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By Jeremy Losek, Sep 14th, 2021

Plastic Injection Molding

Injection molding is a popular manufacturing process for producing large quantities of plastic parts from materials called thermoplastics. It’s the process of melting plastic, injecting it into a mold then repeating the process to make thousands or even millions of the same part. 

ICOMold provide custom plastic injection molding for customers worldwide, in a variety of industries. We are the low-cost leader for high-quality injection molded plastics with a fast turnaround. 

Count on the ICOMold difference. We are the injection molding company of choice for customers worldwide, across various industries. Headquartered in Hartland, Wisconsin, we have been in the plastic injection molding business and a plastic mold maker since 2003. Our customers range from first-time inventors all the way up to Fortune 500 companies. We are an ISO 9001:2008 certified injection molding company.

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Low Cost:We leverage an offshore injection mold manufacturing facility with a lower cost structure in order to offer lower pricing than our purely domestic competitors.

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High Quality:We are ISO 9001:2008 certified, and our process quality systems ensure that your parts are the highest quality possible for your application.

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Quick Turnaround:We offer three different shipping methods, including next day air, to accommodate your timing and budget requirements.

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Online Quotes: Our interactive online quotation system provides INSTANT quotes for plastic injection molding, CNC machining and urethane casting. 

Plastic injection molding is a manufacturing process for producing large quantities of plastic parts. Injection mold manufacturing is typically used when the same part is being created thousands or even millions of times in succession and a plastic mold maker is needed to handle the production of these products. 

ICOMold has been a leading plastic injection molding company since 2003, and is headquartered in Northwest Ohio. We have proudly produced plastic injection molds and plastic parts for companies in the United States, Europe, Australia, Asia and South America. We’ve done custom plastic injection molding projects for various industries including medical, automotive, sports, hunting/camping, electronics, food/beverage, containers, packaging and more. 

Our custom molding process begins with our instant online quote system. The online system also provides quotes for CNC machined plastic prototype parts.ICOMold provides inexpensive custom molds and injection mold plastics, and typically charges 30%–50% less than other plastic fabricators. 

We offer multiple options for types of injection molds based on their expected production cycles and the warranty they carry. There are various options for our “in-house” injection plastic molds – those which you own but we run your parts in our factory – and for our “export” molds – those which we manufacture and ship to you to run your parts in your facility or a facility of your choosing. For a complete description of all our in-house and export injection molds, please visit the Mold Types page. 

We use steel to make a plastics mold as opposed to aluminum for several reasons, including durability and the ability to injection plastic mold complex parts. There are also various types of steel used for a mold for plastic. While P-20 is considered the workhorse of the industry, we will also use other steel types depending on the situation and requirements. 

Whether you need a plastic injection molding company to build molds for plastic injection and produce your plastic parts, or you’re just looking for an export mold to produce plastic parts in your own facility, with your own plastics for injection molding, we’re ready to get the job done correctly and quickly. 

The Plastic Mold Manufacturing Process

Plastic injection molding is a manufacturing process where resin in a barrel is heated to a molten state, then shot into a mold to form a part in the shape of the mold. The resin begins as plastic pellets, which are gravity fed into the injection molding machine through a funnel-shaped hopper. The pellets are fed from the hopper into a heated chamber called the barrel where they are melted, compressed, and injected into the mold’s runner system by a reciprocating screw.

As the granules are slowly moved forward by a screw-type plunger, the melted plastic is forced through a nozzle that seats against the mold sprue bushing, allowing it to enter the mold cavity through a gate and runner system. The injection molded part remains at a set temperature so the plastic can solidify almost as soon as the mold is filled.

The part cools and hardens to the shape of the mold cavity. Then the two halves of the mold (cavity or “A” side and core or “B” side) open up and ejector pins push the part out of the mold where it falls into a bin. Then the mold halves close back together and the process begins again for the next part.

Injection Molding Plastic

When it comes to injection molding plastics, there are a few options available in the industry:

  • Crystalline Plastics
  • Amorphous Plastics
  • Imidized Plastics
  • Polyethylene (PE)
  • Polypropylene (PP)
  • Acrylonitrile Butadiene Styrene (ABS)
  • Polyoxymethylene (POM)
  • Polystyrene (PS)

Choosing the right injection molding plastic for your custom injection molding project is an important decision. Plastic materials look and behave differently based on the characteristics of their chemical makeup. Based on the part’s intended application and functionality, important material properties must be considered, such as durability, flexibility, performance, texture, density and color.

Animation of the Injection Molding Process

Plastic Injection Molds

Steel injection mold

Injection molds, or mold tooling, are the formed halves that come together in the injection molding machine to be filled with molten plastic and produce the plastic parts in their image. The cavity side, or “A” side, is typically the half which will form the “best” surface of the part, and the core side, or “B” side, will typically show the visual imperfections caused by ejector pins because the parts get ejected from this half. 

Injection molds are manufactured by machining or by Electrical Discharge Machining (EDM). Standard machining was the traditional method of building injection molds with a knee mill. Technology advanced the process, and Computer Numerical Control (CNC) machining became the predominant method of making complex molds, with more accurate details, and in less time than the traditional method. 

EDM is a process in which a shaped, copper or graphite electrode is slowly lowered onto the mold surface, which is immersed in paraffin oil. Electric voltage applied between the tool and the mold causes spark erosion of the mold surface in the inverse shape of the electrode. EDM has become widely used in mold making – many injection mold companies now have EDM in-house. The process allows the formation of molds which are difficult to machine, such as those with features such as ribs or square corners. It allows pre-hardened steel molds to be shaped without requiring heat treatment. 

Compared to other plastic manufacturing processes such as CNC machining or 3D printing, injection molding has a high up-front investment because the tooling is expensive. However, for large production runs of thousands or even millions of identical parts, injection molding is typically less expensive in the long run, despite the high initial tooling investment, because of a lower piece price at high volumes. In addition, it is a much faster manufacturing process than the others mentioned. 

Molds can be made of pre-hardened steel, steel that is hardened after the mold is produced, aluminum, and/or beryllium-copper alloy. The choice of mold material is determined in part by the number of parts to be produced. 

Steel molds will generally have a longer lifespan, so a higher initial cost will be offset by longevity – it will be capable of producing a higher number of parts before wearing out. Pre-hardened steel injection molds are less wear-resistant than those hardened by heat treatments after they are machined, so they are used for lower volume part requirements. 

Depending on varying economic conditions and material origin, aluminum molds can in some cases cost substantially less than steel molds. Aluminum molds have a quick build time, and can produce faster cycle times because of better heat dissipation than steel. Beryllium copper can also be used in areas of the injection molds that require fast heat removal, or in areas where the most shear heat is generated. 

Additional complexity can be added to injection molds in order to produce more complex plastic parts. In the basic process of injection molding, the two mold halves separate at the end of the molding cycle and the part is ejected. In this simple case, the part design cannot have any overhanging or undercut part features, because the mold halves would catch on each other when pulling apart. 

So, to accommodate part features such as undercuts, molds can be augmented with side-pull mechanisms called slides. Slides move into a cavity in a perpendicular direction from the draw of the mold halves to form the undercut feature, then stationary angle pins on the stationary mold half pull the slides away when the mold is opened. The pins enter a slot in the slides, and cause them to move backward when the moving half of the mold opens, like a cam. Then the part is ejected, the mold closes, and the slides move forward along the angle pins as a result of the closing action of the mold. 

Multiple-Cavity Molds (Family Molds)

Example of family molded parts that uses multi-cavity mold

Family molded parts

In addition to a single plastic part being produced in a molding cycle, the mold can also be designed to produce multiple numbers of the same part in a single shot. A tool with one impression is often called a single impression (cavity) mold, whereas a custom injection mold with two or more cavities of the same part is referred to as a multiple impression (cavity) mold. The number of impressions in the mold is often incorrectly referred to as cavitation. Some extremely high-volume molds – like those for bottle caps – can have over 128 cavities.

A multiple impression (cavity) mold may also be referred to as a “family” mold. However, a family mold is more accurately defined as one which can produce multiple, like-sized parts in the same quantity, color and material. Family molded parts are often part of an assembly, such as a mating top half and bottom half.


Example of Overmolding

Overmolding is a plastic injection molding process which is very useful for producing multi-material parts with some unique properties. For instance, this process can be used to add a second part, of a different material, for a handle or grip. Picture a power tool with a grip made out of a softer material than the body of the tool. Similarly, a game controller can be manufactured with different textures of plastics on the body of it.

Basically, a previously injection molded part of one material (the substrate) is re-inserted into an injection molding machine, and a different material (the overmold) is injected to form a new layer over the first part.

Some specialized injection molding machines have two or more injection units that can “overmold” in a single molding cycle. For example, “two-shot,” or “multi-shot” injection molds first mold a basic shape in a base color of plastic material, then the second material, of a different color, is injected into the remaining open spaces to produce a one-piece, multi-color plastic part.

Overmolding is not as straightforward as injection molding a part out of a single material, and it is not without its limitations. One consideration is that the two materials must be compatible, chemically and thermally. Since plastic materials have different melt temperatures, the substrate material must have a higher melt temperature than the overmolding material, otherwise the original part would melt and deform when the overmolding material is injected.

Insert Molding

Threaded insert molded into plastic

Metal, ceramic or plastic pieces can be inserted into the molten thermoplastic to form multi-material, robust parts with additional functionality. For example, threaded metal inserts can be molded into the parts to allow them to be attached to other parts.

Insert molding can reduce cost by embedding secondary parts into the plastic injection molding process, as opposed to installing the parts after molding. By integrating the inserts at the time of molding, the parts become more robust compared to staking the pieces in post-molding. Of course, the insert pieces must be able to withstand the high temperature and pressure of the injection molding process.

Insert molding is naturally a more complex process that standard injection molding, so some injection molding companies are more experienced in the process than others. For low-volume production runs, a machine operator may load the inserts into the mold by hand, prior to the plastic injection cycle. For high-volume production runs, however, it is common to use automated machinery to place the inserts into the mold.

Molding Cycle Time

The sequence of events during the manufacturing is called the cycle. It begins when the mold closes and the polymer is injected into the mold cavity. When the cavity fills, the mold maintains a holding pressure to account for material shrinkage.

Then, the screw turns, and the next shot moves to the front of the screw. This causes the screw to react, and, once cooled, the mold opens and the plastic part removed.

A formula can be used to determine the cycle time of injection molding. The time it takes to make a part using injection molding is calculated as:

Total time = 2M + T + C + E

(2M) = Twice the Mold Open/Close Time
(T) = Injection Time (S/F)
(C) = Cooling Time
(E) = Ejection Time (E)
(S) = Mold Size (in3)
(F) = Flow Rate (in3/min)

The mold closing and ejection times of injection molded parts can last from less than a second to a few minutes, depending on the size of the mold and machine. The cooling time, which dominates the process, depends on the maximum thickness of the part.

Process Efficiency

Optimizing the injection molding process is essential because it affects cost, quality, and productivity. Some of various optimization checks include:

• Optimize the holding time by conducting gate seal or gate freeze studies
• Conduct a cooling time study to optimize the cooling time for an injection molded part
• Pressure drop studies determine if the machine has enough pressure to move the screw at the set rate
• Perform viscosity curves to determine injection speeds
• Vary the melt temperatures and holding pressures to optimize the process window

Molding Trial

When an injection molding job is being set up for the first time and the shot size for that mold is unknown, a molding trial will be conducted to get everything “dialed in.” The mold technician will usually start with a small shot weight and fill the mold gradually until it is 95 to 99% full. Then a small amount of holding pressure is applied, and the holding time is increased until gate freeze off (solidification time) occurs on the injection molded part. Gate solidification is important because it determines cycle time, and cycle time is a crucial determinant in the efficiency, and therefore the economics, of the production process. If the parts have sink marks, the holding pressure will be increased until they are minimized and the part weight is achieved. Once the settings are settled in and the injection molding machine is making good parts, a setup sheet is produced for standardizing the process for future production runs.

Equipment for Plastic Injection Molding

Plastic injection molding machines – called presses – consist of a material hopper, a screw-type plunger and a heating unit. Presses clamp the mold to the platen of the machine, where plastic is injected through an orifice to create the finished plastic injection-molded piece.

Plastic injection molding machines are rated by tonnage, which calculates the amount of clamping force the machine can exert. Such force keeps the mold closed during the plastic mold manufacturing cycle. The projected area of the part being molded determines the total tonnage needed. This area is then multiplied by a clamp force of 2 to 8 tons per square inch of the projected areas. The stiffer the plastic material, the more clamp tonnage is required to hold the mold closed. Generally, 4 to 5 tons per inch are used or most plastic injection molding projects.

Today, electric injection molding machines are favored over traditional presses. Electric presses require 80% less energy consumption and offer nearly 100% repeatability. Though the cost of electric presses is 30% higher, greater demand for plastic injection-molded parts has largely closed the gap on cost.

Injection Molds: Aluminum vs. Steel

The choice of material to construct a plastic injection molded part is often a question of budget and the desired end-product.
Historically, injection molds were created from steel and were expensive to manufacture. They were typically only used in large-scale productions when thousands of parts needed to be produced.

Today, aluminum molds cost substantially less than traditional steel injection molds. When higher-grade aluminum like QC-7 or QC-10 is used with modern computer equipment, it can economically mold thousands of parts. Aluminum molds offer faster cycles and quick turnaround because of enhanced heat dissipation.

Steel molds cost more to build, but their longer lifespan often offsets higher initial costs. High-grade steels like H-13 and S-7 are tough metals, withstanding wear and constant pressures of plastic injection molding. Steel grade PAS940 is prime for transferring heat. Stainless steel injection molds are commonly used for larger components or clear parts, like lenses.

Whether we use aluminum or steel to make your mold, ICOMold uses cutting-edge CNC machining and electrical machining to meet your exact specifications.

Cost of Plastic Injection Molding

The cost of manufacturing plastic molds depends on numerous factors, like the size of the mold/parts, complexity of the design and the number of cavities in the mold. While the initial manufacturing cost might seem high, the price-per-piece is relatively low. In short, the final cost correlates with quantity – greater quantities mean overall price decreases. Learn more about injection molding cost.

Variations of Plastic Injection Molding

Most plastic injection molding processes are covered by the methods described above. However, several important molding variations include, among others:

  • Co-injection molding
  • Thin-wall molding
  • Rubber injection
  • Low-pressure plastic injection molding
  • Inset and outset plastic molding

Plastic Injection Molding Companies and Services – Why Choose ICOMold

Whether you’re engineer, industrial designer or a vehicle parts manufacturer, you may have a need for an injected-molded plastic part. And it’s difficult to confidently move forward with the design and manufacture of the part when you have minimal experience.

Count on the ICOMold difference – we have a strong plastic injection molding foundation, which gives us the ability to avoid common mistakes, make smart decisions and execute your next part project with ease.

We’re ISO 9001:2008 certified, which means we’ve streamlined our process and produce fewer defects when compared to the competition. These quality systems also allow us to offer top-notch plastic mold manufacturing at competitive prices.

Contact us by calling (419) 867-3900, or go ahead and request an instant quote for your custom plastic molding project!

Go to our plastic injection molding and CNC machining case studies page to see how we helped customers on their projects.

ICOMold is a Leader in Plastic
Injection Molding for US Companies