Demolding

Reinforced Thermoplastic Sheet. This process uses precombined sheets of thermoplastic resin and glass fiber reinforcement, cut into blanks to fit the weight and size requirements of the part to be molded. The blanks, preheated to a specified temperature, are loaded into the metal mold and the material flows under mol ding pressure to fiU the mold. The mold is kept closed under pressure until the temperature of the part has been reduced, the resin solidified, and demolding is possible. Cycle time, as with thermosetting resins, depends on the thickness of the part and the heat distortion temperature of the resin. Mol ding pressures are similar to SMC, 10—21 MPa (1500—3000 psi), depending on the size and complexity of the part. 

Mold temperatures vary between 150—200°C, depending on the mol ding methods and part size. Parts can be molded in 1.5—10 min depending on the configuration and thickness of the part, the mold temperature, and the desired state of cure at demolding. Since most ethylene—acryflc parts are postcured, it is sometimes possible to demold partly cured articles and complete vulcanization in the postcuring oven. 

Blistering Demolded too soon Improper catalytic action Extend molding cycle Check resin mix for accurate catalyst content and dispersion... 

Add wood furnish (384 g, moisture content 6.02%) to the bowl of a rotary blade paddle mixer (such as a Kitchen-Aid KSM90) and agitate at the lowest speed setting. Add Mondur 541 (7.39 g, 1.9% w/w, a polymeric diphenylmethane diisocyanate of 31.5% NCO, Bayer) dropwise over a 5-min period using a disposable syringe. Continue blending for an additional 10 min and then transfer the blend to an 8 x 8 x 2 -in. metal form at the bottom of which is a metal plate which fits inside. The resin-coated furnish is evenly spread inside the form and another metal plate is placed on top. All parts of the form and plates are presprayed with mold release. The completed form assembly is placed into a hydraulic press (such as a model PW-22 manufactured by Pasadena Hydraulics) with platens heated at 350°F. The furnish is then pressed between the two form plates to a thickness of j in. Press controls are used to ensure consistency of board thickness. The assembly is heated for 4 min. before demolding the cured wood panel. 

A cement paste phantom was prepared from white Portland cement. A thin sheet of copper was placed in the top of the specimen at the time of casting to produce a synthetic crack 300-pm wide, 8-mm long and 3-4-mm deep. The sheet was removed the following day. The specimens were 10 x 10 x 10 mm. At the time of demolding, the specimen was slightly damaged and two cracks formed on the corners of the specimen. The thicknesses of these cracks was estimated to be 100 pm and 60 pm. Prior to the MRI measurement the samples were stored in water to ensure saturation. 

After cooling, the final part is removed and the mold is recharged with resin. Since the mold surfaces are often complex, processors commonly apply mold release agents to allow the part to demold. Mold release agents are lubricants that coat the metal surface with a waxy material preventing the polymer from adhering to the mold s surface. Examples of mold release agents include silicone sprays or olefin based waxes. 

Polyvinyl chloride is also widely used. Rigid polyvinyl chloride is introduced to the mold in powder form. The material is chosen for durable constructions because of its chemical resistance and ease of processing. It incorporates functional additives and demolds easily. Plasticized polyvinyl chloride can be used to produce flexible parts such as balls and soft toy parts. The polyvinyl chloride is introduced to the mold as either a plastisol or powder. A plastisol is a suspension of granules in a plasticizing agent. When heated, the polymer granules absorb the plasticizer and fuse to form a cohesive, flexible material. 

Multiple arm machines rotate molds into the different stations of the process. A three-arm machine would, for example, have one mold filling while another is heating in the oven and the third cooling in preparation of demolding. This is very efficient but requires that each stage takes the same time to accomplish. When there are three different parts, the largest part or the one with the thickest walls defines the speed for the entire process. 

Regardless of the variant of LCM the process can be subdivided into a number of steps. The first step is preforming, which means that dry reinforcement is tailored to the shape of the mold. This can be done in many different ways depending on the complexity of the geometry and the requirements on mechanical performance of the part. The preform is then placed in a mold cavity that is subsequently closed. The preform is usually compressed slightly by the mold. The next step is resin injection into the mold cavity until the preform is fully impregnated. The final step is in-mold cure (i.e., curing inside the mold until the part is sufficiently stiff to be demolded). A controlled postcure is sometimes performed to ensure that optimum properties are obtained. 

Figure 12.1 Truck part manufactured with RTM by Borealis Industrier AB, Sweden. Figure shows demolding step...

A short cure time from completed mold filling to demolding... 

The reactions are seldom allowed to go to completion in the mold. The usual practice is to let the part cure until it is sufficiently stiff to demold and then to perform a postcure either in an oven or at room temperature. If the degree of cure is too low at demolding, then a number of problems can occur It may be impossible to demold without damaging the part permanent deformation of the part can occur work place hazards with toxic fumes may occur, and so on. 

Barone and Caulk performed the same calculation for several plate thicknesses and mold temperatures to predict the time for demolding. They defined demolding time as the time it takes for every point in the plate to have reached at least 80% cure. Figure 8.35 presents the processing window generated, where the time for demolding is plotted as a function of plate thickness and mold temperature. In addition, the... 

Molds must be checked for damage prior to use and preheated to a temperature equal to the maximum that the exotherm will reach when the prepolymer and curative are reacted together. If required, a thin coating of mold release is applied to the mold to assist in the demolding process. It is often found that more that one release can be obtained from the mold that has previously been prepared. 

The degree of difficulty in demolding the parts is a function of the design of the mold and the correct functioning of the mold release. Any levers used must be free of burs and cracks. Compressed air is often used to assist in breaking the seal between the polyurethane and the mold. The air lines and gun must always be in good order. Accidental misuse of the compressed air is very dangerous. 

Before reusing a mold, it should be physically cleaned to remove any grime or protective materials. Buildup on the surface should be cleaned off without damaging the surface of the mold. Location pins must be checked and repaired if needed. Any damage caused during demolding also must be repaired if in a critical area. The repairs must be such that they do not leave any marks on the surface of the part. 

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