What is Injection Molding?
Injection molding is process tailored for producing large volumes of parts. Compared to other manufacturing technologies—like CNC machining and 3D printing—it requires an upfront, capital investment into tooling. But individual piece-part price will be substantially less when compared to other means of manufacturing plastic parts. This cost structure makes it an affordable solution for production runs.
It’s most often used for the manufacturing plastic parts at scale due to its low material waste and low cost per part. It’s an ideal manufacturing process for industries like medical devices, consumer products, and automotive to name a few.
How Does Injection Molding Work?
Tooling fabrication: Once an injection molding design is finalized the first step in the manufacturing process is to mill the tooling, which is typically fabricated from steel or aluminum. In most cases, the metal block of material is placed in a CNC mill, which then carves out a negative of the final plastic part. Additional treatments like polishing or laser etching can then be applied to the tooling to achieve specific surface finishes.
Part production: The actual production of plastic parts begins by loading resin pellets into a barrel. The temperature of the barrel is raised until the resin pellets reach a molten state and are then compressed. Next, the molten plastic is injected into the metal tool through a runner system, which then feed into the mold cavity through gates. The part then cools down, solidifies, and is ejected from the tool with ejector pins.
Types of Injection Molding
The term injection molding encompasses a handful of processes that inject liquid resin into a tool to form plastic parts. Here are four common types:
Thermoplastic injection molding: Thermoplastic injection molding is the most common type of molding. It injects thermoplastic resin into the mold where the material cools to form the final part.
Liquid silicone rubber molding: Liquid silicone rubber uses thermoset materials and a chemical reaction creates the plastic part.
Overmolding: Overmolding is a process used to manufacturing plastic parts with two or more materials. You’ll often find this on parts to improve grip by adding rubber to the handle.
Insert Molding: Insert molding is process that begins with an insert component placed into the mold before resin enters. The material is then injected and flows around the insert, typically metal, to form the final part. This is frequently used for parts that require metal threads.
Basic Design Principles for Injection Molding
Tolerances
With our injection molding process, we can hold about ±0.003 in. machining accuracy. Shrink tolerance depends mainly on part design and resin choice. It varies from 0.002 in./in. for stable resins like ABS and polycarbonate to 0.025 in./in. for unstable resins like TPE.
Wall Thickness
Wall thickness is important because it can lead to defects such as sink and warp. It is best practice to maintain a uniform thickness throughout an injection-molded part. We recommend walls to be no less than 40 to 60 percent of adjacent wall thickness, and all should fit within recommended thickness ranges for the selected resin.
Core Geometry
Core out parts to eliminate thick walls. You get the same functionality in a good molded part. Unnecessary thickness can alter part dimensions, reduce strength, and necessitate post-process machining.
Draft
Applying draft to molded parts is critical to ensure parts do not warp during the cool down process and it helps the part easily eject from the mold. Applying 1 to 2 degrees works well in most scenarios. If there are vertical faces, we advise incorporating .5 degrees of draft.
Side Actions
A portion of the mold that is pushed into place as the mold closes, using a cam-actuated slide. Typically, side-actions are used to resolve an undercut, or sometimes to allow an undrafted outside wall. As the mold opens, the side action pulls away from the part, allowing the part to be ejected. Also called a “cam.”
Undercuts
A portion of the part that shadows another portion of the part, creating an interlock between the part and one or both of the mold halves. An example is a hole perpendicular to the mold opening direction bored into the side of a part. An undercut prevents the part from being ejected, or the mold from opening, or both.
Bosses
A raised stud feature that is used to engage fasteners or support features of other parts passing through them. There can be a tendency to design thick bosses which will increase the likelihood of sink and voids in a part. Consider reinforcing bosses with ribs or gussets for extra strength
Gates
A gate is an opening in the injection mold tool that allows resin to enter and fill the cavity. There are three common types of injection molding gates.
Tab gates are the most common type of gate since it works well with additives and is the most cost effective option.
Hot Tip gate is best for parts that cosmetic appearance is a priority. These gates can also reduce wear on tooling and flash.
Pin, Post, or Tunnel gates are ideal for cosmetic parts that don’t require a vestige. Sometimes not an option depending on material and geometry.
Ribs
A rib is a thin, wall-like feature parallel to the mold opening direction, its used to add strength and support to features like bosses and walls. To prevent sink, ribs should be no more than 60% of the wall’s thickness.
Ejector Pins
Ejector pins are installed in the B-side of the mold and help to release the plastic part from the tool after the part has cooled sufficiently. Designing in sufficient draft can help reduce the need for ejector pins on a part.
Logos and Text
Sans serif fonts will be the easiest to mill into a mold with text. We recommend font larger than 20 pt. and no deeper than 0.010 in to 0.015 in.
Injection Molding Resins
When choosing a material for your part, relevant properties might include mechanical, physical, chemical resistance, heat, electrical, flammability or UV resistance. Resin manufacturers, compounders and independent resin search engines have data online. Here is a quick look at some common commodity and engineering resins.
Common Engineering Resins
ABS: ABS is a great choice for a vast majority of parts. It’s reasonably priced, strong, relatively tough, has a decent appearance and is forgiving even if you don’t follow all the standard design rules for plastic parts.
Acetal: A strong material with good lubricity
LCP: A very strong material that flows well, especially for thin parts. Produces weak knit lines.
Nylon: Affordable cost, strong, and wear resistant. Can be susceptible to shrink and warp, particularly glass-filled nylon.
Polycarbonate (PC): Can tolerate higher temperatures and more durable than a typical ABS but less forgiving when it comes to moldability.
PMMA (Acrylic): An affordable option for clear parts although it can be brittle.
resin pellets
Thermoplastic material comes in pellet form which is heated to a molten state in order to be injected into the tool.
Commodity Resins
Polypropylene (PP): PP is a cheap material and good when cosmetics and rigidity aren’t a priority. Chemical resistant and good for living hinge designs.
Polyethylene: High and low density options. Is durable and chemical resistant.
Polystyrene: A hard thermoplastic that is cheap and clear.
Check out our guide to injection molding materials if you want to dig deeper into plastic selection.
Colorants and Resin Additives for Injection Molding
Stock colors from the resin vendor are typically black and natural. Natural might be white, beige, amber or another color. Semi-custom colors are created when colorant pellets are added to natural resins. For available colors, visit our materials page. There is no added charge for our inventory colors. They may not be an exact match and may create streaks or swirls in parts.
Resin Additives
Short glass fibers are used to strengthen a composite and reduce creep, especially at higher temperatures. They make the resin stronger, stiffer, and more brittle. They can cause warp due to the difference in cooling shrink between the resin and the fibers.
Carbon fiber is used to strengthen and/or stiffen a composite and also to aid in static dissipation. It has the same limitations as glass fibers. Carbon fiber can make plastic very stiff.
Minerals such as talc and clay are often used as fillers to reduce the cost or increase the hardness of finished parts. Since they do not shrink as much as resins do when cooled, they can reduce warping.
PTFE (Teflon) and molybdenum disulfide are used to make parts self-lubricating in bearing applications.
Long glass fibers are used like short glass fibers to strengthen and reduce creep, but make the resin much stronger and stiffer. The downside is that they can be particularly challenging to mold parts with thin walls and/or long resin flows.
Aramid (Kevlar) fibers are like less-abrasive glass fibers only not as strong.
Glass beads and mica flakes are used to stiffen a composite and reduce warping and shrinkage. With high loading, they can be challenging to inject.
Stainless steel fibers are used to control EMI (electromagnetic interference) and RFI (radio frequency interference) typically in housings for electronic components. They are more conductive than carbon fiber.
UV inhibitor for outdoor applications.
Static treatments make resins dissipate static.
Surface Finishes for Molded Parts
Surface finish is another important consideration for injection molding since parts are typically cosmetic and intended for end-use production. Keep in mind that textured surface will require more draft than a more polished finish. This is to ensure the part releases from the mold and so that the bead blast can be properly applied to the tool.
Post-Processing and Advanced Molding Techniques
Ultrasonic welding: uses a high frequency vibration to generate heat and drive an insert into a part. In more complex molding applications it can also be used to fuse parts together.
Part Marking: There two ways to add images, oftentimes logos, to parts: pad printing and laser engraving.
Pad Printing allows for the addition of color graphics to molded parts at scale. This can be used for adding logos, graphics, and instructions on parts. We offer pad printing for ABS, PC, and ABS/PC resins.
Laser Engraving uses a laser to burn a 2D image onto a part or mold cavity. While color is not an option for laser engraving it is a more cost-effective, faster way to mark plastic parts when compared to pad printing.
Pickouts: Pickouts are inserts that are machined separately from the tooling and placed in the mold before material is injected. They are used to achieve undercuts on interior surfaces. Pickouts are ejected along with the plastic part and placed back in the mold. Using a pickout overcomes many shape and positioning restrictions, but is more costly than sliding shutoffs, or using a side-action.
Steel Core Pins: A steel pin is strong enough to handle the stress of ejection and its surface is smooth enough to release cleanly from the part without draft. There shouldn’t be any cosmetic effect on the resulting part; if there is, it will be inside the hole where it won’t be seen.
Injection molds can be categorized in several ways, each affecting various aspects of the production process. Here’s a detailed look at these categories and their significance:
Plate Type
The plate type influences the mold’s cost, the finish of the final product, and overall production efficiency.
Two-Plate Molds: Consist of two main parts—the cavity and the core. They are simpler and less expensive but may have limitations in complex part designs.
Three-Plate Molds: Include an additional plate for separating parts from the runners, improving part ejection and quality. They are more versatile but also more costly.
Stacked Plate Molds: Utilize multiple levels of cavities, doubling the output without increasing the footprint. They are ideal for high-volume production but require precise alignment and complex design.
Runner Type
The runner type impacts mold design flexibility, material waste, and the amount of post-processing required.
Cold-Runner Molds: Use unheated channels to guide the molten plastic into the mold cavities. They are less expensive but require more post-processing to remove the runners from the final product.
Hot-Runner Molds: Incorporate heated channels, keeping the plastic molten until it reaches the cavities. They reduce waste and post-processing needs but are more complex and expensive.
Mold Cavity Design
The mold cavity design plays a crucial role in production efficiency and part quality, directly influencing the speed, cost, and consistency of the manufacturing process.
Single-Cavity Molds: Single-cavity molds produce one plastic part per injection cycle. This gives single-cavity models the slowest production times of the three types of molds we are discussing. However, due to their size and relative simplicity, they are quite affordable to produce, making them a good option for small production volumes in which production speed isn’t the biggest factor.
Multi-Cavity Molds: Multi-cavity molds contain multiple cavities, as the name suggests. These molds produce multiple copies of the same plastic product in each production cycle. They are ideal for mass production purposes, with high production speeds and a low price per part at large production volumes. Multi-cavity molds are complex and expensive to design and produce, however this can be offset when working with large production volumes.
Family Molds: Family molds are injection molds that produce multiple different parts per production cycle. They are useful for manufacturing multi-part products quickly but are very complex to design and more prone to molding defects.
Injection molding benefits：
OEMs across many industries can attest to the benefits of plastic injection molding. It’s ideal for consistent, affordable production of a wide range of high-quality complex plastic parts that can withstand about any environment.
That’s reason enough to rank injection molding high on the list of go-to solutions, but there’s more. To better understand how and why manufacturers use the process, let’s take a look at the individual merits of the top 14 benefits of plastic injection molding (listed in no particular order):
1. ABILITY TO PRODUCE DETAIL/COMPLEX GEOMETRY
With the right tool design and a scientific molding approach to process optimization, injection molding can help manufacturers produce highly complex, detailed plastic parts in large volume with virtually no deviation.
2. EFFICIENCY
An experienced custom injection molding partner provides manufacturers with a decided advantage in terms of efficiency. The molder’s teams — from engineering through production — likely have decades of expertise to draw upon when determining how to optimize part design and manufacturing. Implementing best practices like focusing on design upfront to minimize problems later on, and incorporating value-added services to combine production process steps generally streamline time and cost commitments.
3. STRENGTH
The strength and durability of plastics has greatly increased over the years, and today’s lightweight thermoplastics can withstand even the harshest environments on par with — or better than — metal parts. There are more than 25,000 engineered materials to choose from for constructing complex injection-molded applications. High-performance blends and hybrids can also be formulated to meet very specific part requirements and characteristics, such as high tensile strength.
4. ABILITY TO SIMULTANEOUSLY USE MULTIPLE TYPES OF PLASTIC
It’s not uncommon for complex part designs to require components made of different materials. While seemingly a matter of simple choices, safely combining plastics can be extremely complicated. By ensuring compatibility under all circumstances, plastics expertise from the molder’s project engineers is instrumental in mitigating defects, injury risks, and warranty claims.
5. COST SAVINGS
There are several ways that injection molding can help OEMs experience lower costs, from plastic part consolidation to overmolding. However, the number one way to manage costs is collaboration between OEM and injection molding engineering teams well before production is set. Focusing on Design for Manufacturability (DfM) and other detailed processes during the design phase significantly reduces the number of problems sometimes encountered with moldability — minimizing the need for expensive tooling changes, downtime, and other production delays.
6. PRECISION
For OEMs with complex part designs requiring tight tolerances, injection molders can achieve designs accurate to within +/- .001 inches. These measurements are not only possible, they’re repeatable across production runs and equipment.
7. SHORTEN PRODUCT DEVELOPMENT TIMELINE
Different skill sets of injection molding engineers can help OEMs achieve a shorter product development timeline. Doing so ensures faster production cycles and getting defect-free parts to market faster — a decided competitive advantage for manufacturers.
8. MULTIPLE FINISHES
Most injection-molded parts are produced with a smooth surface finish very close to the desired final look. However, a smooth appearance isn’t appropriate for every application. Depending on the physical and chemical properties of the plastics used, injection molding allows for surface finishes that don’t require secondary operations — from matte finishes and unique textures to engraving and more.
9. HIGH-OUTPUT PRODUCTION AND CONSISTENCY
High-output production of complex plastic parts requires a consistent, repeatable process to achieve designs with tight tolerances. Injection molding helps ensure a consistent quality by repeatedly using the same mold for each part, backed by an injection molder’s continuous improvement practices that incorporate current leading-edge technologies.
10. COLOR CONTROL
From clear to any color an OEM needs, injection molders can make it happen by aligning plastics, additives, and biocompatibility to achieve desired coloration. Often, multiple colors are needed in one product. In that case, overmolding is the solution provided the injection molder has the requisite experience in multi-material injection molding.
11. FLEXIBILITY
Injection molding is all about flexibility, whether ascribed to some plastics’ properties or the ability for OEMs to make custom color choices or meet specific project requirements. Injection molding gives OEMs freedom in design choices, — especially when compared to metal.
12. REDUCED WASTE
At Kaysun, we’re proud to be a part of several green initiatives that positively impact the environment, but our commitment to the planet doesn’t end at corporate social responsibility. We understand the ecological and economic advantages of sustainable practices like using plastic regrind, which minimizes waste and directly benefits OEMs.
13. LOWER LABOR COSTS
Much of the injection molding process is automated by machines and robots, and controlled by a sole operator or technician. This streamlines labor costs and also greatly reduces the risk of rework caused by part defects or human error — both of which save OEMs money.
14. LIGHTWEIGHTING
Although probably most prevalent in the automotive industry, lightweighting is used by OEMs in a number of industries. Simply put, using plastic parts helps reduce the weight of metal parts. Today, high-strength, lightweight thermoplastics can be used to replace metal components with virtually no difference in strength or dependability.
I want to share 6 types of injection molding technology：
1. Thin Wall Molding
Thin wall molding is an injection molding technology that achieves a plastic part with a very thin wall. It is often used in test apparatuses, electronics, vessels, tubes, and other enclosures. Plastic injection molders who do thin wall molding have to consider every aspect of the part design, mold design, and processing in detail to ensure that the thin wall geometry will hold up without quality issues. Here at Micron, we use a sophisticated vision system to examine each completed part to ensure that no cracks have appeared.
2. Gas-assisted Injection Molding
The trouble with any thick plastic injection-molded part is that it runs the risk of distortion as it cools. Gas-assisted injection molding helps solve this issue by shooting gas into a plastic material-filled injection mold. This allows the plastic on the outside of the mold to maintain a smooth and finished appearance while the inside stays porous or hollow. This not only keeps the part from deforming during the cooling stage, but also lessens the cost of the part (as you’re using less material).
3. Metal Injection Molding
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.