Plastics Technology

AUG 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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packing pressure. The 30 parts were weighed, and the ranges was calculated. The results clearly show that higher packing pressures will reduce the varia- tion in the part weights, which will be reflected in the part quality and dimensions. THE FIVE PILLARS Each of the five pillars plays an important role in achieving the goal of reducing variation. Note that the factors mentioned below are not the only factors. Once molders understand the goal of trying to achieve optimum packing, they must investigate means to achieve it. Let's look into some of these: Part Design: One of the important rules in part design is to have flow lengths that will let the plastic be fluid enough not just to flow into the end of fill but also until all the packing of the part is achieved. This is defined for each material by the length-to-thick- ness (L/T) ratio of the part. Thin walls often increase the pressure required to fill the part, and the end of fill suffers. The available pack-and-hold pressure in the section with thin walls is not con- sistent, leading to the variation in the specific volume of fill, which in turn leads to shot-to-shot variation in the part dimen- sions and therefore lower process capability. One guideline to follow is not to have process-capability- required dimensions towards the end of fill if the flow lengths are long. Gating the part in a thin sec- tion and then trying to pack out a thick section can also pose issues of consistency in the thick section. Material: As the melt is filled into the cavity, it cools and reduces in volume. This shrinkage must be consistent from shot to achieve process capability. Naturally, materials with high shrink value will have a higher variation compared with those having lower shrink values. For example, a PP will have more shot-to-shot variation than an ABS since the shrinkage of PP can be 1.5 to 2% or more, whereas it is 0.7% to 1.2% in the case of ABS. A filler can also influ- ence this, usually reducing the shrinkage value. Materials can therefore influence the process capability. For higher process capability, choose a material that has lower shrinkage. It is also important to consider the size of the part. The larger the dimension, the larger is the absolute value of the shrinkage. If a given material shrinks 1%, then the total reduction on a 1-in. part will be 0.010 in., but on a 10-in. part the total shrinkage will be 0.100 in. The variation in the 10-in. part will be higher than the variation in the 1-in. part and therefore may be less process capable. (For that same reason, it is very easy to control dimensions in micromolding, leading to high Cp values .) Mold Design and Moldmaking: Here, the gate location, amount of venting, and cooling are some of the factors that affect the process capability. Remember, in part design the L/T ratio is critical to the fill the part. The gate location should be such that the L/T of the part should not be close to the L/T limit of the material, or that will lead to inconsistency from shot to shot. In some cases, the choice of gate location is dictated by part cosmetics and/or by simple economics of the cost of the mold that can lead to lower process capability. The final customer should be made aware of this. Vents in the mold help push the air out of the cavity and replace it with plastic. If the number of vents and/or the vent depths and/ or the vent lands are not adequate, the air does not flow out of the mold at the required rate, restricting the plastic flow. Air pockets, voids, short shots, and burning can all result because of this restric- tion. This again leads to an inconsistent fill from shot to shot, resulting in lower process capability. The part must reach the material's ejection temperature before it is ejected. Below the ejection temperature, the material has suffi- cient mechanical properties to be ejected from the mold and not be deformed. But the resin molecules may still have enough energy to move and settle at their location of choice, leading to post-mold shrinkage. To have uniform cooling of the part (think warpage again) and efficient cycle times, there must be effective mold-temperature control. Design of the cooling lines is therefore critical. The cooling-line diameters—the distance from the part and between each other— should be such that when the part is ejected there are no "hot spots" that will again lead to inconsistent shrink and warp, affecting the process consistency. The cooling time will depend on the rate of heat removal from the cavity. To achieve a short cycle time, molders may decrease the cooling time. The part dimension may be achieved but the variation from shot to shot may be high, leading to lower process capability. Injection Molding Machine: The melt must be homogeneous and not degraded. The barrel and the screw play an important role in achieving this goal. The molding-machine barrel must be chosen Once molders understand the goal of trying to achieve optimum packing, they must investigate means to achieve it. Results of this experiment show that higher packing pressures will reduce the variation in part weights, which will be reflected in the part quality and dimensions. FIG 2 Effect of Packing Pressures on the Part-Weight Range (Data from 30 parts at 2000 & 8000 plastic psi) Cavity 1 Cavity 2 0.14 0.13 0.12 0.11 0,10 0.09 0.08 0.07 0.06 0.05 0.04 8000 psi R=0.113 g 8000 psi R=0.114 g 2000 psi R=0.056 g 2000 psi R=0.051g @plastechmag 47 Plastics Technology M O L D I N G P R O C E S S C A P A B I L I T Y

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