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<p><strong><u><span lang="EN-US" style="FONT-FAMILY: ??; mso-bidi-font-family: 宋体;"><font size="2"><font face="宋体, MS Song">Mold Bases and Cavities<p></p></font></font></span></u></strong></p><p><span lang="EN-US"><p><font face="宋体, MS Song" size="2"> </font></p></span></p><p><span lang="EN-US"><font face="宋体, MS Song" size="2">The mold base comprises the majority of the bulk of and injection mold. Standard of-the-shelf mold bases are available fro most molding needs. Typical mold bases are outfitted with a location ring and provisions for a sprue bushing in the stationary or "A" half of the mold and ejector assembly in the moving "B" half. Both halves come with clamp slots to affix the mold in the press. The "B' half has holes to accommodate bars that connect the press ejection mecahnism to the ejector plate in the mold. <strong><u><span style="FONT-FAMILY: ??; mso-bidi-font-family: 宋体;">Lead pin</span></u></strong> projecting from corners of the "A" half aligh the mold halves. <strong><u><span style="FONT-FAMILY: ??; mso-bidi-font-family: 宋体;">Return pin </span></u></strong>connected to the ejector plate coners project from the mold face when the ejection mechanism is in the forward (eject) position. As the mold closes, the return pin retract the ejector plate (if not retracted already) in preparation for the next cycle. </font></span></p><p><span lang="EN-US"><font face="宋体, MS Song" size="2">Mold cavities , here meaning core and cavity sets, can be incorporated in the mold three ways: they can be cut directly into the mold plates, inserted in pieces into the moldbase, or inserted as complete cavity units. Cutting cavities directly into the mold base can be the most economical approach for large parts and/or parts with simple geometryies. When doing so, select the mold base steel carefully. The physical properties of standard mold base steels may be inadequate for heavy-wear areas or critical steel-to-steel contact points. Using insert made of appropriate materials in these aras. </font></span></p><p><strong><u><span lang="EN-US" style="FONT-FAMILY: ??; mso-bidi-font-family: 宋体;"><font face="宋体, MS Song" size="2">Part Ejection</font></span></u></strong></p><p><span lang="EN-US"><font face="宋体, MS Song" size="2">Typically, molds ahve ejector systems built into the mving "B" half. The ejection unit of the molding press activate these systems. Rods linking the proess-ejector mechanism to and ejector plate in the mold enable the press controller to control the timing, speed and the length of the ejector stroke. Reverse injection molds eject parts from the stationary side of mold via indeependent ejection mechanisms operated by spring or hydralic cylinders. This configuration faclilitates direct injection onto the inside or back surface of cosmetic parts. The added complexity of reverse injection molds adds to mold cost. 'Specilized ejection components, such as knockout pin, KO sleeves, or striper plate, project from the mold ejector plate to the part surface where they push the part out of the mold. The common, round knockout pin provideds a simple and economical method for part ejection. Manufactured with high surface hardness and a tough core, threse inexpensive, of-the-shelf items resist wear and breakage. The moldmakers selects the desired diameter and shank length from the vast array of standard sizes and machines it to fit. Teh fit of the ejector pin into the round ejector hole must be held to tight telerance to avoid flash. Worn ejector holes can be refitted with .005-inch oversized pin available for standard diameters. <strong><u><span style="FONT-FAMILY: ??; mso-bidi-font-family: 宋体;">Ejector blades,</span></u></strong> Ko pins with a retangular corss section, operate much the same as stanard round pins, but can be more difficult to fit and maintain. Typically, they ae used on the edge of ribs or walls that are too thin for standard round pins. </font></span></p><p><span lang="EN-US"><font face="宋体, MS Song" size="2">KO Pins usually extend to the surface lying parallel to the mold face. IF KO pins push on anagle surface, consider adding the grooves to the part design to prevent the deflection. KO pins extending to narrow walls and edges can be stepped or positioned, so that only a portion of the pin contactsd the molded part. This avoid using small-dimeter KO pins that are more difficult to maintain and can deflect or bend. KO pins leave witness marks, small indetations or ring where the pin contacts the part, that coul be objectionable on cosmtic surface. Addtionally, they can rad-through to the opposite surface if the part is difficult to eject, or if the ejector area is too small. Many factors determine the amount of ejector areas needed, including the part geometry, mold finish, material-release characteristics, and part temperature at the time of ejection. To prevent damage during ejection, thin-walled parts generally require large ejector and greater ejector area than comparable parts with thicker walls</font></span></p><p><span lang="EN-US"><font face="宋体, MS Song" size="2">Drawing Polishing the mold steel in the direction of ejection gnerally helps ejection. Thermoplastics polyurethanes, exception to this rule, usually eject more easily from molds with frosted finishes that limit plastic-to-metal contact the peaks in the mold texture. Core shift and mold flexture can pinch part surfaces, hindering ejection. Top prevent this problem, add support to mold and core, or change the filling pattern to balance the injection forces.</font></span></p><p><u><span lang="EN-US"><font face="宋体, MS Song" size="2">Mold Venting</font></span></u></p><p><span lang="EN-US"><font face="宋体, MS Song" size="2">As molten plastic enters the mold, it quickly displaces air in tightly sealed mold. Although some air escapes though the parting line or loose-fitting ejector or slides, most molds need strategically placed vents for rapid and completed air removal. This section discusses vent design and placemet. Aslo adding a generous amount of mold draft helps ejection. Draft refers t o the slight angle or taper added to part features to ease part ejection. Most material requre at least one degree of draft or easy ejection. Vents should be placed at various location along the runer system and part perimeter, but they are especially needed at last areas of the mold to fill. Typically these aeas are located on the parting line an lie farthest from the gate. When the last area to fill is not vented, air may become trapped in the mold, peventing complete filling of the cavity and causing a gas burn on the part. The trapped air is super heated during compression and in severe cases can pit or erode the mold steel. When feasible, move gates or vary part thickness to change the filling pattern and direct air to parting-line vents. If air-trap area persist, consider using the ejector pin modifed with flats for venting. Ejector-pin Vents usually self clean with each ejection stroke. Air-trap areas not accessible by ejector-pin vents may require vents placed along mold inserts or splits in the mold this type of vent usually rquires periodic disassembly for cleaning. Porous netal inserts can also provide venting for difficutl air-trap areas but may require periodic cleaning. Part feature produced by blind holes in the mold, such as posts and bosses, require venting at the last area to fill, usually the tip or end. Bosses can usually vent along the core insert forming the inside dimeter of boss. Posts usually require ejector-pin vent at the tip of the post. Other venting issues you should address: direct mold filling along the length of the rib so gasses can escape at the end, and round or angle the ends of standing ribs to prevent air entrapment. Air trapped in unvented pockets or recesses in the mold can exit these aras or teardrop-shaped surface defects. Severe weld line often form where flow streams meet head on, especially at the end of fill. You can often improve the strength and appearance of these weld lines by installing overflow wells. Overflow wells are modfied vent features that provide and extra-deep vent channel, usually about one-third the part thickness, that empties into a cylinderical well. Venting air escapes that well around a shortened ejector pin fittted with a 0.002-inch clearance. Cool material at the leading edge of the advancing of flow frontd merges and enters the overflow well leaving hotter material to mix and fuse at the weld line. The overflow well is ejected with the part and clipped off after molding. Overflow wells can alos provide ejector-pin lcation for parts such as clock faces or instrument lenses that cannot to tolerate ejector-pin marks on the part surface. </font></span></p> |
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发表于 2007-6-4 17:40:00
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