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Design Considerations for Overmoulding and Insert Moulding

A guide to two-material moulding and bonding options with plastics, elastomers, and metal inserts

Overmoulding is an injection moulding process that allows an additional layer of resin to be added to an existing moulded part to provide a combination of characteristics that no single material can provide. One of the most common applications is to add a soft , functional, hand-friendly layer of rubber-like material, typically a TPE (thermoplastic elastomer) over a hard substrate. Another is to change or enhance the appearance or “cosmetics” of a part by overmoulding material of a different colour or finish to it. Overmoulded materials can be found on anything from medical devices and hand-tools to toothbrushes and as gaskets and seals within assemblies.

orange overmould part
Insert moulding is an easy way to integrate threaded metal inserts into components like this plastic drone propeller.

As a manufacturing process, overmoulding can provide excellent adhesion between diff erent materials and eliminate the need to assemble materials by hand. Because the process can be automated to varying degrees it can be quite cost-eff ective. And, by reducing the complexity of assembly, it can help reduce costs and accelerate products and devices time to market. But most important, it can greatly enhance the range of material characteristics available to a product developer.

There are two primary methods for overmoulding— two-shot moulding and pick-n-place moulding, with the former using a single production mould while the latter uses two moulds.

Material selection for overmoulding can be complicated. Substrate and overmould resins can complement one another, but to be eff ective, they have to be compatible. Choices vary not just with the application of the overmoulded part, but also with the method being used to produce it. Because process and outcomes are more complex for overmoulding than for single-shot injection moulding, it is helpful to seek input from resin experts when selecting materials.

Two-Shot vs. Pick-n-Place Overmoulding

Let’s fi rst look at two-shot moulding, which involves a substrate that is moulded in one material and then quickly overmoulded with another material. This is generally a highly automated process. The second method is pickn-place moulding in which an entire batch of substrate parts is moulded; the substrate parts are then manually placed, one at a time, into a second mould into which the overmoulded resin is injected to produce the completed parts.

There are three primary methods for two-shot moulding:

  • Transfer overmoulding is a robotic procedure in which substrate parts are mechanically lift ed out of one mould and placed into another, larger mould. The overmoulded material is injected to fi ll the empty space in the second mould, typically while the next substrate is being produced in the fi rst mould.
  • Rotational overmoulding is another robotic process in which the mould itself moves from one injection station to another to allow injection of substrate and overmoulded materials.
  • Core-back overmoulding can be used only with very specifi c linear geometries. The mould is built with a sliding section that is pulled back aft er the fi rst material is injected and set to make room for the second injected material.

All three two-shot methods layer the overmoulded material onto a warm substrate, which aids chemical bonding. All three also require specialised equipment and expensive moulds, but because they are highly automated, they are cost eff ective for high-volume production, typically more than 10,000 pieces, and oft en 100,000 or more.

Pick-n-place moulding uses two completely separate moulds. A batch of substrate parts are manufactured in one mould and allowed to cool. They are then placed, by hand, into the second larger mould, which accommodates the substrate parts and leaves room for the overmoulded material to be injected over the substrate. The process uses less complex equipment and simpler moulds than the two-shot methods and simplifi es and accelerates setup.

Comparing Overmoulding Methods

While the manual placement of substrates into secondary moulds is slower than the robotic processes, it can typically complete production of low- to medium volumes of parts more quickly and at signifi cantly lower cost. The biggest challenge in pick-n-place moulding is the reduction in chemical bonding between overmoulded materials and cool substrates. To maximise adhesion, substrates must be carefully handled to prevent contamination of surfaces that could impact adhesion. In addition, proper material selection also helps ensure good bonding in a pick-n-place application.

Factors favoring two-shot moulding Factors favouring pick-n-place moulding
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
moulds and process
Need to move products and devices quickly to
market or to meet market demand while two-shot
moulds 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

 

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, mould design, and moulding 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 overmoulded 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 overmoulded 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-crystallisation
  • 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 overmoulded materials.

Moulders 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 signifi cantly impact adhesion. This is particularly true in pick-n-place moulding. In this process, the substrate is allowed to cool before overmoulding 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 maximise adhesion— an area that Proto Labs pays close attention to.

Mechanical bonding can be used in place of or in conjunction with chemical bonding. When overmoulded resin fl ows into holes in the substrate, particularly if the holes are “dovetailed” to widen at the bottom, the cooled overmoulded material is locked to the substrate. Other ways to enhance mechanical bonding include wrapping the overmoulded 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 overmoulded material. A raised edge of substrate material can protect the edges of the overmoulded elastomer where peeling could otherwise begin.

Chemical Bonding Compatibility
SUBSTRATE MATERIAL
OVERMOULD MATERIAL ABS POLYLAC PA-717C BS/PC BAYBLEND T65 XF PC MAKROLON 2458 PBT CELANEX 2002-2 PP MOPLEN RP348R NYLON 66 ZYTEL 70G30HSLR
TPU Pearlthane 11T85 C C C C M M
TPV Santoprene 3170 101-87 M M M M C M
TPE Thermolast K TC6 MLZ 101-64 M M M M C M
LSR Elastosil 3003/30 A/B - - M M - M
TPC Hytrel 4068FG C C C C M M

M = mechanical bond C = chemical bond

three types of mechanical bonding
Bonding plays an important role in overmoulding. Three types of mechanical bonding techniques are shown here.
Overmoulding Materials

Thousands of possible combinations exist of substrate and overmoulded 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 identifi ed by material suppliers.

Besides compatibility and adhesion, there are a number of factors that aff ect resin choice for overmoulding. If the goal is cushioning, the thickness of the overmoulded material can be as important as the soft ness of the material itself. Thin layers, typically below 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 overmoulded material and increasing its fl exibility. The actual fl exibility of a material is not directly related to its hardness or durometer. A better measure is fl exural modulus, which measures a material’s resistance to bending. A material with lower fl exural modulus will feel soft er. And while a variety of resins are suitable for overmoulding, there are elastomers such as Versafl ex that can be specifi cally formulated for overmoulding applications.

If the goal of overmoulding is to enhance grip, a material’s coeffi cient of friction indicates how tactile it will be. Thermoplastic elastomers (TPEs), for example, generally have a high coeffi cient 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 specifi c application. 

how insert moulding works
A threaded insert is placed atop a mould core where plastic is moulded over it to form the final component.

Like overmoulding, insert moulding 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 oft en embedded in plastic this way. Similarly, threaded inserts can be moulded into plastic parts for stronger, more durable assembly of plastic components such as device shells. Insert moulding is an alternative to inserting metal parts by either heat staking or ultrasonic welding, processes by which a moulded plastic part is locally melted to allow the insertion of a metal part. Insert moulding is more controllable and allows better encapsulation than the other methods. Mouldedin 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 mould 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 mould, 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.

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

Why Use Overmoulding and Insert Moulding?

While overmoulding requires more complex design, processing, and materials choice than single-shot injection moulding, 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

Overmoulding can reduce production cost. Whereas standard injection moulding can combine multiple parts into a single multi-cavity mould, overmoulding can produce a single part made up of diff erent materials without the need for assembly. Mould production is more complicated, but it eliminates the recurring cost of assembling thousands of parts. There are a variety of ways to produce overmoulded 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

Overmoulding can help speed products to market. Once moulds have been created, overmoulding, in any of its forms, can be a far faster process than those requiring assembly. Two-shot overmoulding processes are best suited for large volume production—more than 10,000 parts. Pick-n-place overmoulding is more economical at smaller volumes in the thousands. While design and manufacturing of two-shot moulds can take a month or more, pick-n-place overmoulding can produce parts within weeks for prototyping, market testing, low-volume production, or bridge tooling while two-shot moulds are being produced. In either case, however, once the mould or moulds have been made, volume part production can be done quickly.

Prototyping and Low-Volume Runs

Pick-n-place overmoulding is ideal for low- to mid-volume production. Because it is more labour-intensive than two-shot overmoulding, 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 moulds. 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 moulds 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 moulds at a fraction of the cost of remaking two-shot moulds.

Range of Applications

Overmoulding 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 sterilisation, 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 overmoulding excels. There are many other reasons to use overmoulding including:

  • One of the most common is for comfort and grip. Soft elastomers are frequently moulded 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 overmoulded 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 fi lled with the overmoulded material in a contrasting colour to create text, a logo, or other design.
  • Overmoulding can change the characteristics of a part’s surface to give it diff erent electrical, thermal, or other environmental qualities.
  • It can also be used to capture or encapsulate something within another material.
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