What is the Draft Angle in Injection Molding

If you work with injection molding, you need to understand draft angles. Without them, removing parts from molds can be difficult, leading to defects and production delays. This guide explains what is the draft angle, why it matters, and how to design it correctly.

What is a Draft Angle?

A draft angle is a small slope added to the vertical walls of a plastic part to help it come out of the mold easily after injection molding.

Every injection-molded part needs draft angles on both sides of the mold (cavity and core). These angles help prevent damage to the part and the mold while making it easier to remove the part after it cools.

When the hot plastic fills the mold and starts cooling, it shrinks slightly and grips onto the mold. Without a draft angle, removing the part can be difficult and may cause scratches or damage.

Importance of Draft Angles in Injection Molding

If you don’t add draft angles, parts can get stuck, scratched, or damage the mold. This can slow down production and increase costs. Draft angles make it easier to remove parts, prevent warping, and improve surface finish.

They also help balance space between ejector pins and cooling channels. With better cooling, you reduce cycle time and lower costs, especially for large production runs.

Benefits of a Draft Angle

• Helps you get a smooth, even finish without scratches
• Prevents damage to parts caused by friction during removal
• Keeps surface textures and finishes consistent
• Reduces wear and tear on the molded part
• Lowers the risk of mold damage
• Prevents warping of the design
• Speeds up cooling time by simplifying the ejection process
• Saves you money by cutting down on production costs

Types of Draft Angles

The draft angle should follow the parting line—the line where the two halves of the mold meet. The larger part of the drafted surface should be near the parting line to make ejection easier. Always use the parting line as the reference point (datum) when designing the draft.

There are different types, based on where they are needed in the mold:

Cavity Draft Angles: These apply to surfaces inside the cavity (the hollow space in the mold).

Core Draft Angles: These apply to surfaces inside the core (the solid part of the mold).

Lifter Draft Angles: If the mold has lifters, these surfaces need a draft angle in the lifter’s movement direction.

Slider Draft Angles: If the mold has sliders, these surfaces need a draft angle in the slider’s movement direction.

Who Designs the Draft Angle?

When making a mold, there is often a debate about who should design the draft angle—the structural engineer or the mold engineer. There are two common approaches:

• The structural engineer adds draft angles to all surfaces during part design, except for some areas that need the mold engineer’s input.
• The structural engineer adds draft angles only to key surfaces (like visible and assembly areas), while the mold engineer handles the rest during mold design.

Each method has advantages and disadvantages:

First Approach (Structural Engineer Handles Most Draft Angles)

Плюсы

• Ensures the part meets design needs, including fit and dimensions.
• Speeds up the mold review process and prevents quality issues later.

Cons

• The structural engineer must understand molds well; otherwise, draft angles might not work properly.
• Applying draft angles to all surfaces increases workload and may delay projects.
• Vertical surfaces turn into slopes, making future design changes harder.
• More interference lines in drawings can lead to annotation mistakes.

Second Approach (Mold Engineer Handles Most Draft Angles)

Плюсы

• Reduces the structural engineer’s workload, saving design time.
• Makes later design changes and drawing annotations easier.

Cons

• Mold engineers might focus only on ejection and overlook functional design needs, like proper fit and strength.
• Mold engineers must modify fillets (rounded edges), which can create differences between the original and final design.

How to Design Better Draft Angles?

There’s no single rule for adding draft angles to your design, but here are some key things to think about:

Deep Pockets and Cavities

If your design has deep pockets or cavities, you need a steeper draft angle. This helps reduce vacuum pressure and friction when the part is removed from the mold.

Surface Texture

The texture of your part affects how easily it comes out of the mold. A smooth surface, like a mirror finish, is easy to remove and helps reduce waste. If the surface has a light texture, you should use a draft angle between 1 and 5 degrees.

For rough or complex textures, like leather or snakeskin, you need a larger draft angle of 5 to 12 degrees or more. These textures create tiny undercuts that make the part harder to release, so adding more drafts helps prevent damage.

Guidelines for Draft on Injection-Molded Parts

When designing a part, add as much draft angle as you can. A good rule is 1 degree of draft for every 1 inch of cavity depth, but this may change based on different factors. Here are some general guidelines to follow:

• Use at least 0.5 degrees on all vertical faces.
• 1 to 2 degrees works well in most cases.
• 3 degrees is the minimum for a shutoff (when metal slides against metal).
• 3 degrees is needed for a light texture (PM-T1)
• 5 or more degrees is needed for a heavy texture (PM-T2)

The table below shows how deep a molded part’s feature is and the least amount of draft you need.

Materials for Draft Angles

The amount of draft angle you need depends on how much the plastic shrinks when it cools. More shrinkage means you need a bigger draft angle. Hard, brittle, or rough materials also need a larger angle than soft, flexible, or smooth ones.

For example, nylon doesn’t shrink much and can be molded with no draft angle, but it’s still a good idea to use a 1- to 2-degree taper.

Here’s a simple guide to minimum and recommended draft angles for common plastics:

Factors Influencing Draft Angle Size

Material Type: Hard plastics need bigger draft angles. Softer plastics might not need them at all because they are flexible.

Shrinkage: If the plastic shrinks a lot, it grips the mold tighter, so you need a bigger draft angle.

Friction: Slippery materials like PA and POM need smaller draft angles. Rougher surfaces need bigger ones.

Wall Thickness: Thicker walls press harder against mold, so they need bigger draft angles.

Shape Complexity: If your part has a complicated shape or lots of holes, you’ll need a bigger draft angle. This helps avoid using too many ejector pins, which must be balanced to prevent warping.

Transparency: If your part needs to be clear or optical, a bigger draft angle helps keep the surface smooth.

Draft Angle’s Impact on Surface Finish

How does a draft affect the finish of a part? Simply put, if there’s no draft, the part will rub against the mold as it’s ejected and cause scratches on the surface.

Since all thermoplastics shrink as they cool in the mold, they create strong surface tension, so it will be harder for the part to release smoothly. This tension can leave small scratches, and it’s even worse for textured surfaces if there’s no draft.

Textures are applied in different ways, but they all create tiny undercuts on the mold surface. Without a draft, these undercuts would lock the part in place. A draft angle helps the part move slightly before the shrinkage releases it from the mold, reducing the risk of scratches or sticking.

Common Mistakes and How to Avoid Them

Undercuts

Undercuts happen when a part’s design makes it hard to remove from the mold. This usually happens if you don’t add enough draft angles.

Mistakes to avoid: If you don’t include draft angles in areas with undercuts, the mold becomes complicated, expensive, and slow to produce.

Insufficient Angles

If a draft angle is too small, the part won’t come out of the mold easily.

Mistakes to avoid: Using draft angles smaller than 1 degree (for most materials) can cause parts to stick in the mold, leading to delays and damage.

Easy Tips for Draft Angles

These simple tips will help you design better parts and avoid production issues:

• For most injection molding jobs, use a 1.5-degree draft angle.
• Apply draft angles to both the cavity and core, making the core’s angle slightly larger.
• Add 1 degree of draft for every extra inch of part depth (depth follows the drawing direction).
• Smooth surfaces need smaller draft angles, while rough surfaces need larger ones—add 0.4 degrees for every  0.1 mm of roughness.
• Always include draft angles on vertical features like walls, ribs, louvers, and undercuts.
• If the outer surface needs to be smooth, use a core cavity approach.
• Hard, abrasive materials need bigger draft angles than soft, flexible ones.

Заключение

Draft angles are essential in injection molding to ensure smooth part removal, prevent damage, and reduce production costs. By designing proper draft angles, you can improve surface finish, avoid defects, and extend mold life. Always consider factors like material type, surface texture, and wall thickness when choosing the right draft angle.

DEK offers high-quality molding solutions with precision engineering. Contact us today to get the best results for your project.

Вопросы и ответы

What is the ideal degree for draft angles?

For most injection-molded parts, you should use a draft angle of 1.5 to 2 degrees. This helps the part come out of the mold easily and prevents damage. However, the ideal angle may change depending on the part’s design, surface texture, and mold depth.

What is a draft angle in vacuum forming?

In vacuum forming, a draft angle is a slight slope on the vertical sides of a mold. It helps you remove the plastic part easily without breaking or damaging it.

How to calculate draft angle?

You measure the draft angle in degrees. Sometimes, it can also be measured in millimeters or inches, but this is rare. To find the draft angle, measure from the vertical axis of the mold. This helps adjust for the shrinkage of the plastic material.