Temparature in Injectiopn moulding
1. Melt Temperature It should be noted that the melt temperature must not necessarily correspond to the barrel wall temperature set at the injectionmoulding machine. This difference is influenced by: a. Screw speed during metering b. Back pressure during metering
c. Residence time in the barrel d. Design and diameter of the screw
e. Viscosity of the melt f. Degree of wear of screw and barrel
A further temperature increase due to friction can be caused, in addition to the shearing of the screw, by a rapid melt flow through a small gate cross section (pin or film gates).
Grillamid, Grivory and Grilon injection moulding grades have excellent thermal stability. The relevant injection moulding material can be processed without problem at the maximum permissible melt temperature for parts with extended flow distances and small wall thicknesses. However, in such cases the influence of point’s a-f given above should be taken into consideration and monitored. Wear of screw and barrel wall have particularly disadvantageous effects. Leakage flow between the screw flight / barrel wall and nonreturn valve / thrust ring, results in quantities of melt remaining in the barrel for long periods of time.
Additional overheating of small amounts of metered melt in these areas of radial screw clearance is not registered by the melt temperature measurements (average temperatures). This in one reason why injection moulded parts produced at correct melt temperature settings
may exhibit discolouration or streaks caused by overheating. A low melt temperature is recommended when producing solid parts with large wall thickenesses, long cooling times and
short flow distances as this reduces thermal stressing of the melt. The choice of a lower melt temperature may also give improved surface quality of thick walled parts made of non-reinforced material.A range of melt temperatures is given in the tables on page 4 – 6. In addition the recommended melt temperatures and melting points are listed.
2. Barrel Temperatures The temperature settings of the heating barrel normally result in a profile where the temperature increases from feed hopper to nozzle. The choice of nozzle temperature is dependant on the design of nozzle used. It should be selected in such a way to avoid filament formation(stringing) at temperatures, which are too high, and cold slug formation at temperatures, which are too low. During long contact times between nozzle and mould, cooling of the nozzle tip from contact with the mould must be compensated by increasing
the nozzle temperature. A low temperature setting in zone 1 (feed zone), together with cooling of the hopper flange, prevents premature melting of the granules and therefore, promotes uniform and trouble free metering. Exceptions:Deviation from these rules is permissible when the maximum volume that can be metered by the plasticising unit has to be used within a short metering time (normally not exceeding 80% of maximum volume). In this case, a higher barrel temperature in the feed zone must be selected in order to create sufficient heat to allow the increased throughput. A temperature profile decreasing from hopper to nozzle is thus created. If production is interrupted, this temperature must be reduced immediately to normal levels in order to prevent melting of the granules in the hopper and feed zone,impeding or preventing restarting of production. Starting up of production runs requiring such high temperatures should be carried out with a normal temperature profile. The temperature in the feed zone is then increased during optimisation of the cycle time.
3. Mould Temperature The mould surface temperature is one of the decisive factors influencing the quality of parts made of Grillamid,Grivory or Grilon. Heating is carried out be a heating unit, which pumps water (up to 95° C, pressurised water up to 160° C) or oil (> 160° C) through heating channels in the mould. Water is preferable as use of a heating medium as it provides better and quicker heat transfer than oil. The heating systems are equipped with a control device, which maintains a constant mould surface temperature throughout production. The control tolerance should not exceed ± 3° C. Using Grilon injection moulding materials at high mould temperatures,parts are obtained which have a high degree of crystallinity and which exhibit excellent mechanical properties and have a low tendency to warp.Optical surface quality of parts made of glass reinforced Grillamid, Grivory and Grilon grades is achieved when mould temperatures above 80° C are used, following the recommendations in the temperature setting tab. If an injection-moulded part is to be sterilised with super heated steam (e.g. at 121° C), the mould temperature selected should be as high as possible. This reduces warping of the part during sterilizing to a minimum or may even prevent it.Large moulds should have separate heating circuits for ejector and nozzle areas. It is important to always have an even temperature distribution over the complete cavity surface of the mould.
1. Melt Temperature It should be noted that the melt temperature must not necessarily correspond to the barrel wall temperature set at the injectionmoulding machine. This difference is influenced by: a. Screw speed during metering b. Back pressure during metering
c. Residence time in the barrel d. Design and diameter of the screw
e. Viscosity of the melt f. Degree of wear of screw and barrel
A further temperature increase due to friction can be caused, in addition to the shearing of the screw, by a rapid melt flow through a small gate cross section (pin or film gates).
Grillamid, Grivory and Grilon injection moulding grades have excellent thermal stability. The relevant injection moulding material can be processed without problem at the maximum permissible melt temperature for parts with extended flow distances and small wall thicknesses. However, in such cases the influence of point’s a-f given above should be taken into consideration and monitored. Wear of screw and barrel wall have particularly disadvantageous effects. Leakage flow between the screw flight / barrel wall and nonreturn valve / thrust ring, results in quantities of melt remaining in the barrel for long periods of time.
Additional overheating of small amounts of metered melt in these areas of radial screw clearance is not registered by the melt temperature measurements (average temperatures). This in one reason why injection moulded parts produced at correct melt temperature settings
may exhibit discolouration or streaks caused by overheating. A low melt temperature is recommended when producing solid parts with large wall thickenesses, long cooling times and
short flow distances as this reduces thermal stressing of the melt. The choice of a lower melt temperature may also give improved surface quality of thick walled parts made of non-reinforced material.A range of melt temperatures is given in the tables on page 4 – 6. In addition the recommended melt temperatures and melting points are listed.
2. Barrel Temperatures The temperature settings of the heating barrel normally result in a profile where the temperature increases from feed hopper to nozzle. The choice of nozzle temperature is dependant on the design of nozzle used. It should be selected in such a way to avoid filament formation(stringing) at temperatures, which are too high, and cold slug formation at temperatures, which are too low. During long contact times between nozzle and mould, cooling of the nozzle tip from contact with the mould must be compensated by increasing
the nozzle temperature. A low temperature setting in zone 1 (feed zone), together with cooling of the hopper flange, prevents premature melting of the granules and therefore, promotes uniform and trouble free metering. Exceptions:Deviation from these rules is permissible when the maximum volume that can be metered by the plasticising unit has to be used within a short metering time (normally not exceeding 80% of maximum volume). In this case, a higher barrel temperature in the feed zone must be selected in order to create sufficient heat to allow the increased throughput. A temperature profile decreasing from hopper to nozzle is thus created. If production is interrupted, this temperature must be reduced immediately to normal levels in order to prevent melting of the granules in the hopper and feed zone,impeding or preventing restarting of production. Starting up of production runs requiring such high temperatures should be carried out with a normal temperature profile. The temperature in the feed zone is then increased during optimisation of the cycle time.
3. Mould Temperature The mould surface temperature is one of the decisive factors influencing the quality of parts made of Grillamid,Grivory or Grilon. Heating is carried out be a heating unit, which pumps water (up to 95° C, pressurised water up to 160° C) or oil (> 160° C) through heating channels in the mould. Water is preferable as use of a heating medium as it provides better and quicker heat transfer than oil. The heating systems are equipped with a control device, which maintains a constant mould surface temperature throughout production. The control tolerance should not exceed ± 3° C. Using Grilon injection moulding materials at high mould temperatures,parts are obtained which have a high degree of crystallinity and which exhibit excellent mechanical properties and have a low tendency to warp.Optical surface quality of parts made of glass reinforced Grillamid, Grivory and Grilon grades is achieved when mould temperatures above 80° C are used, following the recommendations in the temperature setting tab. If an injection-moulded part is to be sterilised with super heated steam (e.g. at 121° C), the mould temperature selected should be as high as possible. This reduces warping of the part during sterilizing to a minimum or may even prevent it.Large moulds should have separate heating circuits for ejector and nozzle areas. It is important to always have an even temperature distribution over the complete cavity surface of the mould.
0 comments:
Post a Comment