SUGGESTIONS ON DESIGN PROCEDURE

SUGGESTIONS ON DESIGN PROCEDURE

Economy in moulded plastics products calls for simplicity of design, a design which permits the most convenient PET preform mould fabrication and which provides for trouble-free moulding. It is not necessary to confine the design to cylinders and rectangles to attain simple designs; but certain conventions, if observed, will help PET preform mould building and moulding technique and need not affect appearance of the product adversely.

In a design procedure for a moulded casting, it may be necessary to make a first draft of the part to indicate the styling and the functional features. In the first draft, it should be possible to watch the progress closely enough to be sure no undercut portions are required, and that the piece can be withdrawn from the mould. However, undercuts are sometimes necessary, and if it appears that certain portions of the first design will need to be undercut, they can be left to be reconsidered. It is good practice to work out the design of a piece with respect to the parting plane, or to a parting surface as much in one plane as possible.

Development of Design

Reconsideration of the design should be a check of necessary function and de-sign features. Then a mould should be designed around the piece. In this way modifications of the first design that are desirable or necessary will be evident. It will be possible to determine where flash will occur, and, if it is in a difficult place to remove, it may be possible that the design can be altered slightly to change the flash line. Edges and corners can be seen in the mould design, and it will be possible to decide whether sharp or rounded corners are more desirable, or are required by the design necessity of the mould.

After considering the outside or outline of the piece, the coring and wall thickness should be given some attention. When the inside of a casting is to be cored out, it is well to watch the wall thickness. Oversight in this matter may result in the core running through the wall, leaving a hole through the casting. Thread fastenings, lettering, decorations, and ejector pin positions should be studied. PET preform mould designers often receive part drawings with no indication as to how threads are to be formed in a piece or where to put ejector pins. When lettering or decorations are to be made on a casting, the method of mould sinking should be taken into consideration. It should be remembered that a design is best engraved in the steel, and not left standing. Thus, if a mould is to be hobbed, the lettering can be engraved in the hob or in the cavity. If the cavity is to be machined, it is better to engrave in the cavity.

Many details of casting design procedure can be cited as accepted methods. Suggestions for simplified mould building are often of real service to prospective users of moulded castings, since a simple change in a mould for easier construction will often bring the cost of a casting down to a price range where it can be sold advantageously in a very competitive market.

Factors that Determine Design

A number of factors determine the design or design recommendations. The total production required influences the quality or type of steel chosen. The rate of PET preform production determines the number of impressions desirable in a PET preform mould and the size of press equipment to be selected. The strength or finish of the product often dictates the type or physical form of plastics material and the tolerances to be used.

The designer must also consider the moulding operation itself. Such factors as the relation of shape of the part to its position in the mould, the loading of inserts and moulding material, the removal of parts from the mould, handling of moulded parts, and the provisions for the protection of loose bars, side pieces or thread plugs, all are properly a part of plastics mould design. The experienced mould press operator can often offer valuable suggestions which should be employed if practicable.

For example, an experienced press operator might be able to offer a suggestion on the ejection device which would make it easier for him to handle and which would speed up the moulding cycle. He may be able to avoid difficulties in moulding preformed or tableted material by his practical knowledge of loading and moulding characteristics