Plastic product design flow molding

Cost When it is necessary to equal the production rates of other processes, the mold cost with RM may exceed that of other processes such as flow molding. The plastics used in RM are generally more expensive than the pelleted plastics used in many other processes, because they must be more finely and evenly powdered, such as to a 35 mesh. However, this process generates low levels of regrind or scrap, even when it is operating poorly. Products can have no flash at all if properly designed molds are used. 

IMM, flow of melt, packing of mold cavity cycle time, to product performances. As an example, parameters that influence product tolerances involve (1) product design, (2) plastics used, (3) mold design, (4) IMM capability, and (5) molding cycle time. Examples of effects due to IMM and plastic material variables are shown in Figures 25 to 29. 

Not all products adhere to this rule, but the product designer must understand the flow of plastic within the mold. The plastic pressure is highest near the gate, and, due to the restriction in the space between cavity and core, the injection pressure drops as the cavity space is filled with plastic farther away from the gate. This means that, by the time the plastic reaches an area where more plastic is required (such as in a thickening of the product), the pressure available to fill that area is low, and it will be more difficult to fill the cavity and to pack out the product to specification. 

Typically, the selection of gate location and the decision between cold runner molds (two-plate or three-plate molds) or hot runner molds are important to the product design when considering the mold cost and the plastic flow within the mold. Today, about 90% of all molds built are still two-plate molds, which are considerably cheaper than three-plate or hot runner molds. For many reasons, in most cases, hot runner molds are better, but these questions must always be asked ... 

Experienced part and mold designers need to understand the flow phenomena of various plastics during the molding process in order to minimize the occurrence of unwanted defects in the final product. But even with the perfect design of part, and the perfect design of mold, the processing parameters of temperatures (of fluid plastic and of mold), fill rate, fill pressures, and in-mold dwell are equally critical in achieving quality parts. 

Molded plastics parts in many ways seem analogous to castings since the mold can be any shape. There are a number of limitations on the part because of the fact that the mold is filled with plastics material and it represents, in addition to the form for the product, the flow path for the plastics material. One factor that is affected is the wall thickness of the part. The viscous melts produced by many plastics require that a minimum wall thickness be used based on the ability to fill the mold rather than on the function of the part. For example, rarely is any injection molded part of any size designed with a wall thickness less than 0.025 to 0.030 inch. This is particularly true if the part has a fairly large surface area because the plastics will not flow to properly fill the part. In addition, as will be seen in Chapter 10, flow conditions can severely affect the properties of the material. 

In order to understand potential problems and solutions of design, it is helpful to consider the relationships of machine capabilities, plastics processing variables, and product performance (Fig. 1-10). A distinction has to be made here between machine conditions and processing variables. For example, machine conditions include the operating temperature and pressure, mold and die temperature, machine output rate, and so on. Processing variables are more specific, such as the melt condition in the mold or die, the flow rate vs. temperature, and so on (Chapter 8). 

Thin to large wall Designing around TP problems is the joint responsibility of the product and mold designers. For example, one way to handle the problem of thin to large area walls is by the inclusion of long ribs into the product in the direction of plastic flow. These ribs are not a functional requirement of the product but they act as auxiliary runners attached to the product to facilitate plastic flow in difficult to fill areas. In some instances the ribs may be used as a surface decoration like a corrugation or they may be on the concealed side of the product where they are stiffeners. 

Jetting Jetting is a condition that results when the mold design has no immediate impediment to flow and the plastics is ejected into a relatively large open volume. This jetted material becomes a weak point on the product and a surface blemish that is difficult to conceal. 

In the reinforced RIM (RRIM) process a dry reinforcement preform is placed in a closed mold. Next a reactive plastic system is mixed under high pressure in a specially designed mixing head. Upon mixing, the reacting liquid flows at low pressure through a runner system to fill the mold cavity, impregnating the reinforcement in the process. Once the mold cavity is filled, the plastic quickly completes its reaction. The complete cycle time required to produce a molded thick product can be as little as one minute. 

Both shape and design details are heavily process related. The ability to mold ribs, for example, may depend on material flow during a process or on the flowability of a plastic reinforced with glass. The ability to produce hollow shapes depends on the ability to use removable cores, including air, fusible or soluble solids, and even sand. Hollow shapes can also be produced using cores that remain in the product, such as foam inserts in RTM or metal inserts in IM. 

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