The extrusion blow mold is mainly composed of two half female dies, which are generally composed of mold body, die neck, die cavity, blank cutting sleeve, blank cutting mouth, guide pillar, etc.
(1) Requirements for parting surface design of the mold
The structure design of the extrusion blow mold the parting surface should be selected to make the mold symmetrical, reduce the blowing ratio, and make it easy to release the mold. Therefore, the location of the parting surface is usually determined by the shape of the blow molding cavity. For irregular shape bottles and containers, the location of the parting surface is particularly important, In this case, irregular parting surface mold is needed, sometimes multi parting surface mold composed of three or more movable parts is used to facilitate product demoulding. For containers with a circular cross-section, the parting surface is set by diameter; for the elliptical container, the parting surface should be through the long axis of the ellipse; The parting surface of rectangular volume can pass through the centerline or diagonal line, the latter can reduce the blowing ratio, but the wall thickness of the corner relative to the parting surface is smaller. For some products, multiple parting surfaces are needed. The container handle should be set along the parting surface. The cross-section of the handle should be square, and the corner should be transitional by arc, to optimize the wall thickness distribution. The handle hole is usually molded by inlay. The handle can also be individually molded by the injection method.
(2) Design requirements of cavity
The shape, size, and appearance of the product are directly determined by the cavity of the blow molding mold. The surface of the mold cavity used for PE blowing should be slightly rough. Otherwise, the poor exhaust of the mold cavity will be caused, and there will be bubbles, which will cause the surface defect of the "orange peel pattern". The cooling rate of the product will be low and uneven, and the shrinkage rate of the product will be different, it is easy to form a rough sandblasting groove on the surface of HDPE, which can be used to blow the rough surface of the blank, LDPE should use fine grain size. The etching cavity can also form rough surfaces and patterns on the surface of the products. When blowing high transparent or high gloss containers (especially PET, PVC, or PP) are blown, the mold cavity should be polished. For blow molding of engineering plastics, the mold cavity can not be sandblasted generally. In addition to etching patterns, it can also be polished or matting.
The size of the mold cavity is mainly determined by the overall size of the product and the shrinkage rate of the product. The shrinkage rate generally refers to the difference between the cavity size at room temperature (22 ℃) and the product size after 24 hours of molding. For example, 80% ~ 90% of the shrinkage rate of the blow molding of the HDPE bottle occurs within 24 hours after molding.
(3) Mold cut
The minimum longitudinal length of the clamp is 0.5 ~ 2.5mm. If it is too small, it will reduce the thickness of the joint seam of the container, reduce its joint strength, and even easily cut the parison without blowing. If it is too large, the tailing can not be cut off, and the mold can not be completely closed, Generally, it is triangular or trapezoidal. In order to prevent the cutting wear, the insert made of cemented carbide materials is usually embedded in the mold. The angle of the end of the cut to the mold surface varies with the plastic variety. LDPE can be 30 ° to 50 ° and HDPE is 12 ° to 15°. The opening and closing of the mold are usually operated by compressed air, and the closing speed can be adjusted to meet the requirements of different materials, It is necessary to establish a balance between the speed and the mold locking so that the clamping part can be fully welded and the flash is not difficult to remove.
A tailing groove is set under the edge of the clamping die, which is located on the parting surface of the die. The depth of the tailing groove has a great influence on the blow molding and automatic trimming of the products, especially for the billets with large diameter and small wall thickness. If the groove depth is too small, the tailing will be squeezed by excessive pressure, and the die, especially the edge of the clamping blank, will be subject to excessive strain, and even the die can not be completely closed, so it is difficult to cut off the tailing, the tail material can not contact with the groove wall and can not be cooled quickly. The heat will be transferred to the joint of the container to make it soften and stretch the joint during trimming. (the depth of the tailing groove of each half of the die is best taken as 80% ~ 90% of the parison wall thickness. The angle of the tailing groove should also be selected appropriately, usually 30 ° to 90 °. When the width of the edge of the clamping blank is large, the larger value is generally adopted.
(4) Inserts of the molds
In order to extrude and seal one end of the parison and cut off the tailing, a separate insert is usually set at the bottom of the blow molding die. When designing the insert at the bottom of the blow molding die, the cutter and tailing groove should be mainly considered, which have an important impact on the molding and performance of blow molding products. Therefore, the following requirements should be met.
① It should have enough strength, rigidity and wear resistance, and bear the pressure produced by extrusion parison melt during repeated die closing.
② In general, the thickness of the sandwich zone is larger than that of the wall of the product, and the accumulated heat is more. Therefore, the material with high thermal conductivity should be selected for the sandwich block. Meanwhile, considering the durability of the insert, copper-beryllium alloy is an ideal material. For soft plastics, the sandwich block can be made of aluminum and can be integrated with the mold body.
③ The joint is usually the weakest part of the blow molding container. A small amount of melt should be squeezed into the joint after the mold is closed but before the tailing is cut off, so as to increase its thickness and strength appropriately.
④ It should be able to cut off the tailing to form a neat cut.
The cutting block is located above the die neck ring, which helps to cut off the residual material and reduce the wear of the die neck ring. The opening of the shear block is conical, and the included angle is generally 60 ° and the die neck ring and shear block are made of tool steel and hardened to 56 ~ 58 HRC.
(5) Exhaust of mold
In addition, the air volume to be excluded is equal to the cavity volume minus the volume of the blank that has been inflated at the instant of complete closing. However, there is still a certain amount of air trapped between the blank and the cavity, especially for large volume blow molded products, the pressure in the blow molding mold is very small. Therefore, the exhaust performance of the blow molding mold is required to be high (especially for the mold with cavity polishing). If the air trapped between the mold cavity and the blank cannot be exhausted completely or as soon as possible, the parison can not be blown rapidly, and the blow can not be in good contact with the mold cavity, resulting in surface defects such as roughness and dent, and the surface characters and patterns are not clear enough, the appearance performance and external shape of the products are affected, especially when there are streaks or melt fracture during parison extrusion. The poor exhaust will also prolong the cooling time of the products, reduce their mechanical properties, and cause uneven wall thickness distribution. Therefore, it is necessary to improve the exhaust performance of blow molding dies.
(6) Mold heating and cooling
In order to keep the mold temperature in a proper range, the mold should be equipped with a cooling device. It is very important to design and arrange the cooling system reasonably.
The general principles are as follows: For large molds, in order to improve the circulation of the cooling medium and improve the cooling effect, a personal nozzle and a water outlet should be set up on the sealed water tank directly behind the blow molding mold. For the smaller mold, the cooling water channel can be directly set on the template, the cooling water enters from the bottom of the mold, and the water outlet is set at the top of the mold. On the one hand, it can avoid the generation of air bubbles; on the other hand, it can make the cooling water flow in the direction of natural temperature rise. In the cooling water channel with a larger die surface, the guide water flow direction can be installed, and the effect of turbulent flow can be promoted to avoid the dead angle in the cooling water flow process.
For some engineering plastics, such as PC, POM, and so on, the mold does not need to be cooled, and sometimes it is even required to raise the mold temperature to a certain extent, so as to ensure the blow out of the parison and the clear pattern. The heating medium can be introduced into the cooling channel of the mold, or the electric heating plate can be used for heating.