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9 Questions About Plastic Injection Molding Answered

plastic injection molding

Everywhere you look, the majority of plastic objects in the world around you have been formed by injection molding. Plastic injection molding has been known as a reliable, well-established manufacturing process for over 100 years. The first injection molded part was created back in 1872 by John W. Hyatt, and the industry has progressed substantially ever since. There are many different use-cases for plastic injection molding throughout a multitude of industries because of the design flexibility it offers, as well as cost-effectiveness. To help you understand how this manufacturing process can be used in your design, we've answered some of the most common questions we get asked about injection molding. 


What is plastic injection molding?

Plastic injection molding is a manufacturing process where parts are produced by injecting molten plastic resin into a mold cavity. The speed and pressure at which the material is injected can affect the desired shape.


How does injection molding work?

Before any injection molding can begin, a mold, also known as a tool, has to be fabricated. Tools are designed with plastic injection molding in mind and are precision-machined out of either steel or aluminum to the exact specification of the part. Next, the tool is inserted into the injection molding machine which starts the process of injection molding.


Thermoplastic resin pellets of the desired resin-type are poured into a hopper that feeds into the injection molding machine. These thermoplastic pellets are then pushed forward in the barrel of the machine by a screw and melted down until they liquefy. This liquid is collected at the front end of the barrel into a chamber, this volume of resin is known as a shot.


The thermoplastic resin shot is injected from the barrel of the injection molding machine into the cavity of the mold. The machine applies pressure until the chamber of the mold is filled completely with resin.


After cooling down, the resin transforms into a solid plastic once again, forming the part. Finally, the part is ejected from the machine and is moved onto the next step of production. This process is easily repeatable for production runs ranging from hundreds to millions of parts.


Why choose plastic injection molding?

In most production runs the molding process produces parts that are typically in finished condition. This means that unless the part needs additional finishing steps, such as painting, the manufacturing process is done, and the part can be packaged up and shipped. This is a huge cost-saving measure as multi-step manufacturing processes can be extremely pricey. The production cycle is relatively short, meaning that the parts have a quick turn-around time for moving on to the next step of manufacturing. Plastic injection molding is one of the most cost-effective and efficient for choices production of large volumes of parts.


How much does injection molding really cost?

Out of all the costs for injection molding, tooling makes up the majority. Tooling makes or breaks the plastic injection molding process because proper tooling is key to having a successful production run. Tooling pricing depends on the complexity of the part, the length of the production cycle, and thermoplastic resin material choice.


Costs also depend on the type of resin and the amount of resin needed to create each part. The size of the part correlates directly to the size of the injection molding machines which are rated by tonnage, or the amount of force needed to hold the mold closed. Injection molding machine cycle time can increase depending on the complexity of the part. These factors all play a part in the costs associated with the plastic injection molding process.


What are the types of plastic injection molding resin?

Injection molding utilizes a wide variety of plastic resins tailored to the parts required specifications. The resins are split up into four categories: commodity resins, engineering resins, high-performance resins, and ultra resins. Commodity resins are the cheapest of the plastics because they are easy to produce at high volumes. Commodity resins are mostly used in disposable plastic products such as plastic cutlery. Engineering resins cost more than commodity resins and offer better mechanical or thermal properties and are used for things like car bumpers. High-performance resins offer the strength of metal at the cost of plastic and are commonly used for car interiors. Ultra resins are the most expensive of the four and are suitable for extreme conditions, like high temperatures. Ultra resins are commonly used in space and military applications.


All thermoplastics are placed into two sub-groups, amorphous and semi-crystalline. Amorphous thermoplastics are easier to form during injection molding than semi-crystalline plastics because they soften easier. Amorphous thermoplastics are usually clear and easily bondable with adhesives. Semi-crystalline plastics are harder to thermoform due to their sharp melting point but have better structural strength than amorphous. Semi-crystalline plastics are used in bearings and heavy-wear applications. Both plastics sub-groups have pros and cons depending on the application of the part and its required specifications. Find out more about which plastic resin is right for your project in our plastic resin selection white paper.


How do you design for injection molding?

When designing parts for injection molding there are specific criteria to follow in order to successfully produce optimal results. Wall thickness is evaluated to make sure that it fits the criteria as material choice and part structure need to be factored in. Tapered sides, known as draft, are evaluated as well. These make the part easier to eject from the injection molding machine by allowing air into the mold. Radii is evaluated to see if it would be beneficial for the part to have rounded corners potentially making the molding process smoother. Coring out is used as a method to save costs by cutting out extra material within a part and making it hollow, while simultaneously introducing ribs for continued strength. Coring out also reduces sink and stresses during the injection molding process.


Parts will be evaluated for design for manufacturability, or DFM, by injection molders prior to heading into the injection molding process. In some cases, a moldflow simulation is used to analyze and predict risks associated with parts in the injection molding process. Simulations provide data that allows designers to adjust tolerances for parts quickly and precisely before tooling begins.


What are the types of injection molding?

There are four different techniques used in plastic molding, each with its own advantage. The techniques are structural foam molding, external gas assist, internal gas assist, and insert molding/overmolding. The injection molder will choose which technique is the best fit for the part depending on cost, function, or aesthetic needs.


Structural foam molding

  • Inert gas or pelletized chemical blowing agent is mixed in with thermoplastic resin inside the barrel of the injection molding machine
  • Makes injection pressures much lower than regular injection molding
  • Reduces the weight of part by up to 15%
  • Since injection pressures are much lower, tooling may be made from aluminum instead of steel


External gas assist molding (EGA)

  • Involves inserting an inert gas into the mold on one surface to help compensate for shrinking on the other surface
  • Ideal for thin-walled parts with large surface areas and textured surfaces
  • Injection pressure is reduced, creating less stress on the mold


Internal gas assist molding

  • Introduces inert gas into the mold after the resin has filled the cavity and displaces resin creating a hollow part
  • Ideal for creating large hollowed sections in thin-walled parts
  • Injection pressure is reduced, creating less stress on the mold


Insert molding / Overmolding

  • A piece is inserted into a material before being molded or one molded material is molded around another molded material.
  • Increases strength of part as well as longevity.
  • Cost-saving because it involves fewer manufacturing steps


What is mold making / tooling?

Mold tooling is the largest investment in the plastic injection molding process because of its critical role in creating the part. Tools are commonly fabricated out of specialty tooling steels, but aluminum can be used, depending on the injection molding technique utilized, to lower costs in the prototype stage. More recently, 3D printed molds are also starting to be used for very early prototypes and low-pressure molding. Before molds are created, the parts need to be finalized completely, with the design frozen, because it is very expensive to change tooling once it has been created. Molders and tool builders must work closely to ensure that the tool design meets the requirements for the part.


Every mold must include callouts for parting lines, gates, and ejector marks. Parting lines are the line of separation on the part between the two mold halves. Gates are openings in the mold where the molten thermoplastic enters the mold cavity from. Ejector marks are the spots where the injection molding machine’s ejector pins will push the finished part out of the mold. Need more information? We explain tooling and mold making for manufacturing in-depth in our tooling guide


What comes after injection molding?

Once the parts leave the injection molding process, they have the option to get finished with a second manufacturing process. The parts can be outfitted with threading using ultrasonic, thermal, or other specialty equipment that welds plastics together. Parts can be marked with logos or other branding by laser engraving. Paint, silk screening, and pad printing may also be applied to finish off the part. Finishing parts with paint or other coatings can prevent the surface of the part from being marked up and increase the longevity of the part. Read more about the different finishing techniques used, in our guide to finishing.


About Synectic Product Development: Synectic Product Development is an ISO 13485 certified, full-scale product development company. Vertically integrated within the Mack Group, our capabilities allow us to take your design from concept all the way to production. With over 40 years of experience in design, development, and manufacturing, we strive for ingenuity, cost-effectiveness, and aesthetics in our designs.  Learn more about our contract manufacturing services and see how we can help your next project.

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