How Plastics Are Made(5)-Thermoplastic and Thermoset Processing Methods

There are a variety of different processing methods used to convert polymers into finished products. Some include:

Extrusion – This continuous process is used to produce films, sheet, profiles, tubes, and pipes. Plastic material as granules, pellets, or powder, is first loaded into a hopper and then fed into a long heated chamber through which it is moved by the action of a continuously revolving screw. The chamber is a cylinder and is referred to as an extruder. Extruders can have one or two revolving screws. The plastic is melted by the mechanical work of the screw and the heat from the extruder wall. At the end of the heated chamber, the molten plastic is forced out through a small opening called a die to form the shape of the finished product. As the plastic is extruded from the die, it is fed onto a conveyor belt for cooling or onto rollers for cooling or by immersion in water for cooling. The operation’s principle is the same as that of a meat mincer but with added heaters in the wall of the extruder and cooling of the product. Examples of extruded products include lawn edging, pipe, film, coated paper, insulation on electrical wires, gutter and down spouting, plastic lumber, and window trim. Thermoplastics are processed by continuous extrusion. Thermoset elastomer can be extruded into weatherstripping by adding catalysts to the rubber material as it is fed into the extruder.

Calendering – This continuous process is an extension of film extrusion. The still warm extrudate is chilled on polished, cold rolls to create sheet from 0.005 inches thick to 0.500 inches thick. The thickness is well maintained and surface made smooth by the polished rollers.  Calendering is used for high output and the ability to deal with low melt strength. Heavy polyethylene films used for construction vapor and liquid barriers are calendered. High volume PVC films are typically made using calendars.

Film Blowing – This process continuously extrudes vertically a ring of semi-molten polymer in an upward direction, like a fountain. A bubble of air is maintained that stretches the plastic axially and radially into a tube many times the diameter of the ring. The diameter of the tube depends on the plastic being processed and the processing conditions. The tube is cooled by air and is nipped and wound continuously as a flattened tube. The tube can be processed to form saleable bags or slit to form rolls of film with thicknesses of 0.0003 to 0.005 inches thick.  Multiple layers of different resins can be used to make the tube.

Injection Molding – This process can produce intricate three-dimensional parts of high quality and great reproducibility. It is predominately used for thermoplastics but some thermosets and elastomers are also processed by injection molding. In injection molding plastic material is fed into a hopper, which feeds into an extruder. An extruder screw pushes the plastic through the heating chamber in which the material is then melted. At the end of the extruder the molten plastic is forced at high pressure into a closed cold mold. The high pressure is needed to be sure the mold is completely filled. Once the plastic cools to a solid, the mold opens and the finished product is ejected. This process is used to make such items as butter tubs, yogurt containers, bottle caps, toys, fittings, and lawn chairs.  Special catalysts can be added to create the thermoset plastic products during the processing, such as cured silicone rubber parts. Injection molding is a discontinuous process as the parts are formed in molds and must be cooled or cured before being removed. The economics are determined by how many parts can be made per cycle and how short the cycles can be.

Blow Molding – Blow molding is a process used in conjunction with extrusion or injection molding.  In one form, extrusion blow molding, the die forms a continuous semi-molten tube of thermoplastic material. A chilled mold is clamped around the tube and compressed air is then blown into the tube to conform the tube to the interior of the mold and to solidify the stretched tube. Overall, the goal is to produce a uniform melt, form it into a tube with the desired cross section and blow it into the exact shape of the product. This process is used to manufacture hollow plastic products and its principal advantage is its ability to produce hollow shapes without having to join two or more separately injection molded parts. This method is used to make items such as commercial drums and milk bottles. Another blow molding technique is to injection mold an intermediate shape called a preform and then to heat the preform and blow the heat-softened plastic into the final shape in a chilled mold. This is the process to make carbonated soft drink bottles.

Expanded Bead Blowing – This process begins with a measured volume of beads of plastic being placed into a mold. The beads contain a blowing agent or gas, usually pentane, dissolved in the plastic. The closed mold is heated to soften the plastic and the gas expands or blowing agent generates gas. The result is fused closed cell structure of foamed plastic that conforms to a shape, such as expanded polystyrene cups.  Styrofoam™ expanded polystyrene thermal insulation board is made in a continuous extrusion process using expanded bead blowing.

Rotational Molding – Rotational molding consists of a mold mounted on a machine capable of rotating on two axes simultaneously. Solid or liquid resin is placed within the mold and heat is applied. Rotation distributes the plastic into a uniform coating on the inside of the mold then the mold is cooled until the plastic part cools and hardens. This process is used to make hollow configurations. Common rotationally molded products include shipping drums, storage tanks and some consumer furniture and toys.

Compression Molding – This process has a prepared volume of plastic placed into a mold cavity and then a second mold or plug is applied to squeeze the plastic into the desired shape. The plastic can be a semi-cured thermoset, such as an automobile tire, or a thermoplastic or a mat of thermoset resin and long glass fibers, such as for a boat hull. Compression molding can be automated or require considerable hand labor. Transfer molding is a refinement of compression molding. Transfer molding is used to encapsulate parts, such as for semi-conductor manufacturing

The formation of plywood or oriented strand board using thermoset adhesives is a variant of compression molding. The wood veneer or strands are coated with catalyzed thermoset phenol formaldehyde resin and compressed and heated to cause the thermoset plastic to form into a rigid, non-melting adhesive.

Casting – This process is the low pressure, often just pouring, addition of liquid resins to a mold. Catalyzed thermoset plastics can be formed into intricate shapes by casting. Molten polymethyl methacrylate thermoplastic can be cast into slabs to form windows for commercial aquariums. Casting can make thick sheet, 0.500 inches to many inches thick.

Thermoforming – Films of thermoplastic are heated to soften the film, and then the soft film is pulled by vacuum or pushed by pressure to conform to a mold or pressed with a plug into a mold. Parts are thermoformed either from cut pieces for thick sheet, over 0.100 inches, or from rolls of thin sheet. The finished parts are cut from the sheet and the scrap sheet material recycled for manufacture of new sheet. The process can be automated for high volume production of clamshell food containers or can be a simple hand labor process to make individual craft items.

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How Plastics Are Made(4)-The Two Plastic Types, Based on Processing

A Thermoset is a polymer that solidifies or “sets” irreversibly when heated or cured. Similar to the relationship between a raw and a cooked egg, a cooked egg cannot revert back to its original form once heated, and a thermoset polymer can’t be softened once “set”. Thermosets are valued for their durability and strength and are used extensively in automobiles and construction including applications such as adhesives, inks, and coatings. The most common thermoset is the rubber truck and automobile tire.  Some examples of thermoset plastics and their product applications are:

•  Mattresses
•  Cushions
•  Insulation

Unsaturated Polyesters:
•  Boat hulls
•  Bath tubs and shower stalls
•  Furniture

•  Adhesive glues
•  Coating for electrical devices
•  Helicopter and jet engine blades

Phenol Formaldehyde:
• Oriented strand board
• Plywood
• Electrical appliances
• Electrical circuit boards and switches

A Thermoplastic is a polymer in which the molecules are held together by weak secondary bonding forces that soften when exposed to heat and return to its original condition when cooled back down to room temperature. When a thermoplastic is softened by heat, it can then be shaped by extrusion, molding, or pressing. Ice cubes are common household items which exemplify the thermoplastic principle. Ice will melt when heated but readily solidifies when cooled. Like a polymer, this process may be repeated numerous times. Thermoplastics offer versatility and a wide range of applications. They are commonly used in food packaging because they can be rapidly and economically formed into any shape needed to fulfill the packaging function. Examples include milk jugs and carbonated soft drink bottles. Other examples of thermoplastics are:

•  Packaging
•  Electrical insulation
•  Milk and water bottles
•  Packaging film
•  House wrap
•  Agricultural film

•  Carpet fibers
•  Automotive bumpers
•  Microwave containers
•  External prostheses

Polyvinyl Chloride (PVC):
•  Sheathing for electrical cables
•  Floor and wall coverings
•  Siding
•  Automobile instrument panels

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How Plastics Are Made(3)-Additives

When plastics emerge from reactors, they may have the desired properties for a commercial product or not. The inclusion of additives may impart to plastics specific properties. Some polymers incorporate additive during manufacture. Other polymers include additives during processing into their finished parts. Additives are incorporated into polymers to alter and improve basic mechanical, physical or chemical properties. Additives are also used to protect the polymer from the degrading effects of light, heat, or bacteria; to change such polymer processing properties such as melt flow; to provide product color; and to provide special characteristics such as improved surface appearance, reduced friction, and flame retardancy.

Types of Additives:

  • Antioxidants: for plastic processing and outside application where weathering resistance is needed
  • Colorants: for colored plastic parts
  • Foaming agents: for expanded polystyrene cups and building board and for polyurethane carpet underlayment
  • Plasticizers: used in wire insulation, flooring, gutters, and some films
  • Lubricants: used for making fibers
  • Anti-stats: to reduce dust collection by static electricity attraction
  • Antimicrobials: used for shower curtains and wall coverings
  • Flame retardants: to improve the safety of wire and cable coverings and cultured marble
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How Plastics Are Made(2)-The Structure of Polymers

As we have discussed, polymers can be homopolymers or copolymers. If the long chains show a continuous link of carbon-to-carbon atoms, the structure is called homogeneous. The long chain is called the backbone. Polypropylene, polybutylene, polystyrene and polymethylpentene are examples of polymers with homogeneous carbon structure in the backbone. If the chains of carbon atoms are intermittently interrupted by oxygen or nitrogen, the structure is called heterogeneous. Polyesters, nylons, and  polycarbonates are examples of polymers with heterogeneous structure. Heterogeneous polymers as a class tend to be less chemically durable than homogeneous polymers although examples to the contrary are numerous.

Different elements can be attached to the carbon-to-carbon backbone.  Polyvinyl chloride (PVC) contains attached chlorine atoms. Teflon contains attached fluorine atoms.

How the links in thermoplastics are arranged can also change the structure and properties of plastics. Some plastics are assembled from monomers such that there is intentional randomness in the occurrence of attached elements and chemical groups. Others have the attached groups occur in very predictable order. Plastics will, if the structure allows, form crystals. Some plastics easily and rapidly form crystals, such as HDPE—high density polyethylene. HDPE can appear hazy from the crystals and exhibits stiffness and strength. Other plastics are constructed such that they cannot fit together to form crystals, such as low density polyethylene, LDPE. An amorphous plastic typically is clear in appearance. By adjusting the spatial arrangement of atoms on the backbone chains, the plastics manufacturer can change the performance properties of the plastic.

The chemical structure of the backbone, the use of copolymers, and the chemical binding of different elements and compounds to a backbone, and the use of crystallizability can change the processing, aesthetic, and performance properties of plastics. The plastics can also be altered by the inclusion of additives.

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How Plastics Are Made(1)-The Basics of Plastic Manufacturing

The Basics of Plastic Manufacturing

The term “plastics” includes materials composed of various elements such as carbon, hydrogen, oxygen, nitrogen, chlorine, and sulfur. Plastics typically have high molecular weight, meaning each molecule can have thousands of atoms bound together. Naturally occurring materials, such as wood, horn and rosin, are also composed of molecules of high molecular weight. The manufactured or synthetic plastics are often designed to mimic the properties of natural materials. Plastics, also called polymers, are produced by the conversion of natural products or by the synthesis from primary chemicals generally coming from oil, natural gas, or coal.

Most plastics are based on the carbon atom. Silicones, which are based on the silicon atom, are an exception. The carbon atom can link to other atoms with up to four chemical bonds. When all of the bonds are to other carbon atoms, diamonds or graphite or carbon black soot may result. For plastics the carbon atoms are also connected to the aforementioned hydrogen, oxygen, nitrogen, chlorine, or sulfur. When the connections of atoms result in long chains, like pearls on a string of pearls, the polymer is called a thermoplastic. Thermoplastics are characterized by being meltable. The thermoplastics all have repeat units, the smallest section of the chain that is identical. We call these repeat units unit cells. The vast majority of plastics, about 92%, are thermoplastics1.

The groups of atoms that are used to make unit cells are called monomers. For some plastics, such as polyethylene, the repeat unit can be just one carbon atom and two hydrogen atoms. For other plastics, such as nylons, the repeat unit can involve 38 or more atoms. When we combine monomers, we generate polymers or plastics. Raw materials form monomers that can be or are used to form unit cells. Monomers are used form polymers or plastics

When the connection of the carbon atoms forms two and three-dimensional networks instead of one-dimension chains, the polymer will be a thermoset plastic. Thermoset plastics are characterized by not being meltable. Thermoset plastics, such as epoxy adhesives or unsaturated polyester boat hulls and bathtubs or the phenolic adhesives used to make plywood, are created by the user mixing two chemicals and immediately using the mixture before the plastic “sets up” or cures.

The formation of the repeat units for thermoplastics usually begins with the formation of small carbon-based molecules that can be combined to form monomers. The monomers, in turn, are joined together by chemical polymerization mechanisms to form polymers. The raw material formation may begin by separating the hydrocarbon chemicals from natural gas, petroleum, or coal into pure streams of chemicals. Some are then processed in a “cracking process.” Here, in the presence of a catalyst, raw materials molecules are converted into monomers such as ethylene (ethene) C2H4, propylene (propene) C3H6, and butene C4H8 and others. All of these monomers contain double bonds between carbon atoms such that the carbon atoms can subsequently react to form polymers.

Other raw material chemicals are isolated from petroleum, such as benzene and xylenes. These chemicals are reacted with others to form the monomers for polystyrene, nylons, and polyesters. The raw materials have been changed into monomers and no longer contain the petroleum fractions. Still other raw materials can be obtained from renewable resources, such as cellulose from wood to make cellulose butyrate. For the polymerization step to work efficiently, the monomers must be very pure. All manufacturers purify raw materials and monomers, capturing unused raw materials for reuse and byproducts for proper disposition.

Monomers are then chemically bonded into chains called polymers.There are two basic mechanisms for polymerization: addition reactions and condensation reactions. For addition reactions a special catalyst is added, frequently a peroxide, that causes one monomer to link to the next and that to the next and so on. Catalysts do not cause reactions to occur, but cause the reactions to happen more rapidly. Addition polymerization, used for polyethylene and polystyrene and polyvinyl chloride among others, creates no byproducts. The reactions can be done in the gaseous phase dispersed in liquids. The second polymerization mechanism, condensation polymerization, uses catalysts to have all monomers react with any adjacent monomer. The reaction results in two monomers forming dimers (two unit cells) plus a byproduct. Dimers can combine to form tetramers (four unit cells) and so on.  For condensation polymerization the byproducts must be removed for the chemical reaction to produce useful products. Some byproducts are water, which is treated and disposed. Other byproducts are raw materials and recycled for reuse within the process.  The removal of byproducts is conducted so that valuable recycled raw materials are not lost to the environment or exposed to populations. Condensation reactions are typically done in a mass of molten polymer. Polyesters and nylons are made by condensation polymerization.

Different combinations of monomers can yield plastic resins with different properties and characteristics. When all monomers are the same, the polymer is called a homopolymer. When more than one monomer is used, the polymer is called a copolymer. Plastic milk jugs are an example of homopolymer HDPE. Milk is satisfactorily packaged in the less expensive homopolymer HDPE. Laundry detergent bottles are an example of copolymer HDPE. The aggressive nature of the detergent makes a copolymer the right choice for best service function.  Each monomer yields a plastic resin with specific properties and characteristics. Combinations of monomers produce copolymers with further property variations. So, within each polymer type, such as nylons, polyesters, polyethylenes, etc, manufacturers can custom make plastics that have specific features. Polyethylenes can be made to be rigid or flexible. Polyesters can be made to be low temperature melting adhesives or high temperature resistant automobile parts. The resulting thermoplastic polymers may be melted to form many different kinds of plastic products with application in many major markets.The variability of the plastic either within plastic family types or among family types permits a plastic to be tailored to a specific design and performance requirements. This is why certain plastics are best suited for some applications while others are best suited for entirely different applications. No one plastic is best for all needs.

Some examples of material properties in plastic product applications are:

  • Hot-filled packaging used for products such as ketchup
  • Chemical-resistant packaging used for products such as bleach
  • Impact strength of car bumpers
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What is Injection Molding

In the 1967 film The Graduate, Mr. Robinson offered a single word of career advice to Benjamin Braddock, the character played by Dustin Hoffman: “Plastics.” It would seem that Robinson had the right idea. The industrial science of transforming plastic resins into useful things through the process of injection molding, has had a tremendous impact on industry and on most of our lives.

The injection molding process was first designed in the 1930s and was originally based on metal die casting designs. Injection molding offers many advantages to alternative manufacturing methods, including minimal losses from scrap (since scrap pieces can be melted and recycled), and minimal finishing requirements. Injection molding differs from metal die casting in that molten metals can simply be poured; plastic resins must be injected with force.

The process uses large injection molding machines, which advance the resins through six major processes to produce everything from computer parts to plastic Halloween spiders. Although an injection molding machine is a complex piece of equipment, it consists of two basic elements, the injection unit and the clamping unit.

The process starts with a mold, which is clamped under pressure to accommodate the injection and cooling process. Then, pelletized resins are fed into the machine, followed by the appropriate colorants. The resins then fall into an injection barrel, where they are heated to a melting point, and then injected into the mold through either a screw or ramming device.

Then comes the dwelling phase, in which the molten plastics are contained within the mold, and hydraulic or mechanical pressure is applied to make sure all of the cavities within the mold are filled. The plastics are then allowed to cool within the mold, which is then opened by separating the two halves of the mold. In the final step, the plastic part is ejected from the mold with ejecting pins. The completed part may contain extraneous bits called runners, which are trimmed off and recycled. The entire process is cyclical, with cycle times ranging from between ten and 100 seconds, depending on the required cooling time.

The injection molding process requires some complex calculations. Every different type of resin has a shrinkage value that must be factored in, and the mold must compensate for it. If this value is not precisely determined, the final product will be incorrectly sized or may contain flaws. Typically, this is compensated for by first filling the mold with resin, holding it under pressure, and then adding more resin to compensate for contraction. Other complications may include burned parts resulting from the melt temperature being set too high, warpage resulting from an uneven surface temperature, or incomplete filling due to a too slow of an injection stroke.

Injection molds themselves can be surprisingly expensive, sometimes upward of $100,000. If the desired part quantity, however, is great enough, the mold cost becomes relatively insignificant, and the resulting plastic parts are very reasonably priced. Some molds are made with more than one cavities; these multicavity molds cost more than their single cavity counterparts, but due to increased production efficiency, the cost per part is minimized.

Injection molding can be used with a variety of plastic resins. The most popular resins for this type of molding include: polypropylene (PP), polyethylene (PE), and ABS. Each resin has its own set of advantages and disadvantages and are chosen based on the desired characteristics of the final part.

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10 Injection Molding Terms Every Engineer Should Know

The plastic injection molding process is extremely complex with (quite literally) thousands of moving parts. As a manufacturing engineer, it’s not critical for you to know every finite detail of mold-closing mechanisms or the difference between every polymeric substance used in injection molding—but understanding the following 10 terms will make a conversation with a potential plastic manufacturing partner much simpler.


In injection molding, precision machining refers to the process by which an injection mold is created with very narrow part tolerances. Creating a snug mold with a tolerance of +/- .0005” keeps the liquid plastic from flashing (e.g. seeping into crevices and ruining the final part).


3D printing is an additive manufacturing process that deposits layers upon layers of material to build up a part. While 3D printing has become mainstream, it still can’t compete with the speed or sheer output of injection molding. However, it does play an important role in the injection molding process, and is often used to prototype a design concept so customers understand what their finished product will look like.


Rapid tooling describes the process of quickly creating a mold with a 3D printer or more traditional machining methods. The issue with rapid tooling lies in part accuracies and tolerances. While a 3D printer can create a mold accurate enough to produce a close replica of a part, the mold won’t have the tight tolerance needed to create hundreds of thousands (or millions) of perfectly shaped plastic parts.


To create a thermoset part, cold material is shot into an extremely hot injection mold. This process cures the part so it can never melt again. This heat resistance is the primary function of thermoset material (most often silicone), but thermoset materials are unable to be recycled.


To create a thermoplastic part, plastic material is melted and shot into an injection mold. Once this part cools, the mold opens and the part drops out. Thermoplastics like styrene and polycarbonate can withstand warm or even hot conditions—but at certain temperatures they will eventually melt again, and thus are able to be recycled.


Transfer molding involves placing a cold, putty-like material inside a cavity in an injection mold. Once the mold is closed, the machine forces the cold material into the hot mold cavity. This transference of the cool material into the hot cavity causes the material to disperse quickly. Once it has cooled, the mold is opened and the part is removed.


Clean room molding is the process of creating plastic parts in a special room optimized to reduce the risk of contamination by dust or other particles. Clean rooms are used for injection molding projects that require a sterile environment, like medical equipment. The room is devoid of any fibrous or corrugated material, uses only electric machines, and filters air through positive airflow to maintain a certain level of cleanliness.


There are two types of plastic injection molding machines: Horizontal and vertical.

In horizontal molding machines, the mold clamps horizontally. Once the plastic part is created and the mold opens, the part drops into a bin and is taken away on a conveyer belt. Or, if the part is sensitive and can’t be dropped, a robot removes it from the mold.


Vertical molds lie flat so the part doesn’t fall out once the mold is opened. Because of this, it must be removed by hand or by robot.

The advantage to vertical molding is that parts can easily be added into the mold. For instance, if you want to add a round washer to your plastic part, simply insert the washer and close the mold—because of gravity, the part stays in place.


Two-shot molding or overmolding are processes used to create parts that require two different kinds of plastic—like a toothbrush or a computer mouse.

In two-shot molding, the more rigid of the two materials fills the mold cavity. Then the top of the mold shifts and the second, more pliable material is injected. In overmolding, the rigid material is injected into a mold cavity, then removed after it has cooled and put into a separate mold, where the second, more pliable material is added.

The primary difference between these two molding techniques is cost, as overmolding takes twice as long as two-shot molding.

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Medical Device Injection Molding: How To Find The Right Partner

Medical device injection molding is used in everything from syringes to IV roller clamps to dialysis machine components.

While you must ensure that your medical device is manufactured to FDA standards and is ISO 13485 compliant, you also need to be certain that the company you select is the right one for your needs. A great medical plastic molding partner is indispensable, as they can help you ensure your part cost stays low while maintaining the highest standards of quality. Below, we’ve outlined a few things to keep in mind during the selection process.


1. Clean Room Requirements

Depending on the function of your plastic part, you may need to ensure that it is manufactured in a clean room. Clean room molding is the process of creating plastic parts in a special room optimized to reduce the risk of contamination by dust or other particles. Clean rooms have a constant positive air flow, use electric (not hydraulic) machines, and are devoid of any corrugated material that could cause dust, all in an effort to ensure cleanliness. It may, for example, be necessary to use a clean room for your medical plastic device if the part is implantable, will come in contact with bodily fluid, or will be used in an operating room.

If your device does not need to be clean room manufactured but does require a more controlled manufacturing environment, look for a partner that is flexible with their manufacturing environments. Micron, for example, has a class 7 clean room, but can also use mobile enclosures over the plastic molding machine (by placing a curtained device over the area that offers positive air flow) and have machine operators wear a hat, gown, and mask.

2. Your Product Performance Requirements

A medical device injection molding partner should be able to assist you in determining what raw materials you need based on the specifications of your product. For example, if your medical apparatus  will not be implanted nor come into contact with a patient’s bloodstream, your plastic injection molding partner should steer you away from a class 6, implantable-grade material and toward something more appropriate for your needs and less expensive. Or if you’re creating a dental tool with a colored handle, the partner you speak with should ensure you select the right FDA-approved, food-grade-contact material.

3. The Injection Molding Company’s Area of  Expertise

Bigger is not always better when it comes to the size of your injection molding company—but attention to detail and an emphasis in the area you’re working with are critical.

Whether you need to create a smaller batch of specialized plastic parts or millions of plastic parts each day, you’ll want to find a medical molding company that has a solid track record for producing high-quality parts similar to yours.

For example, here at Micron, we are very effective at producing high-volume medical disposables due to our robust manufacturing process. In fact, we manufacture billions of parts used in medical devices each year. Our vast experience in process validation for medical plastic injection molding has given us deep insight into the nuances of the production process. We are constantly finding new ways to control the flow of materials, and we use automation throughout the entire manufacturing process (including inspection and packing), which enables us to pass cost savings along to our customers.

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How Does The Injection Molding Process Work? A Breakdown For Product Engineers

As an engineer, your focus is on taking a product idea and figuring out how to get it manufactured so it fits all your specifications and stays within your budget. But before you select an injection molding partner, it’s a good idea to brush up on what the injection molding process looks like.

The process outlined below lists the two most critical phases of the injection molding process; you’ll also find a list of things to consider before you partner with an injection molding company.


The process has come a long way since 1872, when the first injection molding machine was created by American inventor John Wesley Hyatt. Hyatt’s machine was very simple—it used a plunger to push plastic material into a mold.

The plunger was replaced in 1946 by James Watson Hendry, who added an auger in the injection barrel. While machines are now run with far more advanced machining, this same basic mechanism is used in the injection molding process today.

Step 1: Create your injection mold.

Before you can begin the injection molding process, you must have a mold with the shape of the part you want inside it. The injection mold is most typically made of steel (though some parts may require another material) and is created with very narrow part tolerances, +/- .0005 of an inch (to put it in perspective, a human hair is about +/- .0003”). This requirement keeps the liquid plastic from seeping into crevices, which helps avoid potential quality or visual issues with the completed plastic part.

If you are creating a high volume of plastic parts, you may require multiple cavities inside a mold, so every cycle creates many plastic parts. Here at Micron, we have the capability of creating a mold with up to 96 cavities—which can create millions of parts every day.

Step 2: Manufacture your plastic product.

  • The completed mold is placed in the injection molding machine.
  • The plastic pellets are heated until they are liquid.
  • The liquid plastic goes through a dryer, if necessary (as moisture in the plastic might cause splay or hydrolysis in the finished product).
  • The liquid plastic is conveyed into the injection molding machine through a vacuum.
  • The liquid plastic goes through a heated injection barrel, which is attached to a feed throat.
  • The liquid plastic is injected under pressure through the feed throat into a mold.
  • The mold—which is cooler than the liquid plastic—causes the plastic material to cool to a solid state, which forms the plastic part.
  • The mold opens and the cooled plastic part is ejected from the mold either by hand (in a vertical injection molding machine) or by force of gravity (in a horizontal injection molding machine).


If you’re ready to make your product become a reality, you have to find the right partner for your injection molding needs. Below, we’ve outlined a handful of things to consider so you select the right partner.

1. Expertise

When beginning your search for an injection molding partner, narrow the possibilities by honing in on companies with experience in your industry or with similar products. A company with relevant experience probably has both the knowledge and technology necessary to develop or even improve your prototype, create it correctly, and manufacture it on schedule.

From there, you can narrow your search to a specific niche, for example: high-volume molding, over-molding, or two-shot molding. Also, be certain to ask about an exact technology you may need specific to your project. If they don’t have said technology, find out if they’re willing to acquire it.

2. Proximity

Many people wonder whether they should partner with a company in the U.S. or overseas for the injection molding process. If you’re asking this in your organization, consider these two questions:

  • Can the mold-making service you’re considering meet your quality standards? Quality U.S.-based mold-making shops source only the highest quality steel for their molds. If you do not specify the grade of steel you want from an injection molding company in China, you may end up with poorer quality materials.
  • Does the company you’re considering have tooling expertise, and are they willing to make tooling adjustments along the way? Ideally, the company you select to work with you on your injection molding process should be able to create your mold for you, or have the expertise necessary to ensure that your existing mold will hold up to the quality standards you require. Some companies—in the U.S. and in China—outsource for molds, which means less quality control on your end.

3. Capacity

Before choosing a partner for your injection molding process, consider how much volume the company can handle. If you need to produce 10 million units each year and the molder only produced 1 million units last year for all its customers combined, you should look elsewhere. Without that ability, you could end up with increased lead time, quality issues, and stock shortages. Alternatively, if you only need 5,000 units produced each year, you might not want to select a molder that deals primarily with multi-million-unit orders.

4. Capability

Is the molder able to build your mold, test it, help you select materials, assemble the finished product, and package, label, and ship it post-production? We highly recommend looking for an injection molding company that has these capabilities, as it makes your job a lot easier.

5. Customer Service

You’ll be working hand-in-hand with your molder for an extended period of time, so it’s important that the company values honesty, integrity, and transparency. To test this:

  • Watch how quickly the molder responds to small issues you identify.
  • Find out if they have a process for escalation of any issues during the injection molding process.
  • See how long it takes for the molder to return your phone calls or send you updates. If the company is slow to respond at the beginning of your relationship, imagine what it will be like down the road!

6. Precision

The pricing of the plastic injection molding process can be thought of as a combination of renting a company’s injection molding machine in addition to the cost of the operators, the cost of materials, and the cost of tooling and machining. The quicker a machine goes through a cycle time, the more parts you can create.

If a company has a solid injection molding process, it will have stringent standards regarding how to ensure every part is expelled from the mold at the right time to avoid defects, rejects, and cosmetic issues.

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6 Types Of Injection Molding Technology

Plastic isn’t the only thing that can be injection molded—metal can as well. This new technology is substantially more expensive than plastic injection molding and usually serves a niche market. The cell phone market, for example, sometimes uses metal injection molding to shield the cellular electronics from radio or microwaves.

4. Liquid Silicone Injection Molding

The majority of plastic injection molding is thermoset, meaning cold material is injected into an extremely hot mold to create a part. This process cures the part so it can never be melted again. But if you need a part to withstand very high temperatures or chemical agents—as you might with certain medical devices or car parts—you may need thermoplastic injection molding, which frequently uses liquid silicone.

5. 3D Printing

3D printing is a notable injection molding technology because of the role it plays in prototyping an injection molded part. Here at Micron, we create a 3D-printed prototype of a client’s part before we move the design to production. This allows us to discuss potential improvements in more depth than we could while reviewing an online rendering, for example. It’s also worth noting that 3D printing can be used to print actual injection molds using plastic or metal. Currently, the available 3D printing technology does not enable us to print with the narrow part tolerances required in an injection mold—but we imagine it may in the future.

6. Unique Material Formulations

While this isn’t a plastic injection “technology” in the traditional sense, the use of unique material formulations does advance molding capabilities. Injection molding companies may, for example, use a carbon or mineral filler, a blowing agent, and a lubricity additive to add certain properties to a part. For example, here at Micron, we have run 40% carbon-filled ABS (Acrylonitrile Butadiene Styrene) to achieve a degree of electrical conductivity in a plastic stud or sensor. The temperature of the mold and the plastic material are both important when adding a filler, additive, and blowing agent, so we are constantly refining our process to achieve the best advantage for these unique materials.

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Edited by Leafly Mould Provides Injection Mold, Plastic Mold, Injection Molding, Die Casting Mold, Stamping Mold