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Post by Mar 12, 2024 6:01:00 PM · 5 min read

Overmolding vs Insert Molding: A Complete Guide

Insert Molding and Overmolding play a crucial role in creating complex and innovative products. These techniques are subsets of the broader manufacturing process known as Injection Molding, where thermoplastic resin is injected into molds to create various components. In many cases, these two processes are spoken about interchangeably. However, they are different. 

The injection molding process initiates by introducing raw plastic resin, known as thermoplastic, into a hopper. Subsequently, this resin undergoes heating, typically reaching temperatures ranging from 300° to 650° Fahrenheit, causing it to melt. Once in a molten state, it is then pressurized and injected into a sturdy steel mold.

Thermoplastic is the type of plastic that you can heat to make molten. There is another type of plastic called Thermoset, which is a plastic that cannot be reheated once it has cured. A milk jug, for example, is a thermoplastic, while a tire is an example of a thermoset. A thermoplastic can be reheated over and over again and will become molten. The other is like baking a cake; you cannot get the ingredients back to the batter form once the cake is set. That's a thermoset. It all has to do with the chemical makeup of the resin. 

The molds used in insert molding, over-molding, and injection molding are made up of steel plates, water lines, and ejection pins, with other components as needed. A simple open-shut mold is a mold that can be pulled apart without any slides or other components getting in the way. These are a more simple mold and often the type used in insert and overmolding. 

Molding Inserts

Molding inserts require special attention, as not all parts may be suitable, so they need to be able to withstand the manufacturing process. This means that they have to meet strict temperature and pressure resistance requirements.

Molding Inserts Can Be Big and Small as A Dime

Successful insert molding shouldn't alter the structure or the shape of the insert in any way, as the shape of the molding inserts is extremely important. First of all, manufacturers should explore the means to hold and locate these inserts during the molding process.

Second of all, the insert has to have undercuts or bosses in order to provide better retention strength with the molded plastic.

Another consideration is if the mold can close off to the insert so that there isn't any blow-by, meaning the plastic will blow past the insert during manufacturing, causing a flange or additional plastic that will need to be trimmed off. 

What is Insert Molding?

Insert molding is a precision-driven manufacturing technique that involves encasing a part of a preformed part, often made of metal, with a heated thermoplastic resin. The goal here is to create a single, integrated unit. Examples of insert molding applications include manufacturing rods, electrical wire harnesses, threaded components, or knife blades.

The main use of insert molding is to create a strong and reliable molded part with a very low investment of money and time. This process not only lowers assembly and labor expenses but also grants manufacturers greater design flexibility without requiring any sacrifices in the structural integrity of the part. The reduced labor cost and the excellent quality control make it one of the best and least expensive methods to manufacture lightweight and compact parts that are reliable, durable, and fully functional.

Overmolding or Two-Shot Molding

The Two-shot injection molding process, also referred to as double injection molding or overmolding, is widely recognized as a complex manufacturing process that involves the use of two separate materials to produce a molded product. Two-shot molding (often referred to as double injection molding) is still a part of the insert molding family and deals delicately with the molding insert. A common use case for this specific type of molding is cable overmolding or micro molding.

An Example of a Plastic Overmolded Cable

Two-shot molding is an effective, evolving, and innovative manufacturing approach employed by industries to manufacture complicated components, primarily from two distinct (usually plastic) materials, which are far too complicated and uneconomical to produce using conventional molding processes.

Generally, this manufacturing process is carried out on a machine specially designed and built to carry out a double injection in a single cycle. The first stage involves injecting one type of plastic into a mold. On completion of this insert molding process, the machine rotates the plastic mold automatically before injecting another type of plastic into the mold. This is what is known as overmolding.

Benefits of Two Shot or Overmolding

Two-shot plastic molding creates a powerful molecular bond by optimizing the co-polymerization of the two substrates (one of which is usually hard while the other is soft). The ability to create a molded unit from two different types of materials in one process is the biggest advantage of the double injection or overmolding process.

More recently, some 3D printing technologies have been used to overmold objects of two or sometimes more materials. One material is commonly known as the substrate. The substrate is either partially or fully covered by a second material or materials referred to as the overmolded material. The substrate material can be just about anything from metal to plastic or glass.

The over-molded material is often rubber or thermoplastic. Overmolding can be used as a double injection molding process, often resulting in either a chemically bonded part of the materials being mechanically interlocked. The types of products that are overmolded include tool hand grips like screwdrivers, scissors and medical products such as cannulas, needles, tubing, and catheters. 

Key Differences and Similarities

While Insert Molding and Overmolding share the fundamental concept of molding plastic around existing components, they differ in the number of steps involved and the nature of the resulting product. Insert molding is often a single-shot process, whereas Overmolding is a two-shot process.

The Role of Molds

A Mold Being broken Down by Each AspectIn both Insert Molding and Overmolding, molds play a pivotal role. Mold components, including mold cavity design, pins, and alignment tools, guide the plastic during the manufacturing process. Selecting the right mold design is essential to ensure the desired outcome in terms of product quality and structural integrity.


Terminology Disputes

The terminology surrounding these processes can vary, leading to disputes and confusion within the industry. Some prefer to categorize certain processes as "injection molding with a secondary operation" or distinguish between "pre-mold operations" and "overmold operations." Ultimately, clear communication and understanding are important to ensure effective collaboration in the manufacturing industry.

While Insert Molding and Overmolding differ in process complexity and resulting product characteristics, they share a common foundation in the molding of plastic around existing components. These versatile techniques empower manufacturers to create products that excel in terms of durability, aesthetics, and functionality, making them indispensable in today's diverse manufacturing landscape. By grasping the distinctions and synergies between these processes, manufacturers can unlock the full spectrum of possibilities they offer.

In essence, the selection of the ideal molding process is a pivotal step in the journey of bringing your product to life. Whether you opt for insert molding to integrate preformed parts, venture into the world of overmolding to enhance product functionality and aesthetics, or choose the simplicity of injection molding, informed decision-making helps with successful manufacturing. Your product's success story begins with the selection of the right process, and Aberdeen is here to guide you on that path.

We hope this comprehensive guide has clarified the intricacies of Insert Molding and Overmolding, providing valuable insights for professionals in the manufacturing field.

Real Case Study by Aberdeen Technologies: Pioneering Multi-Lumen Heart Catheter with Insert Molding

Unique Challenge: In 1983, a Baxter engineer approached us with a revolutionary product idea: creating a multi-lumen heart catheter using insert molding. At the time this had never been done before. All catheters were assembled using a pre-molded manifold and then gluing the extension lumens in place. A slow and inefficient process resulting in high rejection rates and hazardous adhesives for the medical industry

An Example of some of the catheters aberdeen technologies has helped manufacture using insert molding

Our Innovation: Our proposed solution involved placing stainless steel wire mandrels through individual extension lumens, then positioning them into separate passageways in a multi-lumen extrusion. This assembly was then placed in a mold and polyurethane was insert molded in the shape of a manifold. Afterward the mandrels were removed, leaving unobstructed passageways in the plastic manifold connecting the extension lumens to the multi-lumen extrusion.

Initial Complication: The greatest challenge was ensuring the channels were not touching one another, which would result in leakage between the passageways when fluids were introduced into the assembly during a surgical procedure.The first design featured a flat "Y" shaped manifold. Unfortunately, keeping the mandrels from crossing paths during injection molding proved incredibly difficult. Even minor shifts caused channels to touch, compromising the integrity of the final product.

Insert Molded Solution: An alternative design was formulated which separated the extension lumens into a circular pattern, allowing for more space between the mandrels and preventing them from touching during molding. This proved to be very effective and today it is often combined with insert molding when there are more than 3 lumens in a catheter assembly.

Aberdeen Technologies’ Reputation: Today, Aberdeen Technologies continues to be a leader in complex insert molding parts, such as helping develop devices with smaller footprints and ever-thinning lumen diameters. This pioneering work on multi-lumen heart catheters stands as a testament to our commitment to innovation in insert molding and patient safety. 

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