{"id":97068,"date":"2020-05-22T16:49:23","date_gmt":"2020-05-22T08:49:23","guid":{"rendered":"http:\/\/facfox.com\/?post_type=kb&p=97068"},"modified":"2020-05-22T16:49:23","modified_gmt":"2020-05-22T08:49:23","slug":"the-design-guideline-for-injection-molding","status":"publish","type":"kb","link":"https:\/\/facfox.com\/docs\/kb\/the-design-guideline-for-injection-molding","title":{"rendered":"The Design Guideline for Injection Molding"},"content":{"rendered":"
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Design for injection molding<\/h2>\n

There are several factors that may affect the quality<\/strong> of the final product and the repeatability<\/strong> of the process. To yield the full benefits of the process, the designer must follow certain design guidelines.<\/p>\n

In this section, we outline common defects of injection molding and basic and advanced guidelines<\/strong> to follow when designing parts, including recommendations for keeping the costs to a minimum.<\/p>\n<\/div>\n<\/div>\n<\/div>\n

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Common injection molding defects<\/h2>\n

Most defects in injection molding are related to either the flow of the melted material or its non-uniform cooling rate during solidification.<\/p>\n

Here is a list of defects to keep in mind while designing a part for injection molding. In the next section, we’ll see how you can avoid each of them by following good design practices.<\/p>\n<\/div>\n<\/div>\n<\/div>\n

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Warping<\/h4>\n

When certain sections cool (and as a result shrink) faster than others, then the part can permanently bend due to internal stresses.<\/p>\n

Parts with non-constant wall thickness are most prone to warping.<\/p>\n<\/div>\n

\"IM101-defect-warping\"<\/div>\n<\/div>\n<\/div>\n<\/div>\n
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Sink marks<\/h4>\n

When the interior of a part solidifies before its surface, a small recess in an otherwise flat surface may appear, called a sink mark.<\/p>\n

Parts with thick walls or poorly designed ribs are most prone to sinking.<\/p>\n<\/div>\n

\"IM101-defect-sink_marks\"<\/div>\n<\/div>\n<\/div>\n<\/div>\n
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Drag marks<\/h4>\n

As the plastic shrinks, it applies pressure on the mold. During ejection, the walls of the part will slide and scrape against the mold, which can result to drag marks.<\/p>\n

Parts with vertical walls (and no draft angle) are most prone to drag marks.<\/p>\n<\/div>\n

\"IM101-defects-drag_marks\"<\/div>\n<\/div>\n<\/div>\n<\/div>\n
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Knit lines<\/h4>\n

When 2 flows meet, small hair-like discolorations may develop. These knit lines affect parts aesthetics, but also they generally decrease the strength of the part.<\/p>\n

Parts with abrupt geometry changes or holes are more prone to knit lines.<\/p>\n<\/div>\n

\"IM101-defects-knit_lines\"<\/div>\n<\/div>\n<\/div>\n<\/div>\n
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Short shots<\/h4>\n

Trapped air in the mold can inhibit the flow of the material during injection, resulting in an incomplete part. Good design can improve the flowability of the melted plastic.<\/p>\n

Parts with very thin walls or poorly designed ribs are more prone to short shots.<\/p>\n<\/div>\n

\"IM101-defects-short_shots\"<\/div>\n<\/div>\n<\/div>\n<\/div>\n
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Design rules for injection molding<\/h2>\n

One of the biggest benefits of injection molding is how easily complex geometries<\/strong> can be formed, allowing a single part to serve multiple functions.<\/p>\n

Once the mold is manufactured, these complex parts can be reproduced at a very low cost. But changes to the mold design at later stages of development can be very expensive, so achieving the best results for the first time<\/strong> is essential. Follow the guidelines below to avoid the most common defects in injection molding.<\/p>\n<\/div>\n<\/div>\n<\/div>\n

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Use a constant wall thickness<\/h3>\n

Use a uniform wall thickness<\/strong> throughout the part (if possible) and avoid thick sections<\/strong>. This is essential as non-uniform walls can lead to warping or the part as the melted material cools down.<\/p>\n

If sections of different thickness<\/strong> are required, make the transition as smooth as possible<\/strong> using a chamfer or fillet. This way the material will flow more evenly inside the cavity, ensuring that the whole mold will be fully filled.<\/p>\n<\/div>\n

\"IM101-rules-wall_thickness\"<\/div>\n<\/div>\n<\/div>\n<\/div>\n
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Make the transition as smooth as possible at section of non-uniform wall thickness<\/figcaption><\/figure><\/figure>\n

Wall thickness between 1.2 mm and 3 mm is a safe value for most materials. The next table summarises specific recommended wall thicknesses<\/strong> for some of the most common injection molding materials:<\/p>\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
Material<\/strong><\/th>\nRecommended wall thickness [mm]<\/strong><\/th>\nRecommended wall thickness [inches]<\/strong><\/th>\n<\/tr>\n<\/thead>\n
Polypropylene (PP)<\/td>\n0.8 – 3.8 mm<\/td>\n0.03” – 0.15”<\/td>\n<\/tr>\n
ABS<\/td>\n1.2 – 3.5 mm<\/td>\n0.045” – 0.14”<\/td>\n<\/tr>\n
Polyethylene (PE)<\/td>\n0.8 – 3.0 mm<\/td>\n0.03” – 0.12”<\/td>\n<\/tr>\n
Polystyrene (PS)<\/td>\n1.0 – 4.0 mm<\/td>\n0.04” – 0.155”<\/td>\n<\/tr>\n
Polyurethane (PUR)<\/td>\n2.0 – 20.0 mm<\/td>\n0.08” – 0.785”<\/td>\n<\/tr>\n
Nylon (PA 6)<\/td>\n0.8 – 3.0 mm<\/td>\n0.03” – 0.12”<\/td>\n<\/tr>\n
Polycarbonate (PC)<\/td>\n1.0 – 4.0 mm<\/td>\n0.04” – 0.16”<\/td>\n<\/tr>\n
PC\/ABS<\/td>\n1.2 – 3.5 mm<\/td>\n0.045” – 0.14”<\/td>\n<\/tr>\n
POM (Delrin)<\/td>\n0.8 – 3.0 mm<\/td>\n0.03” – 0.12”<\/td>\n<\/tr>\n
PEEK<\/td>\n1.0 – 3.0 mm<\/td>\n0.04” – 0.12”<\/td>\n<\/tr>\n
Silicone<\/td>\n1.0 – 10.0 mm<\/td>\n0.04” – 0.40”<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/div>\n<\/div>\n
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For best results:<\/strong><\/p>\n