The design requirements of extrusion blow mold are the focus of this post.
The extrusion blow mold is mainly composed of two halves of female die. It is generally composed of a mold body, a molded neck, a mold cavity, a blank cutting sleeve, a blank opening, a guide column, and so on.
There are several aspects to the design requirements of extrusion blow mold.
(1) Design requirements for the parting surface of extrusion blow mold
The position of the mold parting surface should be selected to make the mold symmetrical, reduce the blow-up ratio, and make it easy to demoulding. Therefore, the position of the parting surface is usually determined by the shape of the cavity of the blow-molded product.
Most blow molds are designed as two half-molds matched by the parting plane. For irregularly shaped bottles and containers, the location of the parting surface is particularly important. Improper positioning will cause the product to not be demolded or cause scratches on the bottle.
At this time, it is necessary to use molds with irregular parting surfaces. Sometimes it is even necessary to use a parting surface mold composed of three or more movable parts to facilitate the demoulding of products.
For vessels with a circular cross-section, the parting plane is set by diameter.
For elliptical containers, the parting surface should pass through the long axis of the ellipse.
The parting surface of the rectangular volume can pass through the center line or diagonal line. The latter can reduce the inflation ratio, but the wall thickness of the corner part opposite to the parting surface is small.
For some products, you need to set multiple parting surfaces. The container handle should be set along the parting surface. The cross-section of the handle should be square, and the corners should be rounded to optimize the wall thickness distribution. The handle holes are generally formed by inserts. The injection method can also be used to form the handle separately.
(2) The design requirements of the cavity of extrusion blow mold
The cavity of the extrusion blow mold directly determines the shape, size, and appearance of the product.
The surface of the mold cavity used for PE blow molding should be slightly rough. Otherwise, it will cause poor exhaust of the mold cavity, leaving air bubbles trapped, which will cause the surface defects of the product “orange peel”. It will also cause the cooling rate of the product to be low and uneven so that the shrinkage rate of the product is different.
Due to the lower temperature of the PE blow mold and the lower inflation pressure of the parison, the inflated parison will not wedge into the trough of the rough cavity surface but will be located on or across the crest. In this way, it can ensure that the product has a smooth surface, and provides a tiny mesh channel so that the mold cavity is easy to exhaust.
Sandblast the mold cavity to form a rough surface. The particle size of sandblasting should be appropriate. For HDPE blow molds, coarser particle size can be used. LDPE should adopt finer granularity.
Etching the mold cavity can also form a rough surface, and can also form patterns on the surface of the product.
When blowing high-transparency or high-gloss containers (especially with PET, PVC or PP), the mold cavity should be polished.
For blow molding of engineering plastics, the mold cavity cannot generally be sandblasted. In addition to etching patterns, it can also be polished or matt.
The size of the mold cavity is mainly determined by the external dimensions of the product and taking into account the shrinkage of the product.
Shrinkage generally refers to the difference between the size of the cavity at room temperature (22 ℃) and the product ruler after 24 hours of molding. For example, the blow molding of HDPE bottles takes 80% to 90% of the shrinkage rate within 24 hours after molding.
(3) The design requirements of extrusion blow mold cut
The die part of the blow mold should be a sharp cut to facilitate cutting off the parison.
The minimum longitudinal length of the jaw for cutting the parison is 0.5 to 2.5 mm.
Too small will reduce the thickness of the joint of the container, reduce its joint strength, and even easily cut the parison and not easy to swell.
If it is too large, the tail material cannot be cut, and even the mold cannot be completely closed. The shape of the cut is generally triangular or trapezoidal.
To prevent the abrasion of the incisions, inserts made of cemented carbide materials are commonly used to fit tightly on the mold.
The angle at which the end of the incision expands toward the surface of the mold varies with the type of plastic. LDPE can take 30°～50°, HDPE can take 12°～15°.
The opening and closing of the mold are usually controlled by compressed air. The mold closing speed is best adjusted to meet the requirements of different materials.
For example, when processing PE, the mold closing speed is too fast, and the incision is easy to cut through the parison so that the parison cannot be welded intact. This is to establish a balance between the speed and the mold clamping effect so that the clamping part can be fully welded, and it is not difficult to remove the flash.
A tailing trough is set under the blanking edge of the clamp, which is located on the mold parting surface. The depth of the tailing trough has a great influence on the blow molding and the automatic trimming of products, especially on parisons with large diameters and small wall thicknesses.
If the groove depth is too small, the tail material will be squeezed by excessive pressure, and the mold, especially the blank edge, will be subjected to excessive strain, and even the mold cannot be completely closed, making it difficult to cut the tail material.
If the groove depth is too large, the tail material cannot be in contact with the groove wall and cannot be quickly cooled. The heat will be transferred to the joint of the container to soften it, and the joint will be stretched during trimming. (The depth of the tail groove of each half of the mold is preferably 80% to 90% of the wall thickness of the parison. The angle of the tail groove should also be selected appropriately, usually 30° to 90°. When the width of the edge of the blank is large, Generally take a larger value)
(4) Design requirements for inserts in extrusion blow mold
A separate insert is generally provided at the bottom of the blow mold to squeeze and seal one end of the parison and cut off the tailings.
When designing the mold bottom inserts, the blank cutter and the tailing slot should be mainly considered. They have an important influence on the forming and performance of blow-molded products. Therefore, the following requirements should be met.
① Must have sufficient strength, rigidity, and wear resistance. In the repeated mold clamping process, it is subjected to the pressure generated by the extrusion of the parison melt.
②The thickness of the blank area is generally larger than the thickness of the product wall, and more heat is accumulated.
Therefore, the blank insert should be made of materials with high thermal conductivity. At the same time considering the durability of the insert, copper-beryllium alloy is an ideal material. For soft plastics, the blank insert can generally be made of aluminum and can be integrated with the mold body.
③ The joint is usually the weakest part of the blow-molded container. It is necessary to squeeze a small amount of melt into the joint after closing the mold but before cutting the tail material, and increase its thickness and strength appropriately.
④ It should be able to cut the tail material and form a neat cut.
The inserts forming the neck of the container mainly include a mold neck ring and a shear block. The shear block is located above the mold neck ring, which helps to cut off the neck residual material and reduce the wear of the mold neck ring. The opening of the shear block is tapered, and the included angle is generally 60°. The neck ring and shear block are made of tool steel and hardened to 56-58HRC.
(5) The exhaust of the mold
When molding containers with the same volume, the amount of air discharged from the blow mold is much larger than that of the injection mold.
The volume of air to be excluded is equal to the volume of the mold cavity minus the volume of the instantaneous parison that has been completely closed, of which the latter accounts for a large proportion. However, there is still some air trapped between the parison and the mold cavity, especially for large volume blow-molded products.
Also, the pressure in the blow mold is very small. Therefore, the exhaust performance of blow molding molds is high (especially molds with cavity polishing).
If the air trapped between the mold cavity and the parison cannot be completely or quickly discharged, the parison cannot blow up quickly. After inflation, it can not make good contact with the mold cavity, which will cause defects such as roughness and dents on the surface of the product. The surface text and patterns are not clear enough, which affects the appearance performance and external shape of the product. Especially when the parison is extruded with streaks or melt fracture.
The poor exhaust will also extend the cooling time of the product, reduce its mechanical properties, and cause uneven wall thickness distribution.
Therefore, we must try to improve the exhaust performance of the blow mold.
(6) Mold heating and cooling
During blow molding, the heat of the plastic melt will be continuously transferred to the mold, and the temperature of the mold is too high will seriously affect productivity.
To keep the mold temperature within an appropriate range, in general, the mold should be equipped with a cooling device. It is important to properly design and arrange the cooling system.
The general principle is: the distance between the cooling water channel and the cavity should be consistent everywhere to ensure uniform cooling and shrinkage of the product.
For large molds, to improve the circulation of the cooling medium and increase the cooling effect, a water inlet and a water outlet should be opened directly on the sealed water tank box behind the blow mold.
For smaller molds, cooling water channels can be set directly on the template. Cooling water enters from the bottom of the mold, and the water outlet is located on the top of the mold. In this way, on the one hand, air bubbles can be avoided, on the other hand, the cooling water can flow in the direction of natural temperature increase. In the cooling water channel with a large mold surface, folds can be installed to guide the flow of water, and turbulence can also be promoted to avoid dead angles during the flow of cooling water.
For some engineering plastics, such as PC, POM, etc., the mold does not need to be cooled, and sometimes it is even required to increase the mold temperature to a certain extent to ensure the blowout of the parison and the clear pattern. The heating medium can be introduced into the cooling channel of the mold or it can be heated by a hot plate.
The above is all about the design requirements of extrusion blow mold.
To learn more about the blow molding machine, please continue to pay attention to Yankang blow molding machine blog.