GB1565713A

GB1565713A - Method of making a mould member and method of injection moulding

Info

Publication number: GB1565713A
Application number: GB1637677A
Authority: GB
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1976-05-26
Filing date: 1977-04-20
Publication date: 1980-04-23
Also published as: DE2721393A1; FR2360398B1; JPS6010891B2; JPS52144055A; FR2360398A1; SE391475B; IT1117995B

Description

(54) METHOD OF MAKING A MOULD MEMBER AND METHOD OF INJECTION MOULDING (71) We, INTERNATIONAL BUSINESS MACHINES CORPORATION, a corporation organized and existing under the laws of the State of New York in the United States of America, of Armonk, New York 10504, United States of America do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a method of making a mould member and to a method of injection moulding thermoplastic plastics material employing such a mould member.

The injection moulding of thermoplastic plastics materials has required the production of moulding cavities from materials which are either specialized and expensive materials or which require time consuming processing.

It has been proposed, in Europlan Plastic News of May 1975 to produce a polyimide preform by compression moulding, which preform can itself then be used for compression moulding thermoplastics materials. This proposal however requires a compression press, a pelletizing press and an oven for post curing the preforms.

According to the present invention there is provided a method of making a mould member, the method comprising injection moulding a themosetting plastics material within a moulding cavity formed between a metal male mould half and a metal female mould half, the female mould half having secured thereto retaining means around which the thermosetting material is moulded, the retaining means being shaped to be held captive by the moulded thermosetting material, allowing the thermosetting material to cure, parting the mould halves whilst retaining the cured thermosetting material in the female mould half by means of the captive retaining means, and thereafter grinding the female mould half at the parting surface to compensate for shrinkage of the thermosetting material.

Further according to the present invention there is provided a method of injection moulding a thermoplastics material comprising injecting the material within a moulding cavity formed between two mould halves of which one is constituted by a mould member made in accordance with the method of the preceding paragraph.

The present invention can be used to advantage as a low-cost alternative for the production of prototype and preproduction parts. It can be implemented using machining facilities and a transfer press both of which are common in the production of parts of thermoplastic plastics materials.

The invention will now be described, by way of example, with reference to the accompaning drawings, in which: FIGURE 1 shows two mould halves arranged for the injection moulding of a mould member in accordance with the present invention, and FIGURE 2 shows a modification to the apparatus of Figure 1.

As shown in Figure 1, two mould halves are arranged to receive themosetting plastics material injected therein. A first of the mould halves is a male half 1 consisting of a base form 3 carrying a false plate 4. On the false plate 4 is a pattern 5 secured by cap screws 6 passing through the plate.

The second of the mould halves is a female half 2 constituted by a solid metal form 7. In the alternative the solid metal form 7 may be replaced by a main section 8 and an apertured plate 9 as shown schematically by the dotted line in Figure 1. Cap screws 12, of which only one is shown in Figure 1, secure flanged retaining members 13, of which only one is shown in Figure 1, in the moulding cavity 11 between the two mould halves.

In addition inserts 15 positioned by locating holes in the pattern are clamped between the two mould halves. The inserts 15, of which one only is shown in Figure 1, are provided to be retained in the thermosetting material.

The retaining members 13 are shaped to be held captive by the thermosetting material when injected into the cavity between the two mould halves. The retaining members thereby hold the moulded thermosetting material within the female mould through the medium of the cap screws 12. The inserts 15 are provided either to produce holes in the thermosetting material, as may be required, or to act as ejector pin guides.

As shown in Figure 2 the inserts 15 may in the alternative be flanged, as at 17, and provided with predrilled pilot holes 19 for ejector pins and be held in position by holes in the pattern 5.

In addition bushes 16 to serve as ejector pin guides may be positioned on the pattern using guide pins 18.

The male mould half (1) is positioned in a transfer moulding press (such as Bussman Plastomat, 100S, 100 ton clamp force, or Stoke 751, 50 ton clamp force) in the position normally occupied by the lower mould half. Any inserts (15) for forming subsequent holes in the moulding and inserts (16) to act as ejector pin guide bushes are positioned on the pattern (5). The inserts and bushes placed on the pattern are retained in contact with the face of the pattern by their own weight.

Similarly, the female mould half (2) is mounted in the machine in the position normally occupied by the upper mould half. The cavity (11) in the female mould half is centrally situated directly above the pattern 5.

The moulding press is then closed (manually when possible) to slowly bring the two mould halves together and firmly clamp them against each other. At this point, the pattern and inserts are rigidly held in position within the mould cavity.

When the two mould halves reach the required temperature for transfer moulding the thermosetting material, the thermosetting material is injected into the cavity and allowed to cure. Adequate venting channels are made in the female mould half (2) to allow any air and moulding gases to escape while the thermosetting material is being injected to ensure a complete fill of the cavity.

On completion of the curing, the press is opened slowly to separate the two mould halves and to move the pattern away from the moulded thermoset material.

To allow for shrinkage of the thermoset material, the female mould half is ground along the parting surface between the mould halves. In addition the drilling and seaming operations necessary for the ejector pin holes are carried out. Finally a new runner and gate are machined into the thermoset material and the thermoset material within the female mould half is ready for use itself as a mould member. The mould member or tool is useable for the injection moulding of thermoplastic plastics material into a final moulded part which takes the shape of the thermoset mould member or tool.

The pattern 5 used in moulding the thermoset material has to be overdimensioned relative to the thermoplastic parts desired to be produced from the thermoset mould to allow for shrinkage of the thermoset material and to allow for shrinkage of the thermoplastic material moulded in the thermoset mould.

EXAMPLE The thermosetting plastics material chosen was Fiberite (Registered Trade Mark) FM 4004 marketed by the Fiberite Corporation which has the following properties: Tensile strength = 55 N/mm2 Impact strength (notched) = 12 N.mm/mm Flexural strength = 98 N/mm Flexural modulus = 16800 N/mm2 Hardness (Rockwell M scale) = M100 Compressive strength = 262 N/mm2 Thermal conductivity = 13 x 10-4 Cal/sec/"C/cm Moulding temperature = 143 - 1770C Moulding pressure (transfer) = 6.89 - 20.68 MPa Moulding shrinkage % = 0.15 - 0.30% Coefficient of linear expansion = 1.4 x 10-5 in/in C The advantages offered by this type of material are many. Reinforced thermosetting phenolic plastic is not a new material, but an inexpensive, easily obtained one. Its structure is that of a three-dimensional cross-linked macro-molecule, and offers therefore additional advantages of being durable and capable of withstanding the abrasion and corrosion normally associated with injection moulding processes. At moulding temperatures, its coefficient of linear expansion is very close to that of hardened steel. Furthermore, since the thermosetting material is to be transfer moulded, no special pelletizing equipment is necessary, no flash is produced on the moulding, and no postcuring process and equipment is required. The moulding can be made entirely on location.

The pattern 5 of the thermoplastic part or the prototype to be produced is made in a material that is capable of withstanding temperatures in the range of that of the thermoset material to be transfer molded. Using Fiberite FM 4004, this temperature is in the range of 160 - 1750C, and the material used for the pattern is steel. The pattern must, in addition, be oversized such that allowances are included for shrinkage of the thermoset material after moulding, and shrinkage of the polymer material used in moulding the ultimate thermoplastic part.

Inserts (13, 15, 16) are made from steel, brass or aluminium, and are firmly positioned on the model of the part during the injection of the thermosetting material. The guide pins (18) are removable once the thermoset material has been moulded. The flanged metal inserts (1q) provide for retaining the thermoset material. Retention of the bushes and inserts in the thermoset tool is accomplished through the natural shrinkage of the thermoset material during the curing process, in combination with the flanges (17).

Having transfer moulded the thermoset material, a grinding operation brought the cavity face and the parting face to the same level.

When two separate plates are used, the advantage is gained of being able to use the same main section 8 when making other different moulds requiring only a different cavity (11) volume.

Once a bolster form has been established, a new mould can be produced for a different part by optionally replacing the false plate (4) on the male mould half with a new plate of the same overall dimensions to act as the shut-off face for the male mould half and if necessary mount a new plate (9) for the female mould half. Ejector pin holes in the female mould half can be blanked off with standard steel pins for moulding the new tool and if such holes are suitably positioned, re-used for ejection.

By using the bolster forms and ejector plate systems already made, a new tool can be manufactured, on an average, as follows: Production Step Average Time (hours) Manufacture of model 16 Manufacture of plates for upper and lower mould halves 10 Rebuilding ejector system and benchwork 16 Moulding of thermoset tool cavity in transfer moulding press 1 Total 43 (hours) It is possible to mould the final thermoplastic parts with the gate machined into the thermoset material; however, in a preferred embodiment steel or brass is where possible inserted in the thermoset material where the gate is to be machined.

Whereas the mould sections shown in Figure 1 conform to a modular die set, it should be understood that the present invention is not restricted to any one particular bolster configuration.

WHAT WE CLAIM IS: 1. A method of making a mould member, the method comprising injection moulding a thermosetting plastics material within a moulding cavity formed between a metal male mould half and a metal female mould half, the female mould half having secured thereto retaining means around which the thermosetting material is moulded, the retaining means being shaped to be held captive by the moulded thermosetting material, allowing the thermosetting material to cure, parting the mould halves whilst retaining the cured thermosetting material in the female mould half by means of the captive retaining means, and thereafter grinding the female mould half at the parting surface to compensate for shrinkage of the thermosetting material.

2. A method according to claim 1 wherein the thermosetting material is moulded around a metal insert and the method comprises the further step of machining a gate in the metal insert.

3. A method according to claim 1 or 2 wherein one or more metal inserts are clamped between the mould halves to be retained within the moulded thermosetting material.

4. A method according to claim 1, 2 or 3 wherein the female mould half is formed by an apertured plate backed by a flat plate and the male mould half is formed by a flat plate onto which a pattern is secured.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. produced on the moulding, and no postcuring process and equipment is required. The moulding can be made entirely on location. The pattern 5 of the thermoplastic part or the prototype to be produced is made in a material that is capable of withstanding temperatures in the range of that of the thermoset material to be transfer molded. Using Fiberite FM 4004, this temperature is in the range of 160 - 1750C, and the material used for the pattern is steel. The pattern must, in addition, be oversized such that allowances are included for shrinkage of the thermoset material after moulding, and shrinkage of the polymer material used in moulding the ultimate thermoplastic part. Inserts (13, 15, 16) are made from steel, brass or aluminium, and are firmly positioned on the model of the part during the injection of the thermosetting material. The guide pins (18) are removable once the thermoset material has been moulded. The flanged metal inserts (1q) provide for retaining the thermoset material. Retention of the bushes and inserts in the thermoset tool is accomplished through the natural shrinkage of the thermoset material during the curing process, in combination with the flanges (17). Having transfer moulded the thermoset material, a grinding operation brought the cavity face and the parting face to the same level. When two separate plates are used, the advantage is gained of being able to use the same main section 8 when making other different moulds requiring only a different cavity (11) volume. Once a bolster form has been established, a new mould can be produced for a different part by optionally replacing the false plate (4) on the male mould half with a new plate of the same overall dimensions to act as the shut-off face for the male mould half and if necessary mount a new plate (9) for the female mould half. Ejector pin holes in the female mould half can be blanked off with standard steel pins for moulding the new tool and if such holes are suitably positioned, re-used for ejection. By using the bolster forms and ejector plate systems already made, a new tool can be manufactured, on an average, as follows: Production Step Average Time (hours) Manufacture of model 16 Manufacture of plates for upper and lower mould halves 10 Rebuilding ejector system and benchwork 16 Moulding of thermoset tool cavity in transfer moulding press 1 Total 43 (hours) It is possible to mould the final thermoplastic parts with the gate machined into the thermoset material; however, in a preferred embodiment steel or brass is where possible inserted in the thermoset material where the gate is to be machined. Whereas the mould sections shown in Figure 1 conform to a modular die set, it should be understood that the present invention is not restricted to any one particular bolster configuration. WHAT WE CLAIM IS:

1. A method of making a mould member, the method comprising injection moulding a thermosetting plastics material within a moulding cavity formed between a metal male mould half and a metal female mould half, the female mould half having secured thereto retaining means around which the thermosetting material is moulded, the retaining means being shaped to be held captive by the moulded thermosetting material, allowing the thermosetting material to cure, parting the mould halves whilst retaining the cured thermosetting material in the female mould half by means of the captive retaining means, and thereafter grinding the female mould half at the parting surface to compensate for shrinkage of the thermosetting material.

5. A method of making a mould member, the method being substantially as hereinbefore

described with reference to the accompanying drawings.

6. A mould member when made in accordance with the method of any one of the preceding claims.

7. A method of injection moulding a thermoplastic plastics material comprising injecting the material within a moulding cavity formed between two mould halves of which one is constituted by a mould member made in accordance with the method of any one of claims 1 to 5.