Injection Molding involves heating thermoplastic resin to a molten state and injecting it into a steel mold. There are two ways to accomplish this, insert molding and overmolding.
What is Insert Molding?
Insert molding is a manufacturing technique that involves encasing an established part, typically metal, with thermoplastic resin to create a single, integrated unit. Examples of insert molded applications include electrical wire harnesses, multi-lumen catheters, knife blades, and screwdrivers.
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. Insert molding is revered as one of the best methods to manufacture lightweight and compact parts that are reliable, durable, and fully functional.
The injection molding process starts by introducing thermoplastic resin into a hopper. The resin then undergoes heating, 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 and thermoset resin are the two main types of plastic used for the injection molding process. Thermoplastic can be reheated multiple times while thermoset cannot. For example, a milk jug is a thermoplastic, while a tire is a thermoset.
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.
Molded Inserts
Molding inserts are the established, or pre-formed, parts that the thermoplastic resin will mold around during the insert molding process. These require special attention, as not all parts may be suitable, so they have to meet strict temperature and pressure resistance requirements.
Successful insert molding shouldn't alter the structure or the shape of the insert in any way.
Inserts typically have 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.
Overmolding (Two-Shot Molding)
Overmolding differs from insert molding as it involves the use of two separate materials to produce a single molded product. Overmolding is still a part of the injection 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.
Overmolding, is an effective manufacturing approach employed by industries to manufacture complicated components that are too complicated and uneconomical to produce using a single molding process.
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 Overmolding
Overmolding 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 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 referred to as the overmolded material. The substrate material can be anything from metal to plastic or glass.
Overmolded 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 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
In 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.
Real Case Study: Pioneering Multi-Lumen Heart Catheter with Insert Molding
Unique Challenge: In 1983, a Baxter engineer approached Aberdeen Technologies with a revolutionary product idea to create 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. This slow and inefficient process resulted in high rejection rates and hazardous adhesives for the medical industry
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 was a success and today our solution is still utilized where 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.
Post a comment