Hardened mold surface

When tool steels of high hardness are used they are supplied in the soft annealed condition (hardened mold inserts for cores, cavities, other molding surfaces and gibs, wedge locks, etc are typically hardened to a... 

Once the plastic hardens, you must be able to remove the newly formed item from the mold. For this reason ail injection molds will have at least two sections and more complicated molds can require more sections. The point at which the mold sections come togetlier is called the parting line. It is common practice to place a 2 degree slope on mold surfaces that are perpendicular to the parting line. This is called the draft and its purpose is to make parts easier to remove from the mold. You will find that the draft is not always necessary, but at the very least surfaces must be perpendicular to the parting line. As you... 

For reasons mentioned previously, the molds are made of case hardened steel or a fully coated material with a hardened or refined surface and tough core. All components that do not have molding surfaces and are exposed to wear are made of steel with strengths from 600 to 700 N/mm. Hard chrome plating or a corrosion-resistant material has to be selected to protect the finish and the molding surfaces against wear caused by friction and chemical attack. 

FIGURE 4.91 Flame hardening a mold surface in a wear area... 

The mold surfaces to be grained shall have a polish that is customized to the graining. This polish must completely remove all possible residues from the EDM process and surface hardenings of previous work. To check surfaces and radii in a clean silhouette, sand blasting the surface is recommended. 

In general terms, it can be said that each type of material has its own wear effects. Here, however, it should be noted that in addition to the surface hardness of glass fibers (about 1,400 HV), particular color pigments such as titanium dioxide (about 2,200 HV) can cause a significantly higher wear, because the hardness of hardened mold steels are at about 700 to 900 HV. 

Thin, flat sheets of thermoplastic commonly called sheet molding compounds (SMCs) can be formed into various thin shell shapes by various techniques. One is vacuum forming shown in Figure 10.10. The sheet is heat-softened and pulled by the vacuum onto the cold mold surface where it is cooled and hardened. A vacuum corresponding to a pressure difference of only one atmosphere is all that is usually required. Another process, opposite to vacuum forming, uses positive air pressure. The air, rather than the sheet, is preheated. However, air pressure up to 10 atm is required. 

Molded polyamide surfaces can be hardened by grafting with Ai,Ai-diallylacrylamide [3085-68-5] monomer under exposure to electron beam (159). AijAZ-DiaHyltartardiamide [58477-85-3] is a cross-linking agent for acrylamide reversible gels in electrophoresis. Such gels can be dissolved by a dilute periodic acid solution in order to recover protein fractions. 

Cast molding is an increasingly used manufacturing process for both rigid gas-permeable and hydrogel contact lenses. In this process, two molds, made from a variety of plastics, are used. A female mold forms the lens front surface (convex) and a male mold forms the lens back surface (concave). The plastic molds are made from metal tools or dies that are usually stainless steel, precision lathed, and polished to the specified lens design. A variety of mold materials are used. The polymerized, hardened lens is released from the mated molds and is processed in much the same way as the spin-cast lenses described above. 

It should be recognized that surface finish can be more than just a cosmetic standard. It also affects product quality, mold or die cost, and delivery time of tools and/or products. The surface can be used not only to enhance clarity for the sake of appearance but to hide surface defects such as sink and parting marks. The Society of Plastics Engi-neers/Society of Plastics Industries standards range from a No. 1 mirror finish to a No. 6 grit blast finish. A mold finish comparison kit consisting of six hardened tool steel pieces and... 

This process is used to produce intricate, thin-section parts with great dimensional accuracy, fine detail, and very smooth surfaces. All ferrous and nonferrous alloys can be cast in investment molds. Investment casting begins with expendable wax patterns that are assembled into clusters, then coated with a series of successively coarser ceramic slurries. The assembly is then fired in a furnace to dry and harden the ceramic shell and to melt out the wax, leaving a cavity into which molten metal is poured to form the casting. 

The product from this system can be cured by using a conventional radical initiator such as benzoyl peroxide (BPO). To a sample (100 g) of this product, 1.5 g of BPO in a small amount of styrene was added. The mixture was poured into a mold and heated at 50 °C overnight and then at 75 °C and 100 °C for 2h each, and finally at 135 °C for 3 h in an air oven. The material hardened to a transparent solid with a very good surface. 

Injection molding is employed to stamp out copies of the master discs from polycarbonate. The molded polycarbonate discs are cooled and hardened quickly, within about 4—6 sec, and evenly. The dye layer is then applied. The applied dyes are often proprietary and continually modified in an attempt to get a better dye. The dye must be compatible with the system and adhere to the polycarbonate base. It is applied by spin coating i.e., the disc is spun and the dye is sprayed onto the surface. The dye is then dried and cured. 

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