or weak areas of the molded part.
All of these plastic injection molding terms mentioned are a great start on the vocabulary of plastic injection molding, but they aren’t the only ones you’ll need over the course of a complete project. Every step in the process has unique terms, and in order to become an expert you’ll need a mentor and partner to help. Your best option should be an innovative turnkey production manufacturer that is capable and proven in design, engineering, manufacturing, assembly and delivery. SEA-LECT Plastics has faced the challenges surrounding the global economy, and we are an excellent mentor to partner on your next project. We can offer guidance on how to make your next project operate at world-class levels and incorporate changes to positively affect your daily operations. We have an elite staff that can design, manufacture, and deliver your new product to a global marketplace. If you have a new idea or need help to navigate the global supply chain, call us at (425) 339-0288 or email us at mattp@sealectplastics.mystagingwebsite.com. We can offer you advice on the best technology to use, the best materials to meet your product demands, and how to navigate through each development stage with ease.
Matthias Poischbeg was born and raised in Hamburg, Germany. Matt moved to Everett, Wash., after finishing his bachelor’s degree in business in 1995 to work for Sea-Dog Corporation, a manufacturer, and distributor of marine and rigging hardware established in 1923.
In 1999, Matt took over the reins at Sea-Lect Plastics Corporation, a sister company of Sea-Dog and a manufacturer of plastic injection molded products with an in-house tool & die shop. Matthias Poischbeg is also a contributor to Grit Daily.