Draft Angles in Injection Molded Plastics

The realities of moving a new design into injection molding hit pretty quickly when the part design must be altered to include draft angles.  Steve Jobs famously paid top dollar for expensive molds that removed draft angles from the housing of an early Apple computer.  Most projects cannot afford the luxury of specialized molds to remove draft angles, so a deeper understanding of why they are needed, what they are, and how they work can improve the design and development process.

Draft angles present one of the key aspects of designing plastic parts to for the injection molding process.  Draft angles or tapers on injection molded plastic parts facilitate part ejections from the mold.  Unlike 3D printed parts or machined parts, injection molded parts must consider how the parts will be removed from the mold.  Usually a mold involves a cavity side which is the hollow side, and a core side which is the opposite side.

Why add draft angles?

Before the mold opens, the resin has already begun to cool.  Cooling causes the resin to shrink.  Shrinkage causes the resin to tighten around the core.  When the mold opens, the core and cavity separate.  The part sticks to the core side of the mold.

Part Ejection

The next step in the process is to remove the part from the mold.  Pulling a part off a core with aggressive force can cause stress in the part if done improperly.  Adding draft angles during the design process helps facilitate a smoother ejection process.

Part Removal

Draft angles can be important both on the core and cavity side of the mold because ultimately the part must be removed from both.  If the cavity had no draft, but the core had draft, the part might get stuck in the cavity and pose a greater challenge for removal from the mold.

Deep Draw

The depth of the draw impacts the risk of a part getting stuck.  Shorter draws pose lower risks.  Longer draws pose higher risks if we assume a fixed draft angle with other variables held constant.

Zero Draft Angles

A typical draft angle can range from 2 degrees down to ¼ degree.  However, zero draft angles are possible.  In these cases, decisions of surface finish, material selection, ejection method, and release agents become critical.  For example, syringes with long cores require near zero draft angles because the functionality of the part requires the maintaining of a liquid barrier along the entire length of the part’s inner diameter.

Risk Considerations

Part designers should consider the risks associated with inadequate draft angles.  Parts that do not properly eject from the mold can get damaged.  In more extreme instances, stuck parts can cause damage to the mold which is an expensive consumption of time and other resources.

In addition to draft angles, surface finish, ejector pins, and process cycle time adjustments can help facilitate part removal from the mold.  Collapsing cores, split cavities, and stripper ejectors can also facilitate removal of the part from the core to accommodate inadequate draft angles.  Each of these paths increase the complexity of the mold, which is why these options are more expensive.

Of course, the addition of draft angles changes the design of the part.  This is why the engineers in charge of the design for manufacturability must have good working lines of communication with the engineers in charge of the part design.

The ultimate goal of a well-designed draft angle is to reduce that force required to remove the part from the mold.  Reducing that force, decreases manufacturing risks and cycle times which thereby improves quality and reduces costs.