Injection molding is a process that creates large quantities of plastic parts identical to each other. The method operates by forcing melted plastic into a unique mold cavity. Completing the plastic part comes when the plastic cools, turning into a solid mass.
The process involves a great number of daily-used plastic products such as phones, car parts, and LEGO bricks, which are manufactured in this manner. A high number of parts can be made at once and the costs are kept to a minimum.
This article will cover the complete process. You will find out about the machine, the steps, and some pivotal design rules. Acquiring the basics on how does injection molding work is the first and fundamental step for starting to manufacture good, low-cost plastic products in your company.
The Components of an Injection Molding Machine
To grasp the concept of injection molding, you must first understand the machine. The simplest injection molding machine consists of two primary parts, namely, the Injection Unit and the Clamping Unit. To manufacture the final component, these two systems should work hand in hand, together. People of acknowledged authority assert that it is essential to master the major parts of an injection molding machine.
While the Clamping Unit is the part that closes the mold and assists in the removal of the finished product, the Injection Unit is the part that performs these functions – melting, and pushing the plastic.
The Injection Unit (Melts and Pushes the Plastic):
- Hopper: This is the main entrance for the small plastic piece to enter the machine which resembles a big funnel.
- Barrel: It is the heated part of the machine that is also the housing for the screw which is where the plastic melts.
- Screw: The screw is integral to the function of the barrel. It moves back and forth, pushing the plastic pieces to the front, melting them with heat and pressure, and pushing the liquid plastic into the mold.
- Heating Bands: These are electric heaters that are installed around the barrel and are responsible for providing the heat needed to melt the plastic.
- Nozzle: It is an end of the barrel where the melted plastic gum goes to the mold through the pressing.
The Clamping Unit (Holds the Mold and Pushes Out the Part):
- Mold: It is the main actor of the process. The mold is a metal block made of steel or aluminum which is typically hollow and cavity-shaped along the desired part to be manufactured.
- Clamping System: This is the high-force part of the machine that clamps the two halves of the mold together. During the injection of plastic under high pressure, it uses huge force to keep the mold shut.
- Ejector System: Outwardly, the ejector system is a series of pins that move first in and then out of the mold in order to empty it.
The Steps of Injection Molding: A 4-Step Process
The production is a quick cycle that runs through. Each cycle makes one or more parts in seconds to a few minutes. The process of injection molding is divided into several key stages that occur over and over again. This injection molding cycle is how it works.
The four principal steps in the injection molding cycle are as follows.
Step 1: Clamping
Most importantly, the clamping unit is the one that connects the two halves of the mold, making it complete. The clamping force has to be high. The clamping force keeps the mold shut tightly against the pressure of the hot plastic. If the clamp is not strong, plastic may be leaking. A problem called “flash” is created in this way. Clamping forces can be from a few tons for small parts to over 4000 tons for large parts like vehicle bumpers.
Step 2: Injection
The second step in the work cycle is the one where the injection takes place. After the mold is clamped tight exactly, plastic resin materials from the hopper are melted in the barrel. The screw is now moved by the pressure created by the melted plastic. The exact amounts of hot polymer are pushed through the nozzle and into the cavity of the mold. Injection happens very fast and at high pressure to ensure total cavity filling.
Step 3: Cooling
As the mold cavity fills with hot plastic, it begins to cool. It gets hard during cooling and takes the mold shape. The cooling phase is typically the longest cycle step. The actual time required depends on many conditions, such as: the plastic type; the thickness of the walls; the design of the cooling system. Sufficient cooling is crucial for obtaining the desired part’s form and strength.
Step 4: Ejection
After the part has cooled down and become solid, the closing unit removes and lifts up the two mold halves. The minute the mold is open, the ejector system pushes the finished part from the mold. The ejection part drops down into the bin and at once, the machine is back to a start. The mold is closed and the process is initiated once again.
Design for Manufacturing: 5 Keys to Success
It is not enough to know how does injection molding work; it is equally significant to design a part that can be produced with ease. This is referred to as Design for Manufacturing. According to our observations, these rules are the parts that are commonly overlooked and they result in delayed deliveries along with increased costs.
Sticking to these five rules will really help you in saving time, money, and issues.
- Keep Wall Thickness the Same
It is preferable to design your wall geometry almost equal in thickness. If you have much thicker wall sections, then cooling will be different. This will, in turn, lead to the part shrinking unevenly and would create defects like sink marks (small recesses) or warping (the twisting of the part).
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Add Draft Angles
A draft angle is a small taper that is added to the walls of the part that line up with the mold’s pull direction. This small angle, practically only a degree or two of taper, will increase the ease of removal for the part from the mold. Otherwise, the part’s surfaces may scratch the mold. This may result in damage, or the part sticking. -
Use Rounded Corners
You are advised to stay away from sharp internal corners in the design. They are the reason stress points are formed in the part since they are areas of weakness that make the part more likely to crack. Moreover, they obstruct the free flow of the hot plastic. It is smarter to use round ones instead. This enhances the flow of plastic and the resulting part is much stronger. -
Design Smart Ribs and Bosses
Ribs are the thin wall-like features that are used to add strength without increasing the wall thickness. Bosses, on the other hand, are the rounded parts intended for screws or mounts. Design these features by making sure they are about 50-60% of the main wall thickness. That way, sink marks won’t show up on the opposite side of the component. -
Avoid Extra Undercuts
An undercut is a feature that restricts the part from being directly ejected from the mold. Feature examples can be side holes and clips. These types of features require complex moving parts in the mold called side-actions or lifters. Such features also greatly increase costs and complications in the fabrication de moules par injection process. If you can design your part without undercuts, your mold will be simpler and cheaper.
Problem Solving: Common Problems and Their Causes
A good design does not entirely remove the risk of failure during the process of molding. Minor shifts in temperature, pressure, or speed can result in a variety of problems. Thus, it is important to know these issues in order to manage quality control. Many problems happen when there are issues with the basic principle of how an injection molding machine works, such as wrong pressure or heat settings.
Below is a table with common issues, their looks, and usual solutions for them.
| Problem | What It Looks Like | Common Causes and Solutions |
|---|---|---|
| Flash | An extra plastic layer seen only on the edge of the part. | Insufficient clamping force. The mold does not close properly. Injection pressure or speed are too high. |
| Marques d'évier | Very small craters or holes on the part surface. | The walls are too thick or of different thicknesses. Cooling time is lacking. Holding pressure is too low. |
| Short Shot | Not getting the complete part since the mold did not fill completely. | Definitely not enough plastic. Injection speed is slow or pressure is low. The mold is cold. |
| Déformation | The part gets bent or twisted out of shape while cooling. | The wall thickness is not uniform. The cooling is not even. The part was ejected while still too hot. High Design Stress. |
Materials and Molds: The Building Blocks of Your Part
The success of your project is based on only two types of hardware: the plastic material and the mold. The correct choice of the material and the right design make a huge difference in how your final product works and its cost.
Picking the Right Plastic
There are thousands of plastics available, but they fall into two main groups: thermoplastics and thermosets. Thermoplastics, which can be melted and solidified again many times, are the most common.
Here are a few popular thermoplastics and their common uses:
* ABS: Strong and impact-resistant. Used for LEGOs, keyboard caps, and electronic cases.
* Polycarbonate (PC): Very strong and clear. Used for eyewear lenses, safety goggles, and CDs.
* Polypropylene (PP): Flexible and chemical-resistant. Used for food containers, car parts, and living hinges.
* Nylon (PA): Strong, with low friction. Used for gears, bearings, and zip ties.
The Mold: The Most Important Investment
The injection mold is the foremost investment in your project. It is an expensive element that must be made very precisely. Molds are typically made from high-strength steel or occasionally from aluminum when the productions runs are shorter. Making them is a complex job that requires expertise and is often accomplished with high-precision techniques like China CNC machining services.
Molds also come with different systems for transferring the plastic into the cavity. The two main types are hot runner and cold runner systems. A cold runner is a simple channel in the mold. The plastic in this channel hardens with the part and comes out as waste, called a sprue. It is simpler and cheaper. A hot runner system uses heaters to keep the plastic in the channels melted. This reduces waste and can speed up cycle times, making it better for very high-volume production.
Conclusion: Bringing Your Product to Life with Injection Molding
We have described the plastic part from the very beginning up to the end of the assembly process. It arrives as simple pellets, gets processed by a machine, and goes through a 4-step cycle: clamp, inject, cool, and eject. The end result is a ready, precise, and perfectly manufactured product.
It is vital to keep in mind that the success of a project depends on much more than the machine alone. A smart design, which follows good design guidelines, is the key for achieving quality and being cost-effective. Learning how the injection molding process works is the key to making it at scale and efficiently.
Au Mékalite, we are famous for making complicated designs into high-quality physical products. Our specialized moulage par injection de plastique service can help realize your dream.
Frequently Asked Questions (FAQ)
What is the main difference between injection molding and 3D printing?
Injection molding is a “formative” process. It is greatly efficient in industrial production by using molds to create identical parts rapidly in thousands or millions of units. 3D printing is an “additive” technique to construct the component in layers from a digital file. It is suitable for the manufacture of prototypes, unique custom parts or low to medium scale runs.
How long does an injection molding cycle take?
An average cycle can be very quick, around a few seconds to just about two minutes. The precise amount of time is determined by several aspects. These involve the part’s size, the wall thickness (which controls cooling time), the kind of plastic material, the machine’s efficiency, and the mold.
Is injection molding expensive?
Although the cost upfront is high mostly due to the mold, the overall economic impact is low. A mold can range from several thousand to over one hundred thousand dollars, with huge fluctuations depending on the design and size. Nevertheless, once the mold is made, it only costs just a few pennies to produce each item. This is why injection molding is such a cheap manufacturing option when making large quantities.
What materials can be used in injection molding?
A very wide variety of thermoplastic materials can be utilized. Among the most common are ABS, Polycarbonate (PC), Polypropylene (PP), Nylon (PA), and Polystyrene (PS). The choice of material is dependent on the requirements of the part, such as strength, flexibility, heat resistance, clarity, and overall cost.
How does the injection molding machine know how much plastic to inject?
The machine is programmed with a specific “shot size.” This setting tells the screw how far to pull back. As it moves back, it allows a precise volume of melted plastic to build up in front of it. During the injection step, the screw pushes forward, forcing this exact amount of plastic into the mold. This is a critical part of how the injection molding process works to ensure consistency.
