In practice, some injection molding factories often start from their experience with amorphous plastics, adopt a shorter holding time and a longer cooling time, and also often use this method for POM (polyoxymethylene), PA (nylon), PBT, PET (polyester) and other semi-crystalline plastics. This section discusses some of the key factors that a scheduler can use when choosing the right dwell time.
What happens when you are under pressure?
Once the mold cavity is filled, the plastic molecules begin to crystallize, i.e. the molecular chains begin to arrange in an orderly manner, forming a high bulk density. This process starts at the periphery and ends in the center of the wall (see figure). As with POM, the volume shrinkage caused by this process can reach 14%, requiring a melt to be injected into the cavity under pressure. If so, some small pores (micropores) will be formed, which can adversely affect the properties of the plastic product in many ways.
How to tell if the dwell time is too short
Products manufactured in this way often undergo shrinkage, deformation, dents, voids, and sometimes deterioration of mechanical properties, and even large changes in size. Operators sometimes try to compensate by increasing the cooldown time, but this only adds pointless to the cycle time.
One way to identify the effects of insufficient dwell time for products without reinforcement is to make a cut at the thickest part of the wall. The polished cut surface can be used to inspect voids and bubbles, which can be performed with a magnifying glass or a reflected light microscope. A more precise method is to prepare slices (see picture). In this way, even the smallest defects can be detected with a microscope.
Defects in articles made with reinforced materials can be easily detected through the section at the thickest part of the wall. If a bubble-like structure appears on the cross-section, the fibers exposed to the outside because they are not wrapped in plastic can be seen on the magnified cross-sectional micrograph. Another method is to prepare a photomicrograph of the polished section, from which air bubbles can be clearly detected.
The effective dwell time can be determined by weighing many parts on an injection molding machine (see description for details). For a given mold, this is the best way to determine how long it will hold under actual operating conditions.
The optimal dwell time can also be obtained by comparative method (see table). This method is only suitable for molds with a given wall thickness and does not take into account the effects of other factors such as temperature, nucleating additives or pigments, mold filling time, etc. The dwell time of a mold with a small wall thickness will be shorter, and the dwell time of a mold with a large wall thickness will be longer.
Correct tuning procedures
In order to achieve the best of some of the properties of the module, the holding time should be determined by weighing and the cooling time should be set to the minimum required (slightly above the plasticizing time). This requires that the gate location must be designed accurately (see Plastverarbeiter 46 [1995]6 and 7, Parts 2 and 3). The correct pressure value varies between 60 MPa and 100 MPa depending on the material selected.
