Injection molding is one of the most popular manufacturing processes and for good reason — it is cost-effective, can handle complex parts, and provides consistency during long production runs of identical parts.
Draft angles are among the best practices when manufacturing plastic components through the injection molding process. This design feature has a huge impact on the product quality. Without the right amount of drafting, parts are susceptible to issues such as extended cooling times, difficulties in ejection, and surface finish defects.
This article covers the basics of draft angle injection molding. We’ll explain what is a draft angle, its purpose, design considerations, best practices, and more.
What is a draft angle?
A draft angle refers to the slight taper or slant applied to the vertical surfaces of a molded part. It is usually positioned such that it runs toward the vertical axis or parting line of the mold. Draft is commonly measured in degrees, and less often in millimeters or inches.
Adding draft angles to injection molded parts makes it easier to release them from the mold. Conversely, undrafted components with straight walls are more likely to be damaged due to scraping during the ejection process.
Role of Draft Angles in Injection Molding
Here are some of the benefits of incorporating draft angle into your injection molding design.
Ease of Part Removal
Part ejection is a critical step in injection molding processes and a key reason why we add draft angles. During the manufacturing process, the plastic component is in contact with the mold’s walls. Therefore, the frictional force generated when ejecting the part can lead to significant damage.
A well-designed draft angle can minimize or eliminate friction, aiding mold release actions. Pushing on the ejector pins makes the part pop out of the mold with no hassle. However, forgoing draft angles means that the part rubs against the mold side throughout the release process causing surface finish defects.
Prevents Damage from Part Ejection
As we’ve mentioned in the previous section, inadequate draft angles can lead to serious issues. The part may drag against the mold sides during ejection causing scratches and other surface finish defects. Also, if the finished component gets stuck in the mold, this would destroy the mold leading to unexpected delays and expenses. An injection molding draft angle is an effective solution against such issues.
Reduces Deep Draw
The line of draw or parting line is the point where the two mold halves separate releasing the finished product. The depth of the draw is a crucial parameter that affects the ease of ejecting the part.
With a shorter draw, there is minimal risk of the part getting stuck inside the mold. This risk is considerably higher for longer draws. Drafted walls reduce deep draw and facilitate the easy release of molded parts.
Reduces Warping
Warpage is a common injection molding defect that causes distorted or deformed parts. An omitted draft angle can lead to the formation of a vacuum in the mold cavity during ejecting. Such vacuums have the undesirable effect of causing components to fold in on themselves or warp.
The slant provided by a draft angle eliminates this issue. This is because it allows air to occupy the cavities in the mold preventing the formation of a vacuum. As such, you can manufacture high-quality plastic parts with no warping by incorporating draft.
Manufacturing Costs
Drafting has a significant impact on injection molding costs. From the functions discussed above, we can deduce several ways draft angles can help you save on manufacturing costs.
For starters, applying sufficient drafting allows you to develop parts with no surface imperfections, warpages, or deep drawing issues. This minimizes the costs associated with finishes and damaged parts.
Second, a draft angle extends the lifespan of the mold and reduces maintenance requirements. Thanks to minimal friction during ejection, tooling can last longer. Also, there is a reduced need for frequent repairs and replacements.
Additionally, draft angles speed up the injection molding process allowing you to cut costs further.
How do you design better draft angles?
When it comes to injection molding, there is no blanket rule that can tell you precisely how to incorporate draft during the design process. However, there are factors that injection molding specialists need to be aware of.
Deep pockets and cavities
Deep vertical features are more susceptible to deformation than their shallow counterparts. This is because they have more empty spaces that can create a vacuum during demolding. Fortunately, there is a simple, effective solution.
Deep pockets and cavities can benefit from increased draft angles to counter the vacuum and frictional forces during ejection. In other words, the deeper the cavity, the more draft required.
Part texture
Textures have a big impact on the required draft. The rule is: the rougher the texture, the greater the draft angle should be. The slope should create enough space to release the part without scraping off the texture.
For light textures, a tilt angle of 1-5 degrees is generally appropriate. Engineers can implement extreme draft angles of 5-12+ degrees when working with particularly complex textures.
Factors Influencing Draft Angle Size
Numerous factors influence the ideal degree of draft in injection molding. They include;
Material Characteristics
Your choice of material will affect the draft angle size. For example, soft plastics such as nylon require smaller draft angles compared to hard plastics like polycarbonate. This is because the softer materials demonstrate pliability during part ejection.
Shrinkage Rate
You’ll need to adopt a larger draft angle when using materials with significant shrinkage. A high shrink rate means that the part is more prone to clinging to the mold sides during the cooling phase. Opting for more draft supports smooth ejection of the plastic component.
Friction Coefficient
Materials with lower levels of friction, such as nylon and acetal, require small draft angles. Conversely, plastics with higher friction coefficients demand larger draft angles to ensure successful demolding.
Wall Thickness
As wall thickness increases for injection molding components, the wrapping force on the mold is higher. This calls for the use of a larger draft angle during the product design.
Geometric Complexity
The geometry of a part is an important consideration when determining draft angle size. Complex parts with features such as undercuts, overhangs, and deep cavities require a steeper taper for successful ejection from the mold.
Mold Design
The surface finish of the mold affects the draft angle. If the mold sides are smooth, there is reduced friction during ejection and this allows for a gentler draft. Rougher surfaces necessitate a larger draft to achieve optimum results.
Draft Angles Required For Different Injection Molding Materials
As we’ve discussed, different materials have varying draft requirements. Generally, hard and brittle plastics need a larger mold draft angle due to their rigidity. On the other hand, a gentler taper can be adopted for soft and flexible plastics. Let’s look at recommended draft angles for common injection molding materials.
#1. Polycarbonate (PC)
A draft angle of 1.5 to 2 degrees is usually required for smooth surfaces. If the surface is etched, add 1 degree of draft per 0.001 inch of depth.
#2. ABS
A taper of 1.5 to 2 degrees is appropriate if the part has a good surface finish. Add 1 degree of draft per 0.001 inches of depth for molded components that are more than 2 inches deep.
#3. Liquid Crystal Copolymers (LCP)
For LCP, a minimum draft angle of 0.5 degrees is required. Engineers can implement an additional 0.5 to 1.5 degrees of draft for deep vertical surfaces, etched surfaces, and rough textures.
#4. Nylon
Because of its flexibility, a 1 to 1.5 draft angle is generally enough to eject nylon parts successfully. More draft may be required depending on the complexity of the design.
When do you need to set a zero-degree draft angle？
As the name suggests, a zero draft mold design does not include any draft angle. Simply put, this design features perfectly straight part walls. So, is this possible? Under the right engineering conditions, a zero-degree draft angle is achievable. Such injection molding designs are used when part geometry, functionality, or material characteristics make it impractical to add a draft angle.
To successfully eject plastic parts from a zero draft mold, design engineers must carefully consider the surface finish, injection molding material, mold release agent, and ejection method. For example, syringes with long cores can be molded with a near-zero draft angle. This helps maintain a liquid barrier along the entire length of its inner diameter.
Best Practices For Draft Angle Design For Injection Molding
With plastic injection molding, there is no single draft angle that is suitable for all parts and features. As we’ve discussed, factors such as material characteristics, shrink rate, friction coefficient, and wall thickness influence the suitable draft angles.
However, there are general industry guidelines that can help you when choosing the correct draft for your project. Here are some of the draft angle injection molding best practices.
An injection molding draft angle of 1.5 to 2 degrees is suitable for general injection molding jobs
This recommendation remains applicable as long as the mold depth does not exceed 2 inches. An injection molding draft angle of 1.5 degrees allows for the successful ejection of completed parts from the mold.
Increase the draft angle by 1 degree for each inch increase in part depth
Deeper or larger parts call for additional draft to facilitate an easy non-damaging demolding. The larger degree of draft covers the increased surface area and friction.
Increase the draft angle for textured parts
If your part has an intricate surface design or is textured, it will be harder to release from the mold due to the extra friction generated by these features. It is recommended that you incorporate more draft for such surfaces. A simple rule of thumb is to add 1.5 degrees draft angle per 0.001 inch of surface depth.
Orientation of draft angle
Generally, a draft angle should occur towards the “top” of the mold, following the direction of the mold moving up or away during separation. A good illustration of this phenomenon would be a hollow box with drafted sides. If drafting is executed properly, the open top would be slightly broader than the bottom. This orientation ensures uniform demolding and reduces the risk of the part getting stuck.
Incorporate a draft angle in every component of a piece
Complex molded parts often have features such as louvers, ribs, gussets, and more. It is advisable to include draft angles for all these components. In other words, any surface that is in contact with the mold should have a taper. Even a minimal draft angle can improve demolding efficiency significantly.
Dual-sided draft angles
If a molded component has a parting line in the middle (for example, a solid cylindrical part), consider adding draft angles to both sides of the components. In such cases, there are two mold release actions hence the need for dual-sided draft angles.
Increase draft for metal-on-metal molding functions
This guideline does not relate to injection molding metal. Instead, it applies to mold designs where one metal mold component is in direct contact with another. Due to the increased friction in such applications, incorporate a draft angle of at least 3 degrees. This ensures successful part production.
The minimum advised draft angle on vertical surfaces is 0.5 degrees
This standard is not adopted for general injection molding projects. However, it serves as a guideline when the design calls for a uniform angle from top to bottom of a vertical surface. The minimum half a degree of draft is essentially a compromise between designs dictating straight walls and injection molding requirements.
To reiterate, a draft angle is a requirement in all designs
A draft angle is a crucial parameter for an effective, high-quality production process. Regardless of the complexity or size of the molded part, it’s recommended that you add some degree of draft. Forgoing draft angles can result in damaged parts and molds, extended cooling times, and an overall inefficient production process.