Engineering plastics tolerances

One particular feature of PPO is its exceptional dimensional stability amongst the so-called engineering plastics. It has a low coefficient of thermal expansion, low moulding shrinkage and low water absorption, thus enabling moulding to close tolerances. 

Most of the engineering plastics reproduce faithfully and easily conform to the mold configuration, and when processing parameters are appropriately controlled, they will repeat with excellent accuracy tolerance wise, etc. As an example for the past many decades, we see plastic gears and other precision products made of acetal, nylon, polycarbonate,... 

The pace of development has increased with the commercialization of more engineering plastics and high performance plastics that were developed for load-bearing applications, functional products, and products with tailored property distributions. Polycarbonate compact discs, for example, are molded into a very simple shape, but upon characterization reveal a distribution of highly complex optical properties requiring extremely tight dimension and tolerance controls (3,223). 

Micromolding is precision molding of extremely small parts and components usually in engineering plastics unfilled, filled, and/or reinforced. Parts usually weigh less than 20 milligrams (0.020 g), with some even less that 0.1 mg in products as small as one mm3. Products are measured in microns and have tolerances of 10 microns or less. A micron (pm) is one-millionth of a meter 25.4 pm make up one-... 

In the event of a dispute, the tolerances indicated on the drawing will determine legal liability. Therefore, it is important for the engineer to understand the establishment of plastics tolerances completely, for they are more complicated than for other materials. Plastics tolerances are divided between the toolmaker (one-third) and the processor (two-thirds). Therefore, a dimensional tolerance of 0.006 in would require the toolmaker to build the tool to within 0.002 in. If it is further presumed that the plastic has a shrinkage rate of 0.005 in/in, it is clear that a 1.000 0.005-in dimension requires a precision mold and very careful molding. 

PPS is a crystalline engineering plastic especially known for its high heat performance with heat deflection temperatures in excess of 260 °C. It has exceptional processability when injection moulded on conventional equipment. It also has outstanding dimensional stability and so is particularly suitable for precision moulding to severe tolerances. It can also be used as a replacement for high performance thermosets or metal, especially brass, in some applications. 

Thermal Gradients may be measured or calculated by means of heat flow formulas, etc. After they are established it is likely to be found from the formula that for most cyclic heating conditions the tolerable temperature gradient is exceeded. This means that some plastic flow will result (for a ductile alloy) or that fracture will occur. Fortunately, most engineering alloys have some ductility. However, if the cycles are repeated and flow occurs on each cycle, the ductility can become exhausted and cracking will then result. At this point it should be recognized that conventional room temperature tensile properties may have little or no relation to the properties that control behavior at the higher temperatures. 

Plastic selection ultimately depends upon the performance criteria of the product that usually includes aesthetics and cost effectiveness. Analyzing how a material is expected to perform with respect to requirements such as mechanical space, electrical, and chemical requirements combined with time and temperature can be essential to the selection process. The design engineer translates product requirements into material properties. Characteristics and properties of materials that correlate with known performances are referred to as engineering properties. They include such properties as tensile strength and modulus of elasticity, impact, hardness, chemical resistance, flammability, stress crack resistance, and temperature tolerance. Other important considerations encompass such factors as optical clarity, gloss, UV stability, and weatherability.1 248>482... 

PVC in the feedstock stream in limited amounts can thus be tolerated if a HCl removal process based on calcium hydroxide (slaked lime) injection is used. This is essentially a form of in scrubbing in the pyrolysis reactor itself. This method is used by a number of pyrolysis processes since the fuel oil end products need to be low in chlorine to minimize engine corrosion problems. In the pyrolysis of commingled plastics, lime can be added to the input material and the PVC content reduced down to 2-3% in order to avoid deposits of CaCl2 in pipework. The chlorine content of the final diesel should not exceed 10 ppm. 

---