Mold failure

Short shot n. In injection molding, failure to fill all cavities of the mold completely, caused by too low melt temperature, too low injection pressure, insufficient plasti-cation time, too constricted gates, too viscous resin, inadequate venting of cavities, etc. Short shots are often made deliberately when testing a new multicavity mold to reveal the pattern of runner flow and the sequence of cavity filling. 

For large molds, e.g., for instrument panels, rigid solder joints with different thermal expansion behavior could lead to tension, which would result in premature mold failure. To avoid this tension, large molds are floating mounted to the mold carrier of the machine. 

Depending on the steel s composition, the process temperatures, and particularly the soaking times, layers of varying depths can be achieved. As the alloy content of the steels increases, the surface hardness achievable increases, and the layer thickness achievable declines. It helps to protect against deformation if the substrate is hard enough. Excessively intensive treatment can make the material brittle (nitride formation at the grain boundaries of the steel microstructure), leading to premature mold failure. 

With this, mold failure can be prevented. The objective to increase the lifetime, and thus to increase the benefits, will be pursued. The improvement of safety and increased mold availability is associated with this. The number of failures is minimized. Thus, the operating processes are optimized and a cost control is achieved. 

During some mol ding and extmsion operations, knit line failures, incomplete mold fill, die drag, and excessive heat buildup, ie, scorch, are problems. Many of these problems are reduced or eliminated by the addition of internal lubricants such as low mol wt polyethylene or Vanfre AP-2 Special, a product of R. T. Vanderbilt. 

Slime masses or any biofilm may substantially reduce heat transfer and increase flow resistance. The thermal conductivity of a biofilm and water are identical (Table 6.1). For a 0.004-in. (lOO-pm)-thick biofilm, the thermal conductivity is only about one-fourth as great as for calcium carbonate and only about half that of analcite. In critical cooling applications such as continuous caster molds and blast furnace tuyeres, decreased thermal conductivity may lead to large transient thermal stresses. Such stresses can produce corrosion-fatigue cracking. Increased scaling and disastrous process failures may also occur if heat transfer is materially reduced. 

Basically, in the vicinity of a sharp comer all fringes converge toward the apex. Having a high density of lines at this point indicates the presence of high stress level. At a rounded corner there will be considerably less concentration. Besides the molding problems, sharp corners often cause premature failure because of the stress concentration. To avoid these problems, inside comer radii should be equal to one-half the nominal wall... 

Residual stress There is a condition that develops, particularly in products with thin walls. This is a frozen-in stress, a condition that results from the filling process. The TP flowing along the walls of the mold is chilled by heat transferring to the cold mold walls and the material is essentially set (approaching solidification). The material between the two chilled skins formed continues to flow and, as a result, it will stretch the chilled skins of plastics and subject them to tensile stresses. When the flow ceases, the skins of the product are in tension and the core material is in compression that results in a frozen-in stress condition. This stress level is added to any externally applied load so that a product with the frozen-in stress condition is subject to failure at reduced load levels. 

By extrusion parison control it is possible to minimize the wall thickness variation and the extent of stretching and stretch orientation. These are the province of the processor when the designer is not familiar with BM. Knowledge is required to provide information on what is possible and to select the specific BM process that has the capability to mold the product. The designer should be aware of the possible failure modes and compensate for them in the design. There is little else the designer can do but select the best material and process to make the product. 

Great care is taken that we design blow molds to avoid thin or weak regions that could result in premature failure. Molds are designed to avoid excessive draw into corners that would result in locally thin areas. For this reason, blow molded products invariably have rounded corners. Another potential source of weakness is the pinch-off line. To compensate for this fact, it is common to program the parison to produce a thickened base. 

A part, consisting of a carbon fiber composite tube, was initially adhered to the inside of a short titanium coupling by a silica-filled epoxy. The bond failed and a fluorinated mold release was believed to be the cause of the failure and was the purpose for this investigation. A sample of the epoxy (Sample A) and the part (Sample B) were submitted for X-ray photoelectron spectroscopy (XPS) to analyze for the presence of both fluorine and silicon. 

This case study involved medical diagnostic parts manufactured from PC resin, which were beginning to break too easily. To determine the cause of the failure, a good sample was submitted for comparison to a cracked part. Two possible causes for the failures were postulated. These include brittleness due to an excess level of filler, or the presence of voids due to insufficient drying of the resin prior to molding. 

With his 750,000, Baekeland set up a lab next to his home. He then sought to solve the problem of making the hard material obtained from phenol and formaldehyde soluble. After many failures, he thought of circumventing the problem by placing the reactants in a mold of the desired shape and allowing them to form the intractable solid material. After much effort he found the conditions under which a hard, clear solid could be made—Bakelite was discovered. Bakelite could be worked, it was resistant to acids and organic liquids, stood up... 

Since no difference was found between FR and non-FR formulations in device aging studies, some other cause must account for the relatively early failures observed for devices molded in the electrical grade epoxy material and aged under bias at 200°C. These failures are attributed to chloride contamination present in the non-semiconductor grade epoxy resin. The extractable Cl concentration is a factor of four higher than Br, and this is correlated with a much higher concentration of CHoCl than CH Br in the EGA data below 200°C. The high Br concentration is also attributed to the... 

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