Everywhere you look, most plastic objects in the world around you are created using injection molding. Plastic injection molding has been a reliable, well-established manufacturing process for over 100 years. The first injection molded part was created back in by John W. Hyatt, and the industry has progressed substantially ever since. The design flexibility and cost-effectiveness it offers allow for many different use-cases across various industries. Before deciding whether this manufacturing process is right for your project, here are some things you should know.
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Plastic injection molding is a manufacturing process that produces parts by injecting molten plastic resin into a mold cavity. The speed and pressure with which the material injects into the mold can affect the desired shape.
Injection molding begins with fabricating a tool, also known as a mold, precision-machining it out of either steel or aluminum. A machine operator places the tool into the injection molding machine. The injection molding machine is fed thermoplastic resin pellets via a hopper. When the injection molding machine is activated, these thermoplastic pellets are pushed forward into the barrel of the machine by a screw and melted down until they liquefy. This liquid resin, known as a shot, is collected at the front end of the barrel in a chamber. Then, the machine injects the thermoplastic resin shot from the chamber into the tool cavity. The machine applies pressure until resin fills the tool. After cooling, the resin resolidifies, forming the part. Finally, the part ejects from the machine and moves on to the next step of production. This process is easily repeatable for production runs ranging from hundreds to millions of parts.
In most production runs the molding process produces parts typically in finished condition with the manufacturing process complete unless the part needs additional finishing steps, such as painting. Plastic injection molding is a huge cost-saving measure as multi-step manufacturing processes can be extremely pricey. The production cycle is relatively short, meaning that the parts have a quick turn-around time for moving on to the next steps. Since the molding process is repeatable thousands of times, plastic injection molding is one of the most cost-effective and efficient manufacturing choices for producing large part volumes.
Out of all the costs for injection molding, tooling makes up the majority. Tooling makes or breaks the plastic injection molding process because proper tooling is key to having a successful production run. Tooling pricing depends on part complexity, production cycle length, and thermoplastic resin material. Costs also depend on the type and amount of resin needed to create each part. The size of the part correlates directly to the size of the injection molding machines, which are rated by tonnage or the amount of force needed to hold the mold closed. Molding cycle time can increase depending on the complexity of the part. These factors all play a factor in the costs associated with the plastic injection molding process.
Injection molding utilizes a variety of thermoplastic resins tailored to the part’s required specifications. There are four categories and two subgroups of thermoplastic resin:
Commodity resins are the cheapest plastics because they are easy to produce at high volumes. Common uses for commodity resins include disposable plastic products such as plastic cutlery.
They cost more than commodity resins and offer better mechanical or thermal properties. Car bumpers are an example of a product made with engineering resins.
These resins offer the strength of metal at the cost of plastic. High-performance resins are commonly found in car interiors.
Ultra resins are the most expensive of the four and are suitable for extreme conditions, like high temperatures. Ultra resins are found in space and military applications.
These thermoplastics are usually clear, impact-resistant, and easily bondable with adhesives. They are, however, prone to stress cracking. Common amorphous thermoplastics are acrylic and ABS.
Semi-crystalline thermoplastics are harder to thermoform due to their sharp melting point but have better structural strength than amorphous. Semi-crystalline plastics are used in bearings and heavy-wear applications.
Both plastics sub-groups have pros and cons depending on the application of the part and its required specifications. Find out which plastic resin is best for your project in our white paper on plastic resin selection.
When designing parts for plastic injection molding there are specific criteria to follow to produce optimal results. Wall thickness needs to account for material choice and part structure. Tapered sides, known as draft, are considered as well. Draft sides make the part easier to eject from the injection molding machine by allowing air into the mold.
Engineers review part radii and weigh the benefits of incorporating rounded corners, potentially making for a smoother molding process. Coring out is a cost-saving method where extra material within a part is cut or hollowed out of the design. The designer places ribs throughout the hollowed area to maintain part strength. Coring out also reduces sink and stresses during the injection molding process. Parts are evaluated for design for manufacturability, or DFM, by injection molders before heading into the injection molding process. In some cases, engineers use a mold flow simulation to analyze and predict risks associated with the injection molding process. Simulations provide data allowing designers to adjust part tolerances quickly and precisely before tooling begins.
There are five different techniques used in plastic molding, each with its own advantage. The injection mold manufacturer will choose which technique best fits based on cost, function, or aesthetic needs.
Mold making, or tooling, requires significant capital investment because of its critical role in creating parts. Tool fabrication typically requires specialty tooling steels. However, aluminum tools can lower production costs, especially in the prototype stage but require specific molding techniques. More recently, toolmakers are using inexpensive 3D printed molds for very early prototypes and low-pressure molding.
Before fabricating molds, the parts need to be finalized completely, with the design frozen because any tooling changes are very costly. Molders and tool builders must work closely to ensure that the tool design meets the part requirements. Every mold must include callouts for parting lines, gates, and ejector marks. Parting lines are the line of separation on the part between the two mold halves. Gates are channels in the mold where the molten thermoplastic enters the mold cavity. Ejector marks are the spots where the injection molding machine’s ejector pins will push the finished part out of the mold.
Once the parts leave the injection molding process, they have the option to get finished with a second manufacturing process. Finishing processes include outfitting them with threading using ultrasonic, thermal, or other specialty equipment that welds plastics together. Parts can receive logos or branding by laser engraving. Paint, silk screening, and pad printing may also be applied. Finishing with paint or other coatings prevents part surfaces from abrasion and increases longevity.
If you plan on utilizing plastic injection molding, keep in mind there are specific design feature requirements. We recommend partnering with a product development company intimately familiar with the plastic injection molding process that will initially incorporate that knowledge into designing your product. If you are unsure how to get started or would like some expert advice on plastic injection molding, contact us and we would be happy to help.
About Synectic Product Development: Synectic Product Development is an ISO certified, full-scale product development company. Vertically integrated within the Mack Group, our capabilities allow us to take your design from concept to production. With over 40 years of experience in design, development, and manufacturing, we strive for ingenuity, cost-effectiveness, and aesthetics in our designs. Learn more about our contract manufacturing services and see how we can help your next project.
To some people, DFM (design for Manufacturability) might seem a little overkill or an unnecessary step in the already lengthy process of developing a new product. However, it is one that we at Sofeast take seriously and is a step that pays off time and time again resulting in saving time, money, and headaches down the line. Here’s why tooling DFM is crucial for successful injection molding.
Let’s look at a scenario where you are thinking about skipping tooling DFM: you think you have a great design, and you move straight into mold tool production, however, during mold tool production, unexpected challenges arise. The design needs modifications, leading to delays, rework costs, and a dent in your budget. Tooling DFM would have helped you avoid this nightmare scenario.
By proactively identifying potential issues early on, Tooling DFM saves you from costly fixes later. It analyzes your design through the lens of manufacturability, highlighting areas that could lead to expensive mold modifications or production delays. Addressing these concerns early in the design phase minimizes rework and keeps your project on track, both financially and temporally.
But DFM doesn’t stop there. It also helps you choose the right material for your needs, minimizing waste and reducing material costs. Additionally, a DFM-optimized design translates to smoother production with less downtime and potential machine adjustments, further saving you operational costs. In essence, DFM helps you get the most out of your investment, ensuring every penny counts towards a successful outcome.
DFM also plays a key role in ensuring your injection molded parts meet the highest quality standards. It considers critical factors like wall thickness, draft angles, and gating, minimizing the risk of shrinkage, warpage, and other defects that can impact part quality and functionality.
In the image below you can see our own internal feedback during DFM on a product about the size of the gate and the requirement to add some material on the inside of the rim which shows the level of detail the team goes into during a DFM review to ensure that we have the absolute minimum gate mark on the surface of the product.
We get asked about plastic injection molding cost on every project we manage and it is not always that easy to answer. Let’s break it down into two parts, first, the cost of the mold tools and second, the cost of the injection molded parts.
When it comes to injection molding, the tooling itself is the most expensive part, but what factors contribute to all the cost?
The complexity, size, and material of the mold heavily influence its cost. Here’s a closer look at each factor:
Where would the mold tooling usually be fabricated?
Most mold tooling is fabricated in China; mostly around Dongguan and Ningbo, where there is a very dense network of toolmakers and prices are quite competitive. Here’s our advice for you when requesting quotes from mold tooling shops in China.
Several factors will determine the plastic injection molding cost for your parts with the material selection having a significant part in this equation. We have already touched on some of the plastics commonly used, however, it is not the cost of the material that should influence the selection, it has to be the required properties like strength, heat or chemical resistance, etc.
Another factor that will impact the cost is volume. High-volume production benefits from economies of scale, bringing the cost per part down especially if multiple parts can be molded with a single shot (multi-cavity tooling). Conversely, low-volume projects might have a higher per-unit cost due to mold setup and processing costs.
We’ve written more about the costs of tooling and purchasing process in these blog posts:
As with the question about costs, the answer to how long it takes to make a mold tool is not a straightforward one as no two projects are the same.
Mold complexity is probably the single most influencing factor to the time it will take to fabricate your mold tooling. Single-cavity molds with basic geometries are quicker to create, often taking 2-4 weeks. But intricate designs with multiple cavities, tight tolerances, or complex features like undercuts can push the lead time to 6-8 weeks or even longer. The more intricate the design, the more steps and time required.
The second part of this complexity equation is the mold size. The more cavities involved, the larger the mold itself, the larger the mold, the more machining is involved which increases the time.
Another factor to take into consideration is the mold maker’s schedule. With a busy workshop, your mold may have to wait its turn which adds to the overall lead time. On top of the mold maker’s lead times, you have the supply chain to consider where material shortages or delays in components like cooling systems can add unexpected waiting periods.
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Key factors to remember when planning the build of your mold tool are:
China is still the fastest place to have mold tooling fabricated. The same mold might get made in 3 weeks in China (not including DFM, mold design, first trial, etc.), and 7 weeks in India, simply because Chinese toolmakers have automated the processes much more than in India where a lot of the finishing is done by hand (which is a more labor-intensive process).
Prototype plastic injection molding is a must for prototyping your design before mass production for many businesses, as it is crucial for identifying flaws, refining functionality, and ensuring a smooth transition into the tooling phase. While 3D printing has emerged as a popular prototyping tool, various injection molding-specific methods offer distinct advantages and cater to diverse needs. Here we will look at the different prototype tooling methods, exploring their materials, lead times, limitations, and what they can produce:
This method utilizes simplified, single-cavity molds often crafted from aluminum or epoxy resin. These molds are quicker and cheaper to create compared to their production counterparts, offering faster lead times and lower upfront costs.
PIM leverages inserts made from materials like aluminum or beryllium copper, integrated into a standard production mold base. This offers faster lead times than full production molds while delivering closer quality to the final product.
The 3D printing rapid prototype approach utilizes 3D-printed patterns to create sand molds for casting metal molds. It offers a relatively fast and affordable way to produce prototype molds, particularly for simpler geometries.
In many cases, going straight from 3D printing to hard steel tooling makes sense when one considers the overall lead time of the project. One common exception is product designs that rely on technical assumptions that can only be validated by getting parts in the right polymer coming out of a mold.
Read more: We have created a guide to rapid tooling prototyping which discusses other options for rapid tooling.
Aluminum seldom makes sense if a mold is fabricated in China (which we cover later). Steel is the most common injection mold material, but choosing the right injection mold steel material is similar to selecting the foundation for a building: it dictates the mold’s durability, performance, and ultimately, the quality of your parts. As engineers, we understand the critical nature of this decision.
Let’s delve into the three most common types of hard steel and pre-hardened steel utilized in injection molding:
1. / / H13: This trio offers a workhorse option. After hardening, they achieve a Rockwell C hardness of 49-53 HRC, making them suitable for ordinary hardening molds. Their versatility allows for applications across various projects.
ESR: This steel takes the performance of the previous group a notch higher. It boasts the same hardness range (49-53 HRC) after hardening but excels in applications demanding both durability and a highly polished finish.
S136 / S136SUP / : Don’t be fooled by the “steel steel” reference – these are actually high-performance stainless steels. Their strength lies in corrosion resistance, making them ideal for molds processing materials like POM and PVC, which can be corrosive to standard steels. Additionally, they hold their own when it comes to achieving a polished finish.
Lifespan: Mold tooling made from hard steels like 1.#, #, and # will usually last for around 300k-500K shots, but can reach 1 Million if the mold structure is simple.
S50C / S55C: These steels offer a cost-effective option for mold bases, providing adequate strength and machinability. However, their lower hardness limits their suitability for high-wear applications.
718 / 718H: Renowned for their toughness and ability to achieve a good surface finish with standard polishing techniques, 718 and 718H are popular choices for mold cavities and inserts. Their well-rounded properties make them a versatile option for various applications.
738 / 738H: Offering superior rigidity compared to 718 grades, 738 and 738H excel in core and insert applications. While their polishing capabilities are considered “ordinary,” their rigidity often outweighs this limitation for specific applications.
A Note on P20: It’s important to clarify the perception of P20 steel in China. While technically encompassing a series that might include materials like 718 or 738, the term “P20” in China often refers to a lower-grade steel with potentially less desirable properties compared to the 718/738 series.
NAK80 / XPM: The champion of pre-hardened steels, NAK80 boasts a hardness of 37-43 HRC. This, coupled with its excellent polishing capabilities, makes it the go-to choice for molds requiring high-precision parts from materials like PC, PA+GF, and PC+GF.
Remember, this is just a starting point. Selecting the optimal steel grade requires careful consideration of factors like part complexity, plastic-type, production volume, and budget. Consulting with experienced mold makers and material suppliers is crucial to ensure you make the best choice for your specific project.
Lifespan: For pre-hardened materials the mold life is usually 100K-300K shots.
In particular:
Watch these videos on how to test steel’s properties which may help you to select the types you require.
When we talk about tonnage or the term ‘tonne’ in the injection molding process, it means the injection molding press machine capacity regarding the clamping force it can exert. It has nothing to do with the weight of the machine.
So, if you encounter a statement like “this mold requires a 120 Tonne machine,” it essentially means that the mold needs a press capable of exerting a minimum clamping force of 120 tonnes (metric tons, specifically) to hold the mold halves tightly shut during the injection molding process. This force ensures:
It’s important to understand that injection molding machine capacity isn’t the only factor to consider when choosing a plastic injection molding press. Here are some additional key parameters:
You may also like to read: How To Check The First Production From A New Plastic Injection Mold?
The allure of speed and affordability often leads to the question of soft tooling, particularly aluminum molds. While this approach might seem appealing at first glance, let’s delve deeper to understand why, in many cases, hard steel molds remain the preferred choice for the average plastic injection molding process:
In China, the cost differential between hard steel and aluminum molds is often surprisingly minimal. This eliminates the initial cost advantage that aluminum might seem to offer elsewhere.
Modern injection molding facilities leverage automation extensively. While machining hard steel does take slightly longer due to its inherent hardness, the difference is typically measured in days, not weeks. This automation advantage significantly diminishes the time-saving argument for soft tooling.
Aluminum molds, by their nature, are less durable than their hard steel counterparts. This translates to shorter lifespans, more frequent replacements, and ultimately, higher overall costs, especially for high-volume production runs. Hard steel molds provide significantly more cycles, maximizing production efficiency and minimizing downtime for mold changes.
Soft tools can struggle to maintain the same level of dimensional accuracy and surface finish as hard steel molds over extended use. This can lead to part quality issues, increased scrap rates, and the need for additional finishing steps. Hard steel molds ensure consistent part quality throughout their lifespan, minimizing rework and maximizing yield.
We’ve been approached about soft tooling in the past, and after careful analysis, we’ve consistently concluded that hard steel molds offer the best overall value. Their durability, efficiency, and ability to deliver superior part quality make them the smarter long-term investment, even when considering the slightly longer machining times.
In China, the combination of readily available steel, efficient machining practices, and economies of scale often shrink the cost gap between hard steel and aluminum molds. When you factor in the superior durability, longer lifespan, and higher production efficiency of hard steel molds, they often become the more cost-effective choice in the long run, even for initial production runs.
The world around us is shaped by injection molding, a technology that transforms concepts into tangible realities. This guide has hopefully answered some of your key plastic injection molding questions and equipped you with the knowledge to use this intricate process with confidence.
We’ve covered critical factors like material selection, steel options, runner systems, and gate design, allowing you to optimize production efficiency and achieve superior part quality. We’ve also explored important considerations often overlooked, such as quality control and environmental impact.
But the journey doesn’t end here. Our expertise extends far beyond this guide.
First, delve into how Sofeast can help organize and look after your mold tooling in China: Tooling management for plastic injection molds in China.
Second, get help from our China-based injection molding subsidiary to design and fabricate your tooling and produce your injection molded parts in China.
Third, contact us today! Our team of injection molding specialists in China is here to answer your questions, discuss your project requirements, and help you leverage the immense potential of injection molding to achieve manufacturing success. Together, we can turn your vision into reality.
If you want to learn more, please visit our website Custom Mold Manufacturing.