The flow rate of the melt varies over the wall thickness during filling.
Shear forces arise between the layers of different velocities.
This way a thought volume element is deformed (diamond in the picture).
The time dependent change of the angle (gamma) is called shear rate.
High shear rates lead to shear heating, molecular chains deform and might break.
The quality of the product is reduced.
This term refers to an index named after Albert Ferdinand Shore, which is a measure of the elasticity and hardness of materials such as rubber and plastics.
It is measured using a durometer, which presses a spring-loaded pin into the surface of the material. The Shore hardness is specified in different scales, with Shore A being used for softer materials and Shore D for harder materials.
The measured value ranges from 0 (very soft) to 100 (very hard). The higher the value, the harder the material. The Shore hardness is important for the selection of materials in applications where certain elasticity and hardness properties are required.
Further information can be found in the following standards: DIN ISO 7619-1, DIN EN ISO 868, DIN 53505 and ASTM D 2240.
Depending on the variation of density between processing and ambient temperature in injection molding shrinkage might appear.
Shrinkage is divided into two fields: processing and post processing shrinkage.
Processing shrinkage occurs in the mold due to volumetric contraction, mold restrains, crystallinity and orientation.
Post processing shrinkage is measured usually after 24 to 48 hours after the production.
Shrinkage happens mainly due to the relaxation of residual stresses.
Plastic granules are typically stored in small casks (sack ware), large casks (octabins) or in silos. For storage of larger tonnages of plastic, silos made from stainless steel or aluminum are used. Due to their size, silos are usually erected outside of production areas. From there, the silos are connected to the granulate conveying system of the production plant. The conveyor system first forwards the material from the silo to a central drying system before it is fed to the injection molding machines.
In areas with thick walls and across from ribs, a hollow at the surface might appear.
Sometimes a difference in the gloss to the surrounding surface might be noticed only.
Mass accumulation leads to a higher volumetric shrinkage, which is insufficiently reached by packing pressure.
The surface is pulled to the inside during cooling and a sink mark appears.
Small wall thickness and higher packing pressure in the critical area reduce the risk of sink marks.
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