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By Jeremy Losek, Jul 21st, 2022

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. 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 Basics

The actual process of plastic molding is just an expansion of this basic process. The plastic goes into a barrel or chamber by gravity or is force-fed. As it moves down, the increasing temperature melts the plastic. Then, the molten plastic is forcibly injected into the mold under the barrel with an appropriate shape. As the plastic cools, it solidifies. The plastic molded like this has a reverse shape of that of the mold. A variety of shapes can be produced by the process. The process of plastic molding is cheap due to the simplicity involved and the quality of the plastic material is modifiable by changing the factors involved in the custom injection molding basics process:

  • The pressure of injection can be varied to change the hardness of the final product. Injection pressure causes the material to flow. Pressure increases as mold filling becomes more complex. There is a direct relationship between injection pressure and injection line pressure.
  • The thickness of the mold affects the quality of the part produced. On average, the minimum wall thickness of an injection molded part ranges from 2mm to 4mm (.080 inch to .160 inch). Parts with uniform walls thickness allow the mold cavity to fill more precisely since the molten plastic does not have to be forced through varying restrictions as it fills.
  • The temperature for melting and cooling determine the quality of the plastic formed. In plastic injection molding, the temperature of the melt in the cavity is generally between 200 and 300 degrees, the melt is formed in the cavity, cooled and solidified into a product profile.

Insert Injection Molding

Insert molding is a process that starts with placing metal, ceramic, or plastic parts into the plastic injection mold. After those parts have been properly placed, the mold is filled with molten plastic in the regular injection molding process.

Gas Assist Injection Molding

Gas assist injection molding is a process that utilizes an inert gas to create one or more hollow channels within the injection-molded plastic parts. At the end of the filling stage, the gas is injected into the still liquid core of the molding. From there, the gas follows the path of least resistance and replaces the thick molten ejections with gas-filled channels.

What Materials are Used in Injection Molding?

There are many plastic or resin materials available in the manufacturing industry. Selecting the right match for your custom injection molding project is essential to producing a high-quality part. Each material will have its own set of unique characteristics and chemical makeup. When selecting the best material for your injection molding project, consider the durability, performance, texture, flexibility, density and color.

Here are some of the materials ICOMold offers for injection molding manufacturing.:

Polycarbonate

Sometimes called by its trademarked names Lexan, Makrolon, Makroclear, arcoPlus®, polycarbonate is a commercially available thermoplastic that can withstand strong impacts, is transparent and amorphous. It can be pliable at room temperature and may be reformed without the application of heat. Polycarbonate can be liquified at its melting point, making it easy to use for injection molding applications. Some common applications of polycarbonate include windshields, phone cases, pens, vehicle headlights and more. Its versatility is just one advantage of this common thermoplastic.

Polypropylene

Polypropylene (PP) is the second most widely-produced plastic globally, after polyethylene (PE). Its behavioral characteristics are similar to PE, but it is slightly harder and more heat-resistant. It is a commodity–grade polymer popular in the packaging and labeling industries. PP is tough, fatigue- and chemical-resistant, but vulnerable to UV radiation and is flammable. It is a versatile plastic, as it is easily customized with additives. It is naturally white. Due to the elasticity of PP, it makes a good material for living hinges. It is also used in many common products such as buckets, packaging, bottle caps, toys and many other items.

PVC

Polyvinyl Chloride (PVC) is a popular polymer that is naturally white and brittle. It is available in two forms, flexible and rigid. The first form is a rigid or unplasticized polymer (RPVC or uPVC) commonly used for construction products such as vinyl siding or pipes. The second form, flexible, plasticized PVC, is made bendable by the addition of phthalates. It is used for flooring, insulation, electrical wires and more.

Acrylic

Acrylic (polymethyl methacrylate PMMA), sometimes called plexiglass, is a thermoplastic homopolymer that is often used as an impact–resistant replacement for glass. The light transmittance of acrylic is 92%, which is similar to optical quality glass. Acrylic is UV and light resistant and will not fade. It is ten times more impact resistant than glass. There are numerous benefits to using acrylic for injection molding manufacturing. Common acrylic applications include lenses, medical devices, screens, furniture, paint, beauty products and more.

Resin

Injection molding resins (or thermoplastic resins) are plastics that are easy to manipulate or mold once heated to a certain temperature and become solid once they are cooled. Resins are ideal for the injection molding process because they can be liquified, injected into a mold using pressure and then cooled to form a custom product.  Some common resins used for injection molding include crystalline plastics, amorphous plastics, imidized plastics, polyethylene, polypropylene, acrylonitrile butadiene styrene, polyoxymethylene, polystyrene and more.

Nylon

Nylon (PA) or Nylon Polyamide is a common synthetic thermoplastic polymer that is popular due to its high melting temperature, tensile strength, low friction and resistance to chemicals. It is both durable and flexible. Nylon can be combined with other thermoplastics to achieve increased strength. Nylon can be combined with many additives to create different variants and material properties. Its overall versatility is just one of its many advantages.

Glass-filled Nylon

Glass-filled Nylon is made by adding glass in powder form to Nylon or by extruding the Nylon resin with glass fibers. The result is a Nylon product that has increased stiffness and strength, amongst other enhanced properties. There are additional benefits that make glass-filled Nylon an ideal material for specific projects.

ABS

Acrylonitrile Butadiene Styrene or ABS is a thermoplastic polymer widely used for injection molding. It is durable and can stand up to scratching, is budget-friendly and is easy to machine and fabricate. ABS can be used in conjunction with a metal coating as many metals can adhere strongly to ABS plastic. ABS is perfect for projects that demand strength, stiffness and impact resistance. It can be painted, glued and is easy to recycle. ABS plastic properties are not highly sensitive to temperature or humidity. Molding ABS at a high temperature produces resistance to heat and a nice gloss, while molding it at a low temperature will result in strength and impact resistance. Therefore, its final properties and strength are influenced by the conditions used during the manufacturing process.

Thermoplastic Rubber

Injection molding thermoplastic rubbers are plastics that are easy to manipulate or mold once they have been heated to a certain temperature and then become solid at room temperature or through a cooling process. Thermoplastic rubbers are ideal for injection molding manufacturing because they can be liquified, injected into a mold using pressure and then cooled to form a custom product. There are many advantages and applications of thermoplastic rubber.

Polystyrene

Polystyrene (PS) is a transparent thermoplastic most often used to create single-use products. It is odorless and non-toxic. Polystyrene is available in either solid or foamed. As a solid, it is used to make plastic cutlery, electronics, toys and auto parts. As a foam (Styrofoam) it can be extruded to make packing peanuts, disposable drinking cups and more.

Low–Density Polyethylene

Low–Density Polyethylene (LDPE) is a form of polyethylene (PE) that is made using high pressure and high temperature. LDPE is a flexible type of polyethylene used mainly for packaging. It has superior moisture resistance but is not suitable for products that require weatherability or that must withstand high temperatures. LDPE has good impact strength and chemical resistance. LDPE can be used to make six-pack rings, toys and computer components. In recycled form, polyethylene can be used to make trash cans, tiles, paneling and furniture. Polyethylene products account for 34% of total plastics on the market.

Fiber–Reinforced Plastic

Fiber–reinforced plastics (FRP) is a cost-effective material that results in lighter weight products with increased durability. They are a composite material consisting of a resin and select fibers such as glass or carbon. By adding fibers to resin, the material receives additional support. Fiber–reinforced plastic for injection molding lowers material and labor costs, reduces production time and creates minimal waste. The result is a product that is durable and resistant to warping and cracking.

High Impact Polystyrene

High impact polystyrene (HIPS) is polystyrene that has rubber additives to improve its flexibility and impact strength. While polystyrene is normally transparent, HIPS is opaque and can be painted and labeled.  High impact polystyrene is a cost-effective material that is easy to customize, recycle and manufacture. HIPS is ideal for smaller parts and products that require higher levels of detail and variable thickness.

Acetal/Polyoxymethylene (POM)

Polyoxymethylene (POM, acetal, polyacetal, polyformaldehyde) is a combination of plastic material used to create parts with good dimensional stability, low friction and high rigidity. It is opaque white and is available in many colors. Similar to other thermoplastics, POM becomes a liquid at its melting point, making it an easy material to use in the injection molding process. POM is commonly used for eyeglass frames, fasteners, guns, knife handles, lock systems and other engineering and automotive parts.

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.

Overmolding

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

Where:
(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

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