What is Draft in an Injection Moulding Tool and Why Do Industrial Designers Need It in their Part Design?
- equipedesign
- Apr 27
- 7 min read
Updated: Jun 4
Injection moulding is a highly efficient and versatile manufacturing process for creating plastic parts with complexity and fine detail.
In its basic form the moulding cycle is fairly simple. – close, inject, pack n hold, cool, open eject, repeat.
One of the essential considerations in designing an injection moulded part and the tool that fabricates to achieve the process above is the inclusion of a "draft angle” or commonly called “Draft”.
Draft is a subtle but critical feature that significantly impacts the success of the moulding process and the quality of the final product. We’ll explore what draft is, why it’s important, and how to implement it effectively in injection moulding.
What is Draft in Injection Moulding?
A draft in injection moulding refers to the slight taper or angle applied to the vertical walls of a moulded part. This taper is designed to facilitate the easy removal of the part from the mould after the plastic has cooled and solidified. Draft angles are typically measured in degrees and are applied to surfaces that run parallel to the direction of the mould’s opening and closing.
This direction of open/closure of the tool is commonly referred to as “Line of Draw” or “LOD”
Key Features of Draft:
Positive Draft: Tapers inward from the LOD and part line, making the part narrower at the top and wider at the bottom.
Negative Draft: Tapers outward from the LOD and part line, which causes the part to stick in the tool and can complicate ejection. It. Also called an “Undercut”, this geometry usually requires special considerations of mechanical tooling action to relieve these surfaces prior to ejection
Variable Draft: Adjusted based on part geometry and specific functional requirements.
The primary purpose of a draft is to minimize friction and ensure smooth ejection of the part without causing damage to the mould or the product and aid easy release of the solidified plastic part from the tool.
In simplest terms, draft is in play when we are making sand castles on the beach with our kids.
When we pack moist sand into our child’s toy beach bucket, like injection pack and fill, the sand fills all the nooks and crannies to take the shape of a castle, like a moulding.
We then turn the bucket upside down onto the sand, the part line. As we lift the bucket upwards, in the “Line of Draw”, like ejection, the sand releases from the bucket or cavity.
A well packet bucket with good draft angle on its internal (cavity) surfaces will create a beautifully formed sand castle.
Why Do We Need Draft in Injection Moulding?
The inclusion of draft angles in a mould design is essential for several reasons:
Facilitates Part Ejection:
During the moulding process, the molten plastic fills the cavity and cools, adhering to the mould walls. Without a draft angle, the part may stick to the mould, making ejection difficult and potentially damaging the part or mould.
A draft angle reduces the surface contact between the part and the mould walls, allowing the part to release smoothly.
Reduces Wear and Tear on the Mould:
Friction during ejection can cause scratches or “draw marks”, dents, or wear on the mould surfaces, leading to increased maintenance and reduced tool life.
Draft angles minimize friction, preserving the mould’s integrity and extending its lifespan.
Reducing wear and tear by minimising ejection friction in LOD.
Improves Surface Finish:
Parts ejected without draft can experience drag marks called “draw marks”, scuffs, or deformation.
Proper draft design ensures a clean release, maintaining the surface quality of the moulded part.
Enhances Dimensional Accuracy:
Sticking or dragging during ejection can distort the part’s dimensions.
A draft angle ensures consistent ejection, preserving the part’s intended dimensions and tolerances.
Reduces Production Downtime:
Stuck parts can lead to delays as operators manually remove them or repair damaged moulds.
Draft angles streamline the ejection process, reducing cycle times and improving overall efficiency.
Enables Complex Geometries:
Draft allows for the inclusion of intricate features and designs that might otherwise be difficult to eject.
It expands design possibilities without compromising manufacturability.
How to Determine the Right Draft Angle
The appropriate draft angle depends on several factors, including the material, part geometry, and surface finish requirements. While there is no universal rule, the following guidelines can help:
Material Considerations:
Different materials shrink and behave differently during cooling and ejection.
For instance, rigid plastics like ABS or PC may require larger draft angles compared to flexible materials like LDPE or TPE.
Typical draft angles range from 0.5° to 3°, with larger angles for deeper or textured parts.
1° is a general guide as a minimum for hassle free ejection but the larger the angle possible the better. Don’t just do the minimum if a larger angle is a possibility without impacting part design fit, form and function.
Surface Texture:
Smooth surfaces require less draft than textured or matte surfaces.
Textured surfaces often need additional draft to prevent the texture from scraping or adhering to the mould.
Finer surface finishes require less draft to release from the cavity. 
Courser surface finishes require more draft to release from the cavity.
Part Geometry:
Parts with deep cavities, long vertical walls, or intricate features benefit from larger draft angles to ensure smooth ejection.
Shallow or simple geometries may require minimal draft.
Functional Requirements:
Draft angles should not compromise the part’s functional or aesthetic requirements.
Collaborate with design and engineering teams to balance draft needs with part performance.
Simulation and Testing:
Use mould flow analysis and simulation software to identify potential sticking points and optimize draft angles.
Most common, modern CAD packages with have a draft analysis tool to use as a constant check as the part design evolves. Use this regularly and often to guide your parts design, rather than only a rubber stamp at the end. This will optimise your design.
Common Challenges with Draft and How to Overcome Them.
Insufficient Draft:
Challenge: Parts stick to the mould, causing defects and delays.
Solution: Increase the draft angle or apply release agents to facilitate ejection if the client specification allows. Many medical customers prohibit the use of mould release as it is essentially a contaminant.
If minimal draft is required by the parts design and interaction with mating parts, consider instructing the tool maker to apply a 'draw polish'. This is where the direction of the polishing process/pass is parallel to the line of draw.
Polishing a tool.
Over-Designing Draft:
Challenge: Excessive draft can alter the part’s functionality or aesthetics.
Solution: Use functional draft angles that meet ejection needs without compromising design.
Negative Draft:
Challenge: Outward tapering complicates ejection and may damage parts or moulds.
Solution: Incorporate side actions, lifters, or collapsible cores to accommodate negative draft features. This will increase tool cost and complexity and may impact tool life and maintenance schedules due to increased moving parts in the tool’s architecture.
Impact on Dimensional Accuracy:
Challenge: Draft angles may affect critical dimensions.
Solution: Adjust tolerances and dimensions in the design phase to account for draft. Conduct ‘tolerance stack’ and ‘limits and fits’ analysis and drawing creation after draft has been applied to all mating parts so not to come unstuck later.
Textured Surfaces:
Challenge: Texture can increase sticking and require larger draft angles. Draft will also impact the intensity of textures over time and the ejection process will polish out cavity texture over time impacting the appearance of the part over time.
Solution: Coordinate with mould makers to determine optimal draft for the specific texture.
Shrinkage direction and Draft
Challenge: As all injection moulded plastics will shrink as they cool, features with internal surfaces will shrink down tightly around core pins and similar cavity surface geometry. The external surfaces of the part will tend to shrink away from the tool’s cavity surface. This will vary depending on the shrinkage rate of your chosen material. PP’s 1.6% will shrink more than a PC’s 0.6%. PP though is more mailable compared to PC that is harder, yet more brittle.
Solution: Tailor your modelled draft angle for the location of each surface of the part and how it will be released from the tool.
Benefits of Proper Draft Design.
When draft is applied correctly, it delivers numerous advantages:
Efficient Production:
Smooth ejection reduces cycle times and increases throughput.
Cost Savings:
Simplifying tooling and reducing tool design and fabrication costs.
Minimizing wear and tear reduces maintenance costs and prolongs mould life.
High-Quality Parts:
Preventing drag marks and deformation ensures aesthetically and dimensionally superior products.
Enhanced Mould Longevity:
Reduced friction and sticking preserve mould integrity over time.
Design Flexibility:
Proper draft allows for more complex and innovative designs without sacrificing manufacturability.
Draft is a fundamental aspect of injection moulding part, and therefore, tool design ensuring that parts are ejected smoothly and efficiently.
By understanding its purpose and implementing the right draft angles, manufacturers can enhance part quality, reduce production downtime, and extend mould life. Collaborating with experienced designers, engineers, and mould makers is essential to achieving the perfect balance between design and manufacturability.
Whether you’re producing simple components or intricate parts, considering draft from the outset will save time, money, and resources, ensuring a seamless injection moulding process and a superior final product.
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équipe design & consulting is a Product Design Consultant in Sydney with 20 years experience in design and manufacture of Medical grade moulded parts and product, including 5 years at the coal face as Operations Manager at a world class medical grade moulding facility; we are specialist in Design for Manufacture (DFM).
Please reach out if you feel you need assistance with your part design for plastic injection moulding.
We offer Design Coaching and Guidance to Full Service Design Consulting.
References: ChatGPT, Google Images
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