As an automotive interior designer, have you ever encountered this dilemma: a carefully designed surface is damaged during demolding, or a complex internal structure prevents smooth demolding? These problems often stem from a critical and often overlooked step: draft design.
Automotive interior parts must not only have an aesthetically pleasing surface, but also be richly in reinforced ribs and snap-fit structures. This "soft on the outside, strong on the inside" characteristic makes draft angle design essential.

Draft angle refers to angle designed into side of part to ensure smooth ejection from mold. Without an appropriate draft angle, injection molded parts are difficult to remove from mold. Even forced demolding can result in scratches, pulls, or damage to surface, compromising appearance. A proper draft angle ensures smooth demolding, improves appearance quality, increases production efficiency, and extends mold life.
Grained Surfaces: Adequate Draft Angles
Grained surfaces commonly used in automotive interior parts have strict draft angle requirements. The deeper grain, the larger required draft angle.
Based on practical experience, draft angles required for different grain depths are generally as follows:

This is because deeper grain increases demolding resistance, necessitating a larger draft angle to avoid damaging grained surface during demolding.
Key Points for Draft Design of Ribs
Ribs are a common structural feature in interior plastic parts. Key points for draft design are as follows:
Basic Design Principle: The thickness of rib is generally 0.4-0.6 times wall thickness of main body, and height should generally not exceed three times wall thickness.
Draft Angle Setting: Draft angle of a rib is typically 2°-4°.
Layout Design: Using multiple, lower ribs instead of a single, higher rib can avoid surface concavity. Rib layout should ideally align with melt filling direction.
Draft Considerations for Clip Structures
Clips and mounting points are key components connecting interior parts to body, requiring special attention in their draft design:
Avoid undercuts: Try to avoid designing undercuts that require lateral core extraction to simplify mold structure.
Allow Sufficient Clearance: Allow sufficient clearance around clips to prevent interference during demolding.
Use a Standardized Demolding Direction: Ensure that draft direction of all clip structures aligns with main demolding direction whenever possible.
Practical Design Tips: Consider draft requirements early in product design process to avoid hassles and costs of later modifications.
Gradient Texture Technology: If mold cannot meet required draft angle for texture, consider gradually varying texture depth, gradually shallowing from full-grain surface area toward sidewalls. This can significantly reduce required draft angle. (Applicable to instrument panel bodies)
Rational Internal Structure: Draft angles should also be considered for internal reinforcements, clips, and other structures, otherwise they will affect overall demolding performance.
Utilize Mold Flow Analysis: Use modern CAE software to conduct mold flow analysis to predict potential demolding issues and optimize draft design.
Work closely with mold engineers: Communicate fully with mold engineers during design phase to ensure that design meets product requirements, is easy to manufacture and demold.
Although a minor detail, draft angle is a key factor in determining quality and production efficiency of interior parts. In today's world of pursuing both aesthetic and functional interiors, mastering this seemingly insignificant but crucial aspect of draft angle is crucial for creating both beautiful and functional automotive interiors.
I hope these simple insights will provide you with some inspiration.
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Core Explanation in One Sentence:
Draft is process of adding a slight "slope" to sides of a product (especially plastic or metal castings) to allow it to be smoothly "pulled" from mold, much like taking off clothes or removing ice cubes from an ice tray!
Why do you need draft?
Imagine making ice cubes:
A perfectly straight ice tray: After water freezes, ice cubes adhere tightly to inside of tray. Trying to pull them out? Either you can't, or if you try too hard, ice cubes shatter or tray cracks. (This is disaster scenario of not having draft!)
A slightly sloped ice tray (larger at the top, smaller at the bottom): Once frozen, you give it a gentle push, and ice cubes slide out undamaged. (That's magic of mold draft!)
In industrial design, principle is exactly same:
Products are formed in a mold (injection molding, die casting, casting, etc.);
Molds are typically composed of two or more halves and open in a certain direction (mold opening direction);
If part's sidewalls are perfectly perpendicular to mold opening direction (0-degree draft), part will be stuck tightly in mold. Forcing mold open will either damage part (scratches, strains, deformation) or damage expensive mold (scratches and chipping);
Adding a slight angle creates a gap between part and mold wall, allowing for easier separation during mold opening.

Angle is the Key:
How large should the angle be? There's no fixed standard, but it's usually at least 0.5 to 3 degrees. Specifically:
Material: Plastics (especially soft PE/PP) require a smaller draft angle than hard metals (perhaps 1 degree is sufficient). Hard plastics (PC/ABS) or metals require a larger draft angle (perhaps 1.5-3 degrees or more).
Surface Requirements: For surfaces with high aesthetic requirements (such as front of a phone), angle should be as small as possible (e.g. 0.5-1 degree), but even a small angle is essential! Internal structural parts can be larger (1.5-3 degrees or even larger).
Depth: The deeper sidewall, the larger draft angle required; otherwise, friction will be too great to remove part.
Surface Treatment: If surface is textured (textured), draft angle must be increased! This texture acts as countless small hooks, dramatically increasing friction. 2, 3, or even larger may be required.
Better Large Than Small Principle: As long as both functionality and appearance are met, a slightly larger angle is safer. An angle that is too small makes demolding difficult and requires significant rework.
 
Direction cannot be wrong:
Core principle: Direction of mold removal must align with direction of mold opening.
How to add it? Imagine direction mold is opening. To keep part in desired mold half (usually movable mold), you need
Inner surfaces (holes, grooves): Design them smaller at the top and larger at the bottom (with opening getting larger as you go). This allows mold's "convex" (core) to be pulled out smoothly. (Imagine removing a wine cork; cork should be larger at the top and smaller at tail for easier removal).
Outer surfaces (protrusions, walls): Design them larger at the top and smaller at the bottom (with opening getting smaller as you go). This prevents part from getting stuck in mold's "cavity." (Imagine removing a pullover; collar should be larger for easier removal).
Getting it the other way around is called "inverted": If a part that should be larger at the top is smaller at the bottom, or if a part that should be sloped doesn't have a slope (vertical). This creates an "inverted" shape. An undercut means part is "hooked" by mold. Without a special mechanism (sliders, lifters), it's impossible to remove! This is a major taboo and increases mold complexity and cost.
Parting line is boundary:
Where mold closes, a gap forms, leaving a line on part.
Draft angle is measured from parting line! Location of parting line directly influences how draft is achieved.
Corners at parting line shouldn't be completely sharp (which reduces strength and damages mold). A slight radius (R) is usually required. This radius is crucial for smooth demolding.
Details determine success:
Text/Logo: Embossed or concave lettering on product surface must have draft! Embossed lettering should have a bevel on the sides (smaller at the top, larger at the bottom), and concave lettering should also have a bevel on the sides (larger at the top, smaller at the bottom). Lettering without draft will either fail to form or, if formed, mold will fail.
Reinforcement Ribs/Bone Positions: These thin, deep features are prone to sticking. Their draft angles usually need to be larger (such as 3-5 degrees or even larger), and roots must be rounded to avoid stress concentration, which is not conducive to plastic flow.
Mating surfaces: Where two parts require precise fit (such as snap fits), draft angles must be carefully designed to ensure release from mold without compromising fit accuracy and strength due to excessive angles.
Engineer's Advice (Notes):
Draft is a necessity, not an option! Any vertical (or nearly vertical) surface that needs to be released from mold must be drafted. This should be considered early in design process.
Communicate! Communicate! Communicate again! Industrial designers often strive for vertical, sharp lines (for aesthetic appeal). Structural engineers must thoroughly communicate with them, explaining where draft is necessary, how much, in what direction, and how it will impact visual effect. Together, they should find a balance between aesthetics and manufacturability.
Draft analysis, a core component of Design for Manufacturing (DFM) inspections, is a key component of DFM checks. 3D software (such as SOLIDWORKS, NX, and CREO) makes it easy to perform draft analysis, checking for insufficient draft and undercuts.
Not checking before mold opening can lead to disappointment afterward: Changing draft angles once mold production has begun is extremely costly (redrawing, reworking steel, and delayed delivery). Ensure all draft angles are correct before design freeze.
Texturing = increasing draft angles: If you plan to texture your product's surface (for added texture or anti-slip properties), be sure to inform your mold engineer in advance! They will require increased draft angles based on thickness and depth of texture.
 

Problem: If housing's side walls are perfectly vertical, consequences: When mold is opened, side walls will tightly hug mold core (inner surface), creating significant friction and making demolding difficult. This can lead to scratches ("stretching") on inner wall of housing, or even deformation or cracking during ejection, or damage to mold.
Solution: Add a draft angle to all side walls (outer surfaces), for example, 1.5 degrees. Draft angle should be larger at the top and smaller at the bottom (determined by parting line, with housing opening larger and the bottom smaller). This allows housing to easily release from cavity (outer surface) when mold is opened. Draft should also be applied to inner walls around button holes in the side walls (smaller at the top and larger at the bottom) to facilitate core removal.
 

Problem: For anti-slip and aesthetic reasons, handle surface features dense, granular or striped texture (texture). Designer desires a sharp, crisp texture.
Consequence: If a normal glossy finish (sparkle pattern) is applied with only a 1-degree draft angle, textured mold will experience significant friction due to tiny grooves within mold. This can cause part to become stuck in mold and unable to be removed, or forcefully eject part, resulting in severe damage to textured surface.
Solution: Draft angle in textured area must be significantly increased! Mold manufacturers will provide minimum draft angle requirements based on texture's coarseness (indicated by a designation, such as VDI 3400 standard). For example, fine texture (VDI 18-24) may require 1.5-2 degrees, medium texture (VD 127-30) 2-3 degrees, and coarse texture (VDI 36+) 3 degrees or more. Designers and structural engineers must accept that textured area will appear more "sloped" with an increased draft angle.
 

Practical Application of Draft Angle Design:
In principle, all draft angles should be determined by structural designer and then submitted to mold manufacturer.
Structural designer must draft important mating surfaces. Other non-critical parts, such as ribs, can be drafted by mold manufacturer using reduced glue and confirmed by structural engineer.
Draft angles of mating surfaces of two structural parts must be consistent.
A draft check must be performed before mold opening.
 

Depending on reference datum for drafting, drafting can be categorized as reduced-resistance drafting, added-resistance drafting, and cross-resistance drafting. Unless otherwise specified, drafting generally refers to reduced-resistance drafting.
 

In addition to following aforementioned selection principles and maintaining a general orientation, draft angle for injection molded parts should also be determined based on industry experience. Using ABS as an example, following lists some commonly used draft angles. Within this range, the higher ribs, the smaller draft angle. Draft angles are not required for H ≤ 1mm. For 1<H ≤ 3mm, a draft angle of 3 degrees is used. For 3<H ≤ 5mm, a draft angle of 1.5-2 degrees is used (D-T = 0.2-0.3). For 5<H ≤ 10mm, a draft angle of 1.2-1.8 degrees is used (D-T = 0.3-0.4). For 10<H ≤ 15mm, a draft angle of 0.8-1.2 degrees is used (D-T = 0.3-0.4). For 15<H ≤ 25mm, a draft angle of 0.5-0.8 degrees is used (D-T = 0.4-0.5). For 25<H, a draft angle is selected based on draft drop, generally requiring a D-T > 0.6.
 

For groove-type features in injection molded parts, draft angle should be as large as possible while ensuring both aesthetic and structural functionality. For same product, if clamping force on front and rear molds is comparable, draft angle on front mold side is typically 0.5 to 1 degree greater than that on rear mold side to ensure part does not stick to front mold. A disadvantage is that wall thickness of product body will be inconsistent, but rather will show a gradual change. If clamping force on rear mold side is significantly greater than that on the front mold side, draft angles on both sides can be made same. Below, using ABS as an example, some commonly used draft angle values are provided for reference.
For H ≤ 5 mm, the draft angle should be 3 degrees or more; for 5 < H ≤ 15, the draft angle should be 2.5 degrees or more (S = 0.25 to 0.6); for 15 < H ≤ 30, the draft angle should be 2 degrees or more (S = 0.6 to 1); for 30 < H ≤ 60, the draft angle should be 1.5 degrees (S = 1 to 1.5); for 60 < H, the draft angle should generally be greater than 1.5.
 

For injection molded screw posts, when back of screw is a non-exterior surface, draft is typically omitted from self-tapping thread area (S) to facilitate demolding. Draft angle for non-assembly area (T) should be as large as possible, but hole bottom diameter (D) must be ≥ 1mm, as shown in Figure B. When back of screw is an exterior surface, design according to Figure A—reduce glue depth at hole bottom by 0.5mm and increase corner radius by ≥ R0.4.
 
Factors affecting draft angle include:
Plastic shrinkage, friction coefficient, wall thickness, geometry, and so on. Generally, rigid plastics require a larger draft angle than soft plastics. Transparent parts, complex shapes, or plastic parts with multiple molded holes also require a larger draft angle.
 
If draft angle is too small or too large...
If draft angle is insufficiently designed, product will be difficult to demold and may be damaged. However, an excessively large angle can also affect product's dimensional accuracy and appearance quality. For example, cosmetic surfaces generally require a larger draft angle to avoid scratches, while structural and functional surfaces must consider impact of interference fit and screw support surfaces.