A concise guide for draft angle in injection molding for product designers

draft angle injection molding guide

Draft Angle: A Vital Element in Plastic Injection Molded Product Design

Much like wall thickness, the draft angle plays a crucial role in the design of plastic injection molded products—a facet that demands attention from product designers. Neglecting the draft angle in the initial stages of product design, and deferring it entirely to the mold engineer for subsequent adjustments, can significantly amplify the workload and development cycle. This article aims to provide a concise introduction to the fundamental concepts of plastic product draft angles.

Understanding Draft Angle in Injection Molding

What is Draft Angle?

Draft angle is a critical element in the world of injection molding. It refers to the inclination angle set on the side wall of a plastic product in the demolding direction, which plays a pivotal role in the smooth demolding of the injection molded product. To put it simply, it’s the angle that facilitates the easy removal of the product from the mold.

Expressing Draft Angle

Draft angle can be expressed in terms of angles, typically ranging from 0.5° to 2.0°. However, it can also be represented by the ratio of the indentation in the thickness direction (X) to the height (H). This versatile measurement system ensures that the draft angle is precisely defined, making it a crucial aspect of injection molding design and production.

variables for draft angle

The Importance of Draft Angle in Injection Molding

Why is a Draft Angle Necessary?

During the process of injection molding, products tend to shrink as they solidify and cool inside the mold cavity. This shrinkage leads to a tight fit between the side walls of the plastic product and the corresponding walls of the mold. Consequently, a significant amount of resistance (frictional resistance) is generated during the demolding process.

By incorporating a draft angle, a small gap is created between the product and the mold when the product is partially separated from the mold. This minute gap allows for a smoother and more effortless separation, effectively minimizing or even eliminating the demolding resistance (frictional resistance). The inclusion of a draft angle is thus indispensable in ensuring the seamless and efficient demolding of injection-molded products.

plastic parts without draft angle
Not recommended--without draft angle
plastic parts with draft angle
Recommended--with draft angle

Consequences of Inadequate Draft Angle

Inadequate draft angles can lead to several critical issues, including:

  1. Scratches on Plastic Product Side Walls: When the draft angle is insufficient, it can result in the unwanted friction and scraping of the plastic product against the mold’s walls. This can lead to unsightly scratches on the side walls of the plastic product, compromising its overall quality and appearance.

  2. Surface Damage and Integrity Compromises: The lack of an appropriate draft angle can cause severe damage to the surface and structural integrity of the plastic product. This damage may manifest as whitening of the plastic, deformation, or even localized damage in certain areas. These defects not only affect the aesthetics but also the functionality of the product.

  3. Accelerated Mold Wear and Reduced Lifespan: Inadequate draft angles impose additional stress on the mold. As a result, the mold’s wear and tear increase significantly, causing it to deteriorate at a faster rate. The lifespan of the mold shortens, leading to the need for more frequent replacements or repairs, which can be costly and disrupt production efficiency. Proper draft angles help mitigate this issue, prolonging the mold’s lifespan and reducing maintenance costs.

Common Forms of Draft Angle

1. Outer and Inner Walls: The outer wall is typically formed on the female mold, while the inner wall is commonly created on the male mold.

draft angle of 2 degree for the outer and inner walls
draft angle of 1 degree for the outer and inner walls
draft angle of 0.5 degree for the outer and inner walls

Tip: Preventing Sticking to the Upper Mold:

In the case of a two-plate mold where the upper mold lacks an ejection mechanism, it is crucial to prevent the product from sticking to the upper mold during the mold-opening process. Failure to do so may necessitate manual intervention using tools to remove the part from the mold.

Frequent occurrences of this issue can disrupt smooth production and potentially affect the mold’s functionality. Consequently, the lower mold is often designed as a convex mold, ensuring that the plastic product adheres more tightly to it after shrinkage and is more likely to separate from the concave mold.

2. Reinforcing Ribs and bosses: These components are typically crafted in a tapered form, featuring a smaller upper part and a larger lower part.

draft angle of 0.5 degree for the ribs
draft angle of 1 degree for the ribs
draft angle of 2 degree for the ribs

Note: For ribs, the draft angle will make them thinner at the top as they indent from both sides. The higher the rib, the thinner the top will be. This will cause for the appearance and strength of the part.

Moreover, the higher the ribs, the higher the processing cost will be, and it will increase the difficulty of part ejection.

Generally speaking, it is recommended that the height of the ribs be less than 3 times the wall thickness, but this is not absolute. Sometimes the ribs can be made higher, but then you need to carefully choose the draft angle.

draft angle for boss with blind hole
draft angle for boss with through hole

Note: In blind holes, the top wall thickness differs significantly from the bottom due to opposing inclination angles. Meanwhile, to prevent sink marks at the bottom, maintain a bottom boss wall thickness below about 0.6 times the outer wall thickness (T1≤0.6*T).

Through-holes, however, since the inner and outer walls incline in the same direction, they allow uniform wall thickness throughout the height. At the same time, since the mold core for through holes has support at both ends, the boss can be made taller.

In conclusion, consider using through-holes for taller bosses.

3. Sliders: Draft angles along the direction of the slider’s extraction are essential for facilitating the smooth movement of sliders during the injection molding process.

draft angle for sliders

Note: In the illustrated example, the T pipe’s outer diameter is formed through upper and lower mold clamping without axial extraction. As a result, no draft angle is required for the outer diameter.

However, for the inner holes crafted with three inserts, a 1° draft angle is incorporated in the direction of extraction.

As a product designer, you only need to focus on general draft angle principles. Reach out for assistance; we’re here to help you check and refine draft angles.

Key Factors Influencing Minimum Draft Angle

Several critical factors impact the determination of the minimum draft angle required for successful demolding in injection molding. These factors include:

1. Shrinkage: Products with significant shrinkage necessitate a larger draft angle to facilitate mold extraction from the plastic part.

2. Wall Thickness: Increased wall thickness results in greater wrapping force on the mold, thereby demanding a larger draft angle for effective demolding.

3. Frictional Resistance: Higher levels of frictional resistance call for the utilization of a larger draft angle to ensure smooth and efficient demolding.

4. Complexity of Design Features: The presence of multiple reinforcement ribs, holes, and bosses in the design amplifies the required extraction force, compelling an appropriate increase in the draft slope to ensure successful demolding.

Correlation between Surface Texture and Draft Angle

An influential factor in determining the minimum required draft angle is surface roughness. It’s essential to recognize that when creating surface textures, the depth of the texture directly influences the necessary draft angle. Deeper textures, indicating higher surface roughness, demand a greater draft angle to prevent surface strain during mold ejection.

This table outlines the minimum draft angle required for different surface textures, serving as a guide for optimizing the draft angle based on the intricacy of the surface finish.

Correlation between Surface Texture and Draft Angle

The Innovation of Collapsible Cores

Introduction to Collapsible Core Technology

In certain situations where draft angles must be minimized or eliminated, the innovative solution of a collapsible core comes into play. This mechanism enables the cancellation of draft angles without compromising the smooth ejection of the product. The collapsible core operates by allowing the core to shrink when pulled out, creating a crucial gap between the core and the inner wall of the product.

Directional Options

Collapsible cores come in two primary directions: X single direction and XY direction. The X single direction offers a simpler structure and a more cost-effective solution. However, it only eliminates the draft angle in the X direction, while retaining it in the Y direction. Therefore, in cases where feasible, maintaining a one-way draft is still advisable for optimal design and production efficiency.


In conclusion, understanding the pivotal role of draft angles in plastic injection molding is key for efficient product design. By acknowledging its impact early on, designers can enhance collaboration and streamline the development process, ensuring optimal results in the intricate realm of injection molding.

kitchen table plastic container

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