Types of Injection Molds: Examining Various Categories and Features

Injection molding is an integral process for creating plastic parts for several applications. This technique requires a host of materials as well as different types of injection molds. These injection molds are generally classified based on their features, including feeding system, number of cavities, and mold plate.

Every mold used for injection molding fits a label within each category. For instance, you can have a three-plate mold that has a cold runner system and multiple cavities. Therefore, it is crucial to understand each of the types of molds in injection molding to choose the right one for your projects.

All components have unique designs. Thus, your choice of mold will depend on the plastic parts you need to make. In this article, we will discuss the different types of injection molds and what they entail. Before we dive into that, let’s examine the various components of plastic injection molds.

What Are Plastic Injection Molds?

multi cavity injection mold

Plastic injection molds help carry out the injection molding process. Custom mold tooling may be created with aluminum or steel for efficient results. They often consist of two parts – fixed molds and moving molds.

Fixed molds are installed on stationary plates of the molding machines while moving molds are on movable plates of the machine. Both fixed and moving molds stay closed during the injection molding process to create the injection system and cavity. When opened, these machinists separate the molds to remove the products.

Components of Plastic Injection Tooling

The structures of plastic injection molds often vary based on material properties, types of injection molds, and the structure of final products. However, the basic structure remains the same.

components of an injection mold

Guide Pins

Mold cavities and core halves open and close several times during plastic part production. Therefore, guide pins and bushings in injection mold construction help ensure the perfect alignment of mold halves when the mold is closed. They support mold plates to prevent distortions that may damage the mold or cause injection molding defects

Runner

Sprues and runners work together at the beginning of every injection molding process. The plastic material flows in the runners through the sprue bushing. Individual runners then take the plastic to the gate, the entry point to the molding cavities.

Gate

Injection mold tooling has a gate design that allows the injection of molten plastic into the injection molding cavity. The common gate designs are:

  • Edge gate – is located at the edge of the component, and it is ideal for flat parts. It is also suited to multi-cavity two-plate molds. 
  • Sub gate – automatically trimmed gate that gives more flexibility, allowing you to place it at any location on the component. 
  • Hot tip gate – a hot rubber gate located on top of the part instead of the parting line. It is best for conical or round shapes that require uniform flow. 
  • Direct or sprue gate – a manually trimmed gate for single cavity molds. It is easy to design and provides high strength, making it suitable for large cylindrical components. 

The injection molding gate design, type, and location will affect the parts’ appearance, dimensions, and warping. 

Sprue

A sprue provides support for the injection molding nozzle—the nozzle seats on the surface of the sprue bushing. As a result, it serves as the first pathway for distributing liquid plastic into the mold.

Locating Ring

The locating ring fits over the sprue bushing. It works to place the mold in the right position on the molding machine. Therefore, it ensures proper alignment of the sprue bushing and the nozzle to eliminate leakage.

Mold Cavity

The mold cavity is the inner part of the mold machined to take the shape of the intended final product. The machinist injects the molten plastic into the injection molding cavity to create the finished product. Some molds (e.g., multi-cavity molds) often have several cavities with balanced feed systems to avoid defects. 

Ejector Pins

These precision ground pins combine with ejector plates to effectively remove parts from the mold once they cool. After the components cool, the mold opens, and the ejector pins help push the finished product out of the mold. 

The Shot

This is the volume of plastic material that fills the mold cavity, provides a pressure cushion, and compensates for shrinkages. It helps transfer pressure away from the screw to the mold cavity.

Ejector Plate

Ejector plates provide a base for mounting the ejector pins. The plate moves forward to allow the pins also to move forward and push the component of the cavity or core.

Clamping Plate

Clamp plates help attach the mold halves to the molding machine to begin the injection molding process. The injection molding tooling has two large clamping plates that close when the machinists inject the plastic into the cavity. When the plastic part is cool enough, they open the mold and allow the component to fall out of the mold halves.

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4 Types of Injection Molds Classification

There are various types of injection molds based on different attributes. Choosing the right plastic injection mold types for your project is crucial. The mold will impact production speed, overall costs, and the quality of end products.

a type of plastic injection mold

The different types of molds are classified under four main categories:

1. Based on Feeding System

The first category here is the classification based on the feeding or runner system. The feeding system is a series of channels, including the gate, sprues and runners. This channel directs the molten plastic from the nozzle to the desired parts of the cavity. The types of injection molds under this category are: 

Hot Runner Injection Mold

The hot runner system uses runners that have been internally or externally heated with coils or rods. This type of feeding system is encased within a stationary manifold plate. They remain permanent within the plate, preventing them from ejecting with the part. 

hot runner injection mold

As a result, the molded part will come out clean without any extra plastic waste. This reduces waste production and eliminates any additional processes to remove or recycle the runners. Furthermore, there is usually no need for regrinding or post-processing with hot runner injection molds. Therefore, the production process is much faster. When combined with multi-cavity molds, this injection mold tooling helps with large volumes of complex and fine shapes. 

However, there are some downsides to using the hot runner system. The system is usually costly to set up and heat. The maintenance is also expensive, and it would require skilled specialization to supervise the molding process. Since the runners are hidden, it may be difficult to ensure they are clean and void of leftover plastic from previous production runs. This is often a big issue when dealing with color changes. Finally, there may be sticking problems with internally heated hot runners, especially when working with heat-sensitive materials. 

Cold Runner Injection Mold

The cold runner system delivers molten plastic into the injection molding cavity using unheated runners. Unlike a hot runner injection mold, the cold runner mold is exposed because the operator cuts it into the mold. You would need to make a new runner system for every molding cycle. Depending on the plate type of mold plate used, you may have the molded part, and runner system joined or separated upon ejection.

cold runner injection mold

These types of injection molds are much easier to clean and maintain. They also ensure faster and easier material and color change. Additionally, you can easily incorporate injection molding design iterations due to the fast-changing gate and runner positions. While internally heated hot runner molds struggle with certain materials, cold runners can work with an extensive range of materials.  

However, mold runner injection molds produce lots of waste for each cycle. Most manufacturers often dispose of the runners after use. Although some manufacturing companies find ways to regrind and reprocess these runners for repeated use, doing this lengthens the entire manufacturing process. Regrinding the runners also often alters its physical properties, affecting the end product. As a result, there may be variations from the original design.

Insulated Runner Molds

insulated runner molds

These tools look more like the traditional cold runner molds. However, they use cartridge heaters or other forms of heating to create a surrounding layer of molten plastic. As a result, they form insulated “culls” to create similar effects to the hot runner systems.

Manufacturers often choose this approach because it does not require a temperature controller, thereby making it less expensive than hot runners. Furthermore, it allows easy and faster changing of materials and colors. However, insulated runner molds do not work with every kind of material. They typically are not suitable for demanding engineering-grade plastics.

2. Based on the Number of Cavities

This category consists of three types of injection molds based on the number of parts they can produce per molding cycle. They include the following:

single cavity injection mold 1

Single Cavity Injection Mold

Single cavity molds produce one part per injection cycle. Although it may be a slow production mold type, the tooling costs for this mold type are lower than other options. As a result of the relatively lower cost, low-volume productions are usually more affordable.

A single cavity mold also allows for better control of the molding process. Manufacturers often employ multiple single cavity tools to ensure the continuous running of the process even if one mold breaks down.

Multi Cavity Injection Mold

multi cavity injection mold

The design of a multi-cavity mold allows it to create multiple identical parts in one injection cycle. The initial cost for the injection mold tooling is usually higher than single cavity molds. However, this type of injection mold is more suitable for large volume productions. This is because each mold can produce several components at once. As a result, the production speed is faster, and the cost for a unit part is lower.

Family Injection Mold

family cavity mold

Family molds also have multiple cavities like the multi-cavity mold. However, it is best used to make several parts in one cycle. For instance, manufacturers can use a family mold to create one right and one left component iteration. On the other hand, a simple multi-cavity mold can only produce one iteration in one cycle.

While a family injection mold is expensive, it usually reduces the overall production costs. This is because one family mold can be useful for various components. It also saves a lot of time and operation costs. However, you must note that these plastic injection mold types are only suitable for components made from the same material and color. You will need separate molds for parts that require different materials and color variations.

3. Based on Mold Plate

Another important category to classify the types of molds in injection molding is the number of their plates. As mentioned earlier in this article, the plates make up the entire mold cavity for adequate part production. The injection mold types under this category are:

Two Plate Injection Mold

two plate injection molding

This is the most common mold type in this category because it has low tooling costs. A two-plate injection mold has one parting line where the core plate and cavity plate meet. The gate, runner, and parting line must also align in this type of injection mold. It is compatible with any runner system but is best combined with single cavity molds.

Three Plate Injection Mold

three plate injection mold

This mold’s additional plate (stripper plate) gives it two parting lines. This is placed between the cavity and core plates, automatically separating the runner system from the molded part. This ensures faster production because there is no need for manual separation or recycling of the runner system.

However, the additional plate increases the overall tooling cost because the cutting must be precise to match the other two plates. Note that the three-plate mold is only used for cold runner systems to detach the runner. Hot runner injection molding systems do not require this type of mold.

Stack Injection Mold

stack injection mold

A stacked mold has multiple parallel mold plates. There can be two, three, or four levels of plates to make the process more efficient. Stacked molds require lesser clamp tonnage per cycle. This type of mold has a higher upfront cost because it takes much longer to build. However, the less clamp tonnage requirement reduces the operating costs. The mold can even be designed to accommodate several materials at once.

4. Based on Unscrewing Injection Mold

These are the best molds for making threaded holes within plastic parts. They are automated molds with drive systems made up of:

  • Rack and pinion
  • Electric motors
  • Hydraulic motors

unscrewing injection mold

This unique mold rotates the drive system to create threaded components such as bottle caps, nuts and bolts, automotive parts, shampoo bottles, and cosmetic packaging. These components are often difficult to remove with the standard knock-off method based on the draft angle.

Therefore, the unscrewing injection mold help carries out the removal without damaging the threads. Undercut injection molding is also crucial here because it allows the release of complex parts with damage. It operates at high speed, reducing cycle times and ensuring high volume productions.

Why Are Injection Molds Necessary?

injection molding importance

Custom mold design offers several advantages in plastic injection molding. Some of them include:

Limitless Possibilities

Injection molds make it possible to manufacture virtually anything, ranging from medical devices to high-end mechanical components. Depending on your applications, various molds can be chosen to make your project faster and more efficient.

Material Selection

Although most plastic resins are ideal for injection molding, they don’t have the same qualities. Some plastics are more prone to shrinkage than others. Therefore, mold type and design often take into account the kind of plastic materials in question. The type of mold chosen will greatly impact the final result.

Product Consistency

Injection molding tooling designs can help you reduce manufacturing errors significantly. Most parts of the process are automated, increasing the accuracy of products and eliminating human error. As a result, you can improve the consistency of end products, even in large volume production runs.

RapidDirect – Reliable Injection Molding Services Provider

As we mentioned earlier, the type of mold and its design plays a huge role in your final product. Therefore, selecting the best choice that fits your materials and applications is essential. If you’re having trouble making the right plastic injection mold types for your project, let RapidDirect help you.

RapidDirect injection molding services

Our professional injection molding services encompass mold making, mold design analysis, and plastic parts manufacturing. We can help you manufacture high-quality injection molds to create durable and reliable plastic molded parts. Our engineering team has the skills and experience to deliver the best injection molding solution.

Do you already have a mold design? You can contact our experts to get professional advice for your design and optimize it for better production and cost savings. Once you’re ready, upload your design file to our online quotation platform to get an instant quote and free DFM analysis from our engineers.

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Conclusion

Injection molding is an effective manufacturing technique useful for several applications. However, the choice of mold will determine the result of the manufacturing process. We have discussed the different types of injection molds in this article based on their features, including feeding system, number of cavities, and mold plate. This will help you make the best decision when you’re ready for your project.

Designing injection mold tooling is a technical and skill-intensive task that requires expert handling and process control. Therefore, you must work with a reliable partner to provide high-quality injection molding services. Contact RapidDirect today for reliable and cost-effective production of high-quality plastic parts.

FAQs

What Type of Injection Mold Is Most Commonly Used?

Three-plate multi-cavity injection molds are the most commonly used tooling for plastic injection. This is because of their increased efficiency and high production speed. However, your choice of mold will depend mainly on your material, budget, and intended application of your product.

What Are the Most Common Materials for Injection Molding?

The most common materials for injection molding are acrylic, ABS, nylon polyamide (PA), polyethylene (PE), polycarbonate (PC), polypropylene (PP), and polystyrene (PS).

Are Different Molds Designed for Different Plastics?

Yes, different molds are designed for different plastics depending on the properties and color of the plastic. Plastics have different shrinkage levels. Therefore, you must design the molding tool to account for the shrink rate of your plastic. This will ensure that the dimensions are correct and the plastic part meets tolerance requirements.

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