What is the difference between insert moulding and overmoulding?
Using the injection moulding technique known as overmolding, items made of two or more materials can be produced. “In-mold assembly” is another name for it. Insert moulding and multiple-shot moulding are two separate procedures that fall under the general category of overmolding. Although it refers to the process of coating any prepared object with plastic using injection moulding, insert moulding (sometimes written insert molding) is frequently used to add metal features to plastic parts, such as threaded bosses.
To increase exterior qualities like impact resistance and feel, multiple-shot moulding is used to produce plastic parts from different materials, such as polypropylene and silicone rubber, within a single moulding process. The two types of processes are discussed in this article along with applications where one method might be preferred.
What is insert moulding?
Insert moulding, often referred to as metal insert moulding or plastic insert moulding, is frequently used to add metal pieces to injection moulded parts while the parts are still being moulded instead of after they have dried, reducing the need for post-molding production and assembly processes. Mould cores that are cast into the actual moulds are covered with inserts either manually or automatically.
Following the sealing of the mold’s halves, plastic pellets are plasticized in the injector barrel and fired into the cavity, the flowing plastic material encasing the collected inserts. The inserts are now enclosed in the parts when they are expelled from the mould after the plastic has hardened. One step is taken in the process of insert moulding.
Many times, inserts are threaded nuts that are used in subsequent part assembly, but they can also be bushings, sleeves, pins, blades, or even non-metallic components. From a variety of manufacturers, standard inserts are available. In order to better cling to the plastic, many inserts feature knurled outside surfaces.
A layer of plastic or rubber material is added to an existing metal or plastic object using the overmolding method known as insert moulding. The metal or plastic component is once again inserted into a single-shot injection mould before the overmold material is injected around it. A prefabricated substrate and a single-barrel injection moulding machine are used in single-shot overmolding to add a second layer of material to the product.
Double shot overmolding: what is it?
The thermoplastic elastomer (TPE) is added to plastic substrates in this type of plastic overmolding, also known as two-shot, multi-shot, or double-injection moulding, to enhance the appearance, handling properties, grip, impact resistance, or environmental resistance of injection moulded parts. The overmolding procedure, which adds additional material quickly after the substrate is produced to generate strong bindings between components, is often carried out using multi-barrel injection moulding machines. By using plastic overmolding, manufacturing stages can be cut out while still producing improved injection-molded goods.
Materials may be mechanically or chemically attached, depending on the substrate and overmold materials chosen (via undercuts, for example). For the purpose of strengthening any potential chemical bonds, mechanical bonding is frequently advised. Nylon, polycarbonate, ABS, and HDPE are among the numerous substrate materials that are appropriate for overmolding.
Materials used as overmolds include styrene-ethylene/butylene-styrene copolymers, thermoplastic elastomers such thermoplastic polyurethane (TPU), thermoplastic rubber (TPR), and thermoplastic vulcanite (TPV), and thermoplastic rubber (SEBS). When a softer rubber surface is required for a plastic product, silicone overmolding is also employed.
Comparing insert moulding and overmolding
Both overmolding and insert moulding combine solid plastic or metal components with additional liquid plastic to produce an item that is formed of several different materials. However, overmolding comes in two different varieties, one of which is insert moulding, which uses pre-made inserts.
Contrarily, double shot moulding entails the creation of an injection moulded item and subsequent placement of that part in a second injection mould for the purpose of covering it with a different plastic or the same plastic in a different colour, all in the course of the same manufacturing process.
Applications for Overmolding
Along with its widespread use in generating parts that are assembled with threaded fasteners, insert moulding is a typical manufacturing technique for producing connectors, electronic sockets, etc. Many hand tools, including screwdrivers, medical instruments, etc., have soft grips thanks to insert moulding.
In addition to being helpful for many higher-value products like medical equipment where it can give increased grip, cleanability, moisture protection, and other properties, multi-shot overmolding has many uses in large volume personal products like toothbrushes and disposable razors.
A two-step method that first encases the circuit in low-melt-temp resin and then adds a higher temperature protective shell can be used to overmold delicate electronic circuits. Overmolding is a technique used by automakers to eliminate the need for additional adhesive-based assembly stages. A plastic engine part with an overmolded elastomer seal is an example of a typical use.
Both insert moulding and two-shot overmolding are common services offered by injection moulding manufacturers. Two-shot overmolding is typically utilised for high volume production where the high capital costs are offset by lower labour costs due to the increased cost needed in specific multi-shot machinery. Multi-shot moulding typically makes financial sense at part volumes of above 250,000 per year. Two-shot moulding is also occasionally the only method for forging a reliable connection between dissimilar materials.
It’s a widespread misconception that the softness of the overmold material alone determines how the finished product feels when overmolding is used to create soft grips. The thickness of the layer that is applied also matters a lot because a thin coating will allow the softer top coat to feel through to the tougher substrate.
Injection moulding design for overmolds
Overmolding is a challenging process, especially given the numerous factors at play and the potential for mistakes to be repeated in countless injection-molded duplicates.
Several general guidelines are applicable when designing for overmolded parts: Overmold edges should not feather; instead, they should come to a sudden, full halt against a stop or into an indentation.
The overmold’s surface texture can considerably help the part release from the mould. Additionally, it masks any moulding imperfections by giving the part the appearance of being softer than it actually is. Ample draught angles aid in the release of mould, as well. Gate and vent designs need to be carefully considered by mould designers.
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