Moulding

Transfer Moulding and Injection Moulding

Transfer moulding,In compression moulding where preform ingots are compressed and reduced to a fluid in the mould cavity, it is possible for slender cores to be broken and for inserts to be dislodged. Also, for complex parts with thin sections, the proper flow of plastic is difficult to obtain, To overcome these difficulties transfer moulding has been developed. In transfer moulding the plastic material is forced into the mould cavity as a fluid. It flows freely around inserts and slender cores and fills the mould cavity.

Basically, there are two types of transfer moulds, the conventional sprue type and the positive plunger type. In the sprue type the plastic preforms are placed in a separate loading chamber above the mould cavity, One or more sprues lead down to the parting surface of the mould where they connect with gates to the mould cavity or cavities. Special presses with a floating intermediate platen are especially useful for accommodating the two parting surface moulds, The plunger acts directly on the plastic material, forcing it through the sprues and gates into the mould cavities, Heat and pressure must be maintained for a definite time for curing, When the part is cured the press is opened, breaking the sprues from the gates. The cull and sprues are raised upward, being held by a tapered, dovetailed projection machine don the end of the plunger, They can easily be removed from the dovetail by pushing horizontally. In a positive plunger type transfer mould, the sprue is eliminated so that the loading chamber extends through to the mould parting surface and connects directly with the gates. The positive plunger type is preferred, because the mould is less complicated, and less material is wasted, Parts made by transfer moulding have greater strengths, more uniform densities, closer dimensional tolerances, and the parting line requires less cleaning as compared with parts made by compression moulding.

Injection moulding, Injection moulding is principally used for the production of thermo.plastic parts, although some progress has been made in developing a method for injection moulding some thermo setting materials, The principle of injection moulding is quite similar to that of die-casting. Plastic powder is loaded into the feed hopper and a certain amount feeds into the heating chamber when the plunger draws back, This plastic powder under heat and pressure in the heating chamber becomes a fluid, Heating temperatures range from 265to 500'F. After the mould is closed, the plunger moves forward, forcing some of the plastic melt into the mould cavity under pressures range from 12000 to 30000 psi. Since the mould is cooled by circulating cold water, the plastic hardens and the part may be ejected when the plunger draws back and the mould opens, Injection*moulding machines can be arranged for manual operation, automatic single-cycle operation, and full automatic operation. Typical machines produce moulded parts weighing up to 22 ounces at the rate of four shots per minute, and it is possible on some machines to obtain a rate of six shots per minute. The moulds used are similar to the dies of a die-casting machine with the exception that the surfaces are chromium,plated, The advantages of injection moulding are :