A Beginner’s Guide to Plastic Injection Moulding

Sep 12, 2023

Plastic injection moulding is a versatile manufacturing process that has revolutionised the way products are made in a range of industries. Whether you're an enthusiast curious about the mechanics or an aspiring manufacturer wanting to know more about this field, this comprehensive guide will provide you with a solid foundation in everything to do with plastic injection moulding.

What Is Plastic Injection Moulding?

The plastic injection moulding process can transform granulated plastic into intricate, accurate parts. It involves injecting molten plastic material into a precisely crafted mould which, once cooled and solidified, will take the shape of the mould resulting in a wide range of products.

From everyday household items to complex components, plastic injection moulding has become a huge part of modern manufacturing due to its efficiency, precision, repeatability, and ability to consistently produce high-quality items.

Process Overview

During the plastic injection process, there are many steps involved to ensure the products or components are made to the highest quality.


Before injection begins, purging the material is an important step to ensure there’s no contamination or residue from the previous production cycle. This ensures the quality and integrity of the material being run.

Material Preparation

Preparation of thermoplastic materials will vary depending on whether the chosen thermoplastic material needs drying or not. If it does, it will be completed at the recommended drying temperature and time in a desiccant oven to eliminate any moisture that could compromise the quality of the final product. Once it’s suitably dry, the material is loaded into the hopper, ready for the injection moulding process.


The mould tool is positioned in the moulding machine with a location ring against the fixed platen for first phase fixing either directly or with external fixing clamps. The second phase is to bring the moving platen forward to meet the moving half of the tool. The process is repeated on this side before the ejection system is coupled up or positioned in the correct place.

Temperature Control System Connection

The mould tool’s temperature control system is meticulously connected. The system plays an important role in regulating the temperature of the mould to ensure the plastic cools consistently to achieve the desired properties. The temperature is usually controlled by water, but oil and electric heating can also be used.

Setting Machine Parameters

The injection moulding machine parameters are set and optimised such as injection speed, pressure, temperature, and cooling time based on the material and are fine-tuned to create the perfect conditions for the process.


The material is heated in the barrel of the injection mould press, once molten it is then injected in to the mould tool cavity which is held shut to allow the plastic to cool and produce final form before being ejected from the mould tool.

First Off Approval

With the settings in place and fully optimised, the first shot is produced which is subject to quality inspection and approval. Once approved the full production run can start.

Monitoring for Quality and Consistency

Throughout the production run, rigorous monitoring systems are in place to continuously assess the process and maintain the quality and consistency of each part. Any abnormalities are addressed quickly to ensure exacting standards are maintained throughout the run.

Last Off Approval

As the production run nears its completion, there is a last-off inspection to ensure that the components remain consistent from the beginning to the end of the production cycle.

Release to Next Production Stage

With the last off approval secured, the produced parts are released to the next stage of production this could be for further post moulding operations or directly to packing. Once finished packed and passed final release they will be delivered to the customer.

Types of Plastics

In plastic injection moulding, there are many materials to choose from, and the material selection plays a key role in meeting the specifications stipulated to meet the performance or aesthetic requirements.


  • LDPE (Low-Density Polyethylene): Renowned for its flexibility and versatility, LDPE is often used in applications requiring impact resistance and ease of processing at a low cost. It’s often used for kitchenware, housings, covers, and containers.
  • HDPE (High-Density Polyethylene): A robust material, HDPE has exceptional durability, chemical resistance, and suitability for both rigid and flexible products. It is often used for chair sears, housings, covers, and containers.
  • PEEK (Polyether Ether Ketone): Recognised for its strong, thermal, and chemical resistance properties, PEEK is ahigh-performance thermoplastic ideal for demanding environments. It’s great for aircraft components, electrical connectors, pump impellers, and seals.
  • TPE (Thermoplastic Elastomer): Blending plastic and rubber characteristics, TPE provides flexibility, resilience, and ease of processing, making it a go-to for a range of applications like bushings, electrical components, seals, and washers.
  • NYLONS (PA6, PA66): NYLONS offer a blend of strength, impact resistance, and thermal stability, making them a support for parts subjected to mechanical stress.

Specialised Specifications

  • Glass-Filled Thermoplastics: Adding glass fibres enhances strength and rigidity, making these materials suitable for applications demanding robust structural integrity.
  • UV-Stabilised Materials: Ideal for outdoor use, UV-stabilised thermoplastics maintain their properties and appearance even when exposed to sunlight and environmental elements.
  • Flame-Retardant Polymers: Safety-conscious applications benefit from flame-retardant materials that meet stringent regulations, ensuring a high level of protection.

Why Do People Choose Plastic Injection Moulding?

Among the huge range of manufacturing methods, plastic injection moulding is a preferred choice due to its precision, adaptability, and efficiency. This technique is known for its ability to produce intricately detailed products and replicate design intricacies. Whether it's detailed textures of consumer goods or tight tolerance components, plastic injection moulding ensures a consistent, high-quality output that exceeds design expectations.

Plastic injection moulding is also incredibly versatile. It can be used to produce a wide range of products from medical devices to automotive parts and even electronics. With the range of materials and colours available during this manufacturing process, the opportunities are endless.

Consistency is a hallmark of plastic injection moulding. With tightly controlled parameters, each part is produced to exact specifications to minimise variations and defects. This is a huge benefit and areas on why many people choose to use plastic injection moulding. In the dynamic landscape of manufacturing, plastic injection moulding provides precision, versatility, and efficiency.

The Role of Mould Design

Mould design is important in plastic injection moulding as it’s a bridge between concept and reality. The mould tool is the component in plastic injection moulding that determines the appearance and quality of the final product. The Mould tool design is vital to the successful manufacture of the finished product, this form of precision engineering requires trained expertise and attention to detail.


Designing parts with consideration for their intended application is crucial for the success of the final product. Mould designers will evaluate several factors including wall thickness, rib placement, and feature size to achieve an ideal balance between structural integrity and ease of production.

Draft Angles

Incorporating draft angles into the design is an art that enables the seamless ejection of parts from the mould. These slight tapers on vertical surfaces ensure that the part doesn't become stuck in the mould cavity during the ejection process. Draft angles also contribute to a smoother flow of the molten plastic during injection, minimising the risk of defects and enhancing overall product quality.

High Tolerance

Tolerance is the leeway allowed between the design and the appearance or functionality of the final product. Injection moulds can achieve tolerances of around 0.005 inches; however, this can differ depending on the specifications.

Reduced Defects

Mould design is great for reducing the risk of defects. Plastic injection moulding designers can contribute to the prevention of defects such as surface imperfections, warping, short shots, and sink marks. This proactive approach to manufacturing aligns with the commitment to producing high-quality parts right from the outset of the process.

Types of Products Made by Plastic Injection Moulding

Plastic injection moulding is remarkably versatile and precise, and it plays a role in creating a diverse range of products that fill our daily lives. Here are a few examples of the types of products that are made through plastic injection moulding:

  • Consumer goods - Plastic injection moulding improves our everyday experiences with a range of practical and aesthetic products.
  • Electronics – Plastic injection moulding produces the enclosures that protect our gadgets, the connectors that power our devices, and a range of other precise components needed for the internal and external workings of our everyday electronics.
  • Automotive – Whether it’s interior components, dashboards, or exterior trims and casings, plastic injection moulding plays a huge part in the automotive industry.
  • Medical – The healthcare sector benefits hugely from plastic injection moulding due to the creation of surgical tools, equipment housings, and components for medical devices. These products meet the highest standards of precision and biocompatibility.
  • Furniture – Whether it’s part of a chair, table legs, or other furniture, plastic injection moulding helps produce some of the most used items in your home.
  • Although these are just a few examples, this manufacturing process allows for a huge range of precise and complex shapes, making it a widely popular choice of manufacture for many products.

The Different Plastic Injection Moulding Types

In plastic injection moulding, there are different moulding types that cater to different product requirements and design intricacies. Each method is unique and ensures precision, efficiency, and versatility.

Single Shot Moulding

Single shot moulding is the most common plastic injection moulding and is the processing method that we use delivering efficiency, precision, and versatility. In this technique, a single or sometimes compounded thermoplastic material, is melted, and injected into the mould cavity before cooling and solidifying. It’s ideal for creating single-component plastic parts with the same material properties. It is what makes plastic injection moulding a cornerstone of modern manufacturing.

Over Moulding

Over moulding is a process where one component is moulded over another for products such as screwdrivers or the injection of two or more different materials to create one integrated part. It begins with the injection of a base material, then a second material is injected over and around the first to bond the two materials together.

Twin Shot Moulding

Twin shot moulding is a process that creates complex, multi-material parts in a single operation. It involves a specially designed mould with two or more injection units. This process is used for the manufacture of items such as toothbrushes.

The Importance of Cooling and Ejection

The cooling and ejection stages in plastic injection moulding are hugely important as they shape both the structural integrity and quality of the final product.

Proper cooling ensures uniform solidification, minimising defects like warping and shrinkage while optimising cycle times. Ejection, performed through precision-designed mechanisms, gently removes the solidified plastic part from the mould preserving its form. Efficient ejection not only enhances productivity, but it also prolongs mould longevity by minimising wear and tear.

These stages are crucial to the precision, efficiency, and quality of the products.


The finishing process plays a vital role in determining the final appearance, functionality, and performance of a part. Surface finishes aren’t just about aesthetics, but also about optimising their functionality, durability, and adherence to specifications. At Pentagon Plastics, we offer a range of surface finishes to meet your specifications all listed below with "Ra µm" being the universal measurement for quantifying surface finishes, ideal for specifying on engineering drawings.

  • Polished
  • Milled
  • Turned
  • Ground
  • EDM/ Sparked
  • Bead / Vapour Blasting
  • Chemical Etching

How Pentagon Plastics Can Help

At Pentagon Plastics, we’re a leading plastic injection moulding company that can offer a comprehensive service from initial design review right through to repeat production. Our team have a wealth of experience in technical injection moulding and are dedicated to providing you with precise, high-quality plastic products and components. With a range of cutting-edge tools and technology, we work closely with our customers to fulfil their requirements and exceed their expectations. Get in touch with us now for more information on how we can help you.

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