Originally published on fastradius.com on July 6, 2022
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Injection molding involves injecting molten plastic into a mold, cooling it, and ejecting it. Manufacturers can repeat the process to quickly and cost-efficiently to create thousands of identical parts.
Several subprocesses fall under the general umbrella of injection molding, including insert molding and overmolding (a.k.a. multi-shot injection molding, two-shot injection molding, or double-injection molding). Keep reading to learn the differences and similarities between insert molding and overmolding, their applications, and how to figure out which type of multi-material molding is best suited for your project.
The insert molding manufacturing process involves injecting molten plastic around pre-placed inserts (usually metal), forming a strong bond between the two materials and helping cut back on assembly operations and time. Common inserts include pins, blades, threaded nuts and knobs, sleeves, bushings, and the metal shanks of tools, such as screwdrivers.
Overmolding enables manufacturers to create parts from multiple materials using a manual two-stage process for small production runs or an automated two-stage process for large production runs. Regardless of production volume, the overmolding process works in the following way. First, a thermoplastic injection forms the rigid substrate. After the substrate forms, another shot (generally thinner and more pliable) is injected in, on, or around the substrate. As the materials cool, they bond together, creating a unified, strong, and durable part. Bonds can be chemical or mechanical, depending on the materials as well as the design of the part or product.
Overmolding and insert molding enable manufacturers to create multi-material products without using adhesives, help eliminate secondary assembly steps, and improve the final product. However, it’s important to note the differences between insert molding and overmolding, including:
Overmolding involves injecting two shots of materials to form the substrate and the overmold. Insert molding only involves injecting one shot of molten plastic, though the metal insert needs to be purchased or created separately.
Insert molding involves one shot while overmolding involves multiple, which means the insert molding cycle time is generally faster than the overmolding cycle time. However, that doesn’t necessarily mean that insert molding is always the fastest option for production. In some cases, manufacturers may be unable to find pre-made metal inserts that meet their requirements, meaning they’ll need to create a custom metal insert, which can lengthen production timelines.
Insert molding and overmolding can reduce assembly costs and accelerate production, helping companies simultaneously save on production costs and generate greater profits when producing large quantities of parts. However, overmolding is more expensive than insert molding, as it involves two steps. This is particularly true when it comes to prototype or small production runs, as overmolding requires manufacturing two tools — one for the substrate and one for the overmold.
Overmolding is often used to make toothbrushes, medical instruments, disposable razors, and phone cases, or to enclose electronic circuit boards (e.g. USB flash drives).
On the other hand, insert molding is a popular option for connectors, dash panels, electric sockets and wires, dials, remote control coverings, handles, scissor grips, and surgical implements.
Insert molding and overmolding are both types of multi-material injection molding, but they each have their own benefits and drawbacks and can’t be used interchangeably. To ensure you use the best process for your part, you’ll need to be familiar with each method. If you need some help selecting the best technique to use, contact us to get expert advice on your next injection molding project.
In addition to a team of experts that will help you make your design reality, when you work with SyBridge, you also get access to a suite of online tools that makes designing and ordering parts easy. You can upload your part file, get DFM analysis, and start exploring production and material options, all without initiating a quote — and when the time comes, getting a rapid quote is simple. Contact us today to get the design and manufacturing insight you need to bring your next injection molding project to life.
In this article, we discuss insert molding and overmolding, the advantages of each, and the applications they’re best suited for. We hope these manufacturing insights, along with useful design tips, will help you get great results on your next plastic injection molding project.
Navigate to these sections in the article:
Overmolding is a special form of plastic injection molding. Unlike conventional molding, where a finished product is made in one cycle, overmolding requires at least one additional injection cycle using a dissimilar material. This method is also known as 2K overmolding, two-shot molding, or even multiple material molding.
Overmolding starts with a base substrate, on top of which a relatively softer TPE or TPU material is bonded. The base can be metal or plastic, although, in this article, we will focus on plastic applications primarily.
If the substrate is a plastic base, then it’s customary to mold it in the same production cycle. This is done because it’s more cost-effective to make the entire finished piece at one time, rather than paying to transport and store semi-finished goods. But the more important reason is that the elastomer will adhere much better to a still-warm, semi-cured, and “sticky” substrate.
Overmolding uses a special type of plastic injection molding tool, one that has a cavity for the substrate portion and another separate cavity representing the overmolded section.
The first conventional injection cycle forms the substrate, using a standard thermoforming resin. This resin fills the gate, runner, and cavity system of only one part of the mold, while the section representing the overmold is blocked.
Once the base cools and is semi-hardened, the mold is then rotated to a new position and connected to a separate nozzle, gate, and runner system. The remaining void of the mold tool cavity is filled with TPE or TPU elastomer, creating the overmold that bonds to the substrate.
The process is not limited to just two materials, and can theoretically be expanded to include an indefinite number of additional overmolds. However, in practice, more than two becomes increasingly complex and expensive.
Insert molding involves placing a pre-manufactured component, such as a threaded fitting or an electrical contact, onto a mounting pin or other fixture in a plastic injection molding tool. During the molding cycle, thermoforming resin surrounds this insert and solidifies, thereby permanently sealing it into place.
In this video, Gordon Styles, the founder and President of Star Rapid, goes into greater detail explaining how these processes work, and he gives examples of each. Be sure to check it out for more useful information.
Inserts can be either male or female. Female inserts are used to make circular fittings inside the surrounding plastic. When these types of inserts have internal threads, they’re called nutserts. There are many advantages to using both male and female inserts to enhance a product’s appearance and utility.
Read the 6 advantages of insert molding:
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Featured content:The nutserts themselves add the expense to the build, but that cost is offset by the fact that they help to avoid secondary drilling and tapping, which also takes time and money.
There is no need to invest in equipment upgrades.
This is a great way to protect electrical connections, for example.
This helps to create a complete finished part in one quick and economical operation.
For example, consider a common toggle switch or lever. By using metal inside the body of the switch only where it’s needed to make a mechanical contact, and substituting the remainder of the switch for plastic, the weight of the entire assembly is thereby reduced. This approach, not incidentally, also decreases the reliance on a more expensive, full metal switch.
Now, if that design logic is applied to a fleet of aircraft or millions of automobiles on an assembly line, then the reduction in weight and cost becomes substantial.
While there are many advantages to insert molding, there are some potential drawbacks that designers should be aware of.
are higher when inserted by hand.
will vary when handloading.
Why is this important? For plastic injection molding, the single most important variable that must be carefully controlled for maximum cost savings is machine cycling time or the time it takes to make each part. If this time is variable, it becomes impossible to control costs or accurately predict product ship dates.
For relatively smaller volumes, say below 20K per year, it still makes economic sense to use handloading rather than invest in expensive automation. But as volumes increase it eventually becomes more cost-effective to use automation, which will pay for itself by effectively decreasing the cost of labor.There are many reasons why product designers choose to add overmolding to plastic parts. Here are the main advantages.
But this is done not just for comfort. Some elastomers also absorb unwanted vibration while protecting users from excessive heat or electrical shock.
To get the best results from this process, product developers should be aware of the following limitations and potential disadvantages.
In both cases, this increases the development costs and it takes more time to build these tools compared to conventional single-shot molding.
Which process is right for you depends entirely on your application. Keep in mind that they aren’t mutually exclusive molding methods because they serve different purposes. In fact, it’s not uncommon to find both used on a single product. Think, for example, of a power tool that has overmolded grips on the handle, but that also uses nutserts to bolt the case together.
You should consider using insert molding for a few main reasons. The first is that your product application needs strong mechanical fittings to hold two or more parts together, typically for an enclosure or housing. Threaded nutserts are great for this but there are many types of snap connectors as well.
Insert molding is used to put a rubber or plastic handle on a metal part, such as you might find on hand tools or kitchen knives. And insert molding is ideal for sealing wires and electrical connectors into permanent plastic housings that block out dust and moisture.
Inserts should be used whenever a plastic case needs to be occasionally opened for service and then locked tight again, such as when performing routine maintenance or changing batteries. They cost a little more but that cost is offset by making a much more durable end product.
Product developers should consider overmolding in order to improve the grip and texture on parts that need to be held in the hand or to protect the end user against vibration, heat, or electricity. Overmolding is also a great way to permanently bond rubber to metal, as is found on wheels and casters.
Overmolding also offers more opportunities to be creative by applying colorful designs to improve the appearance of the part as well as its performance.
And overmolding adds cushioning and shock absorption to many common household items that might protect users from accidental injury.
In this section, we highlight important points between designing for overmolding and insert molding.
Remember that overmolding is rarely used to cover the entire base substrate. Instead, apply overmolding in sections. For each of these, keep in mind these useful design tips:
There are some unique engineering challenges that designers might face when using insert molding. The advantages of added strength and versatility must be weighed against the need for a more careful design for manufacturing review. Here are some useful guidelines to consider:
Neither overmolding nor insert molding is restricted in their use to any particular industry or product type. Because both methods are so useful and adaptable, new practical applications are being found all the time that take advantage of their benefits. Here are a few examples:
This is just a partial overview of insert molding and overmolding. Are they the right solutions for you? Not sure how to apply them to your next project? Just contact our technical experts and we can offer helpful advice about how to get the best results from both processes.
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