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The Ultimate Guide to Injection Molding
Everything You Need to Know About Injection Molding
Xometry Image
Ultimate Guides

The Ultimate Guide to Injection Molding

Everything You Need to Know About Injection Molding

Injection Molded Part
Chapter 1
The Basics of Injection Molding
What is injection molding? Learn the basics and background of this traditional manufacturing process.
  Jump to Section
Summary of Chapter 2
Chapter 2
Injection Molding Materials
Learn about the range of materials and their applications.
  Jump to Section
Summary of Chapter 3
Chapter 3
Injection Molding Tooling and Processes
Learn about mold tool types and different molding processes like overmolding, insert molding, transfer molding, liquid silicone molding, and more.
  Jump to Section
Summary of Chapter 4
Chapter 4
How to Optimize Injection Molding Part Design
Learn about tolerances, design-for-manufacturing basics, and advanced design validation software.
  Jump to Section
Mold texturing
Chapter 5
Surface Finishes, Textures, and Post-processing
Once your part is removed from the mold, these are your options for surface roughness, patterns, engraving, image printing, inserts, and more.
  Jump to Section
Injection molding services
Chapter 6
Injection Molding Glossary
Popular terms defined.
  Jump to Section
Injection Molded Part
Chapter 1
The Basics of Injection Molding
What is injection molding? Learn the basics and background of this traditional manufacturing process.
What Is Injection Molding?
Anatomy of an Injection Molding Machine
Applications of Injection Molding
Comparing Other Processes

What Is Injection Molding?

Injection molding is a manufacturing process used for the mass production of identical plastic parts. It involves the injection of molten plastic into a mold to produce a part in the shape of the mold cavity. Injection molding has been around since the late 19th century and continues to be popular. If you look around, injection molded parts are everywhere, from the keys on your keyboard to the dashboard of your car to your X-Wing LEGO set.

Injection molding is so widely used because it is one of the most cost-effective and most repeatable processes for plastic parts at scale, sometimes costing just cents per part. “At scale” typically refers to volumes ranging from thousands to hundreds of thousands of parts. It also has the highest variety of materials, colors, and cosmetics when compared to CNC machining or even 3D printing. 

The basic injection molding process requires an injection molding machine, raw plastic material, and a typically machined mold tool (read more in Chapter 3 mold tool types). The key steps in the injection molding process are clamping, injection, cooling, and ejection.

  1. Clamping: The two halves of the aluminum or steel mold are held together to prepare for the enormous pressure from injecting the molten material.
  2. Injection: Pelletized resin is heated to a melted state and injected into the mold.
  3. Cooling: The part cools and solidifies into the final plastic part. This may take anywhere from a few seconds to a few minutes.
  4. Ejection: The molded plastic part is then ejected, either automatically or manually, and the process starts again for the next part.

Anatomy of an Injection Molding Machine

An injection mold tool consists of three major parts: the injection unit, the mold, and the clamping and ejector unit. We’ll focus on the mold components in the following sections, which break down into the sprue and runner system, the gates, two halves of the mold cavity, and optional side actions.

Injection molding machine diagram

Mold Cavity

A mold typically consists of two sides: an A side and B side. The Core (B Side) is typically the non-cosmetic, interior side that contains the ejection pins that push the completed part out of the mold. The Cavity (A Side) is the half of the mold which the molten plastic fills. Mold cavities often have vents to allow air to escape, which would otherwise overheat and cause burn marks on the plastic parts.

Runner System

The runner is a channel that connects the liquified plastic material from the screw feed to the part cavity. In a cold runner mold, plastic will harden within the runner channels as well as the part cavities. When the parts are ejected, the runners are ejected as well. Runners can be sheared off through manual procedures like clipping with die cutters. Some cold runner systems automatically eject the runners and part separately using a three-plate mold, where the runner is partitioned by an additional plate between the injection point and the part gate.

Hot runner molds do not produce attached runners because feed material is kept in a melted state up to the part gate. Sometimes nicknamed “hot drops,” a hot runner system reduces waste and enhances molding control at an increased tooling expense.


Sprues are the channel through which the molten plastic enters from the nozzle and typically intersects with a runner that leads to the gate where the plastic enters the mold cavities. The sprue is a larger diameter channel than the runner channel that allows the proper amount of material to flow through from the injection unit.


A gate is a small opening in the tool that allows molten plastic to enter the mold cavity. Gate locations are often visible on the molded part and are seen as a small rough patch or dimple-like feature known as a gate vestige. There are different types of gates, each one with its strengths and trade-offs.

Parting Line

The main parting line of an injection molded part is formed when the two mold halves close together for injection. It is a thin line of plastic that runs around the outside diameter of the component.

Side Actions

For simple A and B molds that do not have any undercut geometry, a tool can close, form, and eject a part without added mechanisms. However, many parts have design features that require a side action to produce features like openings, threads, tabs, or other features. Side actions create secondary parting lines.

Side actions are inserts added to a mold that allow material to flow around them to form the undercut feature. Side actions must also allow for a successful ejection of the part, preventing a die lock, or a situation where the part or tool must be damaged to remove part. Because side actions do not follow the general tool direction, undercut features require draft angles specific to the action’s movement. Read more about common types of side actions and why they are used.

Xometry Image
A sprue directly into an edge gate of a part. The perpendicular features are called “cold slugs” and help control material shear entering the gate.

Applications of Injection Molding

Thanks to the high-precision capabilities, repeatability, and cost efficiency of injection molding, the manufacturing process is used to make a variety of products and parts from the smallest micromolded medical tools up to large interior car parts like door panels. Injection molding parts are especially popular in the medical industry. Since plastic molded parts are lightweight, sturdy, and sterilizable, they might be used for tomographic imaging machines at an eye doctor, light and monitor support arms at a dental office, or infusion pumps and EKG machines at a hospital.

Additional applications of injection molding include:

Consumer products

IOT enclosures, handheld items, storage containers, and more.


Turbine housings and blades


Overmolded components like wire cables and connectors

Energy and oil

Insert molded valves


Door panels, engine components

Xometry Image
An example of a consumer product device attachment

Comparing Other Processes

Custom injection molding offers a number of benefits that are not found in other manufacturing processes. Because injection molding has incredible throughput—quickly producing hundreds or thousands of parts a day—it is the cheapest process at scale for plastics. This is especially true for parts with detailed features or complex geometries; if these parts were CNC machined or even 3D printed, the overhead set-up costs for each batch of parts would drive up the per-unit cost. And while 3D printing can be a close competitor in terms of cost (at low volumes), the mechanical properties of injection molded parts are often superior. Injection molded parts produce completely solid, isotropic parts whereas 3D printed parts may have anisotropic or orthotropic properties depending on the process. 


Another key benefit to injection molding is that it offers the highest variety of materials, colors, and surface finishing options—by some estimates, materials options alone include over 18,000 thermoplastics, thermosets, and elastomers. Some manufacturers can match exact Pantone colors and achieve a finish anywhere from a matte finish all the way to a glasslike smoothness. Injection molding also allows for cosmetic addition like different texture patterns, engraving, image printing, and assemblies. 


Different injection molding processes, such as overmolding and insert molding, expand the possibilities of injection molding even further (more on this in Chapter 2). Critically, with all injection molding processes, the parts use the same mold, meaning every part produced with the mold is an identical reproduction.

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How to Choose the Right Material

Injection molding shops can make custom parts in hundreds of materials. In order to choose the right injection molded material for your project, it’s helpful to first think about a few key criteria like: 

  • Location: Will your part live outdoors, indoors, or in a dark, bright, damp, or dry environment? Will it be exposed to chemicals or solvents?
  • Temperature: Will the product perform in freezing, hot, or boiling temperatures?
  • Lifecycle: How long do you expect this part to be used? Does it need to last 5 years, 25 years, or more?
  • Warranty: Do you provide a warranty on your product, and if so, what is the cost of covering its warranty in the future?
  • Cost: Do you need to minimize costs with cheaper commodity plastics or does your budget allow for more expensive engineered plastics? Commodity plastics are widely available and typically are high-density and tolerate less heat. Engineered materials are harder to source and are highly heat tolerant and stiff.
  • Cosmetics: How do you need your product to look? Injection molding allows for a wide variety of colors, finishes, and textures, with cost increasing for more specialized cosmetics.

The above considerations of location, temperature, lifecycle, and warranty relate to material performance. Injection molding materials have been developed to meet certain performance characteristics including mechanical strength, abrasion resistance, low friction, chemical resistance, temperature resistance, clarity, UV stability, hardness (durometer), flexibility, and weight. Another common consideration is shrink rate, or how much a material will shrink during the molding process. This is important to account for in the tool design.

If you’re starting out with injection molding for the first time, you may want to look at the top six most common injection molding materials. These are polypropylene (PP), acrylonitrile butadiene styrene (ABS), polycarbonate (PC), nylon (PA), high-density polyethylene (HDPE), and PC-ABS (a blend of ABS and PC). These are resins that are frequently used because of a combination of their desirable mechanical properties and low cost. High-performance injection molding plastics like PEEK and PEI are more costly but offer the best heat resistance and stiffness. If you're looking for a more specific blend of material characteristics, read on.

Rigid Plastics: Amorphous vs. Semi-Crystalline

When choosing between plastics, it is helpful to understand how different material characteristics arise. The major difference between the two main types of rigid injection molding plastics, amorphous thermoplastics and semi-crystalline plastics, is their molecular structure. Amorphous thermoplastics have randomly ordered structures that will gradually soften then melt and semi-crystalline plastics have highly ordered structures with a narrow transition from solid to melt.

Amorphous plastics are better for parts that need to be clear or colored. They are very dimensionally stable and provide good impact resistance. However, they often have low resistance to fatigue or stress cracking. Examples of amorphous plastics include PC, PMMA, PETG, ABS, PEI, PVC, PSU, and PS.

Semi-crystalline thermoplastics are more wear resistant, making them excellent for bearing, wear, and structural applications. They also offer good chemical and electrical resistance and can often be “self-lubricating” due to their low coefficient of friction. This also can be a downside if looking to bond the parts through chemical processes or overmolding. Semi-crystalline materials are typically opaque or clouded in appearance. Common semi-crystalline materials include PE, PP, PA, PBT, POM, PET, and PEEK.

Sample Comparison of Amorphous and Semi-crystalline Resins
Amorphous ResinsSemi-crystalline ResinsCost


PEI (polyetherimide): high-strengh, high heat and chemical resistance. Used in aerospace.

PEEK (polyetheretherketone): high-strengh, high heat and chemical resistance. Used in bearings and medical implants.



PC (polycarbonate): moderate strength, high heat resistance, high electrical insulation. Used in electrical components and windows.

PA (polyamide, or nylon): moderate to high strength, high chemical and abrasion resistance, low shrinkage and warp. Used in auto parts and textiles.



PS (polystyrene): transparent, low strength, low heat resistance. Used in cutlery and cups.

PP (polypropylene): high flexibility and toughness, high chemical and fatigue resistance. Used in bottles, crates, cases, and living hinges.


Performance, engineering, and commodity examples of amorphous and semi-crystalline resins.

injection molding pellet material
Polystyrene, PVC, and polycarbonate plastic pellets

Guide to Materials Available at Xometry

  • ABS (acrylonitrile butadiene styrene): High-strength general-purpose engineering plastic, used for many commercial products.
  • ASA (acrylonitrile styrene acrylate): A material very similar to ABS with higher resistance to fading and better suited for outdoor use.
  • CA (cellulose acetate): Typically used in eyeglasses and film, CA is a flexible clear material that can be used in food contact.
  • HDPE (high-density polyethylene): Excellent strength-to-weight ratio and chemical resistance. It is often used for fuel tanks, connector insulators, and food containers. HDPE is also used in outdoor equipment like playgrounds.
  • LCP (liquid crystal polymer): LCP has mechanics even at elevated temperatures as well as low dielectric constants. This material provides exceptional features for micromolding and thin walled components. LCP is popular for electrical connectors and interconnects as well as medical devices.
  • LDPE (low-density polyethylene): A flexible and tough material with lower density versus HDPE. LDPE does not react to acids, bases, or alcohols. Useful for trays, snap lids, and general-purpose containers.
  • PA 6 (polyamide 6, nylon 6): Offers increased mechanical strength, rigidity, good stability under heat, and/or chemical resistance.
  • PA 6/6 (polyamide 6/6, nylon 6/6): Offers increased mechanical strength, rigidity, good stability under heat, and/or chemical resistance.
  • PARA (polyarylamide): Often combined with infills such as glass or mineral fibers, PARA creates very rigid parts with low creep and a slower rate of water absorption compared to nylon (PA). PARA is great for structural components in handheld electronics as well as medical devices.
  • PBT (polybutylene terephthalate, Valox): A common electronic insulator with a polyester base. Highly used in automotive as a longer-wear alternative to nylon.
  • PBT-PET (polybutylene terephthalate-polyethylene terephthalate): A compounded blend of PBT and PET.
  • PC (polycarbonate): A clear or colored, lightweight, glass-like plastic that is heavily used across multiple industries. PC is impact resistant, holds great feature detail, and can be used for safety equipment, lenses, electronic devices, and much more.
  • PC-ABS (polycarbonate-acrylonitrile butadiene styrene): The best of both PC and ABS. Higher-strength engineering thermoplastic with slightly more flexibility than standard polycarbonate.
  • PC-PBT (polycarbonate-polybutylene terephthalate, Xenoy): A tough and rigid material resistant to lubricants, solvents, and cleaning agents. Very common in electronic enclosures.
  • PC-PET (polycarbonate-polyethylene terephthalate): A blend of PC and PET provides tough and chemically resistant results and can be used as an alternate to PC-ABS. It endures harsh solvents and cleaners, making it excellent for sports equipment and healthcare applications.
  • PCT (polycyclohexylenedimethylene terephthalate): A thermoplastic polyester that typically outperforms PET due to lower moisture absorption and better environmental stability. PCT is often used for connectors and switches.
  • PE (polyethylene): One of the most common plastics used in the world, PE has high ductility, abrasion resistance, and chemical resistance. PE is often described by its molecular weight, such as UHMW PE (ultra-high molecular weight), LDPE (low density), or HDPE (high density). PE is used in packaging, tubing, films, bottles, and more.
  • PEEK (polyether ether ketone): Offering excellent tensile strength that surpasses most plastics, PEEK is often used as a lightweight substitute for metal parts in high-temperature, high-stress applications. PEEK resists chemicals, wear, and moisture.
  • PEI (polyetherimide, Ultem): Known best for its extremely high heat and flame resistance, PEI is used for many medical applications and is more affordable than PEEK.
  • PE-PP (polyethylene-polypropylene): A resin blend of the polyolefin’s polypropylene and polyethylene.
  • PE-PS (polyethylene-polystyrene): A resin blend of the polyethylene and polystyrene.
  • PES (polyethersulfone): A rigid, transparent plastic that is chemically inert, biocompatible, and sterilizable. PES is suitable for food-contact devices such as coffee machine components as well as aerospace and automotive where chemical exposure is high.
  • PET (polyethylene terephthalate, Rynite): Also abbreviated PETE, this is a clear, strong, and lightweight PE resin heavily used in food packaging, soda bottles, jars, and more. The material is food-safe. PET is recyclable with a resin code of 1.
  • PLA (polylactic acid): A biodegradable and renewable plastic. PLA has a relatively low glass transition temperature and is common in short-use applications.
  • PMMA (polymethyl methacrylate, acrylic): A clear glass-like plastic. Good wear and tear properties. Great for outdoor use.
  • POM (acetal polyoxymethylene, Delrin): Good moisture resistance, high wear-resistance, and low friction.
  • PP (polypropylene): Polypropylene has excellent electrical properties and little or no moisture absorption. It carries light loads for a long period in varying temperatures. It can be molded into parts requiring chemical or corrosion resistance.
  • PPA (polyphthalamide): A subset of nylons (polyamide) that typically exhibit a higher melting point and lower moisture absorption. PPA is typically used in automotive and industrial applications due to its ability to withstand harsh chemicals. PPA is good for fuel and fluid manifolds and headlight housings.
  • PPS (polyphenylene sulfide, Ryton): A high-performance thermoplastic with extreme resistance to solvents.
  • PS (polystyrene): A clear, hard, and brittle material widely used for food packaging, clamshell containers, and even disposable cutlery.
  • PS-PPE (polystyrene-polyphenyl ethers, Noryl): Exhibiting high heat and flame resistance, PPE-PS has high stiffness and tensile strength even at elevated temperatures.
  • PSU (polysulfone, Udel): A rigid, stiff, and transparent plastic which is a higher-performance alternative to polycarbonate.
  • PVC (polyvinyl chloride (Shore D)): A rigid, general-use plastic that is common in plumbing, non-food packaging, and trimming.
  • PVDF (polyvinylidene fluoride, Kynar): A chemically inert, high-temperature material. Due to its low friction, PVDF is used in plumbing parts, bearings, chemical handling, electrical wire insulation, and tubing.
  • SAN (styrene acrylonitrile): A polystyrene that is heat resistant and transparent. Due to its relationship with polystyrene, SAN is low cost and has enhanced clarity and shine. SAN is common in household goods, door handles, and kitchenware.
  • TPO (thermoplastic polyolefin): A flexible plastic with good chemical resistance but lower temperature resistance compared to PP.
  • TPU (thermoplastic polyurethane (Shore D)): A tough, highly abrasion-resistant resin that bridges the gap between rubbers and plastics. TPUs can be formulated to be rigid or elastomeric. TPU exhibits a high flex before break and is ideal for wheels and door panels.

Manufacturing elastomeric parts can be accomplished through injection molding, transfer molding, or compression molding. When getting a quote through Xometry, our team can help you decide the best method depending on the part geometry, estimated annual volume, and the type of material required.

  • EPDM (ethylene propylene diene monomer rubber (Viton)): One of the highest performing rubber elastomers with high heat resistance, chemical resistance, and moisture sealing properties. EPDM is commonly found in automotive seals, gaskets, O-rings, and electrical insulators.
  • PEBA (polyether block amide): A soft, flexible, plastic or elastomer used for medical devices such as catheters. PEBA foams are used for padding, shoe insoles, and sports equipment. PEBA is resistant to moisture and UV exposure.
  • PVC (polyvinyl chloride (Shore A)): A rubber-like, general use elastomer, that is common in outdoor products, protective films, and mats. Shore A rubber-like PVC requires plasticizers to improve its flexibility from its typical rigid state. PVC is flame retardant due to self-extinguishing properties.
  • TPE (thermoplastic elastomer): A broad class of elastomers that behave like a thermoset with high flex and elasticity but process like a thermoplastic through molding. TPE is an umbrella term for many unique elastomer classes.
  • TPU (thermoplastic polyurethane (Shore A)): A tough, highly abrasion-resistant elastomer that bridges the gap between rubbers and plastics. TPUs can be formulated to be rigid or elastomeric. TPU exhibits a high flex before break and is ideal for flexible tires, skateboard wheels, and weatherproof gaskets.
  • TPV (thermoplastics elastomer, vulcanized rubber (Santoprene)): An excellent elastomer with high versatility due to temperature resistance, compression, and elasticity. 
  • LSR (liquid silicone rubber): Offering food and biocompatibility, extreme heat resistance, and excellent flexibility, silicones are a versatile rubber material. LSR is used for medical devices and automotive, aerospace, and consumer products. Liquid silicone rubber molding is a specialized process different from traditional injection molding (more on this below).

Colors, Fillers, and Additives

Injection molded materials can be modified physically, chemically, and aesthetically from their pure resin makeup by melt-blending additives. This process is known as compounding. Fillers are typically inexpensive and inert materials with a variety of uses. Sometimes they are added to lower part cost by adding less expensive bulk, while other times they may be added to change mechanical properties or cosmetics. The list below explains several fillers, additives, and agents that are used to customize a material for its application.

Color masterbatch, or concentrates, is the most common addition to mold resin. Resembling mold resin pellets itself, these materials are added to give molded parts vibrant color and/or increased resistance to weathering. One example is the use of carbon black pigment to help block UV light. Color pellets are added alongside the molding resin in the screw auger, where they fully melt and combine with the carrier resin. 

Glass reinforcement and mineral fillers can increase a material’s stiffness, reduce warpage during cooling, and even stabilize the part for higher heat deflection. Examples of glass reinforcers and mineral fillers include fibers, microspheres, or fused silica. In most cases, a reinforced version of a polymer is compounded by the OEM. A great example is DuPont™ Nylon Zytel®, which is a family of glass fiber-reinforced nylon 6, which typically contains 33% glass fiber by material weight. Glass-reinforced parts may show light grey banding, called splay, on the parts.

UV stabilizers and inhibitors can be added to molding plastics to help slow or prevent yellowing, brittling, or mechanical degradation from exposure to ultraviolet light. UV light is not only emitted from sunlight, but also from all standard overhead lights and fluorescents. This means stabilizers may be necessary in light colored or colorless (clear) materials.

Flame retardants reduce the flammability of the plastic to meet specific UL 94 standards from UL HB to V-0. Common fire retardant additives include alumina trihydrate and calcium sulfate.

Static dissipating (ESD) masterbatches can contain particulates or fibers that increase or decrease the surface resistivity of the polymer. ESD particulates can reduce surface resistivity in a range of 106 to 1012 ohm/sq to give a material anti-static properties.

Blowing agents or chemical-foaming agents can create a foam-like structure in the liquid plastic resin. These agents are typically added to reduce part density by 10-20% or to mitigate sink on thick cross sections. Foaming agents can also help increase throughput by reducing molding cycle times . Blowing agents are also essential for large parts molded with a structural foam molding technique, such as trays, cart bodies, and insulated containers. 

Other common fillers such as talc, clay (kaolin), mica, calcium carbonate, carbon, wollastonite, and more can also be used to increase stiffness, reduce costs, or add dimensional stability. Sometimes, additional fillers may be introduced when troubleshooting a design to mitigate properties like sink, voids, or warp.

Xometry Image
Glass-reinforced nylon often has a light grey or silver texture. This part also features an undercut.
Header of Chapter 3
Chapter 3
Injection Molding Tooling and Processes
Learn about mold tool types and different molding processes like overmolding, insert molding, transfer molding, liquid silicone molding, and more.

Types Tools and Processes

Injection mold tools can be designed and fabricated in many different ways to accommodate budgets, production requirements, and process control. 

A single-cavity tool is typically used for lower-volume batches. Single-cavity tools have one cavity, therefore producing one part every injection cycle. They are sometimes called single impression tooling. 

A multi-cavity tool, also referred to as multi-impression tooling, consists of duplicate cavities for the same part and allows for multiple parts to be produced in a single cycle. Multi-cavity tooling also shares a runner system, which can reduce the percentage of waste or scrap material per part which saves costs in production. Multi-cavity tooling is important when producing a high volume of parts as efficiently as possible. Multi-cavity tooling is typically produced in even numbers, and can have 2, 4, 8 and more cavities. It is important to note that adding cavities increases the complexity and manufacturing time of a tool, which can significantly increase tooling costs compared to single-cavity tools.

Family mold tools are a type of multi-cavity tool where each cavity is a unique part design. Family tooling shares the same tool, reducing overall costs compared to making individual tools per part design. Family molds are popular for low-volume or prototype production. If the parts in a family tool share the same material, range of dimensions, and color, then the tool can share cycle times to produce multiple parts simultaneously. These parts typically run individually using a runner shut-off.

Master Unit Die (MUD) tooling uses a common frame that remains in the molding machine while interchangeable inserts are added or removed to define the shape of the cavity. This is often more cost-effective than a traditional custom-machined mold and can speed up production time. This makes it useful for prototyping and low-volume production. MUD tooling can be designed with single-cavity, multi-cavity, or family tooling if the part designs, gating system, slides, or cores can fit within the exchangeable insert. However, MUD tooling is not used for production of over 100,000 units since this volume of parts often requires advanced molding customization.

Insert molds are a type of tool where a metal component, such as a threaded piece or copper conductor, is placed securely in the mold cavity and bonded to the plastic part. Oftentimes, it is faster to insert mold rather than tap or install inserts as a secondary operation to molding. Insert molds reduce the labor and time required to manually assemble parts via ultrasonic and heat-stake tapping insertion. It also increases the reliability of components by merging two materials mechanically.

Overmolding is when one plastic material is molded over another plastic part, like a toothbrush handle. Overmolds typically involve molding the rigid substrate first and then inserting the part into a second tool to mold the second material over the substrate. Two-shot, or 2K, molds automate the overmolding process using specialized equipment, such as multiple cavities and automated tooling, to heat and feed two separate materials at once. Two-shot molds are best used in production due to high tooling costs.


Liquid silicone molding differs from thermoplastic injection molding in that it uses liquid silicone materials, which are thermoset and cannot be melted once the part is molded. The process is the same in that it requires a machined tool, high injection pressure, and similar ejection mechanisms. 

Compression molding uses a preheated, open cavity side mold into which material, called a charge, is fed. A vertical hydraulic press comes down to form the material into the final part. Compression molding is advantageous for flat or simple parts because it is less expensive due to the simplicity of the mold.

Transfer molding is when a plastic material is forced into a mold in a process similar to compression molding with a vertical hydraulic press. Transfer molding is similar to injection molding, however, in that the mold is enclosed and is transferred into the molding cavity from another chamber.

3D printed molds are possible but most companies that serve businesses do not print molds because of the higher rate of error involved.

Xometry Image
An example of a family mold, with three identical parts and one unique part. This tooling type decreases production time but increases tool cost.

SPI Mold Classifications and Volumes

The injection molding industry often defines molds by their class. This goes from Class 105 rapid prototype molds that produce no more than hundreds of parts to Class 101 tools that produce millions of parts quickly. The number of parts a tool can produce before it wears down is known as the tool life.

Class 105, or a Class V tool, is the simplest type of mold. This mold is a prototype mold only and is produced as quickly and cheaply as possible. This class is typically used for rapid tooling, in which a small batch of injection molded parts prototyped for impact and stress testing (over plastic machined and 3D printed parts). A class 105 tool is not built to create more than 500 parts. An example of this tool is a one-time market test of 100 molded units.

Class 104, or a Class IV tool, is a low-volume production mold that produces under 100,000 parts. It is usually made from aluminum or mild steel and can typically only withstand non-abrasive materials (this means no glass-filled materials, for example). An example of this is an aluminum or MUD tooling, SPI B-3 finish, hand-loaded cores and limited reruns.

Class 103, or Class III tool, is a low- to mid-volume production tool for making under 500,000 parts. It is cost-effective and is a popular class of mool tool. Example: An aluminum or steel tool with automated lifts or slides that will require multiple re-runs with batches between 1,000-5,000 units.

Class 102, or Class II tool, is a medium to high production tool for producing under 1,000,000 parts. This tool is made from a more durable metal—typically steel—so it can be used with abrasive materials and/or parts requiring close tolerance. This is a high-quality, and therefore costly, molding option. Example: Steel tooling with an SPI A-2 finish, automated slides, and scheduled production cycles.

Class 101, or a Class I tool, is the best quality tool possible and is built for extremely high production volumes with a lifetime of over 1,000,000 parts. This is the highest-priced mold. Example: A multiple-cavity steel tool with full automation and overnight production workcenters.

Calculating the Cost of Tooling

There are various reasons why mold tooling can be costly. 

  1. The mold tool that forms plastic parts is typically milled out of steel or aluminum. It is expensive to set-up and cut parts using specialized cutting machines. Generally, the more geometrically complex the parts are, the more machining time the mold tool will require, which increases the price. 
  2. The tool class determines the type and quality of material out of which a mold tool is machined; the lower the class (i.e. production-grade), the more expensive the mold tool will be. 
  3. A more complex molding press set-up, typically for more complex geometries and/or higher production volumes, may require the addition of injection molding machine features. These include side actions, water cooling lines, hot runners, manually trimmed gates, and more. These can dramatically increase tooling costs.

A mold tool may range from $2,000 to $8,000 for a simple, single-cavity mold. For more complex molds with multiple cavities, the price can range from $300,000 to over $1,000,000. Note that molds on the higher end are for very high outputs—an example of this is a completely automated, 80+ cavity mold for the production of a billion contact lens cases.

There is also typically a set-up fee associated with a production run of a tool. This fee is recurring on subsequent runs and is fixed regardless of the job size. As with upfront tool costs, setup fees are often amortized over the course of the production run.

What If My Mold Tool Needs to Be Reworked?

Steel-safe Tool: When creating parts for rapid injection molding, designers may need to change their part design after the mold tool is created. There are two ways to do this: you can modify your mold design by removing metal from the current mold—therefore adding plastic to the final molded part—or you can create an entirely new mold. Creating an entirely new mold is often cost-prohibitive, so it’s best to always machine mold tools to a “steel safe” state. This means the mold is produced in a manner where the tolerances are on the lower end for exterior dimensions and the larger end for interior dimensions. If molding samples show that the initial molded parts need adjustments to hit critical features, then these can be created by removing (milling) additional metal from the mold. It’s easy to remove metal from the mold tool, but very difficult and costly to add metal to the tool when iterating on injection molding designs.

Tooling Checks with T1 Samples: T1 shots, or samples, are the first successful parts produced by the mold and allow the customer to review the parts before the mold is finalized with secondary mold finishing such as texturing. This sampling step is crucial to ensure the production parts meet the customer’s expectations. Once samples are approved by the customer, the manufacturer can finalize the tool and start production.

Process Control: Once all of the above methods are incorporated, the next step to improving tolerance compliance and cosmetic requirements is for an injection molding manufacturer to tweak the process. Controlling the temperature, pressure, and holding time are some of the most common ways of ensuring parts do not contain unwanted knit lines or jetting, or have unresolved features from short shots. Once the ideal set of conditions are determined, the mold can create consistent parts with very little dimensional variability between parts. Learn more about how manufacturers control injection molding tool setup to ensure parts do not have defects.

Header of Chapter 4
Chapter 4
How to Optimize Injection Molding Part Design
Learn about tolerances, design-for-manufacturing basics, and advanced design validation software.

When designing parts that will be injection molded, it is important to use design-for-manufacturing (DFM) principles. A good design will not only ensure that all the features of the part will be moldable, but also limit warping, excessive shrinking, and part misalignment. A good part design also clearly defines minimum functional and cosmetic requirements, which can lower initial tool machining costs as well as machine set-up costs.

The Molding Trinity: Undercuts, Uniformity, Drafts


Undercut features are those that are parallel to the main parting line when it is oriented in the mold, and are such that they need a and may need side actions (often in the form of slides or hand loaded cores) to produce the geometry. However, side actions increase molding production costs, or may not be possible at all. To eliminate the need for these, pass-through coring is recommended.


Uniformity refers to the practice of designing walls of even thicknesses. Uneven wall thickness can cause part deformation that will interfere with tolerances, look, and geometry of neighboring features, holes, and faces. This is because when walls have different thicknesses, they cool at different rates in the mold cavity. This may cause walls to warp, with thicker walls showing a type of warping called sink. Sink is when material cools and condenses faster than the surrounding area, causing a surface depression. 

When thick areas are unavoidable, coring (also known as core outs) should be utilized to maintain an even wall thickness. In instances where thick walls are used to increase strength, it’s better to use ribs and gussets instead. These should be 40-60% the thickness of outer walls and should still maintain draft angles.

Draft Angles 

Draft angles are angles that are designed into vertical feature edges so a molded part can be ejected from the mold tool without scraping or warping the part. Draft angles can vary from 0.5° to 5° depending on the part design and surface finish. They are direction dependent, meaning they are designed in the direction that they are being pulled out of the mold.

In general, add one degree of draft per inch of the feature that is being pulled out of the mold. Textured features require a two-degree minimum draft angle. It is also helpful to minimize the pull directions that are needed to as close to two. This will enable clean draft design.

Xometry Image
The part on the left requires a slide-in mold, while the part on the right has a slot to allow the feature to be molded with only a two-cavity mold.

Advanced Design Strategies

Bosses and Other Features

When designing injection molding part features, the key is to minimize “see-thru,” or sink marks that appear on the surface located on the other side of the feature. Bosses are cylindrical features that rise above the surface of a part and are used to assemble the part with others via a screw or thread. A helpful strategy to minimize see-thru sink of boss features is to position the boss at depth of 30% the wall thickness, then cut out a groove around the exterior with an angle of 30°. This removes the amount of material that needs to solidify in the mold, therefore reducing sink.

Additionally, bosses are more stable if connected to side walls via ribs. As with ribs, boss walls should be 40-60% of the exterior wall thickness. 

Insert Molding

Insert molding involves molding plastic or rubber over a metal part, which typically does not result in a chemical bond. The bond must be secured with physical mechanical features, such as by designing ring grooves, dovetails, lips for the metal insert to lock on to. Xometry can custom manufacture the inserts via CNC machining as well. 


Overmolding materials should be chemically and thermally compatible. Some examples of this include TPE, a thermoplastic elastomer, molded over ABS ((acrylonitrile butadiene styrene).

Similar to insert molded parts, it is helpful to design bonding mechanical features for overmolded parts that experience high use. 

Molded Threads

Threads can be externally or internally can be molded in to reduce assembly time and overall production time. This may be advantageous when a high volume of parts is needed quickly. External threads can be molded without side actions if the main parting line can go lengthwise across the thread. Otherwise, external and internal threads can be molded with the help of hand-loaded or automated winding cores.

Download the Design Guide

Draft on injection molded features
Get in-depth explanantions of injection molding design strategies.

Checking Designs in 3D CAD

In most CAD design software, such as SOLIDWORKS, Autodesk Inventor, and Autodesk Fusion 360, you can use evaluation tools to determine whether your part is manufacturable. Using the draft analysis, undercut analysis, and thickness analysis tools, you can see whether your part’s features pass, could be improved, or would not be manufacturable at all.

The following features are highlighted in red when they are not manufacturable: 

  • When a part is not able to be properly ejected from a part, or is die-locked
  • Parting lines and shutoffs 
  • Sink and see-thru

In addition to the above strategies, it is important to talk to injection molding specialists early on in the design process to prevent costly redesigns later in the design phase.

draft analysis on injection molded part
A draft analysis on this part reveals that it is die-locked in the direction it will come out of the mold, as shown by the red on the hole features. Parts that are die-locked will require some action to pull the feature away at the mold opening.
Mold texturing
Chapter 5
Surface Finishes, Textures, and Post-processing
Once your part is removed from the mold, these are your options for surface roughness, patterns, engraving, image printing, inserts, and more.

Default Surface Finish

Most tools are, by default, finished to some degree to prevent molded parts from picking up any imperfections in the mold tool. In steel and aluminum molds, the machining marks left by the end mills can be transferred to the molded part if these are not removed by bead blasting, polishing, or texturing the mold surface.

On the B side of the mold (the interior, non-cosmetic side), the tooling marks may be left on the mold and will not affect the function or cosmetics of the part. However, for features with a steep draft or longer pull length, some level of polishing may be required to ensure easy part removal from the mold.

Once the A side of the mold tool is polished, additional polishing and texturing options can be added to meet the part’s cosmetic requirements. These options will increase the cost of the mold tool as well as increase the time it takes to complete the mold.

standard injection molding finish
An example of an as-machined finish on the non-cosmetic side of a molded part

Options for Surface Finishes and Textures

Mold finishing grades are typically categorized according to the SPI (Society of the Plastics Industry). SPI finishes range from A-1 to D-3 with three grades per letter. SPI finishes can be achieved directly at the toolmaker location with manual finishing techniques such as media blasting, sanding, and polishing. 

Mold textures are defined according to MoldTech, a leading injection molding manufacturer. Mold texturing references a range of finishes and are achieved through chemical etching, laser engraving, hand finishing, or a combination of these. Although less prevalent today, EDM finishing is also sometimes defined for matte finishes.

The chart below compares some of the standard finishes, descriptions, and associated costs. Finishes can be selectively applied to different features of the tool. It is important to note from a design perspective that smoother finishes are required for part release on low-draft features and a higher draft is required for coarse textures.

Mold Tool FinishFinish DescriptionVisual DescriptionAdded Expense


No additional finishes are required.




Grade #3, 6000 Grit Diamond Buff

High polish parts



Grade #6, 3000 Grit Diamond Buff

High polish parts



Grade #15, 1200 Grit Diamond Buff

High-low polish parts



600 Grit Paper

Medium polish parts



400 Grit Paper

Medium polish parts



320 Grit Paper

Medium-low polish parts



600 Stone

Low polish parts



400 Stone

Low polish parts



320 Stone

Low polish parts



Dry Blast Glass Bead

Satin finish



Dry Blast #240 Oxide

Dull finish



Dry Blast #24 Oxide

Dull finish



A variety of matte surface textures that are more coarse than SPI D-3.

Textured finish


Xometry Image
Mold texturing, such as MT11020 pictured above, can add cosmetic appeal and reduce the visibility of sink marks and fingerprint marks.
Injection molding services
Chapter 6
Injection Molding Glossary
Popular terms defined.

Injection Molding Terms You Should Know

Automatically Trimmed Gates

Gates that incorporate features to automatically cut the runners and sprue from the part as it is ejected from the tool.


Cylindrical protrusions on plastic parts typically used as a mating location for a screw or other threaded part.

Burn Marks (Scorch Marks)

Black or brown burnt areas on the plastic part caused by the overheating of trapped gases.

Cavity (or Tool Cavity)

The concave part of the mold that receives and forms the molten plastic into the part shape.

Core Outs

The portion of a part that is removed to achieve a uniform wall thickness. This portion of the part has no end use function other than lightening the part and reducing warp.


This is a device used to produce internal cavities when the two-sided mold cavity  alone cannot produce these features. The Core may also refer to Side B of the mold, or the interior, non-cosmetic side of the mold.

Clamping/Ejection Unit

The part of the injection molding machine responsible for holding the mold tool together under high pressure and ejecting the part when it is complete.


Angles applied to perpendicular features to the parting line that make it easier to remove from the mold.

Ejector Pins

Features on the clamping unit that detach the new part during cycling.

Ejection Surfaces

The non-cosmetic, interior-side surfaces of a part containing circular ejector pin marks

Family Tool

A multi-cavity tool designed to make multiple, different parts per cycle.


A thin layer of plastic material that appears on the part’s edge along the parting line when excess material escapes the mold.


The location where the plastic enters from the sprue and runners into the mold cavity.

Gate Vestige

Faint markings on a part indicating where the material was injected. This may also refer to the excess protruding material from a gate that is trimmed off by a machine operator.

Hand-loaded Core

A type of side action used to produce undercuts in molded parts. These are manually removed from the mold during the part ejection process.


A surface defect that occurs when molten polymer enters into the tool cavity at a high velocity, causing flow marks.

Knit Lines (or Weld Lines)

Lines that occur when plastic material splits and merges, typically between features such as holes.

Lifter (or Slider)

The mechanism that releases undercuts in a mold and drives the horizontal movement of the mold.

Line of Draw

The direction in which the two mold halves will separate from the plastic part allowing it to be ejected without obstruction.

Manually Trimmed Gates

Gates that require an operator to separate parts from runners and sprues after it is ejected from the tool.

Multi-cavity Mold

A tool with two or more cavities designed to make multiple parts per cycle.

Parting Line (Main)

The line where the tool core and cavity meet, which produces a thin line of plastic around the part.

Parting Line (Secondary)

Parting lines created by secondary features like cores, lifters, slides, and side actions.


The injection molding machine.


Thin internal wall-like features on a part that run perpendicular to a wall or plane, used for strengthening and supporting parts as well as for reducing warp.


A channel that carries the molten plastic from the sprue to the gate to then fill the mold cavity.


A frictional force that causes the plastic layers to flow against each other. This can cause shear stress, or tension between plastic molecules, that weakens mechanical properties.

Short Shot

An amount of material that does not completely fill the mold cavity with molten plastic, typically resulting in incomplete part features.

Shut Off

Locations where the mold tool closes off to prevent excess plastic from flowing through.

Side Action

Any type of feature added to mold cavities to produce undercuts in molded parts. Types of side actions include slides and hand-loaded cores.

Single-cavity Mold

A tool designed to make one part per cycle.


A surface depressions on a molded part, often a result of uneven wall thicknesses.


A type of side action added to mold cavities to produce undercuts in molded parts.


Streak marks left on the molded product caused from excessive heat or moisture, typically near gates.


A channel that links the injection molding machine nozzle to the runner channels.


A mold tool with extra metal left in case the mold needs to be revised, requiring extra milling

Surface Finish

The look, feel, or texture of a molded part surface.

Thin-wall Molding

The molding of plastic parts with wall thicknesses .005" to .060" thick.

Tool Life

The estimated number of cycles the tool can complete before it wears down and can no longer reliably produce parts.


The portion of the designed component where a side action is required to create features that are not in the line of draw.


Small gaps where air can exit while the mold cavity is being filled.

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