RECOMMENDATIONS FOR CASTING technological parameters.

The quality of casting products and performance depend on the properties of the polymeric material, the casting process parameters of the preparation conditions of the material for processing, as well as the technical level of the equipment and tooling (please refer “plasticizing technology. Recommendations organization plasticizing process”, “injection molding technology under pressure. Recommendations on the organization of molding technology “).
The technological casting parameters set based on the type of polymer and its properties (viscosity and thermal).
The technological casting parameters essentially depend on the size and complexity of product configurations.
The technological casting parameters, conditions of preparation of the material to the processing point in the routing.
The technological casting parameters set on the injection molding machine.
The technological casting parameters are:
– The temperature of the casting material or T (° C). This is the temperature at which the material
supplied from the heating cylinder into a mold. Pour point Tlopredelyaetsya temperatures that define zones on the heating cylinder (T1, T2, …).
– The mold temperature Tf. (FROM).
– Injection pressure Px (MPa or kg / cm2). Install hydraulic cylinder in an injection molding machine. Ask also the pressure change graph Pn during the cycle.
– Volume injection rate Q (cm / sec) or parameter is inversely proportional to the Q – time form filling t3 (s). Set the chart also changes as you move the injection screw forward speed during injection of material into the mold.
– Dwell time under pressure tVPD (s).
– Pressure molding or pressure feeding RF (MPa or kg / cm2). This parameter specifies if shaping mode is used with pressure relief. Ask also change the RF-pressure diagram in the cycle.
– Rotational speed of the screw N (rev / min).
– Plasticizing pressure or backpressure RPL (MPa or kg / cm2). Ask also chart mastication pressure change at a set dose.
– The volume of injection VVPR (cm3) or similar setting – stroke screw H (cm).
– The total duration of the cycle T * (s).
The technological casting parameters depend on the size and configuration of products.
Depending on the thickness H molded products can be divided into three groups: The first group – thin-walled products of complex configuration. The second group – the product of general purpose medium size. The third group – thick-walled products easy configuration.
For the molding of the first group, which have increased flow resistance form filling, use the upper value of the operating range of technological casting parameters: temperature T molding, mold temperature Tf, casting pressure Pn, volume injection rate Q. For molding of this group used low-viscosity grade of polymer, which exhibit good formability and easy to fill out forms with complex configurations.
For molding the third group, who have a low flow resistance completing the form, you can use the lower value of the operating range of technological parameters of casting: casting temperature T. Tf mold temperature, injection pressure Px, volumetric rate Q. For injection molding this group can be used high viscosity grade polymer which have increased molecular weight and increased impact strength.
Conditions of preparation of polymer to processing in the determined humidity at which the polymer enters for processing. The routing record:
– Humidity rating (in%) with which the polymer can be supplied for processing. If this humidity is reached and the normal processing of the required quality of products.
– The drying temperature T (0C).
– The drying time tc (hours).
– Acceptable residence time of the dried material under atmospheric conditions is an airtight container or allowed time humidification tuvl (h)
1. The temperature of the material
The temperature T of the material depends essentially on the properties of the polymer material and is defined on the basis of the conditions for obtaining a quality product for a minimum duration of the cycle.
For each polymer has a working temperature range of T molding, above and below which there is a different kind of marriage.
Reference data on the working range of the casting temperature T industrial polymers are shown in Table 3.3.
Polymers with low temperature stability are narrow operating temperature range of the casting, which complicates their processing.
Temperature affects the performance of the material. The higher the temperature T of the material, the greater the duration of a cycle and the lower molding productivity.
When processing nizkotekuchih (high viscosity) material for parties
providing fill in a form it is advisable to increase the casting temperature T.
During the processing of thin-walled products
(Small thickness h) and large with large flow paths (greater ratio L / h) for filling in the form, you can improve the casting temperature T.
T Pour point is determined by the temperatures that are set by zone heating cylinder (see section “Recommendations on the organization of the plasticizing process: the temperature in the zones of heating cylinder”).
2. The mold temperature
For each polymer has a working temperature range of the form Tf, above and below which there is a different kind of marriage.
Reference data on the operating range Tf temperature of the mold for industrial polymers are given in Table. 3.3.
During the processing of engineering polymers requires high temperatures forms of Tf, which forms the heating is used.
mold temperature Tp duration affects the molding cycle and hence performance. The lower the mold temperature Tp, the greater the difference in the casting temperature and the form (T – Tf) and greater cooling speed of the product. In this case the cooling time is reduced, and productivity increases.
During the processing of thin-walled articles (small thickness h) and large with large flow paths (greater ratio L / h) only a very sharp increase in mold temperature Tf (close to the glass transition temperature Tg or a temperature of maximum crystallization rate TM) helps to form filling material. Increasing the mold temperature parameter is less effective for improving the formability of the material than the material temperature rise.
3. Pressure casting
Pressure casting Px depends essentially on the viscosity of the polymer properties (flowability), the configurations and dimensions.
For each polymer has a working range of injection pressure Pn, above and below which there is a different kind of marriage.
Processing of highly viscous polymers (polycarbonates, polysulfones, polyphenylene oxides, polyphenylene sulfides) requires higher injection pressures Pn.
When casting nizkotekuchih polymer batches to ensure the filling of forms, you can increase the pressure Pn casting.
Thin-walled products (small thickness K), and large with large flow paths L / h are characterized by elevated hydraulic resistance, making it difficult to fill. When casting such products Pn injection pressure increase can lead to completion of the form.
4. The volume injection rate (filling time)
Volume injection rate Q or Q parameter is inversely proportional to – fill time t3 essentially depend on the characteristics of injection equipment, configuration and dimensions.
Volume Q injection rate affects the appearance of the product. Q at low injection speeds (large time t3) in the form of melt flow cools. Therefore the products may have a wavy surface on the surface and may be visible line junctions. In an extreme case, very low Q may be incomplete filling.
With increasing volume injection rate Q or a decrease in filling time t3 turn in an operating range in which the disappearing species considered marriage.
With further increase in Q volumetric injection rate above the operating range on the surface of products at certain locations may occur due podgary sudden local compression of the material. At high injection speeds and modulations occur (burr) and hammering.
To improve the formability of the material in thin-walled products (small h) it is advisable to increase the Q. When casting large-sized products with large flow paths (L / h) contributes to an increase Q form filling. This is due to the fact that Q decreases with increasing time of mold filling and cooling of the material in the mold. Therefore nonisothermality filling process has less impact and easier to melt fills out a form.
Operating range of volume injection rate Q for a particular product is determined experimentally, molding products at multiple Q determine the maximum Q, from which are formed Prigara, play and other types of marriage. Q Determine the minimum below which begin to form a wavy surface, junctions and other types of marriage.
5. The molding pressure
Of Russia molding pressure greatly depends on the configuration and dimensions. Russian pressure molding is set in the period of pressure build-up when forming mode is applied with pressure relief. Read more about the purpose and use of modes with pressure relief, see. “Profiles with pressure relief.”
The pressure molding of Russia defines feeding material forms during the holding time under pressure tVPDi therefore the weight and quality of the finished products.
Low pressure molding material makeup of Russia form is insufficient and low quality products. Sink marks and voids are formed, there is distortion products.
With increasing pressure molding material makeup of Russia forms for tVPD increases and disappearing species considered marriage. The molding pressure Rfperehodit in the working range, where it turns out good quality products surface.
With the increase of molding pressure of Russia and an increase in the weight of makeup products increases. Excessive formation of Russia pressure causes compaction of the material and overweight products. This leads to overspray. When high pressure molding as there are big internal stress.
Recommendations on forming a working pressure ranges Rfv depending on the polymer are shown in Table 3.3.
With increasing thickness h shrinkage shaped material increases. Therefore necessary to increase the feeding and forming pressure of Russia. With increasing ratio of length to thickness of the flow path L / h in the form of pressure drop increases. This may result in that the end shape is a low pressure and in this place can be formed sink marks. To address this shortcoming molding pressure of Russia it is advisable to increase with the increase of the ratio L / h.
6. Dwell time under pressure
pressure holding time tVPD mainly depends on the diameter dl of the gate (gate height hl) or diameter d in the inlet. The diameter dl (height hl) or DB determines the duration of the cooling material in the sprue or in the inlet. Therefore, determine the best time of the overflow material from the heating cylinder into a mold under pressure (feeding time), ie, the longest time – tVPD.
With a little time tVPD makeup material form is insufficient and the product formed sink marks and voids, and warping of the products may occur. As the time tVPD weight of the product is increased.
However, after a certain value tVPD weight of the product ceases to grow. Increases the tVPD above this value should not be. If you increase the injection pressure Pn, the weight of the product is increased, but the time tVPD remains practically the same.
Recommendations for an exemplary dwell time under pressure tVPD depending on the gate dimensions are shown in Figure 3.4.
Examples of determining tVPD.
First. The gate has a circular cross section of the flow channel with a diameter dl = 2,2
mm. According to the curve we define tVPD 1 = 4.5 – 6 sec.
Second. The gate has a trapezoidal cross-section of the flow channel with height
hl = 2.5mm. In curve 2 we define tVPD = 14 – 18 seconds.
Operating range dwell time under pressure tVPD for a particular product is determined experimentally by weight of a molded product. Make 4-5 castings consistently increasing tVPD. Celebrating the importance tVPD followed by a subsequent increase tVPD does not lead to a noticeable change in the weight of the product. This value restricts the top tVPD operating range.
7. screw rotation frequency. The pressure of mastication.
Screw rotation frequency N and plasticizing RPL pressure mainly depend on the technological problem of plasticization, namely: Softening unfilled polymers, polymer concentrates dye staining directly during casting, Softening polymer blends.
screw rotational speed is changed from 10 / min to 100-150 / min. Pressure plasticizing varies from 10 to 150 kg / cm2.
With the increase in rotational speed N of the screw plasticizing and pressure RPL plasticizing effect during mixing increases.
Principles for selecting the frequency of rotation of the screw N and the pressure of mastication RPL. To solve the above-mentioned technological problems discussed in section
“Technology mastication. Recommendations on the organization of the plasticizing process.”
8. The injection volume (screw stroke)
QVPR injection quantity or similar parameter – H course depend on the screw size and density of the product polymer material.
casting weight G (in g) is determined by the formula:
G = V * p – (3.1)
where V – volume of the casting, cm3;
polymer density p (g / cm3) at room temperature (20 0C),
p-values ​​are shown in Table. 3.5.
The weight of the casting G and turn screw H connected directly proportional:
GK = NSp1 (3.2)
where S – the cross-section of the screw in cm2;
p1 – ​​resin density (g / cm3) at the casting temperature T / y, the values ​​in Table p1privedeny. 3.5;
K – coefficient allowing leakage of material in reverse during injection and sealing material in the mold and before the cushion material in the screw end position, K = 1,2-1,25. The specific course of the screw (K factor) specify when developing technological regime on the specific injection molding machine. The coefficient K depends on the state of technological equipment, particularly at the end of the locking screw valve. If the course is set screw is less than the required value, there are underfilling, voids, sink marks, warpage articles.
Screw stroke must be such that the end of the hold period under pressure before the screw was not less than 3 – 5% of the material prepared for injection. Auger does not reach its extreme position. Only in this case the transmission is provided in the form of pressure in the pressure during the holding time
tVPD and the flow of material to feed.
9. The total duration of the cycle
The total duration of the cycle duration ttsopredelyaetsya cooling material in the form.
The cycle duration T * (duration of cooling the main tohl-
process variable that affects the productivity of the casting Performance P. P is inversely proportional tohl cooling time.
Tohl cooling time depends on the temperature of the material (injection) mold temperature T and Tf, product thickness h thermophysical properties and a polymer material (a thermal diffusivity).
The lower the temperature T, the material is rapidly cooled product tohl and higher injection performance.
mold temperature Tf affects the cooling rate of the material in the mold. The lower the temperature Tf. the greater the cooling rate. This cooling time is reduced and P tohl productivity increases.
The strongest impact on the duration of the cooling tohl provides product thickness h. The duration of cooling tohl proportional to the square of the thickness of the product h. For example, if the product is cooled with a thickness h1 time t for the product, with a thickness twice during t2kotoroe cooled four times greater than t. To improve performance, you need to, wherever possible, to reduce the product thickness h.