Control Considerations for Plastic Injection Molding

Injection-molded parts have been part of the manufacturing industry for decades. Everything from smartphone cases to toys or airplane tray tables can be manufactured using injection molding. Chances are, you are within 3 feet of an injection molded part right now.

If you are having trouble finding an injection molded part, consider that most plastic and rubber pieces are injection molded. Polymers such as these are the most common injection molded materials.

 

Figure 1. Injection molding machine. Image used courtesy of Brockey

 

The injection molding process has several basic steps: material preparation, injection, mold release, and quality assurance. Even though this is a mature manufacturing process, each step has its challenges and benefits from both automation and increasingly smarter industrial control.

 

The Injection Molding Process

Material Preparation

The first series of steps in the injection molding process involves batching, blending, and heating the raw materials. Plastic beads or shreds of recycled plastic are fed into a mixing hopper. The size, shape, composition, and color of the beads will impact the final product. Bead colors are often mixed to achieve the desired color; however, plasticizers may be added to improve flow properties and uniformity, and so on.

One area that has benefitted from machine vision is color matching and uniformity. Before machine vision, some parts would have minor variations in color, as the colors were selected by recipe rather than through real-time measurements. Conversely, with well-trained machine vision, the colors can be altered during mixing so that the part-to-part color variation is below the detection limits of the human eye.

The beads are mixed and melted using electric heating. The heater controls use a feedback loop from either a thermocouple, thermistor, or RTD to monitor the temperature. This must be tightly controlled because plastics degrade under excessive heat, and colors will be affected differently. Some of the impurities are allowed to burn out, but precision control is needed in this step.

Furthermore, the melting often occurs in the auger barrel, headed toward the mold. This means the temperature profile through the barrel must be well-controlled and tracked to ensure that properly melted and mixed plastic is exiting the auger barrel into the injection molding dies. The system designer must integrate the auger rotation controls and heating profile.

 

Injection

The injection process looks simple: molten plastic flows into a mold and is allowed to cool. As it turns out, this process is significantly more complex. 

 

Figure 2. Video used courtesy of Tooling U - SME

 

Injection nozzle and die design control the success of the entire process. The nozzle is constantly filling up with solidifying plastic, so it must be heated. It also must make a tight seal with the mold to prevent material from squirting out around the nozzle. Often, molds and nozzles are a matched set, meaning a change in mold design requires a nozzle change.

Auger barrel controls must also account for the nozzle design because the injection pressure will relate the volumetric flow rate of material to the injection pressure at the mold. 

The injection pressure is important because too high of a pressure will result in excess material squirting out of multipart molds (called flash) and can also lead to stringers. Stringers, or jetting defects, are when high-pressure jets of plastic shoot into the mold, but because of their narrow width, they freeze on the outside and leave a squiggly defect in the final part. Stringers can also be limited through proper mold design—placing a feature such as a hole or pin near the nozzle will prevent long stringers from forming.

 

Figure 3. What a stringer looks like. Image used courtesy of Progressive Components

 

For most manufacturing facilities, injecting one mold and waiting for it to cool down is inefficient. Instead, there must be a method for lining up a mold, injecting it with plastic, and moving it out of the way for the next mold. Furthermore, the auger control must be linked to this process, often running in reverse to prevent plastic from running out of the nozzle during a mold change.

All of this motion control will benefit from improved artificial intelligence (AI) algorithms, which will adjust for minor changes in heating, flow rate, and so on, to ensure that a mold is in place whenever plastic is flowing.

For example, consider a situation where the temperature is running a little warmer than usual, but not enough to damage the plastic. The increased temperature decreases the viscosity of the plastic, meaning it flows a little faster, and it also increases the volume of the plastic due to thermal expansion. Ultimately, more material is dribbling out of the nozzle. Smart motion control could detect this and speed up the mold changing routine to account for the higher temperatures, while the temperature control reduces the temperature.

It’s all a balancing act.

 

Mold Release

Depending on the facility, the molds may be allowed to naturally cool, or they may be cooled through forced convection—air or water. Once again, mold design is crucial to proper molding. Cooling fins, ports for air escape during injection, and risers, which store excess material to account for shrinkage, are often required for uniform parts.

Outside of mold design, uniform cooling is essential. Without it, parts, particularly thin-walled or flat pieces, will warp during cooling. Temperature control across the mold dictates how much shrinkage and warpage occur, as well as how long it takes to cool the part so that it can be removed from the mold.

Besides temperature control, the mold release likely has a mechanism for separating the part from the mold. This could be mechanical or pneumatic, meaning a ram or an air valve must be controlled and may receive its control signal from the temperature monitoring system in the mold, or perhaps a simple timer.

 

Quality Assurance

The final piece to the injection molding process is quality assurance. Parts typically receive some light machining to remove the sprue, risers, and any traces of flash. They need to be smooth enough not to cut the human hand that uses the product at a minimum, but higher quality parts will show very little mold parting line.

 

Figure 4. What the product should look like versus how it looks with flash. Image used courtesy of Techno-UMG Co.

 

The locations of the sprue, risers, and mold parting line will be the same for each material, so this control will need to be set up for each item, but then requires little maintenance. Cutting surfaces will wear and need to be replaced. Furthermore, a visual inspection that shows lots of flash means that the injection pressure is wrong or the mold surfaces are worn.

Color, stringers, and other visual defects can be detected by machine vision, if the part selling price warrants the investment.

Automation and computer control will improve the quality and throughput of injection molded parts. Advances in machine vision, AI, and sensors will improve the feel and finish of parts, making them more desirable and longer-lasting in the hands of consumers. From the outside, injection molding looks like an easy process; however, there are many small control processes to consider.