Plastic part walls must be uniform in thickness. This is the most basic design parameter, and strict adherence to it will eliminate many manufacturing problems. Parts with uniform walls will not warp, will fill properly and will fit together because variable shrinkage is minimized. Wall thickness variations should not exceed 10% in high mold shrinkage plastics. In fact, even this slight disparity can introduce processing and quality problems.

 

If varying wall thickness cannot be avoided, then designers should provide for proper gate location. If this is not supplied, then attaining uniform pack of the molded part will be nearly impossible. The most effective gate location is when the melt enters at the thickest part of the cavity and then flows to the narrower areas.

 

Theoretically, there is no maximum wall thickness for injection-molded parts. But designers are more concerned with determining the minimum wall thickness because thinner is almost always less expensive. Two factors contribute to this: first, thinner parts require less raw plastic material and second, they cool faster. To determine the most suitable wall thickness, engineers should first consider product requirements. Generally, strength dictates the wall thickness. Engineers can also rely on a finite analysis to select the optimal wall thickness.

 

During injection molding, the molten plastic has to navigate turns or corners. Rounded corners will ease plastic flow, so engineers should generously radius the corners of all parts. In contrast, sharp inside corners result in molded-in stress—particularly during the cooling process when the top of the part tries to shrink and the material pulls against the corners.

 

If the inside and outside radii of a part are each equal to half of the nominal wall thickness, a uniform wall around the corner can be achieved. Both sides of the corner will display equal amounts of shrinkage, and sink marks will be averted entirely. Moreover, the first rule of plastic design—uniform wall thickness—will be obeyed. As the plastic goes around a well-proportioned corner, it will not be subjected to area increases and abrupt changes in direction. Cavity packing pressure stays consistent. This leads to a strong, dimensionally stable corner that will resist post-mold warpage.

 

Draft angles are needed so that a plastic part can be released from the mold without distortion or damage. The high pressures of injection molding force the plastic to touch all the surfaces of a mold's cores and cavities. The cavity becomes so tightly packed that it is often difficult to remove the part. Sometimes, shrinkage will actually make it easier to take the part out of the mold, but in other cases, shrinkage will cause the part to stick to the mold's cores. These natural occurrences call for draft angles.

 

No single draft angle is suitable for all parts. Each individual part requires a unique specification. Large parts call for more draft than small parts. Thin-walled parts that undergo high-pressure injection molding need more draft than parts that are subjected to lower-pressure molding. When calculating appropriate draft angles, the plastic material's shrinkage and physical properties are also considerations. Sizeable draft angles and smooth polish should be used for parts molded in strong, inelastic, abrasive and gluey materials. Smaller draft angles can be utilized on soft, malleable and slippery plastics.

 

From a cost and manufacturability viewpoint, the ideal draft angle is the largest angle that will not lessen the customer's satisfaction with the product. The minimum allowable draft angle is harder to quantify. Plastic material suppliers and molders are the authority on what is the lowest acceptable draft. In most instances, 1i per side will be sufficient, but between 2i and 5i per side would be preferable. If the design is not compatible with 1i, then allow for 0.5i on each side. Even a small draft angle, such as 0.25i, is preferable to none at all.

 

Draft angles must be provided for several part details. For example, the sidewalls that are perpendicular to the mold's parting line must be drafted. Other areas that require draft angles include mounting flanges, gussets, holes, hollow bosses, louvers and other holes. The location of the mold's parting line sometimes remains unknown, however. This lack of information makes it impossible to ascertain whether the part's draft angles should have positive or negative values. As a result, designers commonly draw the part without individual draft angles but with a general specification such as 'allowable draft 1°.' This practice may make the process simpler, but it also encourages misinterpretation. Each individual angle should be provided.

 

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news title creatime Precision Die Design and Manufacture 2009-5-31 （May 13th)Japanese customer come to confirm our moulds 2009-5-16 Top 500 Global corporations--U.S customer visit us 2009-4-18 【Mould Export】Preparation for Mould Exportation 2009-3-17 Example of Injection Mold Designs 2009-2-17 Mould Configuration 2009-2-6 Video for Higrade Moulds 2009-1-9

Author:Shirney Chen

Time:2009-6-9