31Press forms

Injection molds used for making products directly from the melt prepared in the plastication unit TPA. Therefore, their function is to receive the melt and its distribution on a forming objects in molding articles and then pushing them in. Construction of molds are very diverse, which is caused by two main reasons: the widest range of derived products and a variety of recycled plastics. Furthermore, the design of molds influences the type of material (thermal or thermoset) equipment type, the nature of production, special product requirements and so forth.

From the viewpoint of state of the polymeric material during the production cycle of the product molds for thermoplastics are divided into cold and hot runner. The cold runners forms during the entire molding cycle solidifies incoming volume into the mold material. When hot – defined area form hot melt is constantly filled, part of which is periodically fed into the cavity, located in the cold zone.

Moulds for thermosets with cold runners are essentially similar, with the difference that instead of the cooling system they have heating devices that support in the forming chamber temperature required conditions for curing the polymer (160-200 ° C).

According to the number of nests are drawing molds can be single or multi-cavity (720), which in turn determines the particular gating systems.

cold runners form

This type of form is most common in injection molding. Up to 90% of the products produced in the injection molding machine, it is necessary to cold runners technology. Advantages of such molds: relatively low cost, ease of manufacture and maintenance, relatively low cost of the repair operation and versatility in form of processed polymers.

The principal device is cold runners following form.

The molding parts are matrix, punch and sprue bushing, ejector. All these items are in one way or another in contact with the molten polymer to be involved in the casting design and technology are, that is directly involved in the process technology.

Design details, then there are those that provide interaction of the elements of the mold and its attachment to the slide and clamping unit rack, temperature control and other forms of -. It is a center shaft, plate, the plate and the guide column with sleeves. The punch and the matrix made cylindrical channels for the coolant.

Ventilation ducts connect the mold cavity with the atmosphere. They serve to remove air and volatiles from the volume filled with the melt. The maximum channel depth is determined by the material and the product is from 0.04 to 0.06 mm. The number of channels selected constructively. Often, especially in the case of thin-walled products, gaseous substances out of the molding chamber are removed through the gaps in the form of mating elements.

When the plate is bolted to the front desk and Stif TPA, and the plate in the same manner is attached to the slider. Therefore, the matrix of the mold is stationary and the punch reciprocates, opening (thus opening) and locking (closing his) form. When withdrawing the punch left rod rests on the fixed rear rack TPA, whereby the ejector assembly moves relative to the punch, his rods abut the product, and the gate and throw them out of shape.

Hot runner mold

Currently in different countries, depending on their level of technical development, hot runner processed forms from 10 to 30% of thermoplastics. Hot Runner Technology is considered a promising and its use is expanding. The essence of this technology is quite simple. The mold consists of two parts: Cold matrix in which the shaping of products and much more complex hot parts. Heated hot runners form constantly filled with molten polymer material. Hot runner mold part the pre-loaded with advanced nodes with point injection inlet.

The improvement comprises, inter alia, to use autonomously controlled with needle valves or other individual pneumatic actuator. At a given instant the needle valve closes the inlet, which not only stops for the melt, but also allows substantially eliminate the formation of irregularities on the surface of the product gate. When the injection molding machines node constantly link up with the form of action needle valve it agreed with the motion of the kneader.

Advantages of hot runner technology:

1. The complete absence of runner waste.

2. Excluded from the sprue separation operation of the product.

3. The polymer melt was very close to the forming chamber, thereby increasing product quality.

4. The same circumstance allows to obtain on large size articles (furniture plastic) with a minimum wall thickness and, hence, more efficient use of the expensive polymeric material. Disadvantages:
1. The range of recycled polymers is limited by the thermal stability requirements.

2. The polymer melt should be low viscosity (MFR> 8 g ​​/ 10 ‘).

3. Melt Injection requires increased efforts injection kneader.

4. Hot block shape is equipped with high-precision temperature control devices and management.
The design, structure and form of service is much more complicated in comparison with the cold runners. All this is due to high cost goryacheka.nalnyh forms, the use of which requires careful feasibility study (the circulation of products, their market value, the duration of the demand, etc..).

Extrusion blow molding

For the production of hollow and bulky products from thermoplastics – cans, drums, bottles, bottles, toys, etc. -.. The most widely used method for blow molding. Manufacture by this method is carried out in two stages: first a tubular preform at a temperature slightly below the melting temperature, which is then inflated with compressed air. Unlike most of the methods for producing plastic products, where the molding made from a melt in the process is the use of not only a plastic but also predominantly rubbery deformation that is inherent only to polymers is the result of transition rolled into a ball or collected macromolecules packs of elongate shape under the influence of mechanical forces.

Depending on the selected method of producing billets are two method of blow molding: extrusion and injection molding.

In the first case it is formed with an extruder into a preform tube (sleeve), which is then fed into a mold, and where there is the actual product forming process by creating a high air pressure inside the blank.

Due to the high performance and high level of automation of this method is currently the main method of molding hollow articles and, as a result of a number of improvements to make products with volume from milliliters to several tens or even hundreds of liters.

For the preparation of hollow bodies are used, as a rule, a relatively small size extruders, with screw diameter of 50-90 mm. Since the resistance is relatively small head and the main requirement is to obtain a highly homogeneous melt, the screw length is also less than 15-18 D. schematically hollow articles manufacturing process is as follows.

And homogenized in the molten material extruded from the extruder head down into a tubular workpiece which gets into an open shape at this point. After harvesting the length reaches the required value, the mold halves are closed, clamping the lower and upper edge of the workpiece by their sides. This results in welding the lower end of the workpiece and the clearance holes at the upper end (or vice versa; see below). After mold clamping it through IPT nipple mandrel is supplied with compressed air, under the influence of which the softened material adopts a sleeve configuration inside the mold cavity. As a result of contact with the cold walls of the mold resin hardens; Further disclosed form, the finished product is extracted and sent for final treatment (removal of the tides, deburring, and so on. n.). Manufacture of hollow articles made by special units equipped with (besides the extruder) the mechanism displacement, connector and mold clamping with hydraulic or pneumatic actuator. Since the molding process is divided into two unequal duration stage: a short stage of extrusion billets and long – spinning and cooling of products, to improve performance, most units performed a multiposition, with multiple forms, or – particularly in the production of small volume products – supplied two- and a channel forming head, sometimes with several mouthpieces for each channel. In the first case, the processes of obtaining the preform and product design are separated and occur in one form, but in different positions of the unit; -Material of the second extruder is supplied periodically in one or a group of parallel-connected mouthpieces through which the workpiece fall into the mold. During molding and cooling the finished product in this form supplied to other preform forming process starts and t. D. For this special tap connected to the driven mold halves directs melt flow successively into each of the channels leading to the forming head. For normal operation, all the unit rate of extrusion billets should be identical, since all forms occlusion occurs simultaneously.

The molding blank is a major operation, which largely determines the properties and the quality of the finished product. Arriving into the mouthpiece melt should be homogeneous, have a constant temperature around the perimeter of the workpiece and squeezed quite evenly (without pulsation). .. In the preparation of articles having a sectional shape of a rectangle, ellipse, etc., as well as products of complex configuration, the slit forming section makes uneven – to produce a finished product with uniform wall thickness portions of the workpiece that swell more must have a large thickness. It should be borne in mind that the squeezed preform has a thickness greater than the cross section of the molding owing to the gap so-called “swelling” extrudate wearer relaxation character.

“Swelling” depends on the shear rate, the length of the mold channel and the melt viscosity, making it difficult to process control.

Furthermore, it was observed that due to stretching plastic preform under its own weight the wall thickness of the upper part of the product is lower than the bottom of the molding and elongated long products.

Unevenness workpiece depends upon the speed of the melt extrusion, the viscosity and weight of the workpiece. Typically, the preform molding is carried out at the lowest possible temperature and the highest melt extrusion speed. Therefore, in modern machines for controlling the shape, size and thickness of the preform wall microprocessors, allowing by changing the melt feeding rate and the axial movement of the tapered mandrel to form the workpiece with the required polythickness. Minimum gage is achieved by blowing ratio of 3-3.5.

The corresponding program is made in advance based on the geometry of the product, the process parameters of temperature and rheology of the polymer melt. Before the next preform molding mandrel is automatically returned to its original position.

Depending on the design of the product, and a shaping tool for feeding compressed air molding articles can be produced through the mandrel (from above) through a dedicated pin (bottom) or cannula (Fig. 9.2). The latter method is mainly used in the production of closed product (without holes), as forming an opening in this case is very small and after removal of the needle is delayed by heated material.

If necessary, the manufacture of large products, by weight conventional extruder can extrude billets not provide a sufficient rate, and high installation extruder (screw with a larger diameter) is disadvantageous because of the periodicity of the product molding process, it will be used quite unproductive. Therefore, the unit for the production of large-sized products are usually provided with a forehearth – horizontal or vertical hydraulic press, the material of which the cylinder is equipped with heaters.

After filling forehearth extruder is stopped in the press hydraulic pressure forehearth is created, under the influence of which its piston at a predetermined rate through the angle head extrudes molten polymer necessary portion (Fig. 9.3). The tubular billet enters the form, mounted on a special frame; after mold clamping drive includes an extruder that supplies melt to the forehearth new portion, while there is in the form of design and cooling products. Molding made with air pressure to 0.5 MPa, the extrusion billet hydraulic pressure up to 15 MPa.

Finished molded products require an additional treatment, since the end walls (and sometimes on the sides) there is excess material squeezed bead form. Removal of such profits, trimming press edges, necks or nipple by using tools or by hand.

Thus, the process for producing articles by extrusion blow molding consists of the following operations:

– Homogenization of melt extrusion and tubular blanks;

– Blowing the preform to form and forming an article;

– Cooling and removing articles from the mold;

– Final processing of finished products.

With proper choice of design of the extrusion assembly, it provides the necessary quality of the polymer melt (temperature homogeneity). Using the forehearth should only contribute to a higher rate of preform molding large mass.

As mentioned above, the molding preform is associated with its occurrence polythickness resulting tension and die swell under the action of its own weight. Die swell is defined by the geometry of the mold channel, extruding speed of the workpiece and can be largely taken into account in the step selecting technologist forming tool. In general, swelling of the hose decreases with increase in the length of the channel head, the growth temperature of the melt. By increasing the swelling rate of extrusion billet increases. Changing the wall thickness of the workpiece by stretching can be decreased by reducing the melt temperature, but only to a certain extent, since it necessitates increase of pressure molding step and the product is accompanied by the deterioration of the surface quality of finished products.

In the preparation of products of rectangular shape to compensate for differences draw ratio resorted to the use of an elliptic die in combination with a cylindrical mandrel.

The use of microprocessors not only completely automates the blank molding step, but also allows you to achieve high stability of the unit, with the release of high-quality products.

Stage blowing the preform begins after the closing of the mold halves and crushing part of the contour of the future product news notches form; while the tubular preform is separated from the forming head. Blowing workpiece initially has a “free” nature – thickness change occurs fairly evenly, and inflate the sleeve is in the form of “bubble”.

After contact with the shape of an expanding preform this uniformity is broken – the deformation of contacting with form
blank areas stopped by a rapid cooling (wall thickness equal), and forming the rest of the product is only due to the longitudinal orientation when stretched preform in refrigerated units (k-k site). In this case, the wall thickness decreases. The quality of the product at this stage of the molding process greatly influenced by air pressure and temperature of the preform. With increasing pressure improves the surface quality, reduced distortion, reduced cooling time. Note, however, that the pressurized molding requires a significant increase in mold clamping force.

The cooling step starts at the moment of contact with the workpiece surface forms a cooled and finished after cooling products thickest portions adjacent to the neck or bottom. Premature removal of products is accompanied by a softening of the neck adjacent to the land and their irreversible deformation.

To maintain product purchased Gizd forms must be below Tr ·.
A major factor in this step is a mold temperature. It not only affects the quality of finished products, but also determines the productivity of the process, since cooling is from 50 to 75% of the time the molding cycle. Therefore widely used various forms of intensive cooling refrigerants over the cavity of the product cooled air, water mist, liquid nitrogen and carbon dioxide. However, mold cooling below 0-5 ° C is impractical because of the risk of quality deterioration due to the surface of finished articles fogging mold surface.

The cooling time can be reduced lowering the melt temperature (may be accompanied by deterioration of quality of finished product surface) and increased air pressure, which improves the heat transfer from the article to the mold wall.

As a result of cooling product volume decreases due to natural shrinkage. The greatest shrinkage occurs in the diametrical direction because there develops maximum deformation in the molding product. For blown articles with the same shrinkage in the longitudinal and transverse directions, it is necessary that the draw ratio was equal to blow ratio.

Manufacture extrusion blow accompanied by formation of considerable amounts of waste (35%). Most of them are quite suitable. Recycling the same units after the appropriate transformation into granules. The quantity of fresh feed added to the waste should not exceed 30-40%.

The injection method involves obtaining blow molded preform injection molding.

In this case, the molten thermoplastic is injected from a cylinder into a mold, and the tubular preform is made in the gap between the mold walls and the inner hollow core. The blank may be formed into the required shape, the neck, the handle needed for flushing the external surface immediately executed during casting. After completing the molding form is opened and the core together with the hot workpiece is moved to another mold where the mold halves, after closing process is carried out by blowing compressed air into the inner cavity. When this product size increases, and the wall thickness decreases. Since the geometric shape of the workpiece is set in advance, this method allows to obtain complex configuration articles, products of uniform wall thickness and the required ratio of wall thickness in different parts, but due to necessary conversion of injection molding machines, but also because of the high cost of molding and inflatable forms of this method is of limited use.

In recent years, it has been extended version of this technology, when the casting stage of preparation and its blow divided. Harvesting, called preforms molded in multicavity shape on thermoplastic and fully cooled. Often, the preforms are independent commercial products. Blowing the preform is carried out on a special unit air after preheating, followed by cooling in the mold.

Manufacture of hollow articles by blowing them to reduce wall thickness, reduces consumption of expensive polymeric material (eg PET) due to the lack of press edges and tides, increases the strength and improves the appearance of the product.

However, it should be emphasized that all products manufactured by the implementation of rubbery deformation, have the ability to irreversibly lose its shape when the temperature rises above a certain limit, which imposes restrictions on the temperature range of operation. That is the limit of the softening temperature.