Plastic Injection Molds

Plastic Injection Molds

Plastic injection molding is the process used to produce low or high volumes of custom plastic parts for commercial and industrial use. From intricate automobile safety components to simple products like business card holders, the applications span a range of industries.

In combination with skilled mold designers, software technology, and the use of expensive CNC machinery, MSI builds molds for plastic injection that are used to produce high quality molded plastic parts. Our injection molding experts look forward to working with you on your mold build project as the next step toward manufacturing your custom molded parts. If you’re looking for an injection mold supplier, we have you covered.

plastic injection molds

In-House Mold Builds

All of MSI customer molds are built right here inside of our Michigan facility. Our mold build area is staffed with experienced designers and machinists using the most modern software and CNC machinery. After customer part designs are finalized and approved for manufacturing our team will design and build robust trouble free plastic injection molds for your production.

mold for plastic parts

Molds For Plastic Injection

Receiving high-quality plastic parts starts with a quality built mold. If you're curious about how to make an injection mold, we’ll work with you every step of the way to ensure you’re familiar with the different types of molds for plastic injection that are available, and we’ll make sure you’re completely informed before you invest. Below we provide you with a valuable understanding of basic injection mold construction types and the benefits of each.

Insert molds have a cavity side and core side insert that fit inside of a custom mold base here at MSI. They are a great way for us to offer customers economical tooling at very rapid lead times. The insert style design is great for small and medium size parts that require low quantity part orders or fast delivery of parts.

Even though insert molds are low cost in basic design, they are made with the same high quality materials and components as free standing plastic injection molds and provide the same high quality defect free parts.

  • Low cost
  • Average lead times from 5 to 15 days​
  • Good for small parts
  • Good for 1 cavity molds and small order quantities

Free standing molds, also known as stand-alone molds, provide an all-in-one design. They are built to include the mold base, inserts, and all components needed to be fully functional alone.

A properly designed free-standing mold is built for production inside any SPI standard injection molding machine that is sized properly for the mold. Stand-alone molds have a higher cost, but they are a good choice for multi-cavity production and with high-quantity orders.

A summary of free-standing molds for plastic injection includes:

  • Higher cost
  • Average lead times of 3 to 8 weeks
  • Best method for parts that will not fit into insert molds
  • Best choice for multi-cavity molds to reduce part cost

Injection Molding Process

Plastic injection molding requires three primary components – an injection molding machine, a mold, and raw plastic material. Molds for plastic injection consist of high strength aluminum and steel components that have been machined to operate in two halves. The mold halves come together inside the molding machine to form your custom plastic part.

The machine injects molten plastic into the mold, where it solidifies to become the final product. The injection molding process is actually a complex process with many variables of speed, time, temperatures and pressures. The complete process cycle for making each custom part can range from no more than a few seconds to several minutes. Below we offer you a very brief explanation of four steps of the molding process.

Step 1


Before the plastic is injected into the mold, the machine closes the two halves of the injection mold with tremendous forces which prevent the mold from opening during the plastic injection step of the process.

Step 2


Raw plastic, generally in the form of small pellets, is fed into the injection molding machine at the feed zone area of a reciprocating screw. The plastic material heats up by temperature and compression as the screw conveys the plastic pellets through heated zones of the machine barrel. The amount of melted plastic that is conveyed to the front of the screw is a strictly controlled dosage because that will be the amount of plastic which will become the final part after injection. Once the proper dosage of melted plastic reaches the front of the screw and the mold is fully clamped, the machine injects it into the mold, pushing it into the endpoints of the mold cavity under high pressures.

Step 3


As soon as the molten plastic contacts the internal mold surfaces, it begins to cool. The cooling process solidifies the shape and rigidity of the newly molded plastic part. The cooling time requirements for every plastic molded part depends on the thermodynamic properties of the plastic, the wall thickness of the part, and the dimensional requirements for the finished part.

Step 4


After the part is cooled inside the mold and the screw has prepared a new shot of plastic for the next part, the machine will unclamp and open the plastic injection mold. The machine is equipped with mechanical provisions that work with mechanical features designed within the plastic injection mold to eject the part. The custom molded part is pushed out of the mold during this stage and once the new part is fully ejected, the mold is ready for use on the next part.

Many plastic molded parts are fully completed after they are ejected from the mold and simply fall into their final carton to be shipped in, and other plastic part designs require post operations after they are injection molded. Every custom injection molding project is different!

Why Do Plastic Injection Molds Cost So Much?

People often ask why plastic injection molds cost so much? Here is the answer –

Producing high quality plastic parts can only be achieved by using a high quality built mold. Molds for plastic injection consist of precisely machined components made from various metals such as aircraft grade aluminum or hardened mold steels.

These molds are designed and made by highly skilled and well paid people categorically called “mold makers”. They have spent years and possibly even decades being trained in the mold making trade.

Additionally, mold makers need very expensive tools to perform their job, such as very expensive software, CNC machinery, tooling, and precision fixtures. The amount of time that mold makers need to finish a plastic injection mold can range from a few days to several weeks depending on the complexity and size of the end product.

Mold Construction Requirements

In addition to the costs that are associated to molds from the skilled people and machinery that make them, the construction requirements for an injection mold to function properly during the injection molding process is quite astounding. Although the molds are summarized as halving “two halves”, a cavity side and a core side, there are often dozens of precision parts that make up each half.

Almost all of the precisely machined mold components that will come together and function to manufacture your custom molded parts are machined to tolerances of +/- 0.001″ or 0.025mm. A standard piece of copy paper is 0.0035″ or 0.089mm thick. So just imagine slicing your copy paper into three ultra-thin pieces as a reference to how precise a mold maker needs to be to properly build your mold.

Mold Design

And finally, the design of your plastic injection mold has a very large impact on its cost. The plastic injection molding process requires enormous amounts of pressure when the plastic is injected into the mold cavities by the machine. Without these high pressures the molded parts will not have nice surface finishes and potentially will not be dimensionally correct.

Mold Materials

In order to withstand the pressures your mold will see during the injection molding process it must be made with high quality aluminum and steel grades, and be designed to withstand clamping and injection forces that can range from 20 tons for a small precision part to thousands of tons for a residential recycling bin or garbage can.

Lifetime Warranty

Whatever type of plastic injection mold you need, we understand that your injection mold purchase will become a significant asset to your business. For that reason, we warranty the production life of molds we build for our customers for the life of their production requirements.

We hope this information helps you better understand plastic injection mold construction and their cost. Remember the quality of your custom plastic parts will first depend on the quality of your mold. Let us quote your next injection molding project and we will work closely with you to make your project a success!

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