Mold Parts and Their Impact on the Injection Molding Process
Mold parts are crucial to the injection molding process. They can have a great impact on the dimensional stability of a molded part.
The sprue bushing is a tapered hole through which the molten plastic enters the mold. It is usually made of case-hardened medium carbon steel material (CHMCS). It also includes the nozzle.
Ejector Pins
Ejector pins help eject molded parts from the mold. They push against the tops of ribs in the B-side half of the mold to eject them from the mold. When they’re not properly designed, ejector pins can cause a variety of problems, including part sticking to the ejector plate or cams. They also may not have enough surface area against which to push, such as in a grate mold where all that faces into the B-side half are the tops of ribs. To solve this, designers sometimes add bosses to the ejector plates that act as ejector pads.
Another issue with ejector pins is that they often break during the injection process. This is due to the mismatch between the force needed to eject the part and the pin’s strength. A simple solution to this problem is to increase the number of ejector pins or use a larger diameter pin, which will distribute the force over a wider area and reduce the likelihood of breakage.
Another common ejector pin defect is “marking.” This occurs when the ejector pins leave a mark mould parts on the molded product. It’s important for design teams to work closely with engineers to optimize ejector pin size, shape and position. They should also ensure that the ejector pins are properly lubricated to prevent thermal expansion and wear.
Runner System
A runner system conveys the molten plastic from the nozzle through passageways called runners and gateways to reach the actual mold cavities. The design of a runner system can have large effects on part quality, molding efficiency, cycle time and waste generation. Injection molds can use a number of runner system configurations: sprue, main runner, sub runner, cold slug and gate.
Runner systems can also be hot or cold, depending on the type of plastic used. Runners that are heated allow the mold to make larger parts and reduce cycle times. However, they require an additional machine for heating and cooling the runner system. This can increase upfront costs and the cost of maintaining the equipment.
The shape of the runner system is also important. The ideal shape is a circular channel milled into both mold halves that meet at the parting line. This allows the runner system to maintain pressure and flow without losing pressure across the gap between the gate and cavity. Two alternative shapes are the parabola and trapezoid, which only require milling into one mold half. These create a higher pressure drop and are less suitable for most applications.
The diameter of the runner system also has a large impact on the flow rate, with the final runner size determining the amount of material that flows through the gate. Ideally, the last runner should be sized to be 1.5 times the thickness of the gate wall it feeds into. This will help ensure that the gate and cavity fill at the same rate, and prevent premature freeze of the molten plastic.
Parting Lines
The location of the parting line can have a significant impact on molded parts, both cosmetically and functionally. It determines the direction of mold opening, which dictates how well a feature must be drafted to allow for easy ejection from the mold. It also affects where vestiges left by the mating surfaces of the two mold halves will be, which can influence how a finished plastic product looks. Finally, it can impact the cost of machining and other secondary operations needed to finish a molded part.
The choice of a parting line type depends on the product’s design, the desired material properties and production process, and the characteristics mould parts of the molding machinery. Straight lines are the most common, while stepped and contoured lines can be used for aesthetics or increased strength. Venting holes are often placed near the parting line to help displaced air escape during injection.
Getting the parting line right can be challenging, especially on complex products like the plastic toy soldier shown here. The problem is compounded when the parting line needs to pass through or over a functional feature such as a rib. Then the challenge becomes balancing form and function, with designers and molders working together to ensure that the parting line is both attractive and functional. They do this by incorporating holistic design reviews and flow analysis, using both simulation and experience.
Inserts
Inserts are small parts that are inserted into a molded plastic part to provide specific functional features or aesthetics. They can be anything from electrical components to metal reinforcements. They are generally designed with the same geometric parameters as the molded part they’ll be integrated into, and should also have reliable dimensions. This helps ensure that the pressure of molten plastic doesn’t dislodge the inserts and send plastic into spaces it shouldn’t. It’s important to design inserts with reliable shutoffs, too. This is a feature that prevents leaks and cosmetic flash, and helps maintain the longevity of the injection mold.
It’s essential to use the right inserts and tooling for the plastic injection molding process. In general, inserts should be smaller than the finished product and have a small interference fit into the mold. This will help reduce the stress on the mold and improve the quality of the finished part. It’s also best to opt for regular surfaces rather than curved ones, which can cause stress points.
The most common type of inserts are threaded, which means they have threading built into them to hold other components in place. They’re often made of metals like brass or stainless steel, but they can also be made from different types of plastics. The injected plastic will then encapsulate the insert, creating a unified, multi-material product. This technology can save manufacturers time and money by combining the molding and assembly processes into a single step. It also allows for more flexible design options by incorporating different materials into a single, integrated product.
