A BLOW MOLDING METHOD

Technical Field

[0001] The present invention relates to the process of manufacturing hollow plastic parts by using a blow molding method.

Background Art

[0002] Hollow plastic parts, which are derived from a typical blow molding method, will normally have a plastic wall structure in which air is captured. When a hollow plastic part is put into use and there is force on its wall, the plastic part can be easily dented. Similarly, when the hollow plastic part is loaded with weigh in order to perform as a beam, it will collapse easily too. Therefore, to improve the hollow plastic’s bearing capacity, it is needed to increase the thickness of the plastic wall or to change the material to one which has better mechanical properties. Another solution is to design the hollow part to have reinforce bones embedded in the wall which will make the hollow part stiffer than the original one. This method, however, cannot add a lot of embedded reinforce bones into its structure, due to material limitation and other factors such as the design of the product has to be practical for producing the mold, for the production process and for the appearance of the product itself. This invention has been designed and developed by inserting a reinforce part in the production process and by selecting light-weight materials that have an appropriate structure for bearing action force, for example, foam or honey comb paper. This results in a composite hollow plastic part which has a sandwich-like structure. This kind of structure is strong and can withstand bending force. This structure consists of surface part which is made of plastic and the core material which is inserted inside the plastic part. The inserted core material must have a light-weight property but have a high stiffness against high shear force. This composite hollow plastic part will have much more strength than the normal hollow plastic part but have gain only a little more weight. It can be explained by way of engineering equation as follows:

D is flexural rigidity

E _f is elastic modulus of the outer surface and inner surface (facings or skin)

E _c is elastic modulus of core

b is the width of structure

d is the distance between the centroid of the outer surface and the inner surface

c is the thickness of core

t is the thickness of surface

If the thickness of surface is relatively thinner than the thickness of core ( ^d / _t > 6 ) and the surface material is much stronger than that of core ( E _f /E _c ,td ² /c ³ > 17 ), the equation can be reduced to

[0003] For the picture above, it can be seen that the increase of thickness of core can improve the properties of the structure obviously whereby the weight increases only little. If the thickness for the sandwich-like structure increases 2 times, the strength properties will increase 3.5 times and the stiffness increases 7 times. If the thickness for the sandwich-like structure increases 4 times, the strength properties will increase 9.25 times and the stiffness increases 37 times. The important factor for making the sandwich-like structure yielded such a result is the quality of welding the plastic wall with the core material. This is the significant process of this invention.

[0004] This process begins with a unique design of the mold which differs from ordinary blowing mold in that the newly designed mold has holes for air to pass onto its surface. Then, the unique process of blowing the plastic part which results in perfect welding between the plastic surface and the inserted material, which forms the sandwich-like structure. This unique process will not make the inserted material distorted nor surface of the plastic part collapsed because of the insertion of the material. Instead, it will make the surface of the plastic part levelly smooth, thus creating a hollow plastic part which has consistent mechanical properties all over the part. Moreover, this type of hollow plastic is evidently stronger and stiffer than the normal one while it gains only little more weight.

Summary of Invention [0005] The normal process of producing hollow plastic parts is to use a blow molding method which begins with heating up a plastic material until its temperature reaches the melting point. Then, the melted plastic is extruded through the die head which has a cylindrical shape, forming cylindrical shape melted- plastic called a parison. Then, the parison is clamped into a mold which has a hole that is used for air nozzle to move in and for air to be blown into it. When, air is blown into the parison, the parison will be expanded to reach the mold’s surface. Subsequently, it will be cooled down until the plastic is harden in the shape of the mold. Then, the air pressure inside the mold is reduced to be equal to the normal air pressure. The mold is then open and the part can be taken out from the mold.

[0006] In some cases where needs a hollow plastic part which has no hole on the parting line of the mold, it can be done by blowing the air through needle that is lunged through melted plastic wall after the mold has been closed completely in order for the air to expand the parison to the shape of the surface of the mold. Then, it has to be cooled down until the plastic gets harden. Air pressure inside the parison will then be reduced by moving the needle out from the mold until the pressure inside is equal to the normal atmosphere pressure. A clipper on the top of mold will hold the finished plastic, and then take the part out after the mold is open completely. Finally, the part will be placed on a suitable area.

[0007] The plastic part, which is produced by the normal blow molding process as mentioned above, is hollow and has a plastic wall capturing air inside. When the plastic part is put into use and has loads on its wall, the plastic wall will be dented easily. Similarly, when it is used as a beam, it will be easily collapsed or bended. If it is needed to reduce the chance of bending or collapsing when used for work the needs high bearing capacity, the plastic wall has to be thickened. This, however, results in the increase of weight of the plastic part and consume more time to produce it. This is because it will take more time to cool down the melting plastic. Moreover, the thicker the plastic wall, the more chance it is to shrink when removed the plastic part from the mold - it may be bended or deformed.

[0008] The objective of this invention is to solve the bending or collapsing of the plastic part as mentioned earlier by improving the process of manufacturing the hollow plastic part. This is done by inserting a reinforcing material during the process of blow molding in order to create a part which has a splicing structure or a sandwich-like structure. This type of structure consists of a plastic wall and a reinforcing material inside as a core. The reinforcing material can be material which as a strong structure such as foam which can bear load put against the wall and can well be used as a beam. This kind of structure will increase the bending stiffness of the plastic part whereby the weight of the plastic part increase only little. The properties of the plastic part which is made from this innovative process will have a similar quality to the part which is used in the aircraft industry or sea vessel industry where those parts are produced by using the laminating process. For example, hand lay-up, spray lay-up, resin-infused, closed mold, vacuum infusion, and RTM. However, this invention employs the blow molding process which has a relatively higher manufacturing capacity but shorter manufacturing cycle time. It is also easier to form a plastic part which has a sandwich-like structure which is similar to parts used in the aircraft and sea vessel industries mentioned above at a much lower cost.

[0009] The important factor to measure the effectiveness of the part which has a sandwich-like structure is the strength of bonding between the plastic wall and the core material. This is because when the plastic part which has a sandwich-like structure is used for bearing force. The action force on the surface of the plastic wall will be distributed and transferred to the reinforcing material which is at the core and will then pass on to the wall on the opposite side. This is the reason why the plastic part which has a sandwich-like structure is stronger and can bear against more bending force than the common hollow plastic part.

[0010] This innovative process brings about the hollow plastic part having a sandwichlike structure which has a complete bonding between the plastic wall and the reinforcing material core. This is derived from the design of the mold to have holes on its surface for air to be blown into the mold during the process of blow molding. This will push the plastic wall, which is still in a melting condition, to move toward and touch the surface of the reinforcing material made of foam, making it melted and completely bonded with the reinforcing material. This is done by using different air blowing pressure between air blowing pressure inside the plastic part and air blowing pressure on the surface of the mold. The result is that the hollow plastic part is stronger and has a higher bearing capacity. With a thinner surface, the hollow plastic part is relatively lighter but still retains the same bearing capacity when compared with the ordinary non-sandwich structure plastic part. This innovative process of making the hollow plastic part results in the reduction of raw material usage, the reduction of energy used in the plastic melting process, and the reduction of energy used for the cooling machine to harden the melted plastic. When this innovative process is applied to the automobile spare part industry by replacing metals, it will help reduce the total weight of the automobile. This leads to energy saving for propulsion both fuel and electricity and to the reduction of emission of carbon dioxide. Moreover, it will get rid of the need for coating metal parts to prevent corrosion and wearing out too.

Brief Description of Drawings

[001 1] Figure 1 is the hollow plastic part which is formed by using the blow molding method and by inserting a reinforcing material made of foam inside.

[0012] Figure 2 is a cross section view of the plastic part that has a reinforcing material made of foam inside whereby the wall of the plastic part is bonded completely with the reinforcing material made of foam.

[0013] Figure 3 A is a cross section view of the plastic part that has a reinforcing material made of foam which is thicker than the space inside the plastic part.

[0014] Figure 3B shows an enlarged view of thickness of the plastic wall when the reinforcing material made of foam is thicker than the space inside the plastic part.

[0015] Figure 4 A is a cross section view of the plastic part that has a reinforcing material made of foam which is thinner than the space inside the plastic part.

[0016] Figure 4B shows an enlarged view inside the reinforcing material made of foam which is not bonded with the inside wall of the plastic part.

[0017] Figure 5A is a schematic illustration of the process of extrusion a parison to cover the reinforcing material made of foam. [0018] Figure 5B is a reinforcing material made of foam which is puctured at both ends and tied with a plastic tendon.

[0019] Figure 5C is a schematic illustration of the process of closing the mold which has a reinforcing material made of foam inside in the parison which is welded at both ends before the mold is close completely.

[0020] Figure 6A is a schematic illustration of the process ofblow molding of the hollow plastic part which has a reinforcing material made of foam.

[0021] Figure 6B shows an enlarged view of the air pressure against the parison, making it expanded to contact the surface of the mold.

[0022] Figure 7 A is a schematic illustration of the process ofblow molding of the plastic part by blowing air inside the plastic part and into the surface of the mold in which the reinforcing material made of foam is located.

[0023] Figure 7B shows an enlarged view of the air pressure against the inner side and the outer side of the plastic wall when the plastic part is blown inside and outside at the same time.

[0024] Figure 8A is a schematic illustration of the blow molding process when stop blowing air from the inner surface of the mold.

[0025] Figure 8B is an enlarged view of the air pressure inside the inner wall of the plastic part, which pushes the wall of the plastic part back to contact the surface of the mold.

Description of Embodiments

[0026] The hollow plastic part that is formed by the blow molding method (1) has a reinforcing material made of foam (2) inside, which is inserted during the production process. This reinforcing material made of form (2) will be bonded with the plastic wall (1A and 1B) by complete welding which prevents distortion on the plastic wall as illustrated in Figure 2. This is done by designing a unique mold and innovative process which are disclosed as follows. The process begins with the mold having 2 halves (11) are open. The reinforcing material made of foam (2), which is punctured at both ends and tied with plastic tendons, is hanged with the upper clip which is in turn cling onto the core axis (6). The other end of plastic tendon is hooked with the lower clip (12) which is connected to a coil spring (13). This makes the reinforcing material made of foam (2) in line with a die (5) and in the center of the 2 halves of the mold (11) without moving side way. Subsequently, melted plastic is injected through the die (5) and the core axis (6) forming into a parison (8) covering the hanged reinforcing material made of foam (2). When the parison (8) is filled up to the size of the cavity (19), the 2 halves of the mold (11), which are attached to the mounted areas of the molding machine (10), will move to the close position. While the 2 halves mold is closing, the sealing pad (18) which is located at the bottom of the mold will close the lower end of the parison. The air inside (22) will prevent the parison (8) from deforming and melting with the reinforcing material made of foam (2) before the 2 halves mold is completely close. When the mold is close, a blowing nozzle (14) will intrude into the parison (8) and will blow air (20) into the parison (8). The parison (8) will then be inflated until it touches the surface of the mold (19) forming into a shape according to the mold. When the wall of the parison (8) contacts the surface of the mold (19), heat will be transferred onto the surface of the mold (19), making the wall of parison where it touches the surface of the mold (19) getting harden and forming into the shape and details according to the cavity inside the mold. When the wall of the parison (8) gets harden about 20-30% of the thickness of the parison (8), air will be blown (21) through pipes (16) and holes crafted inside the mold (15) onto the surface of the mold. Air that is blown (21) from the surface of the mold (19) will push the wall of the hot plastic part back to contact with the reinforcing material made of foam (2) inside, thus melting the reinforcing material with the inner side of the wall of the parison (8). Air that is blown (21) inside the mold, which makes a perfect bond and complete hardening, must have a higher pressure than the air pressure inside (20) about 0.3-0.7 bar in order to push back the hot plastic part (8A) in which its outer surface is solid but the inner surface is still melting. The air pressure inside (20) is still needed to retain the shape of the soft part of the plastic wall where there is no reinforcing material made of foam (2) in order to prevent the plastic wall around that area from collapsing. [0027] Subsequently, air that is blown (21) on the surface of the mold will blow until the inner wall of the hot plastic part (8A) and the surface of the reinforcing material made of foam are bonded and become solid. Afterward, the air (21) will stop blowing and deflate from the surface of the mold (19) and the surface of the hot plastic part (8 A) until the air pressure is equivalent to the atmosphere pressure. This will make the air pressure inside the plastic part higher, thus pushing back the wall of the plastic, both the part that is bonded with the reinforcing material made of foam (2) and the part that is not bonded with the reinforcing material made of foam (2), to contact the surface of the mold (19) in order to cool down all parts of the plastic wall to become completely hard and solid. Then, the air pressure inside (20) will be reduced to be equivalent to the atmosphere pressure before opening the mold. The hollow plastic part will be taken out from the mold and put onto the specified area. The hollow plastic part will have a levelly smooth surface and have a complete bonded between the reinforcing material made of foam (2) and the inner wall of the hollow plastic part. Best Mode for Carrying Out the Invention

[0028] As explained in the Description of Embodiments.