GB2048158A - Apparatus and method for molding a preform - Google Patents

Abstract

A mold cup for molding preforms for making recorded discs includes upper and lower mold members 24, 26, each having a concave mold cavity 28, 30, in a surface which faces the other mold member. The lower mold member 26 has a plastic material inlet opening 38 therethrough opening into its mold cavity 30 at the bottom 31 thereof. The lower mold member has a portion 32 which can fit into the mold cavity 28 of the upper mold member and the upper mold member is movable toward and away from the lower mold member. To mold a preform the upper mold member is moved until the lower mold member is within the cavity of the upper mold member and the volume defined by the mold cavities is smaller than that of the desired preform, e.g., as defined by an abutment ledge 36 on lower member 26. The plastic material is injected into the mold cavities until they are filled. The upper mold member is then moved away from abutment on the ledge 36 until the volume defined between mold cavities corresponds to that of the preform being molded. Additional plastic material is injected into the mold cavities so that they are again filled. <IMAGE>

Description

SPECIFICATION Apparatus and method for molding a preform The present invention relates to an apparatus for molding a preform which is used in making a recorded disc, and particularly to a bottom filled molding cup which provides an improved preform.

Compression molded recorded discs are generally made by forming a preform of thermoplastic material and placing the preform centrally between a pair of heated molds. The molds are closed against the preform meeting the thermoplastic and compressing it, causing it to flow outwardly to fill the mold cavity and define the record contour. In molding the discs it is highly desirable to prevent, or at least minimize, surface defects, such as voids caused by trapped gasses, which can disrupt the grooves in the surface of the disc. The prevention of such surface defects is more highly desirable in making high density information discs, such as video discs, which utilize packing densities of the order of 10,000 grooves per inch, since even the smallest void can disrupt a large number of the grooves.

The preforms used to mold the discs are generally formed at the molding machine in a two part cup-shaped mold, generally referred to as a "cup". The heated plastic material is injected into the cup through an opening in the side of the cup until the cup is filled with the plastic. The plastic is cooled to harden it forming the preform which is then removed from the cup. When the heated plastic is injected into the cup it extrudes in a snake-like fashion across the interior of the cup and then swirls around in the cup until the cup is filled. This results in a non-uniform temperature distribution across the plastic material which, along with the swirling action, can result in air and gasses being trapped in the preform and in variations in density of the preform. The trapped air and gasses and the variation in density can result in defects in the recorded disc formed from the preform.In addition, the side injection of the plastic into the cup can result in decomposition of the molding composition due to shear, particularly at the edge of the preform adjacent the injection opening in the cup. This also can adversely affect the quality of the record made from the preform.

According to the invention:~ Apparatus for molding a preform, i.e., a preform molding cup, includes a pair of mold members mounted with a surface of each opposed to a surface of the other with each of the mold members having a concave mold cavity in its opposed surface. The mold cavities are of a size and shape so that together they correspond to the size and shape of the preform to be molded. One of the mold members is adapted to fit at least partially into the mold cavity of the other mold member so that when the mold members are so arranged, the volume defined by the two mold cavities is smaller than that of the preform to be molded. A molding material inlet opening extends through one of the mold members into the bottom of the cavity in that mold member. Means are provided for moving at least one of the mold members toward the other.

In molding a preform in the mold cup, the two mold members are moved together until one of them is within the other defining a mold cavity volume of minimum volume with the bottoms of the mold member cavities being relatively close together. The heated plastic material is then injected into the mold cavity where it extrudes against the bottom of the cavity in the other mold member and hardens to fill the mold cavity. When the small volume cavity is filled the mold members are moved apart until they define a volume of the desired volume of the preform. As they are moved away additional plastic material is injected into the cavity to form the completed preform.

In the Drawings:~ FIGURE 1 is a side view of an apparatus incorporating the mold cup of the present invention.

FIGURE 2 is a sectional view of the mold cup of the present invention in separated position.

FIGURE 3 is a sectional view of the mold cup in their completely closed position.

FIGURE 4 is a sectional view of the mold cup in their final position for forming the desired preform.

Referring initially to FIGURE 1 there is shown a molding apparatus, generally designated as 10, which incorporates the mold cup, generally designated as 12, of the present invention. The molding apparatus 10 includes a support frame having vertical slide rails 14 connected together by a horizontal top rail 16. A support plate 18 extends between and is secured to the side rails 14 spaced from and essentially parallel to the top rail 16. A pneumatic cylinder 20 is secured at one end to the top rail 16 and extends vertically downwardly therefrom. A piston rod 22 extends vertically downwardly from the cylinder 20.

The mold cup 12 includes upper and lower mold members 24 and 26. The upper mold member 24 is secured to the end of the piston rod 22. The lower mold member 26 is mounted on the support plate 18 directly beneath the upper mold member 24. As shown in FIGURE 2, the upper mold member 24 has a concave mold cavity 28 in its surface which faces the lower mold member 26, and the lower mold member 26 has a concave mold cavity 30 in its surface which faces the upper mold member 24. The upper mold cavity 28 and lower mold cavity 30 are of a size and shape so that together they correspond to the size and shape of the preform to be molded. The lower mold member 26 has an upper portion 32 which is of a size and shape to permit it to fit within the upper mold cavity 28.The lower portion 34 of the lower mold member 26 projects radially outwardly from the upper portion 32 to form a ledge 36 which limits the movement of the upper portion 32 into the upper mold cavity 28. An inlet passage 38 extends through the lower mold member 26 and opens into the lower mold cavity 30 at the bottom of the mold cavity. An inlet pipe 40 is secured in the inlet passage 38 and is connected to means for injecting heated plastic material into the mold cup 12.

To mold a preform in the molding apparatus 10 the cylinder 20 is actuated to move the piston rod 22 and the upper mold member 24 of the mold cup 12 downwardly until the upper portion 32 of the lower mold member 26 is within the upper mold cavity 28 and the upper mold member 24 is seated on the ledge 36 as shown in FIGURE 3. In this position of the mold member 24 and 26 the volume within the mold cavities 28 and 30 is less than the desired volume of the preform being molded and the bottom of the lower mold cavity 30 is close to the bottom of the upper mold cavity 28. The heated plastic material is then injected into the mold cavities 28 and 30 through the inlet pipe 40 and inlet opening 38 in the bottom of the lower mold member 26.The injected plastic material extrudes across the small space between the bottoms 29 and 31 of the mold cavities 28 and 30 respectively and impinges on the bottom 29 of the upper mold cavity 28 where it cools and hardens to start the formation of the preform. As additional plastic material enters the mold cavities 28 and.30 it impinges and cools onto the previously hardened material so that the mold cavities fill up from the bottom of the upper mold cavity 28 downwardly to the bottom 31 of the lower mold cavity 30.

When the mold cavities 28 and 30 become filled the upper mold member 24 is moved away from the lower mold member 26 until the volume enclosed by the mold cavities 28 and 30 corresponds to that of the preform being molded as shown in FIGURE 4. The upper mold member 24 may be so moved by actuating the cylinder 20 or by means of the pressure of the plastic material being injected into the mold cavities. In the latter case, the distance that the upper mold member 24 is moved can be controlled by the amount of plastic material injected into the mold cavities.

When the mold cavities 28 and 30 in the now more open condition are completely filled with the plastic material, thus forming the preform, the upper mold member 24 is lifted from the lower mold member 26 leaving the preform seated in the lower mold cavity 30. The preform can then be removed for placement between the molds of a disc compression molding machine.

In the use of the mold cup 12, since the distance that the plastic material must initially travel from the inlet opening 38 at the bottom 31 of the lower mold cavity 30 to where it hardens at the bottom 29 of the upper mold cavity 28, is small, there is little or no swirling action of the plastic material. By reducing or eliminating the swirling of the injected plastic material, the amount of gasses trapped in the plastic material is reduced, providing a denser preform. Thus, the preform made in the mold cup 12 is more dense and has less entrapped gasses than conventional preforms, so that the likelihood of forming surface defects in the record discs formed from the preform is minimized. Also, by filling the cavity in the mold cup 12 from the bottom, rather than from the side, the cavity fills more uniformly so that shear decomposition of the material does not occur. Thus, the mold cup 12 forms a preform which results in a better quality record disc.

Claims (11)

1. A preform mold cup comprising a pair of mold members mounted with a surface of each opposing a surface of the other, each of said mold members having a concave mold cavity on its said opposing surface, said mold cavities being of a size and shape so that together they correspond to the size and shape of the preform to be molded, one of said mold members being adapted to fit at least partially in the mold cavity of the other mold member so that when so arranged the volume formed by the two mold cavities is smaller than that of the preform to be molded, a molding material inlet opening in one said mold cavities opening into the bottom of the cavity in said mold member, and means for moving said mold members toward and away from each other.

2. A preform mold cup in accordance with Claim 1 in which the mold members are mounted one above the other.

3. A preform mold cup in accordance with Claim 2 in which the molding material inlet opening is in the lower mold member.

4. A preform mold cup in accordance with Claim 3 in which the lower mold member is adapted to fit into the mold cavity in the upper mold member.

5. A preform mold cup in accordance with Claim 4 in which the lower mold member has a radially extending shoulder which is adapted to engage the upper mold member to limit the distance that the lower mold member can extend into the upper mold member mold cavity.

6. A preform mold cup in accordance with Claim 5 in which the upper mold member is movable to and away from the lower mold member.

7. A method of molding a preform in a mold cup having a pair of mold members and having a concave mold cavity in a surface which faces the other mold member, the method comprising the steps of (a) placing the mold members in a position such that the mold cavities define a first volume smaller than the volume of the preform to be molded, (b) injecting a heated thermoplastic material into the mold cavities from the bottom of one of the mold cavities so that the plastic material extends across the mold cavities to contact and harden against the bottom of the mold cavity until the first volume so formed by the mold cavities is filled, (c) separating the mold cavities until they define a second volume corresponding to that of the preform to be molded, and (d) injecting additional heated plastic material into the mold cavities until the second volume is filled.

8. The method in accordance with Claim 7 in which the mold members are mounted one above the other and the plastic material is injected into the mold cavities through the bottom of the lower mold member.

9. The method in accordance with Claim 8 in which the upper mold member is moved with regard to the lower mold member.

10. A preform mold cup substantially as hereinbefore described and shown with particular reference to FIGURES 1 4 of the accompanying drawing.

11. A method of molding a preform substantially as hereinbefore described.