Introduction: The draft angle of a mold is a big deal when it comes to designing an injection mold. It decides how hard it is to get the product out of the mold. If you have a good draft angle, you can get the product out of the mold without any problems.

If you don’t have a good draft angle, you can damage the product or get it stuck in the mold. This article is going to tell you what you need to know about the draft angle of a mold.

What is the Definition of Draft Angle?

The draft angle is the angle between the mold parting surface and the product axis. The size of this angle determines the size and direction of the external force required to demold the product.

The draft angle (also known as the draft) is the slope designed on both sides of the mold cavity to facilitate demolding. The orientation of the draft angle depends on the internal and external dimensions of the plastic part.

The size of the draft angle of the plastic part is related to the properties, shrinkage, friction coefficient, wall thickness and geometry of the plastic part. Hard plastics have a larger demoulding angle than soft plastics; plastic parts with more complex shapes or more molding holes have a larger demoulding angle.

Plastic parts with larger heights and deeper holes have a smaller demoulding angle; as the wall thickness increases and the force of the inner hole wrapping the core is greater, the demoulding angle should also be larger.

Sometimes, to keep the plastic part in the die or on the core when you open the mold, you intentionally make the slope of the edge less or make the bevel bigger.There’s no hard and fast rule for the size of the draft angle, and most of them are determined by experience and based on the depth of the part.

What are the Types of Draft Angles?

The draft angle of the mold is divided into the front mold surface draft angle and the rear mold surface draft angle. They are mainly distinguished by the parting surface. The parting surface divides the mold core into the front mold and the rear mold. The draft angle that needs to be designed on the surface parallel to the demoulding direction of the front mold is called the front mold surface draft angle.

On the contrary, it is called the rear mold surface draft angle. In addition, if the mold has side core pulling (bevel top and slider), it is called the bevel top surface draft, the slope and the slider surface draft angle, and the draft direction is based on the direction of the slider movement.

What are the Design Principles of the Demoulding Angle?

For products with high appearance requirements, the demoulding angle should be small: In order to maintain the integrity and beauty of the product appearance, the demoulding angle should be as small as possible to reduce the impact on the product appearance during the demoulding process.

For products that require high precision, the demolding angle should be small: A large demolding angle will affect the dimensional accuracy of the product. Therefore, for products that require high precision, the demolding angle should also be as small as possible.

The surface is bright, and the demoulding angle should be appropriately reduced: Products with bright surfaces often have higher requirements for appearance, so the demoulding angle should also be appropriately reduced.

Plastic materials have self-lubricating properties, so the demoulding angle can be reduced.The product has a complex shape, and the demoulding angle should be increased: Complex shapes may make demoulding difficult, so the demoulding angle needs to be increased to ensure smooth demoulding.

Plastics with poor fluidity or added reinforcements should have a larger demoulding angle: These materials have poor fluidity, which may make it difficult to fill the mold or stick to the mold during demoulding, so the demoulding angle needs to be increased.

Make the glue thicker and increase the demoulding angle: Thicker plastic layers shrink more when they cool, and increasing the demoulding angle helps reduce shrinkage stress and prevent deformation.

For plastics with big shrinkage, you should use a bigger demoulding angle: Plastics with big shrinkage shrink a lot when they cool down. If you increase the demoulding angle, it will help to make up for this shrinkage and make sure the product is the right size.

Increase the demoulding angle of transparent plastic parts: Transparent plastic parts often have high requirements for appearance and optical properties. Increasing the demoulding angle properly helps to reduce stress and deformation during demoulding and maintain the transparency and optical properties of the product.

The demolding direction is generally based on the parting surface for demolding, ensuring that the larger end is close to the parting surface after demolding, otherwise it cannot be demolded smoothly.

How to Determine the Appropriate Demoulding Angle?

Selecting the Demoulding Direction

The demolding direction of the injection mold not only affects the size and appearance quality of the finished product, but also directly affects the service life and production efficiency of the mold.

Therefore, before determining the demolding angle, the demolding direction must be determined first. Generally speaking, the demolding direction should allow the finished product to naturally separate from the mold without causing excessive impact and wear to the mold.

Formulate the Demoulding Angle

Once you’ve figured out which way to pull the mold, you need to figure out how much to pull it based on the material and the production conditions. In general, the bigger the pull, the easier it is to get the part out, but it will affect the surface finish of the part.

If the pull is too small, it will wear out the mold and make the part quality inconsistent. So you want to pull it as much as you can without messing up the part quality or the mold life.

Consider the Material

Different plastic materials have different fluidity, shrinkage, and elasticity. When determining the demoulding angle, it is necessary to consider the characteristics of the material to ensure that the product can be demoulded smoothly.

Consider Production Conditions

When determining the ejection angle of the injection mold, you also need to consider the production conditions, such as injection speed, injection pressure, mold temperature, etc.

These factors will affect the surface quality and dimensional accuracy of the finished product, and then affect the selection of the ejection angle. Continuously accumulate experience in actual production, and find the best ejection angle parameters through trial molds and adjustments.

Mold structure: The structure and layout of the mold are also important factors in determining the draft angle. For example, the draft angle of a multi-cavity mold usually needs to be considered uniformly to ensure the balance and consistency of each cavity.

Ejector system: The design and layout of the ejector system will also affect the selection of the draft angle. The magnitude and direction of the ejection force should match the draft angle to ensure smooth ejection of the product.

What are the Key Points in Determining the Draft Angle?

There is no hard and fast rule for how much draft angle you need. Most of it is based on experience and how deep the part is. Also, you have to consider the molding process, wall thickness, and the type of plastic you’re using.

But, in general, you need some draft on every side wall of your part to help it come out of the mold. The amount of draft can vary from zero degree draft angle to several degrees, depending on the circumstances, but 0.5° to 1° is usually a good rule of thumb.

The slope direction is generally based on the small end of the inner hole, according to the drawing, and the slope is obtained from the expansion direction. The outer shape is based on the large end, and the slope is obtained from the reduction direction.

For plastic parts with high precision requirements, a smaller demoulding slope should be used. For higher and larger sizes, a smaller demoulding slope should be used. For plastic parts with large shrinkage, a larger slope value should be used. When the wall thickness of the plastic part is thicker, the molding shrinkage will increase, and the demoulding slope should use a larger value.

Usually, the demolding slope is not included in the tolerance range of plastic parts. The demolding slope of transparent parts should be increased to avoid scratches. Usually, the demolding slope of PS materials should be greater than 3°, and the demolding slope of ABS and PC materials should be greater than 2°.

The side wall of plastic parts with leather grain, sandblasting and other appearance treatments should be added with a demolding angle of 3°~5°, depending on the specific depth of the texture. The required demolding angle for reference is clearly listed on the general texture plate. The deeper the texture depth, the larger the demolding angle should be. The recommended value is 1°.

For the slope of the insert surface, it is generally 1°~3°, and the slope of the demoulding surface of the outer shell is greater than or equal to 3°. The demoulding slope of other features of the shell except the outer shell surface is 1° as the standard demoulding slope.

In particular, it can also be taken according to the following principles: the demoulding angle of the reinforcing ribs below 3mm high is 0.5°, 35mm is 1°, and the rest is 1.5°; the demoulding angle of the cavity below 3mm high is 0.5°, 35mm is 1°, and the rest is 1.5°.

What are the Methods for Adjusting the Mold Demolding Angle?

Adjust the Mechanical Structure

Fix the mechanical structure based on the mold design flaws. Adjust the demolding draft angle while keeping it standardized. Adjust the core pulling draft angle, increase the core pulling angle, or change the template spacing.

Adjust the Temperature

Different plastics have different shrinkage rates and demoulding difficulties. Heating the mold during the injection molding process can reduce the stress inside the injection molded part and improve the plastic fluidity to achieve better forming effects.

Change the Demoulding Method

To better solve the problem of mold in the demoulding process, we can change the demoulding method. For example, we can use the method of translation demoulding, increase the length of the ejector pin, and so on.

Processing the Mold

The degree of draft is reprocessed by processing the mold. Sometimes the demolding injection molding draft angle needs to be adjusted according to the nature and requirements of the product. At this time, mechanical processing or heat treatment methods can be used.

What are the Precautions for Mold Demoulding Slope?

Don’t use too much draft: Too much draft can make the part lean or twist when it comes out of the mold, which can affect the quality of the part.

Pay attention to the coordination between the local and the whole: In complex product structures, you may need to adjust the demoulding slope locally to achieve smooth overall demoulding.

Think about what comes next: Some products need to be processed after injection molding, like hot bending, bending, and so on. When you set the demolding angle, think about what comes next so you don’t mess up the processing.

What are the Functions of the Draft Angle?

Keep improving: In actual production, the mold’s demolding angle design is continuously optimized and improved according to the actual situation to improve production efficiency and product quality.

The demolding angle should be uniform and consistent to avoid local sharp corners or grooves.

The demolding angle should be consistent with the demolding direction of the injection molded part. The demolding angle should consider the shrinkage rate of the injection molded part. The demolding angle should be as large as possible without affecting the appearance and performance of the injection molded part.

Material properties: Different plastic materials have different shrinkage rates and thermal expansion coefficients. Hard plastics like ABS and PC may require a larger demolding angle, while soft plastics like TPR and TPU can be designed with a smaller demolding angle.

Plastic part structure: For plastic parts with complex structures, like parts with deep holes or narrow grooves, the demoulding angle should be increased a bit to avoid the plastic parts from shrinking and getting stuck in the mold during cooling.

Smooth Surface Requirements: Products with high-gloss surfaces need higher demolding angles to make sure the smooth surface of the plastic parts doesn’t get messed up when you take them out.

Mold life and demolding: The mold life will be shortened if the demolding angle is too large, because the larger the angle, the more uneven the force on the mold. Therefore, under the premise of meeting the demolding requirements of the product, try to use a smaller demolding angle as much as possible.

Dimensional accuracy requirements: For plastic parts that require high dimensional accuracy, the demolding angle must be set very precisely, otherwise it is easy to cause dimensional deviation.

Plastic part thickness: Thin-walled plastic parts usually need a bigger demolding angle to avoid deformation, while thick-walled plastic parts need a smaller demolding angle to save materials.

Conditions for the production process of injection molding: The shrinkage deformation of plastic parts is affected by process parameters such as injection speed, pressure, temperature, and holding time, so the size of the demolding angle should be adjusted according to specific process conditions.

Aesthetic considerations: For some products with important appearance design, the design of the demolding angle also needs to take into account the overall aesthetics of the product. Sometimes the demolding angle needs to be cleverly hidden so that it does not affect the appearance of the product.

Functional Requirements: For structures with functions like sliding and snapping, the draft angle setting must ensure that these functions can work properly and not be compromised due to improper demolding design.

What are the precautions for mold demoulding slope?

Don’t use too much draft: Too much draft can make the part lean or twist when it comes out of the mold, which can mess up the part.

Pay attention to the coordination between the local and the whole: In complex product structures, it may be necessary to adjust the demoulding slope locally to achieve smooth overall demoulding.

Consider the needs of subsequent processing: Some products need to be processed after injection molding, such as hot bending, bending, etc. When setting the demoulding angle, consider the needs of subsequent processing to avoid affecting the processing effect.

Conclusion

During the design process, injection molding specialists need to consider as much draft angle, including zero degree draft angle and half a degree draft angles, to ensure smooth mold release actions, thereby optimizing the parting line and core cavity approach to achieve a high-quality production process.

Reasonable draft angle injection molding is the key to high quality production process. By adding draft angles, especially when dealing with complex vertical surfaces, the demoulding effect can be effectively improved.

When you’re making plastic parts with injection molding, the angle of the sides of the part is really important. If you get it right, the parts will come out of the mold really easily. If you get it wrong, the parts will get stuck in the mold and you’ll have to throw them away. So, getting the angle right is a big deal. If you get it right, you’ll make more parts in less time and they’ll be better quality.