Long before any of us designed, produced, or even thought about plastic parts, we experienced draft in the form of an ice lolly or similar frozen treat. An ice lolly's clean, icy surface is achieved the same way we create the unmarred surfaces of plastic part, by drafting—tapering— the sides of the mould so that the surface of the moulded object pulls away from the mould walls during ejection (see Figure 1). If the mould sides were straight, removal from the mould would be difficult and the surface of the lolly could be marred as the ice was pushed out of the mould. It would still taste the same, but would suffer cosmetically.
Figure 1: Without draft (left), the part drags along the entire vertical side; but by adding draft (right), the part falls free from the mould upon ejection.
The moulding of an ice lolly is complicated by the fact that water expands as it freezes, a problem that is overcome by leaving the handle end of the mould open so the expanding ice has somewhere to go. If the mould were closed, the expansion of water as it turns to ice could result in what might be called “ice flash,” much like what occurs when a plastic injection mould is overpacked.
Unlike lollies, many plastic parts include features formed by cores protruding from the B-side mould half (see Figure 2). While shrinkage of the cooling resin could, in theory, cause the outside surface of the part to pull slightly away from the A-side mould half; that same shrinkage can cause the part to tightly grip the core that formed the feature. This is best addressed through the use of a drafted part (resulting in a drafted mould wall), which effectively causes the part to move away from the mould wall as it is ejected.
Figure 2: The purple area represents a plastic injection moulded part; it is formed by the core protruding from the B-side of the mould. The drafted part (right) will move away from the mould and core wall easier as it is ejected.
Moulders will work hard to prevent shrinkage of cooling resin away from the mould half that forms a part’s outside surfaces, as it may result in out-of-tolerance dimensions. They do this by continuing to inject resin into the mould as it cools, forcing the solidifying, cooling resin farther into the mould. In other words, a mould that is “full” with heated resin may be only 95 percent full once that resin starts to cool. Left at 95 percent of capacity, the resulting part might be successfully ejected without standard draft. It would, however, run the risk of surface sink, voids, and failure to pick up proper texture from the mould walls. The addition of the final five percent of the mould’s full capacity reduces these risks at the same time that it reduces shrinkage away from the mould wall. Proper draft prevents this tight fit from hindering ejection while letting moulders avoid the cosmetic problems that come from less-than-optimal filling.
There are two other reasons for incorporating draft into a design. The first is to prevent damage to the mould wherever metal slides against metal, as in a sliding shutoff. The second is to allow end mills to make deep, narrow cuts to create tall ribs. (Check out our sliding shutoff video.)
The reason for drafting sliding shutoffs is simple. Without draft, the metal faces would quickly wear, damaging the mould and allowing flash to form in the spaces between worn mould surfaces. Drafting the metal faces minimises wear as the mould opens and closes.
Deep, narrow cuts require the use of long end mills, and the farther the cutting tip is from the chuck of the mill, the easier it is for the cutter to be pushed out of position as it spins. This can cause chatter, resulting in gouging of the piece being milled, and it can actually break the end mill. A wider rib allows the use of thicker end mills, which can withstand the side-load and maintain stability as they cut. If the rib must be thin, however, drafting its sides allows the use of a tapered end mill, which will be more stable than a straight one with the same size cutting tip. (Note: If your design truly needs tall, thin ribs with minimal draft, our process also supports the selective use of EDM to make that possible.)
Don’t forget to draft what sometimes seems inconsequential. Text and other similar geometry like logos and very shallow features have an amazing ability to stick to a mould and cause pulling. This forms small pieces of standing material that are sharp and can affect the appearance of text and cosmetic details. A little draft goes a long way in releasing resin from the finer detail of the mould.
If you submit parts with insufficient draft, the design analysis in the ProtoQuote® interactive quote will point out areas where greater draft is required. But while you can add draft late in the design process, your design will benefit if you consider the need for draft right from the start. General guidelines for draft are:
- at least 0.5 degrees on all vertical faces
- 2 degrees to provide a margin of safety in most situations
- 3 degrees minimum for a shutoff (metal sliding on metal)
- 3 degrees required for surfaces with light texture (PM-T1)
- 5 or more degrees required for heavy texture (PM-T2)
More information on draft, view our Design Guidelines.