1。4。 Flow orientation effect

Shrinkage is also affected by the direction of material orientation。 During shear flow, polymer molecules align themselves with the flow direction。 The extent of orientation depends on the shear rate to which the material is subjected。 When the material ceases flowing, the induced molecular orientation begins to relax at a rate that depends on the material's relaxation time。 If the material freezes before relaxa- tion is complete, the molecular orientation is frozen。 The shrinkage amount varies in directions parallel and perpendicular to the direction of material orientation。

Molding conditions are perhaps the easiest factors with which to alter the orientation effects exerted on a part。 By increasing melt tem- perature, the viscosity of the material is reduced, as is the frozen layer thickness, indicating that less molecular orientation is frozen in this layer。 The material remains hotter and longer, thereby enabling relaxa- tion to occur; the molecular alignment is randomized, reducing the ori- entation effect。 Increasing the injection speed increases the shear heating in the mold, and thus, the material is highly oriented。 The addi- tional shear heating reduces the viscosity of the material, which remains hotter longer, allowing relaxation to occur and reducing the orientation。 In addition, reducing the thickness generally enhances orientation, but reduces area shrinkage。 Analysis has frequently indicated that the max- imal orientation area is near the gate。 Thus, the number and position of gates should be determined to achieve both uniform and effective shrinkage values at the end of flow  paths。

2。 Literature review

Compared with conventional injection molding, thin-walled injec- tion  molding  forms  a  higher  ratio  of  solid  layers  along  the  depth

Fig。 3。 Sprue–runner–gate system in the injection mold。

Fig。 4。 Melt-front patterns used during mold filling: (a) 85% filling time; (b) 99% filling time。

direction。 In particular, the flowing ability of molten polymer is sensitive to part thickness reduction, and high resistance occurs when melting plastics with decreasing part thickness。 For example, the completeness of thin-walled molding is affected by injection speed, injection pressure, melt temperature, and mold temperature [6]。 Among these, injection speed and pressure are the major factors, followed by melt and mold temperatures。 A high injection speed and injection pressure facilitates effective thin-walled molding without generating a detrimental short shot; however, severe inner pressure deviation occurs along the flowing direction, causing further warpage。 Conventionally, increasing the gate number is a quick solution used to reduce the injection pressure required to complete mold filling, but this method has limitation。 Other approaches, such as high melt and mold temperature settings, are effective, but prolong the cycle time [7–10]。 Nevertheless, a low mold temperature can easily cause hysteresis and further unbalanced flowing behaviors, whereas a high mold temperature stabilizes the fill- ing quality [7]。

Nonuniform temperature and pressure distributions during in- jection molding are known to cause local shrinkage, and associated internal stresses can cause various local strengths, which produce warpage [11]。 Holding pressure can prevent shrinkage and warpage。 A high holding pressure and a high mold temperature has been verified to eliminate shrinkage and warpage [12–13]。 The Taguchi method  and the ANOVA analysis  are  frequently  applied to  optimize

Fig。 5。 Injection-molded portable cover: (a) front; (b) back。

Fig。 6。 Initially designed cooling channels with 8-mm diameter。