Draft on Tap for Stress Release

Long before any of us designed, produced, or even thought about plastic parts, we experienced draft in the form of a Popsicle® or similar frozen treat. An ice pop’s clean, icy surface is achieved the same way we create the unmarred surfaces of plastic part, by drafting—tapering— the sides of the mold so that the surface of the molded object pulls away from the mold walls during ejection (see Figure 1). If the mold sides were straight, removal from the mold would be difficult and the surface of the pop could be marred as the ice was pushed out of the mold. 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 mold upon ejection.

The molding of ice pops is complicated by the fact that water expands as it freezes, a problem that is overcome by leaving the handle end of the mold open so the expanding ice has somewhere to go. If the mold 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 mold is overpacked.

Unlike pops, many plastic parts include features formed by cores protruding from the B-side mold 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 mold 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 mold wall), which effectively causes the part to move away from the mold wall as it is ejected.

Figure 2: The purple area represents a plastic injection molded part; it is formed by the core protruding from the B-side of the mold. The drafted part (right) will move away from the mold and core wall easier as it is ejected.

Molders will work hard to prevent shrinkage of cooling resin away from the mold 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 mold as it cools, forcing the solidifying, cooling resin farther into the mold. In other words, a mold 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 mold walls. The addition of the final five percent of the mold’s full capacity reduces these risks at the same time that it reduces shrinkage away from the mold wall. Proper draft prevents this tight fit from hindering ejection while letting molders 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 mold 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.

The reason for drafting sliding shutoffs is simple. Without draft, the metal faces would quickly wear, damaging the mold and allowing flash to form in the spaces between worn mold surfaces. Drafting the metal faces minimizes wear as the mold 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 mold 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 mold.

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)

Proto Labs
www.protomold.com

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