In plastic injection molding, speed, pressure, and temperature are the three most important parameters that control the quality of each molded part. Specifically, injection speed determines how the molten plastic is injected into the mold cavity and to what extent it’s packed into the facility with high pressure to generate a solid part. The faster a machine injects the molten plastic into the mold tooling press, the more oriented the final part material molecules will be.

Injection speed is one of these parameters that strictly determines overall product performance, and it’s also related to the aesthetics of the product since lower speeds contribute to product surface defects.

Factors Affecting the Choice of Injection Speed

One of the important process parameters that affects the quality of the molded part to a large extent is “injection speed.” The injection speed decides how fast the material is injected into the mold. If it’s too slow, then the material tends to cool down even before flowing completely inside the cavity and results in cold slug formation (cooled/hardened material that flows with/in front of actual material). Also, if low injection speeds are used, then many times complete filling does not take place, i.e., there are unfilled gaps in the part—again affecting its fill and final product use.

1. Material Type and Viscosity

Different plastic materials react quite differently to heat and pressure. ABS, polypropylene (PP), and polycarbonate (PC) all have different flow properties. A high-viscosity plastic will need a slower injection speed to ensure uniform fill of the cavity, whereas a low-viscosity plastic can be injected at higher speeds using a high-speed injection molding machine.

2. Part Geometry & Wall Thickness

The part’s geometry largely determines the injection speed. In particular, thin-walled parts or parts with long flow paths require fast injection molding to prevent the molten plastic from solidifying before all sections have filled.

3. Mold Design and Gate Configuration

A well-designed injection mold is able to determine the appropriate injection molding speed and the amounts along with the locations of gates that directly impact the process of molten plastic arriving into the mold. If they are too small or far from thick sections, higher injection speed will be required to fill up the cavity quickly.

In high-performance plastic mold manufacturing, especially when producing automotive or electronic plastic parts, the flow behavior should be analyzed by Autodesk Moldflow or other similar software before injection speed is set on the machine.

4. Cooling System Efficiency

The cooling stage of plastic injection molding impacts cycle time and product quality. In cases where high precision is required, such as electronic or medical plastic parts production, the cooling rate must be in synchronization with the injection speed to achieve a perfect surface finish and part dimension.

How to Determine the Right Injection Speed

One of the most important factors that impacts plastic parts manufacturing quality is the injection molding speed. The right injection speed affects how molten plastic fills into a mold cavity, cools down, and forms the finished product’s surface. Otherwise, wrong injection speed in the injection molding process may cause defects such as short shots, flash, warpage, or even unacceptable dimensional differences. As a result, engineers and operators need to research and take control over the injection molding process to make it more accurate and productive.

1. Educate material characteristics

Different plastics have different flow characteristics. For example, materials like polypropylene (PP) and polyethylene (PE) have high melt flow rates and can be injected at higher speeds. On the other hand, materials like ABS or polycarbonate (PC) need moderate injection speeds to prevent burn marks or jetting. The plastic injection molding operator should learn about the melt flow index (MFI), viscosity, and cooling property of the material chosen before deciding on the speed.

2. Study the Mold Design and Geometry

The injection mold design should distribute the molten plastic uniformly before solidifying. Flow paths, gate locations, and runner sizes should be uniform to ensure this. Engineers can try different speed profiles with the help of injection mold design software to get the best one. Modifying gate sizes/locations, vent positions, cooling channels, etc. could possibly make that grade better.

3. Evaluate Wall Thickness and Flow Length

Higher injection speed is needed in injection molding when the wall thickness is thinner so as to avoid premature solidification of the molten plastic before the cavity is filled completely. If there are long flow paths in a part with high-speed injection molding machines, the melt can also arrive at the farther section before temperature and pressure decrease. Conversely, for applications with thicker walls, lower speeds may be used since the melt would remain melted for a longer period of time.

4. Utilize Process Monitoring and Information Systems

Today, almost all machine builders offer process monitoring systems as options on their machines. These systems either monitor the cavity pressure, temperature, and fill time in real time or sample the data every few milliseconds or seconds. Based on this information, the injection speed can be adjusted dynamically to maintain part quality.

High-Speed Injection Molding Applications

High-speed injection molding is one of the most advanced and efficient methods to manufacture plastic parts. Manufacturers produce high-precision injection-molded components at high speed with consistent quality if they are produced on a high-speed injection molding machine. Hence, this process is used in the industries demanding accuracy, speed, and repeatability in their plastic parts production.

1. Automotive Industry

Within the automobile industry, industrial injection molding makes use of high-speed plastic injection molding machines for the production of lightweight plastic parts. Dashboards, door panels, bumpers, electrical housings, and interior trims are just some examples of components & parts made by industrial injection molding. High-speed injection molding is required to ensure each part meets exact tolerance and dimensional requirements, as more often than not these components need to be assembled with the help of other precision-engineered parts.

2. Electronics and Consumer Goods

Thin, precise, and heat-resistant parts are needed for the electronics industry. High-speed injection molding is perfect for creating smartphone casings, electrical connectors, and small mechanical switches. These components have a high requirement on dimensional accuracy. It means that the injection molding machine has to keep the best plastic flow speed during the whole production cycle.

3. Medical and Healthcare Devices

High-precision molding produces complex, sterile, and reliable plastic parts for the medical industry. Syringes, IV connectors, diagnostic cartridges, and implant casings are made by manufacturers using high-speed injection molding machines with incredible accuracy.

Conclusion

In the plastic injection molding world, that right balance is how fast or slow you should run an injection molding machine to make parts. The answer is to start slow and gradually get faster until you can fill the part to 99% of its ideal fill rate and complete the cycles consistently from shot to shot and run to run (or as close as possible).

There are a few primary controls in the speed category, but speed over time rated in inches per second or mm per second is typically what we are referring to when we talk about injection molding speed. However, here’s a list of some other speed variables for reference:

• Injection Speed
• Fill time
• Injection Pressure
• Sink Rate

The reason that “speed” is generally considered the most important control in controlling how well melted plastic fills into your mold cavity at high speeds (or even low) is because it tends to affect pretty much everything else, including cooling times.