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4 Step Know About Open Mold Injection Molding

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Open mold injection molding is a widely used manufacturing process in which molten plastic is injected into a mold to create various plastic products. This method offers several advantages, such as high production quantities, versatile design options, and cost-effectiveness. In this article, we will delve into the intricacies of open mold injection molding, exploring production quantity, design frequently asked questions (FAQ), common manufacturing problems, and providing examples of injection molding design.

Production Quantity

One of the key advantages of open mold injection molding is its ability to produce large quantities of plastic parts quickly and efficiently. This method is ideal for mass production due to its high-speed production capabilities. The process begins with the creation of a mold, typically made of metal, which is designed according to the specific requirements of the product. The molten plastic material is then injected into the mold, where it cools and solidifies, taking the shape of the mold cavity. This process can be repeated continuously, allowing for the production of a large number of identical parts with high precision and consistency.

Design FAQ

Designing for open mold injection molding requires careful consideration of several factors:

1. Material Selection:

The choice of plastic material is crucial as it impacts the product’s properties, such as strength, flexibility, and appearance. Common materials used include ABS, polypropylene, and polystyrene, among others.

2. Wall Thickness:

Maintaining consistent wall thickness throughout the part is essential to prevent defects like warping, sink marks, or voids.

3. Draft Angle:

Draft angles are necessary to allow for easy ejection of the part from the mold. Without proper draft angles, parts can get stuck, leading to production delays.

4. Undercuts:

Avoiding undercuts or designing them in a way that allows for mold release is important. Special mold features like slides or lifters may be required for complex parts.

5. Gates and Runners:

The location and size of gates (entry points for molten plastic) and runners (channels that distribute the plastic within the mold) affect part quality and production efficiency.

Common Manufacturing Problems

While open mold injection molding offers numerous benefits, it is not without its challenges. Common manufacturing problems include:

1. Warping:

Warping occurs when there is uneven cooling, leading to deformations in the final product. Proper mold design and temperature control can mitigate this issue.

2. Flash:

Flash is excess plastic that escapes from the mold cavity and forms thin, unwanted edges on the part. It can be minimized through precise mold design and machine calibration.

3. Sink Marks:

Sink marks are depressions on the surface of the part caused by uneven cooling or material shrinkage. Adjusting material flow and mold temperature can help reduce sink marks.

4. Short Shots:

Short shots happen when the mold does not completely fill with plastic, resulting in incomplete parts. This issue can be addressed by optimizing gate locations and increasing injection pressure.

Examples (Injection Molding Design)

Let’s explore a couple of examples of injection molding design:

1. Bottle Caps:

Bottle caps are commonly produced using open mold injection molding. The design typically includes a seal to prevent leaks, threads for easy closure, and a tamper-evident feature. Material choice is crucial to ensure the cap’s durability and sealing properties.

2. Smartphone Cases:

Smartphone cases come in various designs and materials, ranging from rugged, impact-resistant cases to slim, aesthetic ones. The design must accommodate the phone’s dimensions, buttons, and ports while providing protection and style.

Additional Open Mold Considerations

Apart from the discussed points, it’s essential to consider:

1. Mold Maintenance:

Regular maintenance of molds is crucial to ensure the quality and longevity of the production process.

2. Cavity Inserts:

For parts with multiple components or materials, cavity inserts can be used to create specific features or color variations.

3. Automation:

Automation can further enhance the efficiency of open mold injection molding, reducing labor costs and increasing production speed.


Open mold injection molding is a versatile and efficient manufacturing process used for producing a wide range of plastic products. Understanding the production quantity, design considerations, common manufacturing problems, and real-world examples of injection molding design is essential for successful and cost-effective production. As technology continues to advance, open mold injection molding remains a cornerstone of modern manufacturing.

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