Author: Kevin Graham

Plastic injection molding is an ideal production method for high-volume parts with stringent dimensional tolerances. It’s a relatively inexpensive process that can produce millions of identical and consistent parts. Read on Plastic Mold Injection Process Florida for more information.

The two halves of the mold are securely closed by a clamping unit prior to molten plastic being injected. Once the cavity is filled to about 95% the screw changes from speed control to dwell pressure to complete molding and compensate for thermal shrinkage.

Plastic injection molding is a process in which liquidized plastic is poured into a pre-formed mold that takes the shape of the desired product. This is known as the tooling, and it’s a critical component to producing high-quality precision components. The injection molding process requires complex, high-quality, durable and robust tooling. It can cost tens of thousands of dollars to design and build the tools necessary for production, so it’s important that every detail is carefully considered from the initial planning and design phases to ensure the best quality parts are produced.

The type of thermoplastic chosen is crucial to the success of the project, as different materials have varying flow characteristics, shrink rates and thermal properties that must be taken into consideration during the initial tooling design phase. It’s also important that the chosen material is compatible with the injection molding machine, to prevent problems such as warping and sticking.

A skilled machinist will take the concept of the finished product and create an incredibly precise tool to produce the correct shape. This is a much more difficult process than CNC machining, as there is no turning back once the mold is created and is being run on the injection molding machine. It’s therefore extremely important that the machinist and designer work closely together to ensure that the final product will be exactly as intended.

Injection molds are typically made of steel or aluminum and can be used for millions of plastic parts over the course of their lifetimes. In order to ensure that the molds are as durable as possible, they are hardened and tempered. This process makes the molds more resistant to wear and tear, increasing the number of parts they can make before needing to be repaired or replaced.

A variety of methods are available for troubleshooting injection molds, such as industrial CT scanning and flow studies. Identifying issues in the process and taking action can help reduce cycle times, energy consumption and part defects, which leads to lower production costs.

Injection molds are highly versatile, allowing for a wide range of part designs and features. Slides and lifters are a great example of this. These are angular inserts that can be added to the mold, allowing for the creation of undercuts and other recessed surfaces. They can also be swapped out for specialized inserts such as metal inserts for threading and other high-strength features.

Injection

A hopper holds raw plastic material that feeds into a heated barrel. This barrel contains the mechanism for heating and injecting molten plastic into the mold cavity, which is called an injection unit. The injection unit typically uses a reciprocating screw to force the plastic into the mold, but other mechanisms may be used.

A mold’s temperature must be carefully controlled to ensure the molten plastic is thoroughly melted and mixed. Depending on the size and shape of the plastic part, this can require temperatures ranging from 300 to 800 degrees Fahrenheit.

Once the molten plastic is inside the mold, it begins to cool as it makes contact with the interior surface of the mold. During cooling, some shrinkage occurs, and this can cause the plastic to stick to or warp the inner surfaces of the mold. A plastic injection manufacturer can use several techniques to reduce or eliminate shrinkage problems during the cooling process, including using a cooling agent or adding thermal insulation to the injection mold.

When the plastic is fully cooled, it can be ejected from the injection mold by a mechanism that is attached to the back half of the mold. The ejection system can also be triggered by the hydraulically powered clamping motor to keep the moveable platen in contact with the stationary one and prevent it from opening while the injection and cooling processes are underway.

Injection pressures are a major factor in the quality of a plastic injection molded part. Injection pressures are a balance between the main hydraulic clamping pressure of the machine and the injection pressure applied to the screw. Injection pressures should be high enough to fill the entire plastic part, but not so high that it overshoots the mold’s maximum wall thickness and creates a gap between the plastic and the molding surface. The injection pressure also depends on the specific type of plastic and the structure and dimensions of the plastic part. The proper injection pressure can help to improve the dimensional accuracy, surface smoothness and strength of a plastic part.

Cooling

Cooling is a crucial step in the injection molding process. It’s a time-consuming part of the cycle, but it’s essential for quality and accuracy. The cooling phase allows the molten plastic to solidify within the mold cavity, which is necessary for the correct shape and structural integrity of the finished product. If the plastic isn’t cooled properly, it will not take on the desired shape and will instead remain in a liquid state. This can result in poor dimensional accuracy and weak structures.

After the heating injection nozzle has liquified the resin, it is then injected into the mold using either a ram injector or reciprocating screw. The clamping motor then actuates the mold to keep it closed while the injection occurs. Once the injection is complete, it’s time for the cooling to begin.

The molded plastic must be cooled to a temperature where it will retain its shape and can be safely ejected from the mold for inspection and quality control. Ideally, the cooling should be as fast as possible to minimize cycle times. However, it’s important to remember that excessive cooling will add time to the overall cycle time without improving product quality.

There are a few key factors that influence the cooling time of the molded plastic. One of the most important is the thermal diffusivity of the plastic material, which can be calculated from information available on the material data sheet. This factor can be influenced by the complexity and placement of cooling channels in the mold as well as by the type of cooling system used.

Another key factor is the wall thickness of the molded plastic. Thick sections of the plastic will take longer to cool than thin sections. This can lead to stress concentrations at the transition points between thick and thin sections, which can cause warping, twisting, or cracking in the final product. To avoid this, a mold should be designed with walls that are uniform in thickness throughout the entire area of the mold.

A successful plastic injection molding process requires careful consideration of all factors that impact the process. Efficient cooling isn’t usually the first thing that comes to mind for most designers, but there are a variety of methods and designs through which effective cooling can be achieved. Crescent Industries can help you determine the best method for your specific application and create an efficient cooling system that results in improved production efficiency and faster cycle times.

Ejection

During the cooling step of the plastic molding process, molten plastic solidifies into the shape of the final part. It’s important to minimize changes in pressure, velocity and plastic viscosity during this time to avoid mold defects. Some shrinkage of the finished plastic part may also occur during this time, depending on the thermodynamic properties of the material and maximum wall thickness.

Cooling equipment helps reduce deformation of injection molded parts by increasing the speed at which the molten plastic cools. This also ensures that the plastic reaches its melting temperature more evenly, which can help prevent dimpling and warping of the finished product.

Once the molded plastic has cooled sufficiently, it’s removed from the mold by a mechanism known as an ejector. The ejector system uses force to push the solidified product out of the cavity, which can be a challenge because the molded plastic shrinks as it cools and adheres to the mold.

This is where a good design of the ejector system comes into play. Various types of ejector systems exist, such as sprue, gate and ejector pin marks, all of which affect the quality of a finished plastic part. Using a high quality ejector pin that is specifically designed for the molded part can reduce ejector mark formation.

Another way to reduce the risk of deformation is to increase the cooling time or improve the cooling conditions so that the molding material and mold temperature are as close as possible. Similarly, increasing the injection speed can lower the viscosity of the melted plastic to prevent dimpling and other defects.

Finally, it’s important to choose the right plastic material for your injection molded product. Some materials are better suited to certain applications than others, especially those that require transparency or high impact resistance. For example, polycarbonate is useful for products such as eyeglasses and sporting equipment because it offers the benefits of transparency with a strong impact resistance. Other common plastics include polyethylene, nylon and polylactic acid. Each of these materials has different melting temperatures and viscosity, which impacts how easy it is to work with during the plastic molding process.