What is overmolding (over mold)?

Overmolding or over mold is a molding process that is molding one plastic over another matreial to be merged to form a single end-molded product. Plastic overmolding needs to make two separate molds: one is the substrate (the first mold), and the second mold is called the overmold, usually a TPE material is used for the second mold, but not always. If the substrate is metal or brass, this type of overmolding is normally called insert molding, and insert molding only needs one mold (over mold) to finish the process.

The plastic overmolded materials most commonly used are thermoplastics (TPE) and rubber, or the same material but different colors. During overmolding, the activity that happens is either mechanical or chemical, which leaves the two substances bonded.

The overmolding process involves producing the molded part (substrate) from the first mold and moving it to another related over mold. The substance can be used to combine the surface molded with the molding substrate. Otherwise, the molded material can be put back on the surface from which it was extracted, and the overmolded substance can be added. Types of over molding include two-shot sequential over molding, insert over molding, and multi-shot injection molding.

The types of overmolding

Types of overmolding include two shot sequential overmolding, insert overmolding or multi-shot injection molding：

Two shot sequential overmolding

In two-shot sequential plastic injection overmolding, the molding machine injects the first plastic resin into the first mold cavity (both molds in one molding machine); after the material cools and forms the first plastic shape, and then opens the mold, all of those molding processes are the same as the traditional injection molding process.

Once the mold is open, immediately move the first molded part to the second overmold (this can be done by hand or by a robot), then close the mold and inject the second material, simultaneously injecting the first shot. The first component serves as an insert for the second shot, which uses a different material or color. This process requires a special injection machine with two injection units.

Inert overmolding

Insert over-molding uses pre-molded inserts or metal inserts placed into the mold before injecting the second material; if the inserts are metal or brass, then we call it metal insert molding. This over-molding process, which we used a lot, for example, metal cwrew insert molding, filter insert molding, this type of over-molding uses a traditional injection molding machine to proceed, which puts the metal inserts into the mold cavity during the single injection molding cycle.

Like the below picture is showing the insert overmolding with metal inert. This type of overmolding only requires one injection mold; however, if the first insert is made of a plastic part, we will need an additional mold for the first plastic insert part.

TPE overmolding

TPE (thermoplastic elastomer) plastic matrials are used a lots in the injection molding field, specially for the overmolded parts, on the overmolding market, over 80% of overmolded parts made by TPE over molding,

TPE Overmolding is the injection molding process where TPE (thermoplastic elastomer) is molded onto a rigid material (for example PC, PA66, ABS material) according to the specific requirement, the overmolded TPE will strong bond with the first plastic and maintain in the final using purpose. To prevent the TPE material strip off from the second material, material selection and part design is very important.

A TPE overmolding manufacturer will consider all relevant factors when selecting the optimal molding manufacturing method for a plastic injection molding part, choosing between 2K molding and the overmolding process. Critical factors include production capacity, material selection, available equipment, and labor costs.

Normally, the overmolding process is the most popular choice when total production volumes are less than 50K. This number is only a reference and not a definitive one because it depends on the size and complexity of the part design. For higher volume production requirements (total volume over 200,000 units), a 2-shot injection molding process will be a better option; of course, this is still dependent on the part design because some parts can only be molded with an overmolding process, foe example like below part can only be mold with overmolding process.

In every TPE overmolding or 2K injection molding process, the number one issue is to achieve maximum adhesion between the TPE and the substrate. Some TPE overmolding may have significantly different bond strengths between multi-shot and overmolding. Even if an excellent bond is produced with two-shot molding, the same material may have a low bond strength when using overmolding. Thus, to make high-quality finished overmolding and 2K molding products, a thorough understanding of TPEs, part design, engineering plastics, and the specifics of the molding process is important.

TPE Overmolding materials seletion tips

As we know to make high quality TPE overmolded product, both TPE and substrate materials are most important, the number one factor to define the quality of overmolded part is how good the mergeration between two matreials, if the TPE is easy to strip off from the substrate then the material will be issues, below there are some tips for meterials selestion, following thsi tips you will find the best suitable material for overmolded part.

Thickness of TPE overmolded part

Designers frequently request the softest TPE. They don’t realize that a TPE’s soft durometer doesn’t provide much to “cushion” below a particular thickness (usually less than 0.1mm). Thinner TPE overmolded part feel harder—the hardness impact depends on thickness. Multiple closely spaced ribs can produce the illusion of thickness without utilizing much material. Many kitchen utensil handle use this method.

Hardness of TPE plastic material,

There is a softness of TPE material that you need to select when you make TPE overmolding, especially TPE thickless, which is more than 0.5 mm. To have a good touch feel, you may need to test different types of TPE shore A material, unless special function requirements, normally we use in the market will range from TPE Shore A 40 to 60; if too little, that may eash to strip off from the substrate; if hardness is too high, the touch feel may not be good enough.

Substrate material selection tips

Compared to TPE material, substrate materials will be easier to choose; most materials can be substrates, including nylon/PA (PA66 or PA66 GF30, PA6 or PA6 GF30 plastic), Polycarbonate (PC), Acrylonitrile Butadiene Styrene (ABS, PC/ABS, Acetal (POM), PMMA, and so on. The final substrate material selection is dependent on the final purpose. If you are uncertain about the optimal material for your TPE overmolded parts, please reach out to us and we will provide you with some recommendations.

Surface finish on substrate and TPE overmold

Surface finish on the substrate will also affect the adhesion of TPE rubber. The stronger the adhesion, the less likely it will strip off, normally will be good polishing between merge surface between TPE and substrate, on the TPE cavity side, soemtime surface finish will effect to the TPE overmold as well, sometime higi polishing to the TPE cavity mold, the TPE overmolding part will stick to cavity side, add some small VDI texture will improve this.

TPE Overmolding Part Design tips

Part design, as previously mentioned, plays a crucial role in creating a high-quality TPE overmolding product. Generally, the design of the substrate part is similar to that of other plastic injection molding parts. For further details, please refer to the page on plastic part design for injection molding. But there are some factors on the merge area between the substrate and TPE overmoled area; there is no standard design for this era since different parts have different geometries, but there are some key points that you need to think about when you do the overmoled part design. Those factors will be:

How to seal the TPE overmolding well and provent from flash When you do the overmoling part design: 

TPE material easily exhibits flash (0.03 mm gap), and bondable TPE materials meet more stringent criteria than standard TPE polymers. The same holds true when designing parts. Unlike traditional part design, two-component part designs must account for shrinkage from two different thermoplastic materials. Both substrate and over-molding have their own gate and runner systems, which must be tailored to the individual material properties used.

To meet the best cycle time, substrate and overmolding wall thickness should be consistent. In most over-molding applications, 1-3 mm wall thickness ensures satisfactory bonding. Thicker pieces should be cored out to reduce shrinkage, weight, and cycle time. To avoid backfills and gas traps, wall thickness transitions should be gradual. Add radius to the sharp coners to reduce stress issues. Avoid deep, unavailable blind pockets or ribs. Long draws should have a 3-5 degree draft angle for easy demoulding. In overmolded compounds, deep undercuts can be designed if an advancing core is used when the mold opens, the part has no sharp corners, and the elastomer bends during ejection.

Most TPE compounds have significant flow direction mold shrinkage and moderate cross-flow shrinkage. After ejection from the tool, the over-molding compound may contract more than the substrate. This can stretch the substrate, usually in the overmolding material’s direction. This is especially true for long, thin parts or components with a low-modulus substrate or one thinner than the overmold. Use higher-modulus substrate materials and stiffening ribs to mitigate this. Thinner coatings and lower-hardness over-mold grades assist. Relocating the gate to affect TPE flow may also help.

If you can enhance the design to ensure a tighter bond between the TPE material and the substrate, such as by increasing the cross-sectional area, without compromising its functionality or appearance, it would be beneficial. Below is one of exmple of overmolding part design tips.

Avoid designing too many separated areas on the TPE overmolding part, as this can complicate the overmold manufacturing and molding process. Especially many areas are designed to the parting line area; this will be hard to solve the flash completely when you design the TPE overmoulded part, designing as simple as possible, unless some function purpose.

Tips for overmold desgin: when we design injection mold overmolding parts, the first mold (substrate injection mold) we will add shrinage rate according to plastic shrinakge rate, but for the overmolding (second mold), we will not add any shinakge rate on the over mold.

Overmolding cost

The base cost of an overmolding is not a fixed number that will be the same for every application. It has a varying value that can fall between 1 and 10 dollars.

This is an extremely wide price bracket. The right value changes based on several components involved in the overmolding process. These factors that influence the cost are:

Injection molding Equipment

The upfront cost of equipment for injection molding can change greatly depending on its application and type.

There is little-scale injection molding equipment that businesses keep in house. Then there are big overmolding machines generally used by service providers and those in the big-scale manufacturing industry with large production volumes.

Professional industrial overmolding equipment costs between $50,000 and $200,000. There may be shipping costs involved. These machines are not for amateurs and hobbyists, as they need skilled operators.

Over-mold manufacturing costs

While the overmolding equipment is a one-time investment, creating the over mold according to each custom design is an additional cost, and each single unique overmold part design will require a unique over mold. It is an expense for every different part produced. So, over mold manufacturing costs are one of the most vital driving factors for overmolded parts.

This over mold cost can vary depending on the part design, part size, and quality required to create the molds. Generally, three factors are employed for this objective, which are listed below:?

Design complexity

Highly complex designs with more cavities and top mold polish result in extra costs. Such designs need development, research, and technical skill. For this objective, you can hire a professional for the design process. Both options incur expenses.

Part size

Over mold is the same as other injection molds; a large size will require a large moldbase and molding machine, which will increase the over mold cost and unit over moled part cost.

Labor costs

Most steps involved in overmolding are automated and run by software systems. For example, CNC machines are run by computer numerically controlled programs; 3D printers run on their own programs; and injection molding also has mechanical processing.

Final summarize for the overmolding cost

If you are looking for overmolded parts or over mold for your custom injection molding parts, then you do not need to pay any cost on the injection molding equipment, because your over molding supplier should have this, but you need to pay for the over mold cost, overmolding process cost, materials cost, packing cost, and so on. If you want to know the price of your overmolding project, please contact us and we will quote you in 24 hours.

How to reduce overmolding cost

Overmolding is the preferred manufacturing process due to its cost-effectiveness and reliability.

While the process is affordable compared to its alternatives, you can further decrease the costs. For that objective, here are some pointers that you need to take into account:

Optimizing the CAD design

A single part can be approached via different CAD designs. Anyway, not every design idea is perfect. Some designs for the same part may lead to time and resource waste. So, simplifying part complexity via efficient CAD design ensures optimal utilization of resources.

Reducing part size

Bigger parts are not always excellent parts. When the size of the parts rises, so does the expense of the injection molds needed for the parts. If the same process can be accomplished by decreasing the part size, it is a good idea to opt for it.

Resuing over molds

Make full use of the over molds by reusing them for many applications. You cannot just use the same mold for the same part but also for similar parts as well. This can be performed by making adjustments or molding wherever possible.

Using DFM analysis

DFM stands for design for manufacturing. DFM in overmolding refers to producing a part that serves the objective of the customer and is within their stipulated budget.

For DFM, analysts consider many factors based on science, art, and technology to find the most successful design, resulting in reducing injection over mold costs. Go to design for manufacturing page to know more.

Advantages of injection overmolding

1. Compared to 2K injection molding, over molding is easier to make. You can use a normal injection molding machine to make two or three different colors in one molded part or two or three different materials in one end part.
2. For some small quantities of two-color molding part projects, you do not need to invert or hire a 2K injection molding machine; using the overmolded process is the best and most cost-effective way to meet customer requirements.
3. It increases the design diversity and also exalts the end product in many material compositions.
4. With reduced assembly costs, there are fewer secondary activities or processes carried out on the end products. This reduces the cost of labor. Also, after manufacture, no more costs are incurred.
5. The parts have a high level of stability and constitution since, after being left to mechanically interlock, they become one.
6. Products overmolded using plastic have high resistance to vibration and shock due to the plastic resins having been structured perfectly.
7. The plastic molding parts are more reliable because there is no bonding at the production stage.
8. The end items are of the desired standard, such as eye-catching designs and firm components.

Disadvantage of injection over moulding

1. Since the over molding process involves moving the first molded substrate part to another over mold, the tolerance is not as good as in the 2K injection molding process.
2. The production capacity is not as efficient as 2K injection molding, as it requires robots or manual labor to insert the substrate into the overmolded mold. This will take time, and the molding parameter is sometimes not stable, especially when two or more substrates are in one mold. This will lead to additional problems and a higher waste rate, resulting in twice the amount of waste (from the substrate and overmolded material).
3. With the overmolding process, there are fewer available choices in terms of plastic compatibility. Some materials may not bond well together or may not be able to withstand the high temperatures and pressures of the injection molding process.
4. There are no secondary practices carried out on the end products of overmolding. When the molding material becomes cold, activities and adjustments come to a complete halt.
5. In cases where products are scarce, it is expensive to run such an operation. Sincerely, you need someone to put the substrate into over mold, so the cycle time and production cost are increased accordingly.
6. The overmolding process normally requires two molds, one for substrate and one for overmold, so the initial mold cost will be higher.
7. Overmolding is a more complex process than traditional injection molding, requiring precise coordination between the two injection systems and a proper mold design.
8. If there is an issue with the overmolding process, troubleshooting and fixing problems with overmolding can be more difficult than with traditional injection molding.

What is 2k injection molding? (Two-shot molding)

Two-shot injection molding, or 2K injection molding, is a manufacturing process used to mold two colors or materials into one plastic. This two-shot molding technology blends two materials or two different material colors into one plastic part by using a 2K injection molding machine.

The chemical bonding process involved in this process is very important because it is capable of combining two or more materials into one part. When using the 2K injection molding technology process, material selection will be an important factor in the success of the project or not.

Benefits of 2K injection molding 

2K injection molding provides a number of benefits over traditional one-material injection molding. Some of these benefits contain:

Cost-effective

The 2-step process needs just one machine cycle, rotating the primary mold out of the way and putting the secondary mold around the item so that the second, compatible thermoplastic can be inserted in the second mold. Because the method uses only one cycle instead of separate machine cycles, it costs less for any production run and needs fewer employees to make the end product while delivering more items per run. It also ensures a powerful bond between the materials without the need for additional assembly down the line.

Improved efficiency

Two-shot molding permits multiple components to be molded with one tool, decreasing the amount of labor required to run your parts and eliminating the need to join or weld components after the molding process.

Better quality

Two-shot is carried out within one tool, permitting lower tolerances than other molding processes, a high level of accuracy and repeatability, and reduced scrap rates.

Complex molding

Two-shot molding permits the creation of complex mold designs that incorporate different materials for functionality that cannot be achieved via after-molding processes.

Disadvantages of 2K injection molding

2K injection molding has many advantages, but like everything, it has its pros and cons.

A disadvantage of 2K injection molding is that the mold costs are higher than traditional injection mold. because 2K injection molding requires two molds, the first shot and the second shot (that is why we call it two-shot molding), and making 2K injection molds are more difficult than making two separate traditional molds because those two molds will run together in the same machine (a two-shot injection molding machine). so it needs two molds to switch over without any problem.

Additionally, the 2K injection molding process must use a 2K injection molding machine, which also makes the machine more costly and requires a special technical operator to adjust the machine. This is also a higher cost than traditional injection molding. Ultimately, we recover the 2K injection molding costs by reducing labor and assembly costs, as the manual application of, for example, a seal is no longer necessary. This eliminates an assembly step.

Another disadvantage of 2K injection molding is that it makes the recycling of plastic items difficult because often two different plastics are gathered. Even when the plastics are “from the same family,” the quality of return flows will be very low, which makes it hard to reuse the plastic for a high-standard application.

How to choose overmolding and 2K injection molding services

You may have questions about when you need to use over molding and when you should use the 2K injection molding process. Here are some simple suggestions:

1. If the molding part quantity is only a few thousand or ten thousand, then use the overmolded process, because overmolding can save you some mold costs.
2. If you need over a half million parts, then 2K injection molding is the best molding process to save money for you because overmolding has a high labor cost, but 2K injection molding requires high initial cost to both 2K molds and 2 shot injection molding machine and related equipment.
3. Some of the parts can only be overmolded, while others can only be done using the double injection molding process. This depends on the part design structure. If you’re unsure, send your data to info@plasticmold.net. We can check this for you and provide a price for your reference.

Looking for over molding or 2k injection molding service?

Sincere Tech is one of the best injection molding companies in China. Send us your 3D design drawing and your requirements; we will never share your data with anyone else. We are willing to sign an NDA document to ensure the safety of your project.

We will offer you the most competitive price for high-quality overmolded parts, 2K mold and molding, plastic tooling, and plastic parts for the long-term business relationship, and we will suggest the best injection molding process for your products.

More than 18 years of experience providing plastic mold making and custom mold services in overmolding, 2K molding, die casting, machining, and so on, plus 18 years of fluent technical English communication (technical English communication is very important to work with worldwide customers).

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