Draft Angle Guidelines for Injection Molding

Apply proper draft early and often to injection-molded parts to save production time and money

When developing parts for plastic injection molding, applying draft (or a taper) to the faces of the part is critical to improving the moldability of your part. Without it, parts run the risk of poor cosmetic finishes, and may bend, break, or warp due to molding stresses caused by the plastic cooling. Equally important, an absence of draft may prevent parts from ejecting from the mold, damaging not only the parts, but possibly the mold itself—a costly and time-consuming detour.

What is draft in injection molding?

Draft is a taper applied to the faces of the part that prevent them from being parallel to the motion of the mold opening. This keeps the part from being damaged due to the scraping as the part is ejected out of the mold.

Here are five ways that draft can improve part moldability.

Incorporate Draft into Early Prototypes

Ignoring draft early in the design process is a common mistake that occurs when prototyping with 3D printing and CNC machining processes where draft is not required. Because of the layer-by-layer method in which 3D-printed parts are built, nearly any design can be produced with limited concern for moldability. The same can be said for machined parts as part ejection is a purely a molding consideration.

But if a prototype design will eventually move to injection molding, it is befitting to design draft into parts from the very beginning. Draft may alter the form and fit of a part during assembly and its overall aesthetic, so designing in draft, even when it’s not technically needed, can help you avoid any surprises such as costly redesigns and additional prototype development.

Design for the future need, not the current need. When the part is ready to move from 3D printing or machining into injection molding with draft already integrated, design is accelerated and production can begin sooner. Don’t print or machine yourself into a corner by manufacturing a product that needs more design work before it can be molded.

What can be done when draft may negatively impact part performance?

Parts can be designed with 0.5 degrees of draft, or even as little as 0.25 degrees, which is still an improvement over zero draft. Again, the smallest degree of draft that is possible is dependent on material, part geometry, and manufacturer and should be discussed with the manufacturer before finalizing a part with very limited draft.

Many low-volume injection tools are manufactured from aluminum and use CNC machining to mill nearly all of the features in the core and cavities of the mold. With fewer manufactured parts and inserts than a steel production mold, additional draft and wall thickness may be required to produce the same parts with an aluminum mold due to the diameter, length and draft of the end mills that are used to create them. The increase in draft and thickness typically won’t affect the parts, and it may actually improve the performance of eventual production mold

Implement a Core-Cavity Approach

Adding draft to an enclosure can create issues if it isn’t applied correctly. When applying draft to the inside and outside walls, design these walls parallel to each other to avoid deep ribs in the mold that make venting, ejection, mold finishing, and manufacturing more difficult. Consider 

 a core-cavity approach to design as this opens up the mold cavity and core for polishing, accelerates manufacturing speed and, in general, brings added ease during the molding process.

Take advantage of (free) DFM analysis

One of XY-GLOBAL’s most valuable tools is our free Design for Manufacturability (DFM) analysis, which is provided for every 3D CAD model uploaded to our website. Within a few hours, you will receive a quote that highlights the detailed parts of the model that need drafting angles and even provides suggested changes to improve the drafting of those parts. This is a good quality control check and helps avoid future plasticity problems.

Upload a 3D CAD at xy-global.com for a complete analysis. For questions about mold pulling and technical issues, please contact our knowledgeable applications engineers at sales820@xy-global.com or 0086-151 1311 0489

XY-global can apply eight different finishes to thermoplastic molds that range from unfinished to highly polished and even textured surfaces.