One of the most popular forms of custom manufacturing is injection molding. However, this process can be used for many materials including metal (die casting), glasses, elastomers. And most commonly thermoplastic and thermosetting polymers.
To share our readers with real-life insights and knowledge, we’ve interviewed Injection molding technician, Tedi Paduraru.
What Are the Advantages of Injection Molding?
The main advantages are the ability to produce a large number of parts quickly, with fast production cycles and relatively low labor costs (little time and effort to mount the production). In addition, its output production is high, as you’re able to manufacture large part quantities within 1 hour based on cycle time. There’s also low waste during the process. In fact, material waste can even be reused in other machines when processing the material.
What Are the Disadvantages of Injection Molding?
The cost for low production volumes is very high. If you only need a small number of parts. Typically under 100,000. The cost of tuning, testing, and mold design outweighs the benefits of injection molding. But once you exceed 100,000 parts and plan to produce the same design repeatedly, injection molding becomes the most efficient and cost-effective method.
What Scenario Is Injection Molding Better than Other Manufacturing Processes?
It boils down to how many parts you want to produce and within how much time. The main disadvantage is that injection molding doesn't like sharp corners. In principle, if you want sharp corners it's not going to be efficient. There will be weak points. Although you can overcome this, it will be very costly and complicated.
Depending on the design of the part. You can choose injection molding if you’ve got a simple design. But if your part is more complex, then CNC machining may be more suitable.
What Are the Main Components of an Injection Molding Machine?
As a whole, there is a hopper that feeds the granules into the barrel. The barrel contains the extrusion screw, where the plastic is fed and melted. As the screw turns, it pushes the plastic forward while moving itself backward, preparing the plastic to be injected into the mold.
Then there's the mold where the plastic is injected. Once the process of melting and pulling back is ready, the machine is ready to inject the plastic into the mold. It’ll spend an amount of time cooling down into the shape of the mold, after which the mold opens up, and the plastic is injected into the conveyor belt and is now ready to be used straight away once left the machine.
What Is Automatic and What Is Manual in Injection Molding Machine Setup?
Machine programming has a number of parameters that engineers work with. These include speed, time, and temperature. The injection speed is the speed that the screw moves forward and pushes the plastic into the mold. Essentially, the injection pressure is the pressure needed for the screw to achieve and maintain that speed through the whole movement until the mold is fully filled with enough plastic.
The holding pressure, after the sprew reaches its maximum filling point, is used to prevent the plastic from flowing back. As it's around 400–600 miles of pressure, technicians need to make sure it holds enough position until the plastic is solid enough that it doesn't flow back anymore.
Then we have the back pressure, which comes into play when the screw turns and pushes the plastic forward in front of it. Ideally you want it to achieve the optimal density of the plastic before it is injected into the mold, as that´ll give you much more control. So the back pressure is the pressure that resists the screw from moving back until the material reaches the right density. Once that’s achieved, the screw slowly overcomes the pressure and fills the top of the barrel with enough plastic for the next shot.
The temperature is the melting temp of the plastic, which differs depending on which type of plastic, from 60 to 400 degrees (the maximum an injection molding machine can handle). The mold’s cooling system is also important to monitor. Each type of plastic will have to cool down for a certain amount of time to be able to maintain its dimensions.
What Factors Should You Consider for Injection Molding Part Design?
When designing a part, you’ll have to think about what you’re planning to do with this part and the environment the part will be used in.
For example, if the part is used outside in cold conditions, you’ll need a material that’s more flexible and has a lower melting temperature. Materials like polypropylene are flexible. They’re resistant to cold and don’t get brittle. They also stay stable enough in hot weather to maintain their shape.
If the part will be for performing mechanics. Use nylon or pop. These parts are really strong as their melting point reaches 300 degrees. We also use a glass filling which greatly improves the strength of the part. So based on what you want to do with the part, you decide which range of melting temperatures you want to use. As a general rule of thumb. The higher the melting temperature, the more mechanical resistance you have. However, keep in mind that with more mechanical resistance, you’ll also sacrifice flexibility. You need to know what you’re going to do with the part, where you’re going to use it, and how you want it to perform.
What Are the Best Practices for Managing Injection Molding Cycle Time?
Talking about cycle times, once the program is set, we have an average cycle time. There are types of processes that run fully automatic. That means that the machine opens and produces the part, runs continuously, and requires no intervention between cycles. These would be parts without inserts. Basically the technician doesn’t have to do anything on the machine, as it's fully automatically producing parts one after another.
If you have any metal inserts or anything to add, it’ll run a semi-auto cycle. This means that the door will be opened after every shot. The insert is loaded. Then the door closes and the button is manually pressed. This increases cycle time by 10 to 15 seconds at least.
All in all, human factors increase cycle time. The more efficient a machine is, the more automatic it is. The more automation in the process, the more efficient the cycle time will be.
What Are the Steps After the Design Process?
Step 1: Compensate for Material Shrinkage
The manufacturing of the actual mold starts once the design process is completed. When you begin manufacturing, you have to consider the details. For example, how much the plastic is going to shrink after molding. This helps you determine how much extra material needs to be added to the overall parts dimension inside of the mold.
Step 2: Calculate Gas Vents
You also have to calculate gas vents. These are channels small enough that the part doesn’t flow into them. But large enough to let air escape when the plastic is injected.
Step 3: Mount the Mold and Connect Core Systems
The setup process for the molding machine is simple. The mold can be connected using four bolts, along with the injection system and the cooling system, which can use water, air, or oil. In some cases, the oil reaches very high temperatures, while water operates around 190 degrees.
Step 4: Set the Fill Volume, Pressure, and Temperature
After the mold is mounted, it's enough to set it in place and calculate the amount of plastic that goes into the mold, setting the right pressure and temperature. In a few simple steps, you get around 90-95% filling before the pressure is applied.
Step 5: Apply Holding Pressure for Part Integrity
After you’ve confirmed the part is 95% filled, you apply pressure to the plastic. That pressure gives the part its final details, consistency, density, and dimensions.
Step 6: OptimIse the Forward Hold Time
After applying the pressure, you have the timing. It’s about how long the screw needs to stay forward until the plastic is solid enough that it doesn’t flow back. These timings can be sensitive. If you add just 2–3 seconds per cycle, over 100,000 cycles that adds up to a lot of time.
You’ve got to find the sweet spot when the plastic is just solid enough not to flow back, and set it at exactly the right moment.
Conclusion: Injection Molding Explained by a Technician
Injection molding remains one of the most powerful tools in manufacturing. It's fast, efficient, and capable of producing thousands of identical parts with minimal waste. Like any process, it comes with trade-offs: high upfront costs mean it’s best suited for larger production runs, and design details like sharp corners can create challenges if not properly managed. But when the material, design, and machine settings are all dialed in, injection molding delivers impressive strength, consistency, and speed.
If you’re planning your next injection molding project or just want a second opinion on your part design. Feel free to reach out to us at Haizol Global. Our team is happy to offer advice and help you get the best results.