Plastic injection molding is the commonest process of fabricating plastic parts. Many parts that vary in size, application, and complexity are produced using injection molding. From computer parts to automotive sections and home tools used today, the chances are that they have been made using plastic injection molding. The main question that everyone keeps asking is; how does plastic injection molding work?
In this post, we look at the plastic injection molding cycle, the main tools, and considerations for clients.
The plastic injection molding cycle
The cycle for injection molding is usually very short; about 2-120 seconds. The cycle can be seen in 4 main stages.
- Stage one: Clamping- This is what happens before injection of the plastic material into the mold. First, two halves of a mold are clamped together using a carefully calculated force to ensure that it can withstand pressure during injection. The time required for closing and clamping the molds depends on the machine being used. If the machine is larger, more time is required.
- Stage two: Injection –At this stage, the raw plastic in the form of pellets is added to the injection molding machine and moved towards the injection unit. Then, it is heated, and pressure exerted to the molten plastic. Within a very short time, the molten plastic is driven into the mold to occupy the all the cavities. A lot of care is required to avoid defects.
- Stage three: Cooling – Now that the molten plastic is inside the mold, it is cooled to the right temperature to take the shape of the expected part. During cooling, some shrinkage takes place in the mold. The packing of materials at the injection stage is carefully calculated to cater for this shrinkage. The cooling time can be calculated using thermodynamic properties of the respective plastic and wall thickness.
- Stage four: Ejection – Once the molten plastic has cooled enough, it is ejected to release the part that was being fabricated. Some force has to be applied to release the parts because shrinking might make the part to hold firmly unto the mold. Once the part is released, the process is repeated to make more parts.
Note that once the molding cycle is completed, an additional procedure might be required. If some material in the channels solidify and remain attached to the channel, some trimming might be needed. This is done using special cutters. The trimmed materials can be recycled back to the injection mold process.
The main equipment for injection molding
While injection molding has a lot of parts that come in varying configurations such as vertical and horizontal models, all of them must use a power supply injection unit, mold assembly and an appropriate clamping unit. These are the main equipment that runs the outlined 4 stages.
- The injection unit: This is a vital equipment that is used for heating and injecting the molten plastic into the predesigned mold. This part should have a hopper for receiving the raw plastic pellets.
The hopper contains an open bottom that releases the plastic into the heating barrel. It also has a ram injector/reciprocating screw that forces the plastic via the heated section crafted with a hydraulic. The latest reciprocating screws used by advanced manufacturers such as Sositar Plastics melts the plastic through friction, pressure, and heat. The molten plastic is rapidly forced into the mold before cooling starts.
- The cramping unit: Before the molten plastic is injected into the mold, two halves of the mold are secured together using a clamp. The Mold cavity (front part) and the mold core (rear part of the mold) are held in a movable pattern that allows sliding along tie bars. The clamp exerts ample pressure to withstand the force of the molten plastic. The clamp is only opened after the parts have cooled enough.
- Tooling: The Plastic Injection Molding manufacturer you select utilizes molds designed from aluminum or steel. Each half of the mold is fitted inside the injection molding machine while the rear part can easily open and close on the parting line. The main tools involved here include;
- The mold base: This and the mold cavity are mounted at the base of the mold. Then, they are fixed to the system platens in the injection molding machine. The front half of the mold base must have a support plate for attaching the mold cavity and the spruce brushing that allows the plastic to flow. On the rear half of the mold base are a support plate and ejection system.
- Mold channels: For the molten plastic to run through the mold cavities, some channels are included into the design. The plastic enters via a spruce while more channels referred as runners take the plastic to the cavities. A gate is located at the end of every runner.
The main considerations for injection molding
If you are considering fabricating parts via plastic injection molding, here are the main things to consider.
- The cost involved: The process of plastic injection molding can be very expensive. Make sure to factor the cost of design, testing, production, assembly, and distribution before deciding to move on. The more the parts involved, the lower the overall cost. If you only want a few parts, ensure to discuss with the selected plastic injection molding company on methods of keeping the cost low.
- Design considerations: Starting from the early stages of the part design, it is important to be very clear about the process of injection molding. The target at this point is simplifying the geometry and focusing on the tools to minimize any defect.
- Production considerations: Here, you need to think about two things; cycle time and assembly. The target should be minimizing the cycle time and assembly. These two considerations are perhaps the most crucial when picking the plastic injection molding manufacturer. Top notch companies such as Sositar Plastics have made a name because they use experts, advanced equipment, and always help clients to keep the cost minimal.
The process of plastic injection molding is used in making almost every item used for home and industries. Whether you are looking forward to creating a completely new part or want to redefine your current items in the market, the secret is the manufacturer you pick. Make sure only to pick the manufacturers with experience in the plastic injection molding and that are committed to walking with clients for better results.