Principles of Injection Molding 255

This class will familiarize you with injection molding and the design concerns associated with injection molding.

  • Difficulty Intermediate

  • Format Online

  • Number of Lessons 20

  • Language English


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Course Outline
  • Objectives
  • What Is Injection Molding?
  • Steps to Creating a Plastic Part
  • The Injection Molding Machine
  • Injection Molding Screw
  • Gas-Assisted Injection Molding
  • Structural Foam Injection Molding
  • Flow Rate
  • Colorants
  • Cost Factors
  • Mold Materials
  • Molecular Orientation
  • Multi-Cavity Molds and Family Molds
  • Gating
  • Weld Lines
  • Shrinkage and Warpage
  • Cycle Time
  • Ejecting the Part
  • Troubleshooting
  • Summary
  • Describe the injection molding process.
  • Describe the design steps for injection molding.
  • Describe the injection molding machine.
  • Describe the injection molding screw.
  • Explain gas-assisted injection molding.
  • Explain structural foam injection molding.
  • Describe flow rate concerns.
  • Describe colorant concerns.
  • Describe the cost factors of injection molding.
  • List mold materials.
  • Describe molecular orientation in injection-molded parts.
  • Compare multi-cavity molds and family molds.
  • Describe gates.
  • Describe weld lines.
  • Distinguish between shrinkage and warpage.
  • Describe cycle time for injection molding.
  • Describe ejection methods for injection molding.
  • Identify possible injection molding errors.
Vocabulary Term


Any substance that is added to a plastic during manufacturing in order to improve or change the plastic's properties. Colorants are an example of additives.


A nonferrous, silvery white metal that is soft and light. Aluminum is often used to make injection molds because it is easy to machine and inexpensive.

anisotropic shrinking

Shrinking that is uneven or not uniform. Anisotropic shrinking is undesirable in plastic products because it often leads to warpage.


To heat a material and then let it cool gradually. Annealing is done to plastics to allow polymer chains to recoil and relieve internal stresses.


The chamber of an injection molding machine that holds the resin as it melts and mixes. The screw is located within the barrel.

butt weld

The point where streams of resin meet within a mold, but do not re-entangle their molecules. A butt weld is also called a cold weld.

check valve

A device located at the end of the injection screw. The check valve makes sure that resin does not flow back into the machine after it is pumped into the mold.

clamping unit

The part of an injection molding machine that physically opens and closes the mold to eject the solidified plastic part.

cold weld

The point where streams of resin meet within a mold, but do not re-entangle their molecules. Another term for cold weld is butt weld.


An additive that is used to change the color of plastic.

compressive strength

A material's ability to resist a squeezing force.

cooling circuit

An enclosed path in an injection mold through which water or other cooling materials run, aiding in the cooling and solidification of the plastic part.

cooling stage

The stage in the injection molding process during which the plastic part cools and solidifies in a mold.

corrosive chemical

A chemical that can erode certain materials. Designers must be aware of a plastic's reaction to corrosive chemicals before using it for a particular product.

cycle time

The time it takes to close the mold, fill the cavity with resin, cool the resin, open the mold, and release the part.

draft angle

An angle incorporated into a wall of a mold so that the opening of the cavity is wider than its base. Draft angles allow for easier ejection of the part from the mold.

ejector pin

A long pin that extends and retracts to force the solid, molded part out of the cavity. To avoid part deformation, the part must be effectively cooled before ejector pins are activated.

family mold

An injection mold that contains various shaped cavities to mold all the plastic pieces for one particular part. Family molds can be used only when the different plastic parts are made from the same material.

fan gate

A gating strategy in which the diameter of the gate opening is wider than the diameter of the runner. The gate looks like an open fan.

feed zone

A section of the injection molding screw that moves resin through the barrel.

flow rate

The speed at which resin enters a mold. Flow rate determines if resin will successfully fill the mold.


The resulting material after mixing plastic resin with foaming agents. Foam contains tiny air bubbles that give it unique qualities.

foaming agent

Any chemical that releases a gas, causing foam to form.

gas-assisted injection molding

A variation of the traditional injection molding method in which gas is injected into the mold after the mold is partly filled with resin. The gas forces the resin to the walls of the mold, thereby reducing voids or sink marks.


The entryway for the resin into the mold cavity. It is best for resin flow if gates are circular.

gate shear

A condition caused by plastic entering a mold gate too fast or entering a gate that is too small. Gate shear results in burn marks or varying surface finishes on a plastic part.


A large, funnel-shaped device located on the top of the barrel on extrusion and injection molding machines. The hopper serves as the entryway for resin into the barrel.

impact loading

The application of stressful pressure to an object. Impact loading tests explain how an object will likely react to sudden, heavy loads.

impact resistance

A material's ability to resist the force of a sudden impact.

injection molding

A molding process in which resin is heated in a barrel and then injected into a mold by a reciprocating screw. The resin then cools in the mold and is ejected as a solid part.

injection screw

A rotating device in the barrel of the injection molding machine that melts, mixes, and moves the molten resin through the machine to the mold.

injection unit

The part of an injection molding machine in which the molten resin is melted, mixed, and pumped into a mold by the injection screw.

internal stress

A force from within the material of an object that attempts to deform that object. Plastic products often experience internal stress near the gate, where polymers are oriented due to the flow of material.


The condition of resin entering the mold as a stream, instead of fanning out. Jetting leads to heavy, unwanted weld lines.


An aluminum and zinc-alloy used to make injection molds. Kirksite is often used for prototypes or short runs.

living hinge

A flexible strip of plastic that connects two larger, thicker pieces of plastic. The bend of a living hinge should be perpendicular to the orientation flow of the plastic molecules.

metering zone

A section of the injection molding screw in which the resin is pushed into the mold area. In the mold area, the injection screw pushes the resin into the mold cavity.

mold insert

A device that holds the injection mold to the support plate. Mold inserts allow the mold to be removed from the support plate.

mold unit

The part of an injection mold machine in which the molten resin fills the mold, cools, and solidifies in the shape of the mold.

molecular orientation

The manner in which polymer chains position themselves in the mold cavity. Polymers near the wall of the mold orient themselves by straightening out, while polymers near the center tend to stay coiled.

multi-cavity mold

An injection mold that contains more than one cavity of the same shape. Multi-cavity molds have a symmetrical layout, producing many parts of the same design.


A device through which resin exits the barrel and enters the sprue channel.

polymer chain

A very long strand of repeating molecules linked together by primary bonds. Polymer chains orient themselves in lines as they enter a mold, but may be annealed so they can recoil.


A channel through which resin enters the gates of the mold cavity. The runner connects the gate and the sprue channel.

shear stress

A force that causes the internal structure of a material to slide against itself.


The reduction in size that a plastic part experiences as it contracts while cooling within a mold. Ideally, shrinkage is uniform throughout the part to avoid warpage.

side coring

A special mold device used to form an internal design feature on a part. Side coring devices must separate from the plastic part in an additional retraction movement before the part can eject from the mold.

sink mark

A surface blemish in which the outer wall is depressed in areas of the plastic product. Sink marks are often caused by unwanted voids within the plastic material.


Plastic resin that is left in the sprue channel while resin fills a mold. This resin solidifies in the sprue channel and can be reheated and recycled.

sprue channel

The passageway for molten resin that connects the nozzle to the runner.

structural foam injection molding

A variation of the traditional injection molding method, during which foaming agents are mixed with resin. When resin hits the mold walls, thick foam walls form in the part. This process is ideal for large, thick parts that require flexibility.

support plate

The device that holds molds in the injection molding machine. Molds are attached to support plates with mold inserts. Ideally, this allows for one support plate to be used for many different molds.

surface finish

The degree of roughness and variation on the surfaces of the plastic product. Surface finish is a concern for designers because of its aesthetic value, but also because surface finish affects how plastic parts fit together.

tab gate

A type of gate that is positioned at a right angle to the mold. Resin does not enter the mold straight-on, which lessens the chances of jetting.

tensile strength

A material's ability to resist forces that attempt to pull apart or stretch it.

thermal conductor

A material that serves as a good transmitter for heat. For example, aluminum is used to make molds because it is a good thermal conductor.


An unwanted but acceptable deviation from the specified dimension.

tool steel

A carbon alloy steel that is used to make injection molds. Tool steel is expensive, but is very strong and durable.

transition zone

A section of the injection molding screw in which resin is melted and mixed within the barrel.

unscrewing mechanism

A special mold device that shapes threads within a part. The unscrewing mechanism must unscrew out of the part before the part can be ejected.


An empty space between the plastic material and the mold. Voids are undesirable because they compromise the integrity of the finished product.


The physical twist or turn within a part caused by internal stress. Warpage is a particular problem for parts with tight tolerances.

weld line

The place on a plastic product where two streams of resin join together. Weld lines are an aesthetic concern for part designers.