Understanding Mould Components
If you are looking to produce plastic parts, it is crucial that you understand the various mould components. This will help you select the right thermoplastic for your application.
These components are built from materials that withstand high temperatures and pressures, such as hardened tool steels or self-lubricating bronze alloys. They are also designed to promote proper venting of the molding process.
Core/Cavity or “Inserts”
A core and cavity are the structural portions of a mold that shape the interior dimensions and features of the finished plastic part. Whether the desired result is a complex internal thread or a sliding mechanism, these are essential components of a successful mould.
A solid metal or other non-plastic material is inserted into the cavity of the plastic part to form specific structural and functional characteristics. This process is known as insert molding. A variety of materials can be inserted into an injection-molded product, including metals to improve wear resistance or conductivity.
The resulting combination of these metals and plastic is often used to create a part that is lighter and stronger than an individual metal component, but still provides the benefits of a thermoplastic. This is an important consideration, especially when a design requires high strength or durability.
In some cases, the insert can be designed to be collapsible during the forming process. This can be useful when a large internal feature must be formed in the same injection cycle, but it can also lead to unwanted dimensional variations and warping of the finished product.
To ensure proper insertion of an insert, the locating surfaces should be drafted to the opposite direction mould parts of the parting surface. This will help prevent the insert from sticking to the A-side of the mold and bending or tearing off when the part is ejected.
Runners
The runner system distributes the molten plastic from the injection point to the individual cavities in the mold. It’s critical to the quality of the molded part, the operational efficiencies of the molding process and the cost of production. Injection molding systems are designed with either a hot or cold runner. Both serve similar functions, but they are tailored to address different facets of the injection process.
The shape and size of the runner system directly impacts the performance of a multi-cavity injection mold. For instance, the runner size has an impact on filling pressure, material viscosity, warping and shrinkage. It also has a significant influence on the cycle time and ejection force of the finished parts.
A well-designed runner system should have a uniform diameter to ensure a consistent flow of plastic to each cavity during the injection process. This helps the press to maintain a steady flow-front velocity between each cavity, which reduces the potential for defect and improves part quality.
A runner balance analysis can help the designer resize the runners to optimize the filling balance between the cavities and stay below a target injection pressure. It’s also important to make sure there are no sharp corners on the runner and at all runner-to-gate connections, as these can lead to stress cracking during ejection.
Ejector Pins
Ejector pins are used to force parts out of the mold. These are usually located on the B-side of the mold (mounted to the moving side of the molding machine). Finished products often have a dent or “pin mark” from these ejector pins. The design goal is to position these pins to minimize cosmetic witness marks on finished product.
The ejection pins require considerable force to push a part out of the mold. The pins can be broken by this if they are too small, so it’s important to choose the right size. Also, location is critical—placing pins in hard areas like ribs or pillars helps to distribute the force over a larger area and prevent breakage. It’s also helpful to have a draft angle in the design of the part to facilitate smooth ejection.
Choosing the right ejector pins can save time, money, and resources. Generally, the larger the pin, the better as this will increase ejection force and reduce damage to the mold and finished product. It’s also a good idea to have a selection of different types of ejector pins available, including shoulder, sleeve, and blade pins. Local distributors or specialized online marketplaces that carry mold components can be convenient sources for sourcing these. These vendors typically offer a wide selection of sizes to choose from and can provide guidance on selecting the best options for specific mold requirements.
Parting Lines
The parting line is a feature that separates the core and cavity of a plastic molded product. It is often (but not always) found along the edges of a plastic product’s outer surface, like a cup’s brim. A well-placed and managed parting line can improve the quality of an injection molded product. A poorly placed or managed parting line, on the other hand, can lead to problems with the way a product functions. If a parting line is not properly aligned or managed, the two mold halves may not fit together correctly or the ejector pins might have trouble pushing the molded product out of the mold.
A plastic product’s parting line can vary with the geometry of the design. For instance, more complex shapes may require a more complicated parting line layout to accommodate injection moulding parts manufacturer the shape. Additionally, the injection molding process can play a role in where a parting line should be placed. For example, insert molding requires a particular type of parting line layout that is different from other injection molding processes.
There are several types of parting lines, including vertical, beveled, curved, and stepped. These parting lines can be used with either hot or cold runner systems. Parting lines are also sometimes angled to better balance the force of injection on both mold halves and prevent sliding between the two sides of the mold. These angled parting surfaces are typically only used in very complex parts, and they can be difficult to machine.