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PBT thermoplastic injection molded part warpage

While full pack is important, even when fully packed and molded perfectly, glass filled PBT thermoplastic is extremely difficult to mold without warpage due to its inherent nature to shrink differently for several reasons.

1) Being relatively short glass fibre reinforced, the fiber tends to align itself in direction of flow. The level of orientation of the glass depends on fibre length vs section thickness and on turbulence in the flow of the material. Both factors need to be considered when analyzing this. Mold flow often misses the turbulence thing and maybe the level of orientation thing. The change in shrinkage due to orientation is extreme and can cause severe warpage depending on plastic part geometry and gate position.

2) PBT thermoplastic is semi crystalline. The number and nature of more or less crystalline areas is somewhat dependent on hold pressure and additive packages and even color, ut is very dependent on rate of cooling and rate of transition from liquid to solid and on second order transitions. Plastic part and injection mold geometry can have a strong influence on this and frankly detailed analysis takes a first hand look at drawings, injection mold and plastic parts and a real knowledge of the characteristics of the material.

Tools like moldflow and tools to help with heat flow and temperature gradients can help as can detailed data re the materials shrinkage characteristics under a wide variety of conditions, but the tools are useless unless you know how to apply them, what is possible from a mold making point of view and where to suggest to the software that the gate should be. It does not tell you. It only analyses your suggestions.

On the plastic part warpage can be classified into 2 kinds, the first is the whole part warpage, the second is the partial warpage. both of them are caused by the uneven shrinkage after molding, the root reasons normally are uneven part surface temperature, inner stress, and fiber ordination of the fiber filled resin, for temperature reason, check the injection mold mold surface temp and make sure the deviation be within 5 degree, or need to adjust the water circuit linkage, for inner stress reason, make sure from the press nozzle to the cavity fill end there's no sharp corner and big change on the plastic part thickness, for the fiber ordination, just adjust the gate location, but first of all, make sure it is a whole warpage or partial warpage.

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