Feed throat temperature is a variable that is often overlooked in the molding process and provides many benefits in plastic injection molding companies in USA. When an empty hopper is not the reason for a flow restriction in the throat, there is a possibility that the restriction comes by a plastic clogging at its inlet.
In general, this occurs due to overheating derived from a prolonged stoppage of the injector. The programming comes on a very high temperature in the feeding zone. It can be also due to a bad programming in the drying conditions. Even for maintaining a long dosing career. In general, the most current injection machines have a cooling system in the throat, in order to keep it fresh at all times.
Throat temperature is a variable that we often overlook in the molding process except plastic injection molding companies in USA. If we control it, the conditions improve when transforming hygroscopic plastics and / or of high thermal behavior. Programming a correct temperature in the throat section promotes the free flow of the plastic that passes through it.
In a robust molding process, the recovery time of the material has to be repetitive to achieve this condition. The supply of the material has to be constant; therefore, the tempering that we maintain at the entrance of the cylinder will be decisive so as not to restrict the flow. Throat tempering is programmable in the vast majority of machines.
For hygroscopic polymers plastic injection molding companies in USA recommend that the temperature is not below 55º C. On the contrary, the humidity present in the air and the volatiles expell from the cylinder can condense on the cold area. The accumulation of water on the surface of the throat can cause aesthetic defects and in other polymers it can generate hydrolytic degradation.
Surely on some occasion you have heard the term rheology, with which most of us who are dedicated to the transformation of plastic are not familiar. However, it is important that we understand some concepts in this topic to relate the phenomena that happen to plastics when we process them.
The polymeric rheology is the study of the flow of the material mainly liquid, but also the deformation of the solid state in respons e to an applied force.
Plastic injection molding companies in USA say viscosity is opposition to flow, higher viscosity increases resistance to flow. Therefore, a plastic that exhibits a higher viscosity will have a greater resistance to flow. Water is a very easy flowing substance, therefore it is considered to have a low viscosity. Gases have even lower viscosity compared to water, therefore they flow even easier.
Other substances, such as honey, have high viscosities, which prevent them from flowing easily under normal circumstances. Viscosity depends on several factors and is an inherent property of the fluid but can be influenced by external forces.
In a Newtonian fluid such as water, the viscosity remains constant and is only affected by pressure and temperature, its viscosity being independent of the shear rate. In non-Newtonian fluids such as plastic, the viscosity changes as a function of modifying the cutting speed, therefore, plastic injection molding companies in USA during their processing its viscosity will always be changing.
The viscosity of a polymer is influenced by its average molecular weight. A sample of polymers comes up of a mixture of homogeneous polymers with different chain lengths. Therefore, plastics determine an average molecular weight, which in turn has a direct relationship with the properties of the plastic.
The relationship between molecular weight and viscosity is relative, since there are often cases of incorporating fillers or additives in the plastic formulation without altering its molecular weight. "Viscosity is proportional to the molecular weight of a plastic and inversely proportional to its temperature and flow rate"
There are different methods common in plastic injection molding companies in USA to measure the viscosity of plastics. One of them is through a capillary rheometer, which characterizes the viscosity of the material. It comes in a wide range of cutting speeds and temperatures.
However, it is one of the least common methods by the industry due to the complexity of interpreting the data obtained. Another method to interpret the viscosity of a plastic is also famous. And with which most processors are familiar is through the Melt Flow Index (MFI) test performed with a plastometer. The MFI test was not intend to measure viscosity. People adopt it by the industry for this purpose.
The capillary viscometer or rheometer is a laboratory instrument. It is common to measure the rheological properties of a plastic. Moreover, it comes in a wide range of cutting speeds and temperatures in plastic injection molding companies in USA, which resemble the conditions in which a plastic will be exposed during the injection molding process.
The MFI test will sound familiar to plastic injection molding companies in USA. Since it consists of introducing pellets into a cylinder. It heats to a particular temperature. The molten material comes to pass through a nozzle with a hole of known diameter. The force common for this action comes through a weight.
The test method standardizes under ISO 1133. The result we record comes on how many grams of plastic came out of the nozzle in a 10 minute period. Therefore, the units common for the test give in "grams / 10 minutes".
There is a relative relationship between the MFI and the molecular weight of the plastic. MFI values are inversely proportional to molecular weight. The higher the melt index, the lower the molecular weight. However, there are many other factors that can alter this relationship and make interpretation difficult.