Experimental researches regarding the influence of the injection moulding temperatures on quality plastical with complex shape.
Injection process knowledge involves work procedures, chemical structure, thermo plasticity material properties and individual factor reciprocal influence knowledge. That is why mould temperature adjustment issue has to be solved function of these individual factors, which have an important role in injection process. This is the reason why the injection process will be shortly explained, for a better figure out about moulds temperature adjustment.
Because of the heat, grain material is melting, then get in system. In this stage it has to overcome sewer resistance and resistance of the moulds drain. By the time of this above all stage, the injection, the thermo plasticity material relieves the heat out, with other words cooling process is starting, the metal near the system is heating.
The heat give out can be that big, that in case of an long sliding way of the piece, the much cooled material will not be able to completely fill the mould cavity. The reasons of this thing are more than clear if we think about injecting the material in the mould, talking about an Newtonian fluid, it will immediately get to the mould wall and it is strengthtly in the edges. (fig.1)
When the drain channels expands, the solidified layer of the interior wall of the mould expands too, and the channel which serves to fuel the material in the direction of the running diminish, in order to fill up the mould and even in the case of long draining channels, several measures must be considered:
--increasing pressure and injecting speed
--increasing the temperature of the plastic material (change in viscosity)
--The greatest efficiency is obtained by increasing pressure and injecting speed
[FIGURE 1 OMITTED]
2. DETERMINING THE MOULD GENERAL THERMAL BALANCE EQUATION
The mould temperature is the decisive factor for cooling speed and the injected reference point its properties. It is established according to the amount of heat that is exchanged in the mould:
--between the thermoplastic material injected into the mould and the mould material Q;
--between the mould and the between the mould and the environment QE;
If we consider the thermal fluxes that enter the mould as positive, and the fluxes that exit the mould as negative, then it can be write the thermal balance equation:
Q= -[Q.sub.t] - [Q.sub.e], (1)
Q+ [Q.sub.e] + [Q.sub.t] = 0, (2)
[FIGURE 2 OMITTED]
3. HEAT TRANSFER BETWEEN THE PLASTICS MATERIAL AND THE MOLD
Plasticity material inserted into the mould center, field, during an injecting cycle, to the mould body, an amount of heat Q, which can be calculated using:
Q+ [Q.sub.E] + [Q.sub.T] = 0, (3)
m--is weight of the injected piece, including the network [Kg]
[i.sub.1]--the enthalpy of the plasticity material upon removing [Kj/Kg]
[i.sub.2]--enthalpy of the plasticity material upon insertion into the mould
The enthalpy of the plasticity material is calculated using this:
Di = [i.sub.2] - [i.sub.1] = [c.sub.p]([T.sub.Mp] - [T.sub.D]), (4)
[C.sub.p]--specific heat of the plastic material
[T.sub.mp]--the temperature of the material in the center
Conductor in the mould. The quantity of heat evacuated by the piece is taken through conductor by the mould and transported into the temperate environment. We can consider the phenomena of conductor stationary transfer in a plane homogenous wall (Stefanescu et al., 1982).
The quantity of heat Q is determined using this function:
Q = [lambda]M/[delta] S([T.sub.pc] - [T.sub.pT]) (5)
--[[lambda].sub.M]--thermical conductibility to the mould [W/mK];
--[delta]--the channel distance of temperature beside the mould surfaces [m];
--S--the transversal surfaces of the mould
--[T.sub.pc]--the medium temperature of the wall cavity
- TpT - the medium temperature to the temperature wall channel [degrees]K.
4. CONSTRUCTIVE MOULD TEMPERING SOLUTIONS FOR INJECTING HIGH PRECISION THERMOPLASTICITY MATERIAL PARTS
4.1 Methods of cooling down complex high precision single centered moulds
Only two methods will be presented. One used in real life (fig. 3) and the other proposed by the author which substantiate to be superior (fig. 4).
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
Follow that the analysis of a finished part, we can see that when cooling down a product of high precision, with thick walls, that have high contractions, it is recommended to use more cooling circuits, but respecting the constant distance between the outline of the part and the cooling circuit, (fig. 4) so that a higher amount of heat can be removed.
4.2 Methods of cooling complex high precision moulds utilizable the thermosiphon principle
This temperature adjustment method applies in general to high complexivity moulds where there can be critical areas which cannot be cooled down with other methods. These solutions are based on the thermo siphon principle. (fig.5)
This concept of isobaric superconductivity is based on using a metallic tube, 1 made of copper into which another metallic tube, 2 of special composition with capillary structure is clamped. In tube 1's interior and between tube 1 and 2, a fluid is circulating, fluid which can be both liquid and vapor. The liquid take part of the heat from the exterior, passes through tube 2 in the interior and vaporizes. A pump effect is achieved at A end towards B end where the fluid vapors go through tube 2 and transform into liquid, giving heat to the environment. The liquid enters the circuit towards B end of the tube in order to take heat from the outside environment.
In this system case, thermical transfer is very fast and constructive solutions that use this system become very efficient.
[FIGURE 5 OMITTED]
A,B-tube ends; QA absorbed heat; QB abstracted heat ; 1-exterior tube; 2-interior tube
Optimizing the mould temperature has a very important role both in the future quality of the product, as in productivity. Cooling conditions from the mould have a great influence upon injected piece warping, no matter the size and complexity .
Moulds temperature influences directly cooling time, injecting cycle time, the efficiency of the product formed inside the mould, crystallinity and internal tensions.
As a conclusion, we must say that the solutions presented contribute significant to optimizing temperature in the active part of the moulds, particularly for products of medium size with thick walls.
De Laney, D.E., Reilly J.F. (1998). A new approach to polymer rheology for process and quality control. Plastics Engineering, June,
Fetecau C. (2007). Plasticity injection material. Second edition. ISBN 429-2637-28.Ed Pedagogical and didactic style. Bucharest,
Losch, K. (1997). Thinwall molding: demanding bul rewarding. Modern Plasticity International,.
Seres, I. (1999). Moulds for injection. ISBN 973-8195-42. West Publishing printing Oradea
Stefanescu, D., Marinescu, M., Danescu, A. (1982).--The transfer of heat in the technical, 1st volume, conductor, convection, radiation, global exchange. ISBN 392-5219-14.Technical Publishing House, Bucharest,
Zemanski, M.W.Bazic (1995). Enginering Termodynamics Mc. Grow Hill Book, Co New York
***PLASTPRACTICE"--Temperature Control by Means of Fluid Media.
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|Author:||Mihaila, Stefan; Fazecas, Marius; Chira, Danut; Pop, Alin; Porumb, Camelia|
|Publication:||Annals of DAAAM & Proceedings|
|Date:||Jan 1, 2009|
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