Custom Plastic Injection Molding: A Guide for Beginners

custom plastic injection moulding

This article will provide an overview of custom plastic injection molding, a versatile and cost-effective manufacturing process that can be used to create a wide range of plastic parts and products. We will cover the following topics:

What is custom plastic injection molding and how does it work?

Custom plastic injection molding is the process of making plastic parts with an injection-molding machine. Injection molding is an economical and efficient manufacturing process that can be used to make large quantities of the same or similar parts. The process starts by making a metal mold of the part, usually from steel or aluminum. The mold has two halves that fit together and form a cavity that matches the shape and size of the desired part. The mold also has channels and vents that allow the molten plastic to flow into the cavity and air to escape.

The injection-molding machine consists of three main components: the hopper, the barrel, and the clamp. The hopper is where the plastic resin pellets are stored and fed into the barrel. The barrel is where the pellets are melted and mixed by a rotating screw. The screw also pushes the molten plastic forward into the nozzle, which injects it into the mold cavity under high pressure. The clamp holds the mold halves together and applies a force to keep them closed during the injection cycle.

The injection cycle consists of four stages: clamping, injection, cooling, and ejection. During clamping, the mold halves are closed and locked by the clamp. During injection, the molten plastic is injected into the mold cavity until it is completely filled. During cooling, the plastic solidifies and takes the shape of the mold cavity. During ejection, the mold halves are opened and the part is ejected by pins or air blasts.

The cycle time depends on various factors such as the size and shape of the part, the type and amount of plastic used, the temperature and pressure settings, and the cooling system. Typically, the cycle time ranges from a few seconds to a few minutes.

What are the benefits and applications of custom plastic injection molding?

Custom plastic injection molding offers many benefits for producing plastic parts and products, such as:

  • High accuracy and consistency: The parts produced by custom plastic injection molding have tight tolerances and uniform dimensions, which ensure high quality and performance.
  • High efficiency and productivity: The parts produced by custom plastic injection molding can be made in large quantities in a short time, which reduces labor costs and waste.
  • High flexibility and versatility: The parts produced by custom plastic injection molding can be made from various types of plastics with different colors, finishes, textures, and properties. The parts can also have complex shapes and features that are difficult or impossible to achieve with other manufacturing methods.
  • High durability and strength: The parts produced by custom plastic injection molding have high resistance to wear, tear, impact, heat, chemicals, and corrosion.

Custom plastic injection molding is used for producing many items, including medical device components, specialized adapters for the telecommunications industry, window and door locks, handles, accessories and tools used in the construction industry, and point of purchase displays . If the product or parts of the product are made of plastic, chances are they were made with plastic injection molding.

What are the main factors to consider when designing a custom plastic injection molded part?

When designing a custom plastic injection molded part, there are several factors to consider that can affect its quality, functionality, appearance, cost, and manufacturability. Some of these factors are:

  • Material selection: The choice of material depends on the requirements and specifications of the part, such as its mechanical properties (e.g., strength, stiffness, toughness), thermal properties (e.g., melting point, heat resistance), chemical properties (e.g., compatibility with other materials), optical properties (e.g., transparency,
    color), electrical properties (e.g., conductivity), environmental properties (e.g., biodegradability), cost, availability, etc. There are many types of plastics available for custom plastic injection molding, such as ABS (acrylonitrile butadiene styrene), PC (polycarbonate), PP (polypropylene), PE (polyethylene), PS (polystyrene), PVC (polyvinyl chloride), etc. Each type has its own advantages and disadvantages, and some may require additives or modifiers to enhance or alter their properties.
  • Mold design: The design of the mold is crucial for ensuring the quality and accuracy of the part. The mold should have a smooth and polished surface to prevent defects and facilitate the flow and cooling of the plastic. The mold should also have adequate draft angles, which are the angles between the mold walls and the parting line, to allow the part to be easily ejected from the mold. The mold should also have appropriate gates, runners, and vents, which are the channels that connect the nozzle to the mold cavity and allow the plastic and air to enter and exit the mold. The size, shape, location, and number of these elements can affect the pressure, temperature, flow rate, and distribution of the plastic in the mold, as well as the occurrence of defects such as sink marks, flash, weld lines, etc.
  • Part design: The design of the part should take into account the limitations and capabilities of custom plastic injection molding. The part should have a uniform wall thickness to ensure even cooling and avoid warping, cracking, or shrinking. The part should also have rounded corners and edges to reduce stress concentration and improve strength. The part should also avoid undercuts, which are features that prevent the part from being ejected from the mold, such as holes, slots, threads, etc. If undercuts are necessary, they can be achieved by using side actions, such as slides, lifters, or hand-loaded cores, which are movable parts of the mold that create or remove undercuts during the injection cycle. However, these elements can increase the complexity and cost of the mold.

How to choose a reliable and experienced custom plastic injection molding service provider?

Choosing a reliable and experienced custom plastic injection molding service provider is essential for ensuring the success of your project. Here are some tips on how to choose a suitable service provider:

  • Do your research: Before contacting any service provider, do some research on their background, reputation, portfolio, certifications, reviews, etc. You can use online platforms such as Xometry, ICOMold, or The Rodon Group to find and compare different service providers based on your needs and preferences.
  • Request a quote: Once you have narrowed down your options, request a quote from each service provider. The quote should include information such as the price per unit, tooling cost, lead time, minimum order quantity, payment terms, shipping options, etc. You should also provide your part design files (preferably in CAD format) and specifications (such as material type,
    color, finish, etc.) to get an accurate quote.
  • Ask for feedback: Along with the quote, you should also ask for feedback from each service provider on your part design and mold design. The feedback should include suggestions on how to improve or optimize your design for custom plastic injection molding. You should also ask for samples or prototypes of your part if possible to verify its quality and functionality.
  • Compare and choose: After receiving all the quotes and feedbacks from different service providers, compare them based on various criteria such as quality,
    cost, speed, reliability, communication, etc. Choose the service provider that best meets your expectations and requirements.


Custom plastic injection molding is a widely used manufacturing process that can produce high-quality plastic parts and products with high accuracy, consistency, efficiency, productivity, flexibility, versatility, durability, and strength. Custom plastic injection molding can be used for various applications across different industries. However, custom plastic injection molding requires careful consideration of several factors such as material selection, mold design, part design, and service provider selection. By following the guidelines and tips provided in this article, you can design and produce custom plastic injection molded parts that meet your needs and specifications.