June 28, 2023

The Beginners guide to Injection Molding

medical Class 7 Clean-rooms

Injection molding is a process in which a thermoplastic polymer is melted or plasticated, and injection molded into a mold containing a cavity in the shape of the desired article.  With thermoplastics the mold is kept at a temperature below the solidifying point of the plastic, causing the injected polymer to freeze, forming the article.   After a short cooling period, the mold is opened, and the finished part is ejected from the press or removed by an operator.

The main pieces of equipment typically used in this process are the injection molding machine, the mold, water heater, material dryer, and occasionally side arm robotics; often used to facilitate the movement of completed parts from the press to a staging area or to perform secondary operations such as inspection, de-gating, palletization, etc.

In this guide, you’ll find everything you need to know to get started with injection molding, including:

  • How the injection molding process works
  • The various Quality standards that regulate injection molding
  • The different types of injection molding services
  • Why you should choose injection molding for your projects.


How does plastic injection molding work?

Each project has a separate set of tools that have been machined from either Steel or Aluminum that will be used over the course of the project’s life.  Steel molds tend to be on the more expensive side but can run millions of parts in their lifetime.  Aluminum molds are typically used for smaller batches and prototyping jobs.

To develop these molds, engineers use specialized equipment to machine these molds into the specific dimensions and geometries a customer wants. These can include CNC milling machines, an EDM (Electrical Discharge Machine), surface grinders, etc.

Once a mold is completed, they undergo various testing processes to ensure that the mold is functioning properly.  Small prototyping runs are also typically performed to test that the parts are being produced to the customers specifications.

Injection molding can be summarized in 5 stages:

  • Clamping: Applying tonnage to the two mold halves between two platens, either with a hydraulic cylinder or a toggle clamping unit. This is to achieve shut off, a seal between the parting line and other details of the mold, so that the force of the molten plastic does not override the force of the two mold halves being held together.
  • Injection: A two or three stage process in which the screw is driven forward by either a hydraulic cylinder or electric servo motor (sometimes both), forcing plastic through the barrel’s nozzle into the mold’s sprue (an opening between the back of the mold and the cavity). The first stage of injection is controlled by velocity, while the 2nd and 3rd are utilizing a pressure control called pack and hold. This delineation of the injection process is known as decoupling and is intended to maximize throughput while maintaining the highest level of control. Pressurization of the molten plastic is necessary due to shrinkage. Without packing and holding after injection, the plastic would shrink away from the details of the molded area and lack form, structural integrity, or mechanical properties specific to the material.
  • Recovery or Plasticization: Plastic pellets fed into the barrel through the feed throat are brought to their melting point by a series of heater bands, attached to the outside of the barrel, and the mechanical shearing of the rotation of the screw. The flights of the screw allow the molten plastic to travel in front of the screw assembly, where it awaits its next injection cycle. Once a process has been “cycled in”, the majority of the heat maintained through manufacturing is due to the mechanical heat of the screw compressing and shearing the material.
  • Cooling: In order to maintain the cosmetic, dimensional, and mechanical integrity of the molded parts, cooling is necessary. The mold will have cooling channels near the molded area with the purpose of circulating water or a heat transfer fluid to remove heat from the tool. Efficient cooling is highly important as this stage accounts for the majority of the cycle time.
  • Demolding: The process of freeing the molded part from the mold. This typically consists of the return of any slides, lifters, etc. followed by ejection. Ejection is the removal of a molded part from a mold by mechanical means, compressed air, or human assistance.

Once we are ready for molding, the mold is installed onto an injection molding machine. Plastic pellets are loaded into a hopper which feeds automatically into the machine’s feed throat and is controlled by the controller which runs the manufacturing software. Once inside the hopper these pellets move down to the feed throat and into the barrel. Once in the barrel the pellets are heated by a series of heater bands and the mechanical rotation of the screw in a process called plasticizing. The molten plastic is churned in front of the screw through the flights on the screw tip through the open non-return valve, which acts similarly to a plunger in a syringe, forming our shot. The shot is a specified volume of thermoplastic that is used to fill the mold cavity.  Once enough material has been collected, it is injected into the mold at a specified velocity and then pressurized to maintain the form of the mold’s coring and cavities.  Once cooled, the mold opens and ejects the completed part and begins the process all over again.


Quality Standards:
Quality is a tremendously large portion of any injection molding process.  Depending on the end application of your molded parts, you will have to adhere to certain conditions or regulations to ensure that the parts were manufactured in accordance with specific quality standards.

ISO is an independent organization that brings together experts to develop International Standards that support innovation and provide global solutions. The ISO 9000 family of standards is the most widely used quality management standard in the world. It provides a framework for managing quality and continuous improvement in any organization.

There are many other ISO standards that cover different aspects of quality management, such as ISO 14001 for environmental management systems, ISO 13485 for Medical Devices ,ISO 45001 for occupational health and safety management systems, and ISO 27001 for information security management systems.

At MOS Plastics, we adhere to the ISO: 13485 certification requirements for all projects. ISO 13485:2016 specifies requirements for a quality management system where an organization needs to demonstrate its ability to provide medical devices and related services that consistently meet customer and applicable regulatory requirements

KS Manufacturing maintains ISO: 9001 certifications in Tijuana and San Leandro. ISO 9001 sets out the criteria for a quality management system and is the only standard in the family that can be certified.  It can be used by any organization, large or small, regardless of its field of activity.  This standard is based on a number of quality management principles including a strong customer focus, the motivation and implication of top management, the process approach and continual improvement.


Different types of Injection Molding:

The process of Injection molding isn’t isolated to only one type.  Here are 3 types of injection molding that are the most common.

  • Thermoplastic injection molding: Thermoplastic injection molding is a manufacturing process that creates fully functional parts by injecting plastic resin into a pre-made mold. It has several subcategories.
  • Insert Molding: Insert molding is the process of molding or forming plastic parts around other, non-plastic parts, or inserts. The inserted component is most commonly a simple object, such as a thread or rod, but in some cases, inserts can be as complex as a microchip or battery.
  • Overmolding: Overmolding is an injection molding process used to mold one plastic over top of another component (often referred to as the substrate). The substrate is usually an injection-molded plastic part, but it could also be made from various other materials.
  • Micro injection molding: Micro Injection molding is a molding process for plastics components for shot weights of 1 to 0.1 grams with tolerances in the range of 10 to 100 microns. This molding process permits the manufacture of complicated geometries with maximum possible accuracy and precision.


Why choose Injection Molding?

Plastic injection molding is a popular parts manufacturing process for numerous reasons, including:

  • Consistency: The injection molding is repeatable and can quickly produce thousands of moldings with consistent quality. Since every part you have will be practically identical to the first one, you will develop a reputation for brand consistency and part reliability in high-volume production.
  • Efficiency: After setting up and testing the process, your injection molding machines can produce thousands of items per hour relatively energy-efficiently.
  • Flexibility: Companies can choose any mold design and a specific thermoplastic for each component, including the most complex and detailed.
  • Lower scrap rates: When compared to CNC machining, during which a significant amount of material is machined away, injection molding produces low scrap rates.
  • Cost-effectiveness: Molds are an expensive element in the injection molding process, but after they are manufactured, the cost per part is low, mainly if produced in high numbers.
  • Quality: No matter what a manufacturer is looking for in a component—strength, flexibility, or detail – the injection molding process can repeatedly produce them in the highest quality.


Injection Molding: The Bottom Line

Injection molding is an excellent technology for finished production on a considerable scale, but it is also helpful for prototyping and product testing. Although the process offers lower costs at higher volumes, aluminum tooling helps to make prototyping economical.

The many benefits of plastic injection molding—including its repeatability, exceptional surface finish, complex parts, and plentiful selection of materials—have made it a favorite of the consumer, medical, and automotive industries, to name a few.

Looking to learn more about Plastic injection Molding? Connect with the experts at MOS Plastics to learn more about how we can bring your plastic medical devices to life.