Part Warpage

The optimization of the moulding parameters can be difficult because of the delicate balance between the blowing agent decomposition and the part shaping. Part warpage and shrinkage are sometimes difficult to predict. 

Plasticizers and waxy additives can give good initial color and physical property results yet cause the finished product to have poor heat deflection properties that can lead to latent part warpage. Much of this delayed defect phenomenon is associated with crystalline or semicrystalline products that must recrystallize after melting in order to reach their full property potential. Waxes and plasticizers can hinder recrystallization, and the effects can be different in thick sections that cool slower than thin sections. 

To obtain dimensionally correct parts, warpage must be estimated, then reduced as far as possible by means of structural design, material selection and processing. 

In injection molding of semicrystalline thermoplastics, for example polyamide, polyoxymethylene, ultrahigh molecular polyethylene (very high melt viscosity), linear polyesters (PET, PBT), local variations in molecular orientation result in dimensional variations in the part (warpage, memory effect), especially if the toothed gear is exposed to a thermal stress load. 

Understanding Thermoplastic Part Warpage, LNP Technical Times, LNP Engineering Plastics, Inc., Exton, Pa., Spring 1995, pp. 1—3. 

One area of recent interest is in the combination of glass fiber with other fillers. Glass fiber and mica are commonly used to reduce part warpage. Another reason for glass and mineral fiber combinations is cost reduction. One type of mineral fiber that can be used is a wool fiber from Lapinus. This fiber is composed of... 

During this period two principal tasks were carried out. Evaluation of water-milled silicon nitride compositions required several qualification runs where test tiles from water-milled powders were CIP ed and densified by a Norton proprietary HIP ing process. Concurrently, an experimental evaluation of fixturing approaches to eliminate part warpage during HIP ing was conducted with both CIP ed and injection molded tensile rods. Both densified tiles and tensile rods were submitted for testing. 

Lower part warpage. Nanoparticles are much smaller than other particulate additives, thus are more insensitive to shear. The random distribution results in a reduced chance of part warpage. 

Figure 11-23. A mismatch in alignment of two molded parts (such as a box and cover) usually is traceable to part warpage or difference in shrinkage the misalignment at the parting line appears to be improved when a bead or a designed mismatch is utilized.

SWIS (Moldflow, Kalamazoo, MI). Predicts linear shrinkage and, with unreinforced materials, part warpage. Planned for future commercial release as part of a refined software system. 

The fill time is the time necessary to fill the mold. The fill time is dependent upon the shot size or dosage stroke and the injection rate. Plastic materials have different injection rates associated with them. The pack or hold time is needed to control the part warpage. Likewise, cool time is needed to minimize part warpage due to shrinkage of the plastic material. The time for filling the mold, packing the part, and cooling the part can be optimized to improve the quality of the plastic part and to minimize cycle time. 

Less strength and modulus efficient Lower fiber volume than tape Greater scrap rates Warp and fill properties differ Fabric dishxtion can cause part warpage... 

Ticona s COC (cyclic olefin copolymer) (Topas ) has been noted to significantly reduce the part warpage of fiberglass filled polypropylene. Commercial utility in automotive apphcations has been noted. 

The results show that the external-internal cooling reduces significantly the cycle times particularly for the moulding of thick plastic products. According to Chen s off-line experimental results (13), the part warpage starts from the crystalline melting temperature,. An early separation... 

A preliminary study shows that the onset of part warpage at different degree of part solidity is independent of the total cycle time for the rotational moulding process with external-internal cooling. For the external cooling process, this warpage formation for thicker parts does have a considerable influence on prolonging the total... 

Moldflow Plastics Insight software from Moldflow Corporation was used for simulation of filling, packing, cooling stages, and part warpage. 

The computer simulation software was a very usefid tool for simulating the whole injection molding process and predicting part warpage. 

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