About Injection Molding - Xcentric Mold & Engineering

About Injection Molding

Injection molding is a manufacturing process for producing plastic injection molds from both thermoplastic and thermosetting plastic materials. Material is fed into a heated barrel, mixed, and forced into a mold cavity by a reciprocating screw or a ram injector, where the injection molded part cools and hardens to the configuration of the mold cavity. After a part is designed, usually by an industrial designer or an engineer, molds are then manufactured by an injection mold company, where it is assigned to a mold maker (or toolmaker). Injection molds are usually constructed using either steel or aluminum, and precision-machined to form the features of the desired parts. Injection molding is widely used for manufacturing a variety of parts, from the smallest component to entire body panels of cars. Injection molding is the most common method of production, with some commonly made injection molded items including computer components to outdoor furniture.

Injection Molding Machine

The Process and Characteristics of Injection Molded Parts:

Utilizes screw-type plunger to force molten plastic material into a mold cavity

Produces a solid or open-ended shape (typically a cavity and core shape) which conforms to the contour of the injection molded part

Injection molds require the use of thermoplastic or thermoset materials

Parting line, sprue, and gate marks are present

Ejector pin marks are usually present on injection molds


How did the world of injection molded parts begin? In 1868, billiard ball maker Phelan and Collander, John Wesley Hyatt invented a way to make billiard balls by injecting celluloid into a mould. Hyatt improved the celluloid so that it could be processed into a finished form. In 1872 John and his brother Isaiah patented the first injection molding machine. This machine was relatively simple compared to the machines used by today’s injection molding companies. It contained a basic plunger to inject the plastic into a mold through a heated cylinder. The industry progressed slowly over the years producing injection molded products such as plastic collar stays, buttons, and hair combs. In the 1940’s the concept of injection molds grew in popularity. This is because World War II created a huge demand for inexpensive, mass-produced products.

In 1946, James Hendry built the first screw injection molding machine, revolutionizing the plastics industry with an auger design to replace Hyatt’s plunger. The auger is placed inside the cylinder and mixes the injection molded material before pushing forward and injecting the material into the mould. This allowed colored plastic or recycled plastic to be added to the virgin material and mixed thoroughly before being injected. Today screw injection molding machines account for 95% of all injection machines companies. The industry of injection molds has evolved over the years, from producing combs and buttons to a diverse array of custom injection molded products for the following industries: medical, aerospace, consumer, toys, plumbing, packaging, automotive, and construction.

Applications of Injection Molds

Plastic injection molding is the preferred process for manufacturing plastic parts. Injection molds are used to create many things such as electronic housings, containers, bottle caps, automotive interiors, pocket combs, and most other plastic products available today. Plastic injection molds are ideal for producing high volumes of plastic parts, due to the ability of making multi-cavity injection molded parts, where multiple parts are made with one cycle. Some advantages of injection molding are high tolerances, repeatability, a wide range of material selection, low labor cost, minimal scrap losses, and little need to finish parts after molding. Some disadvantages of this process include an expensive tooling investment and the need to prototype, as some custom complex parts may encounter problems during the injection molding process such as warp or surface defects. Therefore, injection molded parts must be designed with careful molding consideration.

Examples of Polymers Best Suited for Injection Molds

Most polymers may be used for molds, including all thermoplastics, some thermosets, and some elastomers. There are tens of thousands of different materials available for injection molds and that number is increasing every year. The materials can mixed with alloys or blends of previously developed materials. This allows product designers to choose from a vast selection of materials so they can choose exactly the right properties for the injection molded part or parts they need. Mold materials are chosen based on the strength and function required for the final part and each material has different parameters for molding that must be considered. Common polymers like Epoxy and phenolic are examples of thermosetting plastics while nylon, polyethylene, and polystyrene are thermoplastic.

Guidelines for Designing Custom Injection Molds

See our Design Guidelines for tips on designing a plastic injection molded part.


Injection Molds Machinery

Injection molding machines, also known as presses, consist of a material hopper, an injection ram or screw-type plunger, and a heating unit. The molds are clamped to the platen of the molding machine, where plastic is injected through the sprue orifice to create injection molds.

Presses are rated by tonnage, which is the calculation of the amount of clamping force that the machine can exert. This force keeps the mold closed during the injection molding process. Tonnage can vary from less than 5 tons to 6000 tons, with the higher figures used in comparatively few manufacturing operations. The total clamp force needed is determined by the projected area of the custom part being molded. This projected area is multiplied by a clamp force of from 2 to 8 tons for each square inch of the projected areas. As a rule of thumb, 4 or 5 tons/in can be used for most injection molded products. If the plastic material is very stiff, it will require more injection pressure to fill the mold, thus more clamp tonnage is needed to hold the mold closed. The required force can also be determined by the material used and the size of the part, larger plastic parts require higher clamping force.

Today, electric presses are taking over the typical hydraulic injection molding machines. Companies who produce injection molds prefer them as they offer 80% less energy consumption and nearly 100% repeatability, by utilizing electric servo motors. While the cost of an electric molding machine is typically 30% higher than a hydraulic press, higher demand for injection molds is closing the gap on cost. It is estimated that in the next 20 years hydraulic molding machines will be a thing of the past, as more molding companies are making the switch to stay competitive.

Mold Image


Mold or Die are the common terms used to describe the tooling used to produce injection molded plastic parts.

Traditionally injection molds have been expensive to manufacture. They were usually only used in mass production where thousands of parts were being produced. Molds are typically constructed from hardened steel, pre-hardened steel, aluminum, and/or beryllium-copper alloy. The choice of material to build an injection molded part is primarily one of economics. Steel molds generally cost more to construct, but their longer lifespan will offset the higher initial cost over a higher number of parts made before wearing out. Pre-hardened steel injection molds are less wear resistant and are used for lower volume requirements or larger components. The steel hardness is typically 38-45 on the Rockwell-C scale. Hardened steel molds are heat treated after machining. These are by far the superior in terms of wear resistance and lifespan. Typical hardness ranges between 50 and 60 Rockwell-C (HRC).

Today, aluminum molds cost substantially less than steel injection molded parts. When higher grade aluminum such as QC-7 and QC-10 aircraft aluminum is used and machined with modern computerized equipment, they can be economical for molding hundreds of thousands of parts. Aluminum molds also offer quick turnaround and faster cycles because of better heat dissipation. It can also be coated for wear resistance to fiberglass reinforced materials. Beryllium copper is used in areas of the injection molds which require fast heat removal or areas that see the most shear heat generated. Today’s Mold companies use CNC machining and Electrical Discharge Machining (EDM) in the manufacturing processes.

Custom Mold Design – Injection Molds and Ejector Molds

The amount of resin required to fill the sprue, runner and cavities of a mold is called a shot. Trapped air in the mold can escape through air vents that are grinded into the parting line of the mold. If the trapped air is not allowed to escape, it is compressed by the pressure of the incoming material and is squeezed into the corners of the cavity, where it prevents filling and causes other defects as well. The air can become so compressed that it ignites and burns the surrounding plastic material. To allow for removal of the injection molded part from the mold, the mold features must not overhang one another in the direction that the mold opens, unless parts of the mold are designed to move from between such undercuts when the mold opens (utilizing components called Lifters or slides).

Mold Illustrations

Sides of the molded part that appear parallel with the direction of draw (the axis of the cored position [hole] or insert is parallel to the up and down movement of the mold as it opens and closes) are typically angled slightly with (draft) to ease release of the part from the mold. Insufficient draft can cause deformation or damage to the injection molded part. The draft required for mold release is primarily dependent on the depth of the cavity: the deeper the cavity, the more draft necessary. Shrinkage must also be taken into account when determining the draft required. If the skin is too thin, then the molded part will tend to shrink onto the cores that form them while cooling, and cling to those cores or part may warp, twist, blister or crack when the cavity is pulled away. Injection molds are usually designed so that the molded part remains securely on the ejector (B) side of the mold when it opens, and draws the runner and the sprue out of the (A) side along with the parts. The part then falls freely when ejected from the (B) side. Tunnel gates, also known as submarine or mold gate, is located below the parting line or mold surface. The opening is machined into the surface of the mold on the parting line. The molded part is cut (by the mold) from the runner system on ejection from the mold. Ejector pins, also known as knockout pin, is a circular pin placed in either half of the mold (usually the ejector half) which pushes the finished molded product, or runner system out of a mold.

The standard method of cooling is passing a coolant (usually water) through a series of holes drilled through the mold plates and connected by hoses to form a continuous pathway. The coolant absorbs heat from the mold (which has absorbed heat from the hot plastic) and keeps the mold at a proper temperature to solidify the plastic at the most efficient rate.

To ease maintenance and venting of injection molds and ejector molds, cavities and cores are divided into pieces, called inserts, and sub-assemblies, also called inserts or blocks. By substituting interchangeable inserts, one mold may make several variations of the same part.

More complex plastic parts are formed using more complex injection molds. These may have sections called slides, that move into a cavity perpendicular to the draw direction, to form overhanging or undercut part features. When the mold is opened, the slides are pulled away from the plastic part by using stationary angle pins or horn pins on the stationary mold half. These pins enter a slot in the slides and cause the slides to move backward when the moving half of the mold opens(like a cam). The part is then ejected and the mold closes. The closing action of the mold causes the slides to move forward along the angle pins.

Some injection molds allow previously injection molded parts to be re-inserted to allow a new plastic layer to form around the first part. This is often referred to as overmolding. This system can allow for production of one-piece tires and wheels.

2-shot or multi-shot injection molds are designed to “overmold” within a single molding cycle and must be processed on specialized injection molding machines with two or more injection units. This process is actually an injection molding process performed twice. In the first step, the base color plastic material is molded into a basic shape. Then the second material is injection molded into the remaining open spaces. That space is then filled during the second injection molding step with a material of a different color.

Injection molds can produce several copies of the same parts in a single “shot”. The number of “impressions” in the mold of that part is often incorrectly referred to as cavitation. A tool with one impression will often be called a single impression (cavity) mold. A custom mold with 2 or more cavities of the same parts will likely be referred to as multiple impression (cavity) mold. Some extremely high production volume molds (like those for bottle caps) can have over 128 cavities.

In some cases, multiple cavity tooling will mold a series of different parts in the same tool. Some toolmakers call these molds family molds as all the parts. Frequntly asked questions on tooling.

Effects on the material properties

The mechanical properties of an injection molded part are usually minimally affected. Some parts can have internal stresses in them. This is one of the reasons why it’s good to have uniform wall thickness when molding. One of the physical property changes of an injection molded part is shrinkage. A permanent chemical property change is the material thermoset, which can’t be remelted to be injected again. Material to Wall Thickness Guidline.

Tool Materials

Tool steel or Aluminum are often used on injection molds. Mild steel, nickel, or epoxy are only suitable for prototype or very short production runs.

High grade aluminum is fast becoming the material of choice for injection molds, as it offers substantially lower tooling costs and better molding process conditions. Aluminum is used in both prototype and production molds. With high grade aluminum, you can expect to get ten to hundreds of thousands of parts. It is recommended to shop injection molding companies that only use high grade aluminum such as QC-7 or QC-10.

Geometrical Possibilities

Geometrical Possibilites of Injection Molded Parts

The most commonly used plastic molding process, injection molding, is used to create a large variety of injection molds with different shapes and sizes. Most importantly, this molding method can create injection molded parts with complex geometry that many other processes cannot. There are a few precautions when designing something that will be made using this process to reduce the risk of weak spots. First, streamline your product or keep the thickness relatively uniform. Second, try not cramming too many details into one part may cause visual defects in show surfaces or the inability to fill some of the details without sacrificing others. It may be better to make multiple injection molds for your process. A good injection molding company can steer you in the right direction.

The size of an injection molded part will depend on a number of factors (material, wall thickness, shape, process etc). The initial raw material required may be measured in the form of granules, pellets or powders. Here are some ranges of the sizes.

MethodRaw MaterialsMaximum SizeMinimum Size
Injection Molding (thermo-plastic)Granules, Pellets, Powders700 oz.Less than 1 oz.
Injection Molding (thermo-setting)Granules, Pellets, Powders200 oz.Less Than 1 oz.



Injection Mold Companies use two main methods to manufacture molds: standard machining and EDM. Standard Machining, in its conventional form, has historically been the method of building injection molds with a knee mill. With technological development, CNC machining became the predominant means of making more complex molds with more accurate mold details in less time than traditional methods.

The electrical discharge machining (EDM) spark erosion process has become widely used in mold making. Most injection mold companies have EDM in house, as it is essential to the mold build process of complex molds. EDM allows the formation of injection molded shapes which are difficult to machine, such as square corners or ribs. The process allows pre-hardened molds to be shaped so that no heat treatment is required. Changes to a hardened injection molds by conventional drilling and milling normally require annealing to soften the steel, followed by heat treatment to harden it again. EDM is a simple process in which a shaped electrode, usually made of copper or graphite, is very slowly lowered onto the mold surface (over a period of many hours), which is immersed in paraffin oil. A voltage applied between tool and mold causes spark erosion of the mold surface in the inverse shape of the electrode.


The cost of manufacturing injection molds depends on a very large set of factors ranging from number of cavities, size of the parts (and therefore the mold), complexity of the pieces, expected tool longevity, surface finishes and many others. The initial cost is great, however the piece part cost is low, so with greater quantities the overall price decreases. With global competition, companies with an ISO-Quality system usually will have better pricing as they have streamed lined their process and produce less defects. Mold Cost Guidline.

The Process of Producing Injection Molds

With injection Molding, granular plastic is fed by gravity from a hopper into a heated barrel. As the granules are slowly moved forward by a screw-type plunger, the plastic is forced into a heated chamber called the barrel, where it is melted. As the plunger advances, 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.

Injection Molding Cycle

The sequence of events during the injection molding of a plastic part is called the injection molding cycle. The cycle begins when the mold closes, followed by the injection of the polymer into the mold cavity. Once the cavity is filled, a holding pressure is maintained to compensate for material shrinkage. In the next step, the screw turns, feeding the next shot to the front screw. This causes the screw to retract as the next shot is prepared. Once the part is sufficiently cool, the mold opens and the injection molded part is ejected. Molding companies typically use the formula below to determine a cycle time of injection molds.

Time Function

The time it takes to make a product using injection molding can be calculated:

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 total cycle time can be calculated using tcycle = tclosing + tcooling + tejection

The closing and ejection times of injection molded parts can last from a fraction of a second to a few minutes, depending on the size of the mold and machine. The cooling times, which dominate the process, depend on the maximum thickness of the part.

Different types of Processes for Injection Molds

Although most injection molding processes are covered by the conventional process description above, there are several important molding variations including:

  • Co-injection (sandwich) molding
  • Fusible (lost, soluble) core injection molding
  • Gas-assisted injection molding
  • In-mold decoration and in mold lamination
  • Injection-compression molding
  • Insert and outsert molding
  • Lamellar (microlayer) injection molding
  • Low-pressure injection molding
  • Microinjection molding
  • Microcellular molding
  • Multicomponent injection molding (overmolding)
  • Multiple live-feed injection molding
  • Powder injection molding
  • Push-Pull injection molding
  • Reaction injection molding
  • Resin transfer molding
  • Rheomolding
  • Structural foam injection molding
  • Structural reaction injection molding
  • Thin-wall molding
  • Vibration gas injection molding
  • Water assisted injection molding
  • Rubber injection
  • Injection molding of liquid silicone rubber

Injection Molds: Process Troubleshooting

Optimal process settings are critical to influencing the cost, quality, and productivity of plastic injection molds. Process optimization is done using the following methods. Injection speeds are usually determined by performing viscosity curves. Process windows are performed varying the melt temperatures and holding pressures. Pressure drop studies are done to check if the machine has enough pressure to move the screw at the set rate. Gate seal or gate freeze studies are done to optimize the holding time. A cooling time study is done to optimize the cooling time for an injection molded part.

Molding trial

When filling new or unfamiliar injection molds for the first time, where shot size for that mold is unknown, an injection molding company technician/tool setter usually starts with a small shot weight and fills gradually until the mold is 95 to 99% full. Once this is achieved a small amount of holding pressure will be applied and holding time increased until gate freeze off (solidification time) has occurred on the injection molded part. Gate solidification time is an important as it determines cycle time, which itself is an important issue in the economics of the production process. Holding pressure is increased until the parts are free of sinks and part weight has been achieved. Once the parts are good enough and have passed any specific criteria, a setting sheet is produced for people to follow in the future.

Molding defects

Injection molding is a complex technology with possible production problems. They can either be caused by defects in the molds or more often by injection molded part processing (molding).

Molding DefectsAlternative NameDescriptionsCauses
BlisterBlisteringRaised or layered zone on surface of the Plastic partTool or material is too hot, often caused by a lack of cooling around the tool or a faulty heater
Burn marksAir Burn/ Gas BurnBlack or brown burnt areas on the plastic part located at furthest points from gateTool lacks venting, injection speed is too high
Color streaks (US)Localized change of colorPlastic material and colorant isn’t mixing properly, or the material has run out and it’s starting to come through as natural only
DelaminationThin mica like layers formed in part wallContamination of the material e.g. PP mixed with ABS, very dangerous if the part is being used for a safety critical application as the material has very little strength when delaminated as the materials cannot bond
FlashBurrsExcess material in thin layer exceeding normal part geometryTool damage, too much injection speed/material injected, clamping force too low. Can also be caused by dirt and contaminants around tooling surfaces.
Embedded contaminatesEmbedded particulatesForeign particle (burnt material or other) embedded in the partParticles on the tool surface, contaminated material or foreign debris in the barrel, or too much shear heat burning the material prior to injection
Flow marksFlow linesDirectionally “off tone” wavy lines or patternsInjection speeds too slow (the plastic has cooled down too much during injection, injection speeds must be set as fast as you can get away with at all times)
JettingDeformed part by turbulent flow of materialPoor tool design, gate position or runner. Injection speed set too high.
Polymer degradationpolymer breakdown from oxidation etcExcess water in the granules, excessive temperatures in barrel
Sink marksLocalized depression (In thicker zones)Holding time/pressure too low, cooling time too short, with sprueless hot runners this can also be caused by the gate temperature being set too high
Short shotNon-fill / Short moldPartial partLack of material, injection speed or pressure too low
Splay marksSplash mark / Silver streaksCircular pattern around gate caused by hot gasMoisture in the material, usually when resins are dried improperly
StringinessStringingString like remain from previous shot transfer in new shotNozzle temperature too high. Gate hasn’t frozen off
VoidsEmpty space within part (Air pocket)Lack of holding pressure (holding pressure is used to pack out the part during the holding time). Also mold may be out of registration (when the two halves don’t center properly and part walls are not the same thickness).
Weld lineKnit line / Meld lineDiscolored line where two flow fronts meetMold/material temperatures set too low (the material is cold when they meet, so they don’t bond)
WarpingTwistingDistorted partCooling is too short, material is too hot, lack of cooling around the tool, incorrect water temperatures (the parts bow inwards towards the hot side of the tool)

Tolerances and Surfaces

Molding tolerance is a specified allowance on the deviation in parameters such as dimensions, weights, shapes, or angles, etc. To maximize control in setting tolerances there is usually a minimum and maximum limit on thickness, based on the process used. Injection molds are typically capable of tolerances equivalent to an IT Grade of about 14. The possible tolerance of a thermoplastic or a thermoset is ±0.008 to ±0.002 inches. Surface finishes of two to four microinches or better are can be obtained. Rough or pebbled surfaces are also possible.

Molding TypeTypicalPossible

Lubrication and Cooling

The temperature of injection molds must be maintained in order for the production to take place. Because of the heat capacity, inexpensiveness, and availability of water, water is used as the primary cooling agent. To maintain temperature of an injection molded part, water can be channeled through the mold to account for quick cooling times. A consistent temperature mold is more efficient because this allows for faster cycle times. However, this is not always true because crystalline materials require the opposite of a warmer mold and lengthier cycle time.

Power Requirements

The power required for producing injection molds depends on many different factors and varies based on what materials are used and what the injection molded parts will be used for. The Manufacturing Processes Reference Guide states that power requirements depend on “a material’s specific gravity, melting point, thermal conductivity, part size, and molding rate. Below is a table from page 243 of the same reference as previously mentioned which best illustrates the characteristics relevant to the power required for the most commonly used materials.

MaterialSpecific GravityMelting Point (°F)
Epoxy1.12 to 1.24248
Phenolic1.34 to 1.95248
Nylon1.01 to 1.15381 to 509
Polyethylene0.91 to 0.965230 to 243
Polystyrene1.04 to 1.07338

Insert Molding

Insert Molding

Metal inserts can also be injection molded into the work piece. It is best to choose a company that has experience in insert molding as there are many variables to consider. For large volume parts the inserts are placed in the mold using automated machinery. An advantage of using automated components is that the smaller size of parts allows a mobile inspection system that can be used to examine multiple parts in a decreased amount of time. In addition to mounting inspection systems on automated components, multiple axial robots are also capable of removing parts from the injection molds and will place them in latter systems that can be used to ensure quality of multiple parameters. The ability of automated components to decrease the cycle time of the processes allows for a greater output of quality injection molded parts. In low volumes, a machine operator will remove the parts by hand.

Specific instances of this increased efficiency when it comes to injection molds include the removal of parts from the mold immediately after the parts are created and use in conjunction with vision systems. The removal of parts is achieved by using robots to grip the part once it has become free from the mold after in ejector pins have been raised. The robot then moves these parts into either a holding location or directly onto an inspection system, depending on the type of product and the general layout of the rest of the manufacturer’s production facility. Visions systems mounted on robots are also an advancement that has greatly changed the way that quality control is performed in insert molded parts. A mobile robot is able to more precisely determine the accuracy of the metal component and inspect more locations in the same amount of time as a human inspector.