injection process. Surface luster and finish relate to mold surface, wear, plastic material brand and quality, and injection molding conditions. The molding surface quality must be higher than the surface requirement for the plastic parts. Transparent plastic parts typically require a very high surface finish, which is normally a Ra of0. 025 um or bette 1.2.2 Wall thickness Plastic parts require a reasonable thickness. It should not be too thin, as it needs to have sufficient strength for its application, fastening during its assembly, filling out its cavity during molding, and impact during ejection. It should not be too thick, for it increases the shrinkage, further varies dimensions, prolongs the cooling time, and wastes the material. It may also cause porosity, shrinkage cavity, dent and warpage. The thickness decision needs to carefully strike a balance between economics and part quality and strength, though it also depends on the material type, part size, and molding conditions. thermoplastic materials are good for parts with a think wall, which usually varies between 1.5-4 mm. though it could be as thin as 0.6-0.9 mm or even 0.25 mm at times The thickness should be homogeneous all around the part, to avoid residual forces and defects, caused by uneven shrinkage during the solidification and cooling process. Fig 1-1(a) shows an undesired part structure, while Fig 1-1(b)is a reasonable structure. It is a practice to vary the thickness of a part design, in order to probably locate the welding mark. Fig 1-2 shows an effort of assuring the part quality on the top, by increasing the thickness of the top area, thus avoid leaving welding marks in the top area. Fig 1-1: wall thickness design of plastic parts Welding marks 接痕 4:>1湖门 Gate Fig 1-2: non-uniform wall thickness of plastic parts
injection process. Surface luster and finish relate to mold surface, wear, plastic material brand and quality, and injection molding conditions. The molding surface quality must be higher than the surface requirement for the plastic parts. Transparent plastic parts typically require a very high surface finish, which is normally a Ra of0.025μm or better. 1.2.2 Wall Thickness Plastic parts require a reasonable thickness. It should not be too thin, as it needs to have sufficient strength for its application, fastening during its assembly, filling out its cavity during molding, and impact during ejection. It should not be too thick, for it increases the shrinkage, further varies its dimensions, prolongs the cooling time, and wastes the material. It may also cause porosity, shrinkage cavity, dent and warpage. The thickness decision needs to carefully strike a balance between economics, and part quality and strength, though it also depends on the material type, part size, and molding conditions. thermoplastic materials are good for parts with a think wall, which usually varies between 1.5~4 mm, though it could be as thin as 0.6~0.9 mm, or even 0.25 mm at times. The thickness should be homogeneous all around the part, to avoid residual forces and defects, caused by uneven shrinkage during the solidification and cooling process. Fig.1-1(a) shows an undesired part structure, while Fig.1-1(b) is a reasonable structure. It is a practice to vary the thickness of a part design, in order to probably locate the welding mark. Fig.1-2 shows an effort of assuring the part quality on the top, by increasing the thickness of the top area, thus avoid leaving welding marks in the top area. Fig.1-1: wall thickness design of plastic parts Fig.1-2: non-uniform wall thickness of plastic parts Welding marks Gate
1. 2.3 Ribs Ribs are often used to enhance the plastic part strength and stiffness without a thicker wall. Ribs could also improve material flow conditions during molding Air bubbles Fig 1-3 adding ribs to reduce wall thickness Fig 1-3(a)shows a thick and uneven wall design. Fig 1-3(b) shows a wall design of even thickness It saves material and enhances its strength and stiffness while avoiding air bubbles shrinkage cavities dents and warpages. Rib dimensions are shown in Fig. 1-4. Their thickness is usually smaller than the wall thickness When considering adding ribs to a plastic part design, the focus is to minimize the concentration of the material in an area, to avoid air bubbles and shrinkage cavities. Fig. 1 -5 shows a rib arrangement Fig 1-5(a) shows an undesired arrangement where material concentrates in one area. Fig 1-5(b)shows a better arrangement. Rib should not be too big; they should be short and more in quantity. The distance between ribs should be equal or greater than twice the wall thickness. As shown in Fig. 1-6, a good lesign can avoid shrinkage cavities and increase part strength and stiffness. The orientation of rib arrangement should be in line with the material flow direction, to ease filling out the cavity and to avoid disturbing the flow. There should be a gap between rib's end and the part support surface <(0.50. R=t/8
1.2.3 Ribs Ribs are often used to enhance the plastic part strength and stiffness without a thicker wall. Ribs could also improve material flow conditions during molding. Fig.1-3: adding ribs to reduce wall thickness Fig.1-3(a) shows a thick and uneven wall design. Fig.1-3(b) shows a wall design of even thickness. It saves material and enhances its strength and stiffness, while avoiding air bubbles, shrinkage cavities, dents and warpages. Rib dimensions are shown in Fig.1-4. Their thickness is usually smaller than the wall thickness. When considering adding ribs to a plastic part design, the focus is to minimize the concentration of the material in an area, to avoid air bubbles and shrinkage cavities. Fig.1-5 shows a rib arrangement. Fig.1-5(a) shows an undesired arrangement where material concentrates in one area. Fig.1-5(b) shows a better arrangement. Rib should not be too big; they should be short and more in quantity. The distance between ribs should be equal or greater than twice the wall thickness. As shown in Fig.1-6, a good design can avoid shrinkage cavities and increase part strength and stiffness. The orientation of rib arrangement should be in line with the material flow direction, to ease filling out the cavity and to avoid disturbing the flow. There should be a gap between rib’s end and the part support surface. Fig.1-4 Rib dimensions Air bubbles < (0.5~0.7) t < 3 t t R = t / 8
Fig 1-5: rib arrangement Fig 1-6: rib design 1. 2.4 Support Surface When plastic parts need to have a support surface, it is not desirable to use the whole part bottom surface as the support. Fig 1-7 shows the bottom surface becomes uneven, when the plastic part deforms a bit. A Frame, three or four bottom feet are usually a better support surface Fig. 1-7: design of support surface 1. 2.5 Draft Draft is used to protect plastic part surface from scratch during ejection. It is a draft reserved along with the ejection direction. The draft depends on the material shrinkage, part shape, wall thickness, and ejection location For inner holes on a plastic part, the smaller end of core is used as reference. Its draft is shown along with the rising direction. For exterior of a plastic part design, the larger end of cavity is used as reference. Draft is dimensioned along with the reducing direction Please refer to Table 1-3 for various draft designs. A typical draft ranges between 30-1 30. Normally a tall or large plastic part design has a smaller draft. When there is a special need or higher precision requirement, a smaller daft is used
Fig.1-5: rib arrangement Fig.1-6: rib design 1.2.4 Support Surface When plastic parts need to have a support surface, it is not desirable to use the whole part bottom surface as the support. Fig.1-7 shows the bottom surface becomes uneven, when the plastic part deforms a bit. A Frame, three or four bottom feet are usually a better support surface. Fig.1-7: design of support surface 1.2.5 Draft Draft is used to protect plastic part surface from scratch during ejection. It is a draft reserved along with the ejection direction. The draft depends on the material shrinkage, part shape, wall thickness, and ejection location. For inner holes on a plastic part, the smaller end of core is used as reference. Its draft is shown along with the rising direction. For exterior of a plastic part design, the larger end of cavity is used as reference. Draft is dimensioned along with the reducing direction. Please refer to Table 1-3 for various draft designs. A typical draft ranges between ' ' 30 1 30 ° − . Normally a tall or large plastic part design has a smaller draft. When there is a special need or higher precision requirement, a smaller daft is used
External draft can be as small as 5. Internal draft can be as small as 10 -20. A complex plastic part which is difficult to eject, should adopt a larger draft. Usually a draft of 4-5 on each side is considered for a plastic part design with ribs or bosses. Larger draft should be chosen for a plastic product with large wall-thickness; during mold opening, in order that the product is retained on the side of the moving mold, draft of the inner surface should be smaller than that of the outer surface and the other way round, draft of the outer surface should be smaller than that of the inner surface so as to retain the product on the side of the fixed mold Fig 1-8: draft Table 1-3: drafts for common plastic materials Draft Draft Plastic Types Plastic Types Cavity PMMA PP&PVC-F 20′~50 PA PVC-U 50′~1°45′ 30′~50′ 30 ABS 30′-1°35′-1°30 1.2.6 Hole design k Holes may exist on a plastic part, including through holes, blind holes, screwed holes, and irregular shaped holes. In principle, holes should be as simple as possible. a complex hole increases the difficulty of mold making. As shown in Table 1-4, a sufficient gap should be reserved between holes or between a hole and the wall. The diameter of a hole also relates to its depth as shown in Table 1-5. When the distance between two holes or from the hole to the edge is smaller than the one specified in Table 1-4 please refer to the pattern design shown in Fig 1-9 Table 1-4: holes'pitch, space to wall and diameter of thermoset plastics Hole diameter mm 3~6 6~10 10~18|18-30 Holes pitch, space to wall /mm 3~4 4~5 5~7 Table 1-5: relations of hole's diameters and depths Molding types Hole depth Through hole Blind hole Horizontal hole 2.5d <1.5d Compression molding Vertical hole Extrusion or injection molding 4~5d
External draft can be as small as ' 5 . Internal draft can be as small as ' ' 10 − 20 . A complex plastic part, which is difficult to eject, should adopt a larger draft. Usually a draft of ° ° 4 − 5 on each side is considered for a plastic part design with ribs or bosses. Larger draft should be chosen for a plastic product with large wall-thickness; during mold opening, in order that the product is retained on the side of the moving mold, draft of the inner surface should be smaller than that of the outer surface and the other way round, draft of the outer surface should be smaller than that of the inner surface so as to retain the product on the side of the fixed mold. Fig.1-8: draft Table 1-3: drafts for common plastic materials Draft Draft Plastic Types Core Cavity Plastic Types Core Cavity PE 20′~45′ 25′~45′ PMMA 35′~1° 35′~1°30′ PP&PVC-F 20′~50′ 50′~1° PA 20′~40′ 25′~40′ PVC-U 50′~1°45′ 50′~2° PC 30′~50′ 35′~1° PS 30′~1° 35′~1°30′ CPT 20′~45′ 25′~45′ ABS 35′~1° 40′~1°20′ POM 30′~1° 35′~1°30′ 1.2.6 Hole Design Holes may exist on a plastic part, including through holes, blind holes, screwed holes, and irregular shaped holes. In principle, holes should be as simple as possible. A complex hole increases the difficulty of mold making. As shown in Table 1-4, a sufficient gap should be reserved between holes or between a hole and the wall. The diameter of a hole also relates to its depth as shown in Table 1-5. When the distance between two holes or from the hole to the edge is smaller than the one specified in Table 1-4, please refer to the pattern design shown in Fig.1-9. Table 1-4: holes’ pitch, space to wall and diameter of thermoset plastics Hole diameter mm ＜1.5 1.5~3 3~6 6~10 10~18 18~30 Holes’ pitch, space to wall /mm 1~1.5 1.5~2 2~3 3~4 4~5 5~7 Table 1-5: relations of hole’s diameters and depths Molding types Hole depth Through hole Blind hole Horizontal hole 2.5d ＜1.5d Compression molding Vertical hole 5d ＜2.5d Extrusion or injection molding 10d 4~5d
白④ Fig 1-9: improvement design for too small holes' pitch and space to wall The holes used for fastening on the plastic products as well as other holes under stress should be reinforced with a convex edge designed thereon, as indicated in Fig. 1-18. The fixing hole can be designed in the form of a screw hole with sinking head as illustrated in Fig. 1-19a), whereas that in Fig 1-19b)is generally not recommended. The form as shown in Fig. 1-19c)can be used instead so that the core can be better set m山 Fig1-10: reinforcement of holes 国國 Fig. 1-11; types of fixing holes 1.2.7 Screw Screws on a plastic part can be formed during the injection process. They may be created by machining after the injection process. For a plastic part which is subject to frequent assembly and For design of screws on a plastic part, the following principles are followed 1. The diameter of injected external screws should not be smaller than 4 mm. Internal screws sl not be smaller than 2 mm. Length of fit should be reduced to less than 1.5-2.0 times of the diameter. in order to minimize the accumulative error on the screw pitch
Fig.1-9: improvement design for too small holes’ pitch and space to wall The holes used for fastening on the plastic products as well as other holes under stress should be reinforced with a convex edge designed thereon, as indicated in Fig. 1-18. The fixing hole can be designed in the form of a screw hole with sinking head as illustrated in Fig. 1-19a), whereas that in Fig. 1-19b) is generally not recommended. The form as shown in Fig. 1-19c) can be used instead so that the core can be better set. Fig.1-10: reinforcement of holes Fig.1-11: types of fixing holes 1.2.7 Screw Screws on a plastic part can be formed during the injection process. They may be created by machining after the injection process. For a plastic part which is subject to frequent assembly and disassembly, the part may be inserted with a metal screw. Fig.1-12: screw design on plastic parts For design of screws on a plastic part, the following principles are followed: 1. The diameter of injected external screws should not be smaller than 4 mm. Internal screws should not be smaller than 2 mm. Length of fit should be reduced to less than 1.5~2.0 times of the screw diameter, in order to minimize the accumulative error on the screw pitch