Injection molding sink marks

As discussed in the previous section on injection molding, a sink mark almost always occurs in extrusion on a flat surface that is opposite to and adjoining a leg or rib, because of unbalanced heat removal or similar factors. As with injection molding, sink marks can be concealed by adding a design feature, such as a series of serrations on the area where they occur (see Fig. 11-104). 

Because of low injection pressure, some cost savings are possible in mold and press constmction. Mol ding cycles are somewhat longer than for injection molding. The part must be cooled in the mold long enough to be able to resist swelling from internal gas pressure. In stmctural foam parts there is almost a total absence of sink marks, even in the case of unequal section thickness. Stmctural foam has replaced wood, concrete, sohd plastics, and metals in a variety of appHcations. 

As discussed in the previous section on injection molding, a sink mark almost always occurs in extrusion on a flat surface that is... 

MPX functionality is arranged into three modules, the first of which is the Setup Xpert, a module that allows users to perform a variety of injection-velocity- and pressure-phase-related setup routines to fix certain defects, such as short shots, flash, burn marks, sink marks, etc. The objective of Setup Xpert is to achieve one good molded part with no defects. The basic process is that a user molds a part, then provides feedback to the MPX system regarding molded part quality. The MPX system then processes this feedback along with data being collected from the machine and (if necessary) determines a process change that will improve the result. 

Chang, R.Y. Tsaur, B.D. Experimental and theoretical studies of shrinkage, warpage, and sink marks of crystalline polymer injection molded parts. Polym. Eng. Sci. 1995, 35, 1222. 

Compression Molding of Low-Density Foam Casting of Soft Plastisol Foam Extrusion of Structural Foam Injection Molding of Structural Foam Injection Molding Elimination of Sink-Marks... 

Sink Marks Injection Pressure too Low Excessive Injection Rate Short Injection Time Low Melt Temperature Low Mold Temperature Cooling Rate too High Gate Size too Small Incorrect Pad Size... 

With GIT injection molding (c) hollow parts with a defined smooth outer surface can be produced. Areas of application include suitcase handles, gripping handles in car interiors, garden tables, hollow-seamed ribbed feet (avoidance of sink marks). 

Injection compression molding (ICM) is, similarly to the gas injection technique and thermoplastic foam injection (TFI), particularly well-suited to making thick-walled moldings with little warping and few sink marks. However, thin-walled parts with long flow paths can also be produced efficiently by means of ICM. 

In common with the gas-injection- or structural foam molding processes, injection-compression molding also aims at producing thickwalled articles, free of warping or sink-marks. It is presently even employed for producing moldings with long flow-paths. 

With conventional injection molding where a solid part is being molded, CBAs also can be useful. With about 0.5 percent CBA, by weight, surface imperfections such as voids, sink marks, and so on, can be eliminated. The CAB has no effect on part density. 

Sink mark Machine 1. Shot size 3. Mold temperature 2. Injection pressure 5. Melt temperature 4. Holding pressure 6. Injection speed 7. Back pressure 8. Screw speed... 

Blowing agents are used in plastics for several reasons weight reduction, savings in cost and material, and achievement of new properties. The new properties include insulation against heat or noise, different surface appearance, improved stifl iess, better quality (removal of sink marks in injection molded parts), and/or improved electrical properties. 

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