Plastics specifications and drafting practices

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Plastics specifications and drafting practices

There are several aspects of plastics specifications and drawings (or files) which are particular to this family of materials and processes and which deal with realities which must be addressed.

Cavity identification. When more than one cavity is built, they must be identified in order to evaluate their adherence to quality specifications. Each will vary in actual dimension due to the effects of location in the mold base on melt flow and to construction differences. The number or letter should be located in a place where it will not interfere with a fitment or the appearance of the part. The following notation is used: Each cavity must contain an identification number- location to be approved by engineering [1].

Draft indication. With the exception of extrusion, machining, and pultrusion, all of the plastics processes require draft. It is critical that the part be drawn with draft in order to determine what the draft will do to the design. Draft can cause wall thicknesses to double or to disappear altogether. Failure to draw the part with draft can lead to fitments which do not fit and molds which cannot be repaired.

Draft is usually specified as x° side (or x° side) and placed on the dimension. (1.000 1°/S) [2, p. 34]. Thus, the designation 1°/S indicates that a 1° draft is intended to increase from the point of the dimension so indicated. Conversely, the designation 1°/S would indicate that the draft decreases from the point dimensioned by 1°/S.

The draft specification controls the way the mold is built as the direction of draft normally indicates the direction of draw (removal of the part from the tool) since reverse draft would be an undercut condition. For injection molding, the mold designer will attempt to locate the core on the back, or movable, half of the mold. That is the side where the molding machine's ejector bars are located, and locating the core on that half makes the ejector mechanism available to push the part off the core. Part designs with cores from both sides of the part may require slight undercuts to keep them on the side of the mold which has the ejector mechanism. The draft specification will also control the location of the parting line, which should be indicated [2, p. 54].

Ejector locations. Ejection devices for plastic parts can range from screwdrivers used to pry parts out of a hand mold to mechanized stripper plates and elaborate mechanisms which also retract collapsible cores. All of them share one common characteristic: they exert pressure on a newly formed part. That pressure can distort the part to the point of disturbing its function or appearance if it occurs while the part is still too soft to withstand it. Therefore, the processor must delay ejection until the moldment can endure it. The more ejectors there are, the more ejection surface there is to distribute that pressure and the sooner the part can be removed from the mold, thereby shortening the molding cycle. However, ejectors cost money and leave marks on the surface of the moldment. Therefore, there is a mold cost associated with a faster molding cycle. (Differences between bidders on a project are often based on variations in cooling and ejection systems.) Additional ejectors leave more marks on the surface and their number and location may be limited by functional and appearance concerns. Neither the molder nor the mold maker is intimately familiar with the product or its application. Therefore, ejector location should be controlled with the following notation: ejector locations must be approved.

The stripper plate is a variety of ejection system which need not leave a mark on the part. It also permits ejection of the part in a much warmer state because it distributes the force of ejection uniformly around the parting line of the part. This system can reduce cycle times by as much as 35%; however, there is a considerably greater tooling cost for a stripper plate and not all designs permit its use [3, p. 47].

Flatness control. The very nature of plastics processing makes absolute flatness a virtual impossibility. Therefore, flatness specifications must take this factor into account. However, most plastics are flexible to some degree, thus they will conform to the shape of the mating part, making absolute flatness unnecessary. Nonetheless, plastic parts are process sensitive and there must be limits or the parts can go out of control and proper fitments can be jeopardized [4, p. 174].

Gate location specification. Gates can interfere with the function or the appearance of the part. Therefore, the engineer or designer must approve their location. Furthermore, they should specify surfaces where gates may not be placed (or where they must be trimmed flush) so the mold designer can lay out the mold accordingly. This issue can normally be addressed with the following specification: Gate location must be approved by engineering.

Knit or weld lines specification. Parts from processes in which material flows in the mold nearly always have knit lines and these knit lines will be the weak points in the part. Therefore, a specification regarding the acceptability of knit lines is necessary. For most applications, it is enough to indicate where knit lines are unacceptable. Knit lines which cannot be seen with the naked eye are always regarded as good; however, a good knit line can sometimes be faintly visible on a glossy surface. Thus the use of the specification which permits no visible knit line requires highly subjective decisions on the part of those charged with quality assurance. The only sure way of learning the strength of a weld line involves destructive testing. To reduce the level of subjectivity, weld-line examples can be tested and limit samples provided for quality assurance. In that case, the specification should read: Limit samples for acceptable weld lines [5, p. 277].

Material specification. The material specification is, perhaps, the most critical of all the specifications. A material deviation can lead to a variety of problems in the molding of the part, its properties, and its performance, both short and long term. That is true, not only of the resin itself, but of the other additives, such as fire retardants, ultra-violet inhibitors, fillers, lubricants, and pigments as well. [6, p. 117].For applications with low safety factors, no material substitutions should be permitted without prior testing. Strange things do happen, even with applications which appear to be significantly overspecified. The following material specification is recommended: Material is to be (name of manufacturer) (exact number of resin). Part is to include 20% additive (name of manufacturer) (exact number of additive). No substitutions permitted without written authorization.

The pressure for substitutions arises from market conditions. Material shortages and price increases force processors to seek means of relief. They will sometimes offer an «equivalent» material. Equivalent is, however, an ambiguous term when referring to plastics. It cannot mean precisely the same resin because resins are covered by patents and, therefore, each one is somewhat different than the others in its behavior and properties. Therefore, whether the alternative resin is close enough to the specified material to be acceptable depends on the application. Product engineers find it difficult to rely on processors' recommendations because they are primarily concerned with producing the part to a level which will achieve acceptance by the purchaser and are usually innocent of the product's performance requirements and characteristics, particularly in the long term. It is, therefore, wise practice for the end user to test at least two alternatives for each of the resins and additives. This should be done in advance of the need because there is usually no time for testing when the need for an alternative arises. Finally, the material supplier may not be the actual resin producer. In fact, the material supplier may use several sources. That needs to be established and resin from all possible sources tested for critical applications.

Mismatch specification. Closed molds are subject to misalignment when the parts are put together. This condition is referred to as «mismatch» and should be controlled according to how much it affects appearance or function in order to avoid any kind of dispute as to whether any misalignment is or is not included in the tolerance. When misalignment occurs between a core and cavity, a variation in the wall thickness between the two sides results. The flow of the melt will be altered since the cavity space is different between the two sides of the mold. This could affect the strength and location of a weld line [8, p. 74].Misalignment between core and cavity is also controlled by placing a tolerance on the wall thickness. The following specification can be used to control mismatch: Maximum allowable mismatch is XXX in.

Regrind limitation. Thermoplastics are capable of nearly 100% material utilization by regrinding sprues, runners, and reject parts and mixing that «regrind» in with virgin resin. However, the material suffers some degradation when it is raised to elevated temperatures. Regrind, therefore, reduces some of the physical properties of the material; the degree of drop varies between plastics and according to molding conditions. It is necessary to perform actual tests to determine the degree of property loss; however, readily visible signs of degradation are an increase in brittleness and a yellowing or darkening of color.

Physical properties drop according to the amount of regrind permitted. This is magnified by the multiplier effect which takes place. For example, if 20% regrind is used, then the batch will contain 4% (20% of 20%) which has been through twice, 0.8% of which has been through 3 times, 0.16% which has been through 4 times, etc. The processor may presume the use of all the regrind is acceptable unless it is controlled with a specification like the following: XX% regrind acceptable.

The engineer should be alert to the fact that the use of regrind runs the risk of contamination of the resin from a variety of sources. If a contaminant will pose a significant risk for the application (medical product) or if the need for all of the physical properties is critical, it may be necessary to prohibit the use of regrind. It is still possible to obtain near 100% material utilization in a closed runner system (hot or insulated runner system), which results in a significant increase in tooling cost [2, p. 207].

References

indication plastic cavity

1. Cadillac Plastic & Chemical Co., Troy, Mich.

2. John L. Hull, «Design and Processing of Plastic Parts,» Handbook of Plastics Elastomers and Composites, 2d ed., Charles A. Harper, ed., McGraw-Hill, New York, 2001. - P.83.

3. Terry A. Richardson, «Machining and Finishing,» Modern Industrial Plastics, Howard W. Sams & Co., New York, 1999. - P. 69.

4. «Joining of Composites,» in A. Kelley, ed., Concise Encyclopedia of Composite Materials, The MIT Press, Cambridge, 1999. - P. 295.

5. «Engineer's Guide to Plastics,» Materials Engineering, May 2002. - P. 369.

6. «Surface Preparation of Plastics,» in Adhesives and Sealants, vol. 3, Engineered Materials Handbook, H.F. Binson, ed., ASM International, Materials Park, Ohio, 2004. - P. 169.

7. J.O. Trauernicht, «Bonding and Joining, Weigh the Alternatives, Part 1, Solvent Cements, Thermal Welding,» Plastics Technology, August 1999. - P.147.

8. D. K Rider, «Which Adhesives for Bonded Metal Assembly,» Product Engineering, May 25, 2001. - P. 299-300.

9. «Mechanical Fastening» Handbook of Plastics Joining, Plastics Design Library, Norwich, NY, 2001. - P.259.

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