Warpage avoidance

In mould design, flow balance avoids warpage. 

The SiH4/02 and SiH4/N20 routes to Si02 thin films are perhaps the most widely studied photochemical CVD of dielectrics [21], mainly due to their importance in VLSI technology. Photo-CVD of Si02 provides a suitable route to deposition at low substrate temperatures, thereby avoiding potential thermal effects of wafer warpage and deleterious dopant redistribution. In addition, other low temperature methods, such as APCVD and PECVD, often provide poor purity of films. 

Avoid large, plain surfaces. Break up these areas with ribs or serration to avoid warpage and distortion. 

Tolerance for board warpage and previously assembled components. Avoids need to compensate for height. 

Currently, multiple-component, internal gas-assisted and decorative processes are being used more and more frequently in combination with thin-wall techniques. A good example of this is the trend to highly individualized cell phone designs. Decorative processes such as inmold decoration or insertion of preformed, printed foils makes variable coloration of even small run volumes possible. The internal gas-assisted method is used for example to reduce the mass of thick-walled parts and to avoid sink marks minimization of warpage is thus a primary requirement for thin-walled parts. A wide variety of functions can be realized by means of the multiple-component technique, including seals, napped surfaces, improved antishp properties and functional elements, some of which are now still being mounted in complex and expensive additional steps. 

Figure 11-14. Avoid having uneven sections, which cause distortion, warpage, cracks, sinks, and strains because of differences in shrinkage from one section to another. Where this situation exists or at least usually exists the problem is normally eliminated by changing the process controls which in turn usually requires a longer cycle time.

Factors—part shrinkage, dimensional tolerances, warpage, drafts and undercuts, location of holes, fillets and radii to promote resin flow and reduce stress concentration, wall thickness, weld and parting lines to improve appearance and avoid areas of high stress concentration, and the introduction of ribs, bosses and studs, and metal inserts to improve strength—are treated. Recommended safety factors as a function of load are given. 

Nelson C. Baldwin, How to Avoid Warpage Problems in Injection Molded Parts. Part 1 — Part and Mold Design , Plastics Design and Processing, 14, No. 1 (January 1974). 

Although the rotational molding process is carried out with very little shear on the polymer melt, the cooling rate of the polyethylene after the fabrication step remains important in order to avoid excessive shrinkage and warpage. This is due to the crystallization process that takes place as the polymer melt cools from an amorphous, less dense state to a solid, semicrystalline state, higher density solid. 

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