Effective injection molding draft angle for part designs for molding is a critical factor in component modeling.
Draft angles are the features that allow parts to be withdrawn from a mold tool cavity, without destructive levels of adhesion and surface damage. Parallel faces don’t allow easy withdrawal, slight tapers are needed to make ejection easy.
There are a variety of types and requirements for draft angles, so this article will lead you through a basic understanding of part design for the quality of the molding, the implications in tool design and the easy ejection of the part.
In a number of blogs and articles, we have talked about what is best practices for the design of plastic parts for injection molding. While some are nice-to-have and optional there are some that are absolute requirements. Draft angles are one of those, topping the list.
No part, no small detail of a part can avoid the need for draft angles to be applied. Injection moldings must be designed for easy processing, good welding, full feature replication and above all clean and damage free ejection.
No step in the draft angle analysis and its integration into the part can be neglected.
Your parts may form perfectly, when the mold fills, but if they won’t eject without damage, it was all for nothing. This article can be read as a primer in the importance of and perfecting of draft angles, to mold better parts.
What are Draft Angles?
The draft angle is the taper designed into the walls/features of every injection molded part, to allow it to be ejected from the tool.
Draft angles are applied along the axis of the tool – features that run in the direction of tool opening/closing must be able to come out of the tool without rubbing along their length, so cavity and core tooling features must both be drafted, generally (but not necessarily) at the same angle.
This allows the molding to withdraw from the cavity side when the tool starts to open, then it allows the ejector pins to push the part smoothly off the force/core, as opening completes.
Draft angles are, in many regards, the most important consideration in good part design for moldability. When the draft angles are right, many molding problems are avoided.
When they’re wrong in the original design, the consequences can be devastating – up to and including high cost of tooling alterations.
The picture can become more complex, for real world parts:
- Textures on the cosmetic faces of parts can cause hang-up and scuffing ejection damage, so textured areas require more draft than smooth areas.
- Blanking or shutoff is another factor to consider, even though it’s primarily a tool design issue, it will impact the part design in intricate ways.
- Where parts must be parallel, the tool features will require polishing and they will wear faster.
5 Ways That Draft Can Improve Part Moldability
Good drafting is a key to good moldability – so developing a thorough understanding of the how-to and where-to of both molding drafting and tooling drafts is a serious responsibility that rests on the designers shoulders.
This is closely related to a range of other best-practice design approaches that can make better products.
Incorporate Draft Into Early Prototypes
Design for easy ejection and high quality molded components with good surface finish should not be an afterthought. Incorporating draft angle planning in the earliest phases of product/component design is a necessary discipline.
Draft angles are imperative to ensure that molded or cast parts can be easily ejected from their molds or dies without getting stuck or damaged.
But draft angles can also become critical aspects in the aesthetic and functional design, offering opportunities and challenges that are integral to an effective design process.
In designing for effective drafting from the earliest development phases, consider these proposed guidelines:
- Understand the detailed requirements of the molding and mold tooling processes. Various areas and tasks within a part/tool different need localized variations in draft angle. It is a good ideal to consult with the tool designer and molding manufacturing team early, to get the best support possible in delivering a good-to-tool design. Surprises that require significant redesign, after the ‘release for tooling’ milestone are a burden you don’t need.
- Start with the draft angle in mind. It’s as important as deciding the part line for each component. In early stage design, draft angles are a key design parameter that will function to drive the detailing of many aspects of the part design. Operating in this way prevents a lot of pain in intricate problem solving later, when the design is more fixed and solutions will be more time consuming. For experienced designers of plastic moldings, the consideration of draft angle is so intrinsic that they’re not even aware they’re solving problems.
- Use the design software that’s intended for the task. If you’re designing in a 3D CAD package, then draft tools are right there, so use them. Some will even automate aspects of the design, although it pays to validate this. All CAD software will perform draft analysis, and the better packages do this really well, highlighting the problem areas that may have developed in your model.
- When you think you’re doing well, run draft analysis again! Incorporating draft angles into your design doesn’t mean you’ve not backed yourself into a corner that will scuff at ejection, or hang and not come off the core. Looking for errors either finds some – which is a very real benefit – or it shows you how well you’ve done, which is a reward in itself.
The keys to good design are delivering function, durability and easy manufacturing. And in plastic molded parts, draft angle is one of the prime drivers of easy moldability.
Basic Guidelines for Draft on Injection-molded Parts
Basic guidelines for drafting and draft related features in injection molded parts:
- Maintain draft angle discipline throughout the design process Appropriate drafting of features enables easy ejection of undamaged parts. Draft angles can vary, but avoid parallel faces, except where a sleeve ejector or stripper plate can be used to localize the high ejection forces.
- Minimize undercuts and bumped features Undercuts and bumps can become hang-up points, making difficulties in ejection. Undercuts require expensive additional tooling features like slides or lifters. Where undercuts cannot be achieved by shut off or blanking features, try to group them to operate off common sliders to reduce costs. Take great care over draft angles around shut offs and sliders.
- Design with uniform wall thickness as a constant target, varying smoothly where you must. Varying wall thicknesses can cause uneven cooling and shrinkage, leading to distortion and excessive binding on the core, resisting ejection.
- Make sure to use appropriate filets and radii, as sharp corners can create stress points in molded parts, resulting in drag/ejection related damage.
- Use appropriate drafts for the selected materials. It’s important to select the appropriate material for the application and draft the part accordingly.
- Be careful in choosing the part-line location. Part-line location can have significant effects on the part drafts, and all of this can be planned for at the start of each component design.
Leverage (Free) DFM Analysis
Design for Manufacturability (DFM) analysis is a useful tool in product design. Using free (and/or CAD package included) DFM analysis tools can bring these benefits:
- Fast detection and correction of potential manufacturing issues, as these tools can be used on early stage designs to identify faults. Draft analysis is a key aspect of this DFM process. Early detection enables necessary adjustments to designs, without the disruptions that can be implicit, later in the process.
- Reduced manufacturing costs and improved product quality often result from effective DFM analysis. Factors such as poorly specified draft angles, overly complex geometry, material wastage and hard to mold/eject features are best addressed early in the design process.
- Closer linkage can result between design and manufacturing teams, by sharing the DFM analysis tools outcomes. Designers can draw on the manufacturer’s experiences to solve manufacturing issues in design. In particular, seeking tool designer and molder input in draft angle assignment can be very effective in improving moldability.
Factor in Surface Finish
Surface finish is a key factor in making durable and appealing products. Changes in surface finishes across a product speak subconsciously of quality in design and manufacture.
Textured surfaces benefit from much greater cosmetic durability, showing imperviousness to light damage that quickly marr polished plastics.
There are strong reasons to texture surfaces and these textures come in many forms, imposed on the cavity side surfaces by various methods.
But a price of these textures is the need for increased draft angles. Typically, drafts should increase by 2° for every 0.025mm (1 thou) of texture depth.
It’s also important to select non-jagged texture forms that release easily from the tooled texture elements. Scuffing of textured surfaces that are insufficiently drafted is an ugly problem to solve.
Benefits of a Draft Angle
With good draft angle design, the product will benefit from:
- Improved cosmetics, as surfaces will not be scuffed by ejection and ejector pins will not bend or crack the parts.
- Draft angles used sensitively can improve the overall appearance of the parts – and they can combine to make the parts feel disjointed in themselves and in their fit to other parts, if selected without good care and understanding.
- Draft angles used smartly can solve design issues and create cost savings, for example in creating undercuts that require no costly slides.
- Care with drafting of ‘internal’ features that must attach to ‘external’ faces will reduce the risk of cosmetic sinking and part distortion
Draft Angle Design Guide and Best Practices
These are general guides and best practices for draft design, although context is key, so watch out for pitfalls, particularly where multiple features interact, or where there are blanking/shut off faces tangled up with complex features:
- Most faces require 1.5 to 2° of draft as a minimum. This is a generalized guide that applies to parts of up to 50mm (2”) depth. With shallow features (say <10mm deep) that have no texture, 1° should be safe. This will be enough to release parts out of the cavity or off the force/core without dragging or hanging up and distorting.
- Draft angles should typically increase by around 1° for each additional 25mm (1”) of molded depth. Deeper parts need larger draft angles to compensate for increased surface area and therefore drag risk.
- Drafts must follow the line of draw – the deeper into the cavity side, the smaller the drafted size. This allows the part to move ‘up and away’ as the tool opens, releasing the art from all contact with the cavity walls as the core pulls it from that side of the tool.
- Surface textures require more release from the walls to avoid damage at ejection. Firstly, select textures that are specified and designed to release well. Sand blasted surfaces are sharp and can hang up. Shot peened surfaces can be much better, as the features are rounded. Best of all are the photo-etched surface textures that are highly geometry controlled for release from the cavity walls. Think coarse matte and leather textures, as examples. For textured surfaces, draft angle should increase at about 1° for each 0.025mm (1 thou) texture depth.
- Nothing can escape draft angle consideration and analysis. Complex geometries or ribbed features should all have draft angles applied. Features like gussets, louvers etc must all conform to the draft standards. Where possible, an extra 0.5 – 1° on internal gussets at ribs will reduce the risk of external face damage from ejection stress at these features.
- Internal and external faces of a feature both require drafts, but at times this cannot be accommodated. An example of this is a pillar intended to receive a screw directly into the plastic. The screw hole MUST be parallel. This will result in a high extraction force for the pin that firms the hole.
In such a case, parallel sides are required, but this is facilitated by three factors:
- the pin will wear, but it’s a simple service item that can be replaced easily
- pins can be electropolished to reduce adhesion, easing withdrawal forces
- the pin can be surrounded by a tubular or sleeve ejector pin which applied ejection force directly to the top of the pillar, removing damaging offset loads
- Where external surfaces must be near vertical, the cavity face in this area should be polished in the line of draw/ejection (i.e not textured). 0.5° of draft is an acceptable compromise for designs that require ‘straight’ exterior regions.
- Feature depth influences the selection of draft angles considerably. Other factors may also be influential; wall thickness, feature depth, shrinkage rate, material, surface texture and supplier skill. Implement a Core-Cavity Approach.
Feature Depth and Minimum Draft Angle in Injection Molding
The basic guidance in draft angle selection is:
|Depth of feature (mm)||Feature thickness|
|Minimum draft angle (add 1-2° for textured surfaces)|
|Shut-off features require 3-5°|
Effective use of draft angles makes better products, with better cosmetics and easier manufacture. The problems that arise from adverse selection of draft angles are serious, but on the whole they’re easily avoided.
If you would like to consult with Kemal about molding issues, either in new parts or in existing tooling, our experienced team will be glad to hear from you and we will strive to guide you with the best knowledge and most applicable advice.