Overmolding and Insert Molding Design Guide

Factors favoring two-shot molding Factors favoring pick-n-place molding Large production volume—typically over
10,000 parts Modest production volume—typically under
10,000 parts Certainty of the design and materials you
will use for the entire production run Need to prototype design or test materials, or
possibility that the design will change Time and money to invest in costly two-shot
molds and process Need to move products and devices quickly to
market or to meet market demand while two-shot
molds are being made Need for maximum chemical bonding
between layers A design that specifies chemical bonding, and,
through proper material selections, can provide a
strong chemical bond

 

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The Role of Bonding

Bonding between resin layers helps keep the layers from separating. Depending on the part geometry, bonds can be subjected to several forces that can pull the layers apart.

These include:

• direct tensile pull causing separation at a butt joint
• shearing caused by a pull parallel to the bonded interface causing separation at a lap joint
• peeling that typically begins at an edge and propagates along the interface between materials

Bond strength is particularly important when one of the materials is an elastomer, which can flex and be pulled away from the substrate. This applies to both thermoplastic elastomers, which soften when reheated, and thermoset materials, which do not soften.

There are two primary ways in which layers bond. One is actual chemical bonding at the interface of the two resin layers; the other is mechanical bonding, which depends on the physical geometry at the interface. Acceptable bonding is achieved through a combination of part design, material selection, mold design, and molding process.

Chemical bonding takes place at the molecular level and is affected by several factors. The first is the wetting ability of the substrate by the injected overmolded material. Better wetting allows more contact between the two materials and more opportunity for bonding. This is a factor of temperatures of the materials, viscosity of the overmolded material, and texture and porosity of the substrate surface. At the interface, adhesion can take place in three different ways:

• mechanical entanglement of the polymer molecules
• co-crystallization
• chemical reaction between the polymer chains

Adhesion can also entail a combination of the three, and a textured surface increases the area over which adhesion can take place. In addition to the need for compatibility of the resins themselves, the presence of additives, fillers, and certain surface treatments can reduce the chemical interactions of substrate and overmolded materials.

Molders and material suppliers can be a valuable resource in identifying resins that both meet the designer’s performance requirements of the completed part, and resins that are compatible with one another and able to provide maximum adhesion. Processing can also significantly impact adhesion. This is particularly true in pick-n-place molding. In this process, the substrate is allowed to cool before overmolding and is both exposed to the environment and handled during the process. It is critical that the manufacturer prevent the accumulation of impurities on the substrate surface during storage and handling in order to maximize adhesion—an area that Protolabs pays close attention to.

Mechanical bonding can be used in place of or in conjunction with chemical bonding. When overmolded resin flows into holes in the substrate, particularly if the holes are “dovetailed” to widen at the bottom, the cooled overmolded material is locked to the substrate. Other ways to enhance mechanical bonding include wrapping the overmolded material around the substrate or increasing surface area of the interface with grooves, pickets, posts, or bosses. A porous substrate provides tiny holes into which an elastomer can migrate to create a mechanical bond. To prevent peeling, avoid exposed edges of the overmolded material. A raised edge of substrate material can protect the edges of the overmolded elastomer where peeling could otherwise begin.

Chemical Bonding Compatibility

SUBSTRATE MATERIAL OVERMOLD MATERIAL ABS LUSTRAN ABS/PC CYCOLO C2950-111 PC LEXAN 940-701 PBT VALOX 357-1001 PP PROFAX 6323 NYLON 66 ZYTEL 103 HSL NC010 TPU-Texin 983-000000 C C C C M M TPV-Santoprene 101-87 M M M M C M TPE-Santoprene 101-64 M M M M C M LSR-Elastosil 3003/30 A/B - - M M - M TPC-Hytrel 3078 C C C C M M TPE-Versaflex OM 1060X-1 C C C M M M TPE-Versaflex OM 6240-1 M M M M M C TPE-Versaflex OM 6258-1 M M M M M C TPE-Versaflex OM 1040X-1 C C C M M M

M = mechanical bond C = chemical bond

Overmolding Materials

Thousands of possible combinations exist of substrate and overmolded material. A few of the more common possibilities are included in the aforementioned chart, but if you require special characteristics, there are many others that can be identified by material suppliers.

Besides compatibility and adhesion, there are a number of factors that affect resin choice for overmolding. If the goal is cushioning, the thickness of the overmolded material can be as important as the softness of the material itself. Thin layers, typically below 0.40 in. (10mm), will feel hard regardless of material choice. For this reason, many consumer products will have rows of taller ribs, to increase perceived thickness while reducing the amount of overmolded material and increasing its flexibility. The actual flexibility of a material is not directly related to its hardness or durometer. A better measure is flexural modulus, which measures a material’s resistance to bending. A material with lower flexural modulus will feel softer. And while a variety of resins are suitable for overmolding, there are elastomers such as Versaflex that can be specifically formulated for overmolding applications.

If the goal of overmolding is to enhance grip, a material’s coefficient of friction indicates how tactile it will be. Thermoplastic elastomers (TPEs), for example, generally have a high coefficient of friction. As in the case of cushioning, durometer is not a reliable measure of a material’s grip. Since many resins including both thermoplastics and thermosets can have a range of characteristics, it may be useful to consult experts in choosing the right resin grade for a specific application. 

Like overmolding, insert molding injects a resin over another material, but instead of a plastic substrate the other material is typically metal and the injected plastic material is typically a rigid plastic. Metal electrical components or custom-machined metal parts are often embedded in plastic this way. Similarly, threaded inserts can be molded into plastic parts for stronger, more durable assembly of plastic components such as device shells. Insert molding is an alternative to inserting metal parts by either heat staking or ultrasonic welding, processes by which a molded plastic part is locally melted to allow the insertion of a metal part. Insert molding is more controllable and allows better encapsulation than the other methods. Molded in inserts also eliminate the need for a secondary insert installation process, saving time and money.

Because inserts are metal, they must be placed into a mold in which they will be encapsulated in plastic. This can be done robotically for high volume production, but for low- to mid-volume production insertion into the mold, pick-n-place is a manual process. There is no chemical bonding between metal inserts and plastic, so the insert and resin components must be designed for mechanical bonding.

Protolabs accepts pre-fabricated inserts, including PEM, Dodge, Tri-Star, Spirol and Tappex.

Why Use Overmolding and Insert Molding?

While overmolding requires more complex design, processing, and materials choice than single-shot injection molding, it offers significant benefits:

• It allows materials to be combined to provide characteristics that no single resin can deliver.
• It can eliminate assembly steps, saving both time and money.
• It can meld materials in a way that assembly processes cannot match.
• Inserts add strength and durability to parts.

Cost Reduction

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Overmolding can reduce production cost. Whereas standard injection molding can combine multiple parts into a single multi-cavity mold, overmolding can produce a single part made up of different materials without the need for assembly. Mold production is more complicated, but it eliminates the recurring cost of assembling thousands of parts. There are a variety of ways to produce overmolded parts, and choosing the right one for your needs—time to market, total production volume, and likelihood of product change—can help determine which will be most efficient.

Quick-turn Production

Overmolding can reduce production cost. Whereas standard injection molding can combine multiple parts into a single multi-cavity mold, overmolding can produce a single part made up of different materials without the need for assembly. Mold production is more complicated, but it eliminates the recurring cost of assembling thousands of parts. There are a variety of ways to produce overmolded parts, and choosing the right one for your needs—time to market, total production volume, and likelihood of product change—can help determine which will be most efficient.

Prototyping and Low-Volume Runs

Pick-n-place overmolding is ideal for low- to mid-volume production. Because it is more labor-intensive than two-shot overmolding, production cost per part using this technique will tend to be higher, but it eliminates the extremely high cost and delay of producing complex two-shot molds. Total cost of pick-n-place will tend to be lower than two-shot below production volumes of 10,000 parts. The process can also be used to produce prototypes before investing in two-shot molds for high-volume production. If speed to market is critical, it may make sense to use pick-n-place to deliver product to market
while waiting for full-scale production to begin. And, in markets where parts are subject to frequent redesign, pick-n-place reduces risk by allowing redesign of molds at a fraction of the cost of remaking two-shot molds.

Range of Applications

Overmolding is widely used in industries ranging from consumer products to automotive and electronic components, but it is particularly suited for medical and health care applications. Devices that contact, enter, or are inserted into the body may have to meet stringent requirements and have challenging functions. They may have to stand up to heat for sterilization, endure chemical exposure, and meet standards including FDA, USP Class VI, ISO 10993, and biocompatibility. In many cases, no single resin can meet all of the requirements. For safety and sterility, multiple materials may have to mate virtually seamlessly, and this is an area in which overmolding excels. There are many other reasons to use overmolding including:

• One of the most common is for comfort and grip. Soft elastomers are frequently molded over a hard substrate to create a safe, non-slip grip on a variety of hand-held items ranging from hand tools to devices.
• Because the overmolded material is typically an elastomer, sealing, shock absorption, and vibration damping are also common applications.
• Another common application is aesthetic; the substrate can have an indented pattern that is filled with the overmolded material in a contrasting color to create text, a logo, or other design.
• Overmolding can change the characteristics of a part’s surface to give it different electrical, thermal, or other environmental qualities.
• It can also be used to capture or encapsulate something within another material.

Plastic injection molding embodies numerous subsets of versatile manufacturing processes. Two distinct methods of plastic fabrication include overmolding and insert molding, each offering its own advantages. Insert molding is the process of molding plastic over an entirely different material, and overmolding is a method that adds a layer of material over an existing part. Though, this is not where their differences end. Let’s take a more in-depth look at the difference between overmolding and insert molding.

What Is Insert Molding?

Insert molding’s aim is to create a single part using two or more materials. Usually, a metal piece, such as pins or threaded rods, is combined with a thermoplastic, making a product that benefits from the characteristics of each material. The metal components are placed into the mold cavity before the molten plastic is injected. The plastic is molded over these “inserts,” where it then hardens to create the part. This eliminates the need for fasteners, as the metal components are securely held by the bonded plastic.

What is Overmolding?

Overmolding is essentially a type of insert molding, as the process uses plastic to mold over another part. However, with this technique, the initial component is also made inside a mold. Once the first piece of completed, it’s placed in a second mold, where the over-molded material is added. This technique combines multiple different plastics to create parts with varying practical purposes or appearances. For example, one might use this method to mold a softer plastic over a stiff plastic to make the part easier to grip.

Is Overmolding or Insert Molding Right for You?

Choosing which plastic injection molding method is right for you depends on what type of part you want to create: a part that utilizes two vastly different materials or one that combines the characteristics of various thermoplastics. It’s also important to look at the additional attributes of each technique.

The Pros and Cons of Insert Molding

Insert molding has the advantage of using different materials to create a part, but it also comes with some other benefits, some of which include:

• Lower Assembly Cost: Assembly is a bottleneck when it comes to production. With insert molding, assembly is taken care of during the injection molding process, which results in cost savings.
• Part Performance: With insert molding, you can combine the robust qualities of metal with the design flexibility of plastic to create a lighter weight and more cost-effective part.

However, there are a few drawbacks to insert molding, as well. For instance, insert molding may involve a two-step manufacturing process: requiring both a metal-forming process and plastic injection molding. Also, you will have to rely on a skilled part designer, as this process can be very complex.

The Pros and Cons of Overmolding

Overmolding is a very versatile process that allows manufacturers to leverage the benefits of multiple different thermoplastics. Some further benefits of this technique include:

• Embedded Seals: Instead of adding elements to a part where a component can be installed later, such as an O-ring, you can save costs by permanently molding the seal.
• No Adhesives Needed: Overmolding fuses the materials in the mold, eliminating the need for permanent bonds or glues to combine plastics. This increases durability and cuts assembly costs.

However, overmolding does require a two-step molding process, which can lengthen cycle time. Also, bonding two plastics together can sometimes result in delamination.

Understanding the difference between overmolding and insert molding can help you choose the best design for your next product. Both techniques take advantage of the benefits of multiple materials, ensuring your part contains all the qualities you need from it. At Midstate Mold, we have experience working with both methods, and we can carry your project from the prototyping phase through production. If you require an experienced manufacturer for your next project, contact us today.

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