The Basics of Mold Design: Understanding Injection Molding

In the first few sections of the basics of mold design, we will mainly look at the theory behind mold design. In the subsequent sessions, you will be capable of designing the mold using SolidWorks.  

Here is the table of contents for this part down below: 

  • What is injection molding?
  • Mold cycle
  • Thermoplastic and thermosets


What Is Injection Molding? 

Injection molding is one of the types of large-scale manufacturing processes. In this process, a material is melted and poured (or injected) into a mold and allowed to cool. The hot molten material starts solidifying as the temperature drops to take the shape of the mold. 



The raw material used for the injection molding manufacturing process is small plastic pellets, which are known as resins. These resins are dropped in a hopper through which they go into the injection barrel.  

The plastic pellets in the injection barrel are driven forward by the rotation of a screw. These plastic pellets get converted into molten plastic due to friction. External heating may also be provided in some cases to melt the raw material.  

The molten plastic is then injected through the nozzle into the mold. It then fills the region between the core and the cavity to take the shape of the mold. 

The core is the convex region (also called the male part of the mold), which forms the inner surface of the component which is being molded. The cavity is the depression or the concave region (also referred to as the female portion of the mold) and forms the component's outer surface. 

The material (molten plastic) filled between the core and the cavity is allowed to cool so that it solidifies before ejection.  

The other components that primarily affect the mold are: 

  • Runner
  • Sprue
  • Gate

From the nozzle, at the end of the injection barrel, the material first enters the sprue. The sprue is a large channel in the mold, which receives the molten material from the nozzle. This sprue channel gets divided into several smaller channels called runners.  

The runner carries the molten material to each section of the mold. The connecting channel between the runner and the part is called the gate. 


Advantages of Using Injection Molding Manufacturing Process 

Although there are numerous methods of manufacturing plastic parts, there are certain distinct advantages to using injection molding as a manufacturing method for plastic components. 

  • It can create very detailed and complex geometries with high accuracy.
  • This method is relatively fast, thus allowing for higher production rates.
  • An additive can be added in the feed to impart greater strength to the plastic.
  • Multiple plastic types can be used simultaneously.
  • The entire process can be automated, which brings down the manufacturing cost.

Microinjection molding is one of the recent innovations in this field, which allows manufacturing plastic parts in micro tolerances. This technique will be useful in the medical industry or in building high-precision devices like watches. 


Mold Cycle 

The mold cycle can be defined as the amount of time it takes to complete a single injection molding process. The calculation of this time does not have a well-defined formula and depends on several factors like: 

  • The Number of Cavities: It is the number of parts that will be produced in each cycle. It is possible to create multiple components using a single tool in a single cycle. More cavities translate to longer cycle time.
  • Type of Material: Several materials are available for injection moldings like ABS, polycarbonates, and more. Different materials will have different properties like densities, which will affect the cooling time. The more the cooling time, the more is the mold cycle time.
  • Runner Removal: Since the runner is not part of the required component, they have to be removed, which increases in the mold cycle time. They can be removed manually or with the help of robots. The use of robots is preferred for lower mold cycle times.
  • Runner Type and Gate Type: There are various types of runners and gates available, which will also affect the molding cycle.
  • The Geometry of the Part: The thickness of the manufactured part will influence the mold cycle time. Greater thickness means a higher cooling time, which increases the cycle time.
  • Design of the Mold: Aspects like the length of the runner channel come into the design considerations. Overshooting the runner channel length increases the cycle time.
  • Type of Injection Molding Machine: The machines are primarily classified on the type of driving systems used. They can be hydraulic, mechanical, or electric. Electric molding machines are generally faster and more accurate but also more expensive.


Thermoplastics and Thermosets 

All plastics can be divided into two major types: 

  • Thermoplastics
  • Thermosets


  Thermoplastics  Thermosets 
Shape  Can be reshaped by remelting Cannot be reshaped once formed
Strength  Low High
Recyclable  Yes No
Melting point  Low High
Surface finish  Very high Poor
Example  PET, polypropylene, polycarbonate  Vinyl ester, epoxy resin 


When thermosets are heated, the polymer chains are attached through a process known as cross-linking, which gives them a permanent 3D shape and makes it more resistant to melting. However, since thermoplastics do not form such cross-links, they can be quickly melted. This property of thermoplastics makes it easier to shape them, which is why they are used to make plastic components using injection molding.

Most of the plastic car components (about 90%) are thermoplastics. As thermoplastics can be remelted and reshaped, they are also recyclable. In applications where excellent surface finish is required, like that of the interiors of luxury cars, thermoplastics will be used.

Generally, thermosets are more durable than their counterparts, thermoplastics. Hence, you can find thermosets in safety equipment like helmets and also in heavy industrial applications like load-bearing.  

One of the disadvantages of using thermoplastics is that they cannot withstand high temperatures. So, applications where high-temperature resistance is required, like in engine parts, aircraft landing gears, and satellite structures, thermosets will be preferred.  



In the first part, we have covered the basics of mold design in three topics, as discussed in the beginning. In the next section, we'll take a look at: 

  • Plastic processing methods
  • Current scenario of the mold industry
  • Engineering plastic in automobiles
  • Plastic material properties

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