If you are always curious about how things get made, at some point you would have taken a look at a plastic product. You would have wondered how it’s made. For those with some knowledge of plastic processing techniques, some might be obvious. Looking at a simple straight pipe one would immediately think of extrusion. A screw cap of a water bottle is most likely injection molded. You might have a product you want to manufacture. Could be that the design doesn’t quite fit into any of the typical processes. In such a case, you’ll need to use a combination of processes to achieve the final design. These processes could include machining, gluing, or welding parts together. In manufacturing, having more stages and more components introduce more risk. At every stage, there is a risk of failure. You might also want to cut the number of components used in a product for different reasons. For example, if the product gets exposed to heat or solvents. These may damage the glue used. A process that removes the need for more components works better. You’d also want to keep the number of stages involved to the least. This is where overmolding comes in as a variation of injection molding.
The Overmolding Process Explained
In a typical injection molding process, the polymer melt gets injected into a mold. The mold closes and cools through cooling lines. The product hardens within the mold cavity as it cools. Once the product hardens, the mold opens and the formed products get ejected. The next cycle begins. Overmolding takes this up a notch by making it possible to have more than one part in a single mold. These parts are combined to form one product. Here’s how it works.
In overmolding, a substrate is first formed. This may be by injection molding or machining. The substrate uses materials like plastic or metal, usually a rigid material. The substrate is then placed in the mold and the mold closes. The substrate fits within the mold in a manner that allows fluid to flow around it. The polymer in liquid form is then injected into the mold. The polymer can be either in melt form as thermoplastic or TPE or a thermoset elastomer liquid. This is the overmold. The overmold is then allowed to either cool or cure. Once cooled/ cured the overmolded part gets ejected as a single article. The result of overmolding is a product that comprises two parts bound together. This binding does not make use of methods like joining or gluing. Rather the parts joined by physical binding and or thermochemical bonds
Note that overmolding does not only apply to thermoplastic injection molding. Overmolding is also used in the production of thermoset parts. The fundamental principles of the process are still the same. The difference is that the polymer injected could be a liquid at room temperature. This is then left to cure for some time after which it hardens within the mold. The time it needs for curing could be up to 48 hours depending on the type of material used.
Material compatibility is an important factor to consider is overmolding. Bearing in mind that different materials have different chemical and physical properties. There are several factors to consider in the overmolding process of a product. This includes shrinkage and melting point. If the plastic molded over shrinks this could exert pressure on the plastic itself. It could also exert pressure on the material underneath. A mismatch between the shrinkage % could result in disfiguring or breakage. In some cases, the processor might want a level of shrinkage to improve fit or seal. By designing the process the shrinkage becomes an advantage.
Taking into consideration the properties of different plastics. Some polymers are more common in overmolding. As the industry continues to expand, more manufacturers and processors explore more materials. materials in overmolding. A broad range of plastics gets listed in products made using overmolding. Examples include PEEK, ABS, PET, and Acrylic thermoplastics. PEEK finds a lot of use in biomedical applications, ABS finds use in automotive engine parts. PET is most common in food and beverage applications. Acrylics like PMMA make good substitutes for glass. These tend to serve as rigid substrates in overmolding injection molding. Elastomers such as silicone and EPDM are often used as the overmolds.
The image below is an illustration of an overmolding process.
Why Use Overmolding
There are several reasons why one would want to use overmolding injection molding. The general reason is to combine the properties of two different materials in a single part. With that in mind, we can think about many reasons why one would need this is in a product. For example where one has the desired texture for the surface but is too expensive. So it can’t be the sole material for the entire product.
Another example is where the product needs to be a particular weight. The material to meet the weight sometimes is not resistant to environmental factors. A lighter more durable material is then used as overmold in such situations. Examples of such are exercise dumbells. The heavier metal makes up the substrate while plastic or elastomer serv as overmold. This way the product has the desired weight and it also protects the metal from rust.
A product might need a rigid core with a soft outer layer. Such products allow for a good grip and soft feel while remaining rigid. The rigid core might be necessary for its function. The soft outer layer also provides impact resistance in case of falls and drops. It could also be a combination of a rigid, non-reactive part and an insulating layer. Below is a list of some of the reasons overmolding is the right option for a product.
- Aesthetics
- Improved resistance to impact
- Thermal or electrical insulation
- Chemical and water resistance
- Better grip
- Bonding
Some Notable Applications of Overmolding
The injection molding process improves through the innovation of overmolding. This has brought about a reduction in production cost and improved efficiency. Products that would usually need many stages now get produced using overmolding. This reduces the manpower and time required, hence reduces cost. The application of overmolding injection molding extends across several industries. This section describes some example applications.
Filter Cartridges
Whether it’s for water or air, filters help maintain a level of sterilization. Often a filter cartridge comprises more than one layer of the filter. These combine to form a filter cartridge. Using a combination of filters helps achieve higher efficiency. Higher than is achievable with one filter alone. The efficiency of the filter is evident in how well it can prevent the passage of particles or microbes. It is all very well to produce an effective filter. The effort gets thwarted when during assembly of the filters there are holes. Also if the filter is going to be used for example water, glues might be out of the question. These may be toxic and may leach into the water. Overmolding is a means to achieve effective sealing of filter cartridges.
Electronics Casings and Housing
In many electronic devices, components need protection from the environment. Elements like dust, water vapor, and air can reduce the life span of electrical components. Metals will generally rust when exposed to water and oxygen. This is what leads to many parts needing replacement. The metal’s rust and the connections that allow electricity conduction gets lost. So the circuit fails. This also applies to electrical components that need isolation from the environment. For example lightings in pools, waterproof devices like watches amongst others. Overmolding injects the plastic melt and holds it under pressure while it cools. This process ensures that no gaps exist between the two parts. This gives a better sealing than typical glues can achieve. Using overmolding in electronics can prolong the lifespan of the product. There is increasing concern around the implication of electronics waste on the environment. Using methods such as overmold injection molding contributes to having longer-lasting electronics. This keeps them away from the waste stream longer.
Medical Devices and tools
The medical environment calls for a high level of hygiene. Sometimes a sterile environment. Many of the tools and devices get sterilized before and after use. Different methods exist for sterilization depending on the type of tool or device. Methods used include heat, chemical, and radiation. If the device or tool is not well-designed gaps and holes can serve as a hiding place for microbes. This could render sterilization ineffective. Imagine if the sterilization is by radiation. A hole on the surface will shield the microorganism from the full effect of the radiation. This is like sitting in a shade when the sun is out. Examples of medical devices where overmolding gets used are probes and syringes. Another is as teeth for gap control in insulating sealing plates in surgical forceps.
Packaging
When designing a packaging material there are several considerations. The need to contain the product is a basic need. Other requirements depend on the type of product in the package. Taking food or beverage packages as an example. The material in direct contact with the food or drink must not be toxic. It all must not react with the food. This could cause altered organoleptic properties and devalue the product. A material that is food safe might not make a tasteful and practical packaging. For example, it might be transparent and not protect from light. Or it might not be a material that adheres well to printing ink.
Overmolding allows the combination of materials in different layers. This helps to achieve effective packaging. That is it combines properties of each material to meet the packaging requirements. The material in direct contact with food is next to a material with a good insulating property. Or one that can be better for printing or have better grips. An example of this is in some new design of reusable water bottles. The water bottle itself could be of a material like a polycarbonate. This is a more rigid plastic that serves as the substrate. The overmold is then a material like a silicone elastomer. This can then serve to give a better grip and also absorb impact in case the bottle drops. Polycarbonate is likely to get cracked or scratched. The elastomer is more flexible and impact resistant.
Improved Sealing with Adhesives
Where two different types of polymers get joined, adhesives are sometimes used. For example a thermoplastic and an elastomer. Even where the elastomer is a thermoplastic elastomer, the two might not bind well. If the design does not allow the overmolding to create a sealed bonding then an adhesive gets added. For small components applying adhesives can cost more time and manpower. So this process needs to get integrated into the manufacturing process. The adhesive also requires heat and pressure for the best adhesion. The process design is such that the adhesive layer gets injected first. Next, the thermoplastic gets injected. The mold is then closed and the parts cool and harden. Overmolding is also done in such a way to protect the adhesive from degradation. This happens when the overmold completely covers the adhesive layer. For this to work better it may also adhere to the substrate layer. This way elements from the environment like water and dust do not come in contact with the adhesive.
Conclusion
Overmolding creates products that combine two different types of materials. One of the materials serves as the substrate while another as the overmold. Several combinations of hard and soft materials for overmold exist. Manufacturers should consider The properties of each material for compatibility. There are various reasons overmolds get used. The general idea is to combine the properties of two or more materials. This variation of injection molding offers more possibilities. It’s application cuts across many industries.
