May 16, 2021

Injection Molding Machines

The Basics

Injection molding machines use a processing method in mass production of plastic parts with complex shapes. The advantages of the injection molding machines are fast production speed, high efficiency, automatic operation, variety of colors, shapes from simple to complex, the size can be from large to small, and the size of the injection molded part is accurate, the product is easy to be replaced, and injection molded parts with complex shape can be molded.

According to Wikipedia, “Injection molding uses a special-purpose machine that has three parts: the injection unit, the mold and the clamp. Parts to be injection-molded must be very carefully designed to facilitate the molding process; the material used for the part, the desired shape and features of the part, the material of the mold, and the properties of the molding machine must all be taken into account. The versatility of injection molding is facilitated by this breadth of design considerations and possibilities.”.

The “tonnage” is the measure of force that is required to keep the tool closed during the injection process. This force is the main factor in determining the injection moldier machine size required for producing the part.

Plastic injection molding machines are classified or rated based on tonnage, or more specifically the clamping pressure or force. Smaller presses have tie bar spacing too narrow to accommodate larger products. Lower tonnage machines such as 10 to 100 Ton injection molding machines are great for smaller plastic parts such as those used in medical devices and can also accommodate a wide variety of parts in the aforementioned industries as well.

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Calculating the Tonnage required based on size of part to be Molded

Most injection molding machines will recommend 2.5 times the surface square inches of the part and an additional 10% as a safety factor. If you have a part that is 120 square inches, you would need a press size with 300 tons of pressure. When you add the 10% safety factor, the required press size would have 330 tons of clamping force.

While large-tonnage injection molder machines require some big investments in infrastructure, the markets that require large parts offer some great business opportunities to Injection molders. These Plastic injection molders are classified or rated based on tonnage, or more specifically the clamping pressure or force. Larger presses can accommodate larger molds and multi-cavity molds often reducing the cost per part.

A machine rated for 100 tons can deliver 100 tons of clamping pressure. This pressure keeps the mold closed during the injection process. Too much or too little pressure can cause quality issues. Too much or too little pressure can also cause flashing, where excess material appears on the part edge. Pressure also impacts the viscosity of the plastic being used in the project. Melt Flow Index or MFI is a measure of the ease of flow of the melt of a thermoplastic polymer. Plastic compounds react differently to pressure based on their MFI. The higher the MFI, the higher the pressure needed.

Other factors to consider when selecting an Injection Molding Machine

Shot Size

The injection machine needs to insert the right amount of material into the mold otherwise known as the shot size. The shot size is the maximum amount of plastic injection molding material that can be injected in one cycle. For the best quality the parts must use about 60 to 70% of a machine shot size.


Shrinkage of the plastic part is the size change that occurs in the molded part as it cools. The majority of the part shrinkage occurs while still within the mold during the cooling stage, but their is also a small amount of shrinkage after ejection as the part continues to cool.

Types of Injection Molding Machines

Other than the standard injection molding machine there are many different types of injection molding machines. Below you will find some of the most popular.

Electric Injection Molding Machine

The electric press, also known as Electric Machine Technology (EMT) or Electric Injection Molding Machine, reduces operation costs by cutting energy consumption and addresses some of the environmental concerns surrounding the hydraulic press. Electric presses have been shown to be quieter, faster, and have a higher accuracy, however the machines are more expensive.

Horizontal Injection Molding Machine

Horizontal injection molder machines are the workhorses of numerous plastics-driven industries. Plastic parts made on 10 to 1000+ ton injection molding machines are utilized to make parts in consumer goods, electronics and telecommunications, food and beverage, medical, automotive, etc. A manufacturing process in which melted plastic is injected into a mold to form a part.

Liquid Type Injection Molding Machine

Liquid injection molders use an industrial fabrication method that molds stock materials into a range of components and products. It differs from the standard reaction injection molding process in that it relies on mechanical mixing rather than pressurized impingement mixing, and it focuses specifically on liquid silicone rubber and similar elastomeric materials. It is capable of efficiently producing many versatile items, including seals, o-rings, isolators, and electronic contacts.

Rotary Type Injection Molding Machine

Rotary table vertical injection molding machines rotates the mold first to a load station, then a molding station, and finally an unloading station. Rotary tables allow for complex parts to be formed and maximize production.

Shuttle Type Injection Molding Machine

This method allows two bottom mold halves to be used with one top half. While one bottom half is molding with the top half, the other bottom is free to be loaded with inserts. When the press opens, the bottom half that was just loaded with inserts shuttles into location with the top half, while the bottom that was just molding shuttles out for part ejection and insert reloading.

Thermoset Type Injection Molding Machine

A material, which when heated, is pressed or molded into a shape. The heating process changes the structure of these materials, and for this reason they cannot by re-heated.

Tie Bar Less Type Injection Molding Machine

Bars which provide structural support to the mold in the press. The spacing between the tie bars dictates the size of the mold that can be placed into the injection machine. The mold opens and closes riding on the tie bars.

Two Color Type Injection Molding Machine

Multi-shot molding is a process where two or more plastic substances are injected into the mold to form a part. Toothbrushes are often manufactured using this technique. A two-shot process, in which two plastic substances, are injected into a mold sequentially, usually a harder base material with a coating of softer material.

Vertical Type Injection Molding Machine

The majority of machines are horizontally oriented, but vertical machines are used in some niche applications such as insert molding, allowing the machine to take advantage of gravity. Vertical injection molders are used to create molded plastic pieces that are attached to or inserted in other materials. Vertical injection molders feature a clamp which holds the mold from which the end product will be made.

Vertical/Vertical Insert Molders

Vertical/Vertical insert molding machines are more flexible in terms of efficient production cycles, cost, and types of products it can handle. Vertical clamp units on vertical molding machines are robust and allow maximum flexibility with associated mold tooling. Vertical injection molding machines are particularly suited for insert molding applications. Components can be easily loaded into the mold and held in place by gravity prior to over molding. Vertical injection molding machines often incorporate a shuttle or rotary table allowing loading and demolding within the machines cycle time.

So which Injection Molding Machine is right for my project?

While plastic injection molding machines will help you determine the size of the machine needed to get the best result, a project designer or engineer can get a good estimate based on some basic information. By knowing approximately what size machine will be required, you can better source a plastic injection molder that will meet your needs.

In the end, your plastic injection molding machine will determine which machine would be best suited for your project. Larger presses can accommodate larger molds and multi-cavity molds often reducing the cost per part. However, larger molds are more expensive. Choosing the right press size can balance the upfront tooling expenditures with long-term manufacturing costs.

Need more help?

The Machinery Center, also known as MCI was founded in 1990 by both Richard Stone and Richard Todd. They have over 60+ years’ experience combined in both the Plastics and Metal Industry. They are the leading providers as well as the largest “stocking” dealer of quality pre-owned equipment for the US, Mexico, Canada and Overseas Market. Call us at (561) 953-9100 and we will be happy to help you.

Injection Molding Machines at Machinery Center

We carry a wide assortment of injection molding machines from many different tonnages and manufacturers.