Understanding the Plastic Injection Molding Process - Dalal Plastics

Plastic injection molding is one of the most widely used methods for mass-producing plastic parts. However, many people who choose this process are still unaware of how it works.

This guide is your comprehensive material for understanding the intricate art and science of plastic injection molding. We cover every aspect of the process, ensuring that you thoroughly understand what happens after you hand us your part design.

What is plastic injection molding? 

Plastic injection molding is a process used to make plastic parts by injecting hot, melted plastic into a mold. The mold used is a hollow shape that gives the plastic its final form.

To help you understand the injection moulding process, let us start with understanding how a plastic injection moulding machine operates.

Key Components of an Injection Molding Machine

A plastic injection moulding machine can be divided into three main parts: the injection unit, the mould and the clamping unit . Below is a detailed breakdown of how the machine works:

Injection Unit

This unit is used for melting and injecting plastic into the mold cavity. It includes:

• Hopper: The starting point of the process where plastic pellets are fed into the machine.

• Barrel & Screw: The pellets move from the hopper to the barrel, where they are heated and mixed correctly with a screw.

• Injection Nozzle: The molten plastic is forced inside the mold cavity through an injection nozzle at high pressure.

Mold

The mold shapes the final product and ensures proper cooling and solidification. It consists of:

• Cooling System: A water or oil-based system circulates and solidifies the plastic inside the mold. The cooling time depends on the part thickness and material properties.

• Runner System: The molten plastic travels through either a hot runner system (keeps the plastic molten) or a cold runner system (plastic solidifies in the runner).

• Mold Halves: The mold consists of two parts: one is stationary on the machine, and the other one moves with the machine. The moving part holds the mold tightly during injection to prevent any leaks.

Clamping unit:

The clamping unit in an injection molding machine consists of several key components that work together to secure the mold, apply the necessary force, and facilitate the opening and closing of the mold.

• Mold Securing System: This ensures the mold halves stay in position.

• Clamping Mechanism: Applies the necessary force to keep the mold shut.

• Ejection System: After cooling, the mold opens, and ejector pins push the finished part out.

The success of plastic injection molding services relies significantly on mold design and several crucial pre-production steps. Here are a few pre-production steps that injection molders follow:

Pre-Production Phase: 

• Finalizing the mold design: Completing all necessary features according to part design, such as draft angles and wall thickness, with the help of an expert moulder.

• Prototyping & Testing: Create a prototype mold if necessary to validate the design.

• Material Selection: Polymers (such as ABS, Polypropylene, or Nylon) are chosen based on the application.

• Ensure Design Readiness: Confirming that the mold design meets all functional and manufacturing requirements

• Run mold flow analysis: To detect potential issues in the mold, such as air traps, weld lines or warpages and ensure an even flow of plastic with proper gate placement.

• Fine-tune: Fine-tune gates, vents, and parting lines based on test results.

• Approving for Production: The mold is ready for full-scale manufacturing once everything is agreed.

To ensure high-quality, defect-free plastic parts, a plastic injection molder follows certain design principles, such as:

• Maintain consistent wall thickness to prevent warping, shrinkage, and uneven cooling

• Apply a draft angle to vertical walls to facilitate easy ejection from the mold.

• Increase the draft for deeper cavities or textured surfaces

• Avoid sharp corners by using fillets and radii

• Position gates in areas that allow uniform material flow

• Ensure proper venting to release trapped air

After completing the Pre-Production Phase, the Injection Molding Process begins, where the selected material is melted, injected into the mold, cooled, and ejected as a finished product. Let us understand this process in more detail:

Injection Molding Process:

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• Melting: The plastic pellets in the hopper feed into the barrel. Inside the barrel, a screw pushes the pellets forward and melts them.

• Injection: Just like a syringe plunger, the mold cavity is filled with melted plastic. (That's why the name)

• Cooling & Solidification: The plastic cools and takes the shape of the mold cavity.

• Ejection: The plastic molded part is ejected directly from the mold.

• Cycle Repeats: The machine resets for the next injection cycle

  
  

  Each cycle can take anywhere from a few seconds to a few minutes, making it one of the fastest production methods.

What material pellets are used in plastic injection molding? 

Depending on the application, durability, and flexibility required, various plastic polymers can be used in injection moulding. Here are some of the most commonly used materials:

• Polypropylene (PP)

• Polycarbonate (PC)

• Polybutylene Terephthalate (PBT)

• Polyethylene (PE)

• ABS (Acrylonitrile Butadiene Styrene)

• Nylon (PA 6, PA 66, Unfilled/ Filled)

• Polyether Ether Ketone (PEEK)

As a custom plastic injection molding solution provider based in Mumbai, India, at Dalal Plastic, we have the capacity to mold over 200 different polymer grades.

Along with plastic pellets, various additives are incorporated based on the final application of the plastic part. These include colorants or pigments for desired aesthetics and processing aids such as lubricants, plasticizers, impact modifiers, and stabilizers to enhance performance, durability, and manufacturability.

What products are made via plastic injection molding?

The possibilities are endless! This process is commonly used to manufacture a wide range of products, including:

• Automotive Parts – Bumpers, dashboards, and even small clips inside your car.

• Medical Equipment – Syringes, surgical tools, and casings for medical devices.

• Consumer Goods – Toys, phone cases, toothbrush handles, and kitchen utensils.

• Electronics – Computer keyboards, remote controls, and connectors.

• Packaging – Bottle caps, containers, and lids.

Well, to put it simply, if it’s made of plastic and produced in large quantities, there’s a high chance it was injection molded! Here’s a secret tip to spot injection-molded plastic parts: Look for small circular or rectangular ejector pin marks—marks left when the part is pushed from the mold.

Have a plastic part in mind to manufacture? Towards the end of the blog, we’ve covered whether custom plastic molding services are the right choice for your manufacturing needs. Keep reading to find out!

Types of Plastic Injection Molding

While the core process remains consistent, it is tailored to specific polymers and end applications.

• Thermoplastic Injection Molding – Uses thermoplastic materials that can be melted and reshaped multiple times, making them highly recyclable.

• Thermoset Injection Molding – Uses thermosetting plastics that harden permanently after being heated, making them heat-resistant and durable.

• Overmolding – Combines multiple materials in a single part by molding one layer over another, often used for soft-touch grips on tools and toothbrushes.

• Insert Molding – Embeds a metal or non-plastic component inside the mold before injecting plastic, commonly used for electrical components and fasteners.

• Micro Injection Molding – Produces tiny and precise parts, often used in medical and electronics industries.

• Liquid Silicone Rubber (LSR) Injection Molding – Uses liquid silicone rubber for flexible, heat-resistant, and biocompatible products like medical devices and baby bottle nipples.

Challenges in Plastic Injection Molding

Plastic injection molding does come with its own set of challenges, such as:

• High Initial Costs: Designing and making the mold can be expensive

• Design Limitations: Some complex shapes may be difficult or impossible to mold in one piece.

• Defects: Warping, bubbles, sink marks, or incomplete filling can occur if the process isn’t controlled properly.

Fortunately, with advanced technology and experience, manufacturers can overcome these challenges.

Is Plastic Injection Molding Right for You?

If you need to produce thousands (or millions) of identical plastic parts, injection molding is likely the best option. It is a cost-efficient process for manufacturing precise & high-quality plastic products in bulk. However, alternative methods like 3D printing or CNC machining can be considered if you're making only a small batch.

Conclusion: 

Custom plastic injection molding services are the base of modern manufacturing. It’s responsible for countless everyday products. To put things into perspective, the Indian injection-molded plastic market was valued at USD 27.84 billion in 2024 and is projected to grow at a CAGR of 5.5% from 2025 onward.

If you're still wondering whether injection molding is the right choice for your needs, revisit the key factors discussed in this blog—or better yet, consult with experts from our team. At Dalal Plastics, a plastic injection molding manufacturing company, we offer custom plastic parts tailored to your needs, ensuring high quality and precision through in-depth client discussions and precise manufacturing. Contact us Today