Choosing the right plastic injection mold for your requirements is far from a straightforward task, owing to the different types of choices available. Plastic injection molds are categorized based on the feeding system, cavity design, mold plates, and unscrewing mechanism.
It’s vital to consider the complexity and size of the component, thermoplastic material, budget, aesthetics, and expected precision level when selecting the mold. You should also be able to produce your part efficiently with a minimal defect rate.
With that in mind, let’s look at the structure and function of plastic injection molds and go into detail about the four main types of injection molds.
What Are Plastic Injection Molds?
A plastic injection mold is a custom tool created to achieve the desired shape of the end product. It is a hollow component into which molten plastic is injected for curing.
The mold is fitted to the injection molding machine and is usually made of metal like aluminum or steel for enhanced durability.
A plastic injection mold typically consists of two halves. The cavity half, also known as the A side, is attached to the stationary plate, while the ejector half, known as the B side, is attached to the moving plate.
Both halves remain clamped shut when pouring the plastic. The B side opens after the shape is solidified, ejecting the completed product.
Components of Plastic Injection Tooling
Here is the basic anatomy of a plastic injection mold tool. Some features may vary depending on the type of injection molding machine used for the process.
Guide pins locate the core and cavity sides of an injection mold and align them precisely. It will ensure the molding process happens accurately without any errors. Guide pins are essential to prevent potential damage or misalignments that occur when the mold cavity halves open and close repeatedly.
Runners are passages that transport the molten plastic to the mold cavity gates. There are cold runners and hot runners. Cold runners maintain the same temperature as the mold cavity, while hot runners are heated components that deliver better precision.
The gate is essentially the entrance to the mold cavity that lets the molten plastic into the mold. The shape, location, and gate size play a crucial role in the formation of the end product. There are six main types of gates in injection molding machines.
- Edge gates: They are located at the edge of the product and are ideal for forming flat parts or medium sections. Edge gates are very common and often leave an imprint.
- Hot tip gate: Built for hot runner systems, these circular-shaped gates on the A side maintain the same temperature and pressure to ensure plastic is in a liquid state until it goes into the cavity. It is ideal for circular-shaped parts.
- Direct/sprue gates: With a lack of a runner, direct sprue gates allow the plastic to flow directly from the sprue to the mold. It is a simple and economical option ideal for single-cavity molds.
- Submarine gates: These angled gates are below the mold parting line and allow automatic trimming when the product is ejected. As molten plastic flows from the bottom, it eliminates the chances for imprints.
- Pin gates: These narrow gates are on the B side of the mold and are used for producing thin-walled parts with a high level of precision. You can have multiple gates here, allowing the plastic to flow into the cavity from many channels.
- Tab gates: These are similar to edge gates but have an extra tab section to absorb shear stress and allow for a cleaner trimming process. Manufacturers prefer them for creating flat products.
Sprue is the passageway through which molten plastic flows into the mold. It is the primary channel connected to the injection molding nozzle and is usually larger in diameter. Runners can further divide the distribution of the plastic.
Fitted over the sprue bush, the function of a locating ring is to position the mold correctly on the injection molding machine. It prevents leakage by ensuring the nozzle and sprue bush are aligned correctly.
While the locating ring is a simple component, it is vital for the smooth operation of the mold system.
The mold cavity is a hollow-shaped component made according to the product’s shape. The melted plastic is injected into this cavity and allowed to cool.
It is the primary component responsible for forming the exterior of the product. Depending on the complexity, single or multi-cavity molds can be used.
Ejector pins, also known as bouncers, are on the B side of the mold and are responsible for pushing the solidified part out of the cavity. As they exert force to remove the part, ejector pins can leave an imprint on the product.
A shot is the maximum volume of molten plastic injected into the cavity, accounting for shrinkage. It takes the pressure away from the screw to the cavity with a cushioning effect. In some cases, multiple varieties of resins are injected simultaneously, which can be dual-shot or multi-shot.
An ejector plate holds the ejector pin heads and stops them from removing during molding. Two ejector plates are used for the task, and they move together with the pins to push the solidified product out of the cavity.
An injection molding machine consists of two wide clamping plates designed to hold the two halves of the mold. They hold the halves tightly shut together when molten plastic is injected, preventing the mold from being opened due to pressure. Once cured, these plates open to eject the product.
4 Types of Injection Molds Classification
Injection molds can be categorized based on how they function. They vary in speed of production, quality, and precision of the final product, and cost of operation, and also demand different levels of expertise.
Based on Feeding System
The purpose of the feeding system is to transport the molten resin from the barrel’s nozzle to the mold cavity effectively.
The features of the feeding system and its components, like the sprue and gates, determine the quality, precision, and finish of the end product. Here are the main types of feeding systems available today.
Hot Runner Injection Mold
Hot runner systems aim to keep the melted resin in its state until it reaches the mold cavity. A hot runner injection mold has a manifold to maintain the temperature of the feeding system, similar to that of the heating barrel.
There are both externally heated and internally heated runner systems. External systems are used for heat-sensitive polymers, while internal heating systems are utilized to control the resin flow effectively.
The ability to control temperatures of hot runner systems has allowed manufacturers to use a wide range of resins for injection molding and produce parts with high precision and structural integrity.
Cold Runner Injection Mold
Cold runners come in two-plate and three-plate systems. While two-plate is ideal for most molds, three-plate is preferred for parts with complex geometries. The feeding system in a cold runner injection mold is unheated, and the temperature is the same as that of the mold cavity.
The channels used for transporting the resin are larger than the part to prevent underfilling. Cold runners are also an affordable choice and are easy to maintain.
The benefit of cold runner systems is that you can use resins regardless of their heat sensitivity. However, they require a higher volume of material, increasing the cost per unit. The lack of flow control can affect the quality and precision of the end product.
Insulated Runner Molds
Although insulated runner molds look similar to cold runner systems, they imitate the functionality of hot runner systems. They have thick-walled runners to keep the resin melted when transferring it to the mold cavity.
The molten polymer is injected into the runners and allowed to cool and form a layer around the runner walls. It helps maintain the temperature and ensures the next shot of resin flows in its liquid form.
Insulated runners are inexpensive compared to hot runners because the feeding system is not heated with a manifold. But, they are not ideal for producing extremely durable products due to issues like thermal degradation. The starting process of insulated runners is also quite complicated.
Based on the Number of Cavities
We can also categorize injection molds based on the number of cavities in a mold.
Single Cavity Injection Mold
As the name implies, a single cavity mold can only produce one unit at a time. It is ideal for large-sized parts, products that demand high precision and quality, and low-volume orders.
Designing single cavity molds is cost-effective, and you can achieve any shape. There’s also greater control over the processing parameters, and achieving the desired result is easier. However, the production time can be longer.
Multi Cavity Injection Mold
A multi-cavity injection mold can produce multiple units of the same part in a single production cycle. The number can range from two to hundreds. It is faster, more efficient, and is a good choice for high-volume production. It’s ideal for producing small parts.
However, multi-cavity production is riskier, as a mishap in a cycle can lead to multiple failed units. Also, the mold and the feeding system are costly to design and may require more investment than single-cavity molds.
Family Injection Mold
Family injection mold can produce different components in a single production cycle. It is helpful when manufacturing components that are later assembled into one product.
This type of mold helps you achieve a consistent color for all the components. It is cheaper compared to designing single-cavity molds for each part of the product.
But, designing a family injection mold is very complex as you may have to use different parameters to achieve different wall thicknesses and volume requirements within the same mold. Also, it’s only suitable for parts produced using the same resin in the same color.
There’s also a loss of control over the consistency, and it may affect the quality of the final product.
Based on Mold Plate
Injection molds can also be set apart based on the number of mold plates.
Two Plate Injection Mold
A two-plate mold is the most common type used for producing a wide range of components. It has a single parting line for the moving and stationary plates to meet. In a cold runner system, the part has to be removed from the mold manually.
Two-plate injection molds are an economic choice because they are simple in design and easier to construct. In a multi-cavity mold, the runner and gate need to be near the parting line to ensure proper ejection.
Three Plate Injection Mold
A three-plate injection mold has a runner plate between the top and base plate and has two parting lines. It is usually used for parts that are difficult to achieve from two-plate molds. These plates separate during opening where one plate allows for the part ejection while the other enables sprue ejection.
You can gate a three-plate mold from any side of the cavity and also can use pin-point gates. Also, it can cut the gate automatically during ejection and offers a better-quality output.
Three-plate injection molds are only necessary for cold runners. As the design is complex and needs to be highly precise, it may incur a higher construction cost than its counterpart.
Stack Injection Mold
Stack injection mold refers to a group of interconnected and same-sized molds stacked together. This specialized injection mold is the most complex structure of the three. It has multiple parting lines, with each surface having one or many cavities.
Stack injection molds are usually available in 2, 3, or 4-level configurations. They reduce production time significantly and are ideal for high-volume production purposes. However, you will require experienced operators to handle the system efficiently.
Based on Unscrewing Injection Mold
Unscrewing molds are specially designed for threaded parts that are difficult to produce using standard injection molds. These components cannot be removed by the knock-off methods effectively.
The technology combines movement and rotation powered by a hydraulic cylinder to produce the desired part. After the injected plastic cools in the mold, the mechanism in the mold gradually unscrews the threaded core. Once fully unscrewed, the mold opens and ejects the part.
Some parts produced using this mold type include,
- Bottle caps and lids found in bottles
- Threaded parts for vehicles
- Screws, nuts, and bolts
- Medical supplies
- Threaded components in consumer goods and appliances
- Light fixtures
- Construction equipment
Why Are Injection Molds Necessary?
Here are some benefits of choosing injection molds for your manufacturing requirements.
Plastic injection molding has enabled manufacturers to design and produce parts with complex geometries with ease. The shape and size of your product are no longer a barrier because of custom mold tooling.
It has also made high-volume production with consistent quality and faster lead time possible. Different types of molds give you endless possibilities as long as the design is within the constraints posed by injection molding.
The availability of both hot and cold runner systems means the choice of resins is wide and varied. Plastic injection molding also supports materials that are costly to machines using other manufacturing processes.
It means that you can produce high-performance and heavy-duty products cost-effectively. You can review the properties of suitable thermoplastics and their shrinkage rates and select the best option for you. It’s also noteworthy that the mold type will determine the final output.
You can produce millions of a unit easily with injection molding while keeping the defect rate minimal. With the right setup and material choice, you can expect all the molded parts to be consistent and reliable in quality.
As most injection molding processes are heavily automated, the chance for human error is also less. It will reduce material wastage and other costs related to defects and help you increase your profit margin.
Kemal – Your Reliable Injection Molding Services Provider
Selecting the right injection mold can be a challenging task, especially if you have less experience. Don’t worry. We can help with the decision and ensure your manufacturing requirements are fulfilled satisfactorily.
Kemal is a leading manufacturer in China offering high-quality injection molding solutions. We are equipped with cutting-edge solutions and long-standing expertise to cater to your manufacturing requirements effectively.
Our expert team members can guide you every step of the way and recommend the best resin and mold type for your product!
Have questions? Get in touch with our support team for assistance.
We hope our guide helped you understand the different types of plastic injection molds available and their unique functionalities. It’s important to consider how the feeding system, cavity design, and mold plate system will affect your product requirements before selecting the right mold type.
We highly recommend you get in touch with an expert manufacturer to optimize your injection molding process.