From Plastics Wiki, free encyclopedia
Mold A hollow form or cavity into which molten plastic is forced to give the shape of the required component. The term generally refers to the whole assembly of parts that make up the section of the molding equipment in which the parts are formed. Also called a tool or die.
Moulds separate into at least two halves (called the core and the cavity) to permit the part to be extracted; in general the shape of a part must be such that it will not be locked into the mould. For example, sides of objects typically cannot be parallel with the direction of draw (the direction in which the core and cavity separate from each other). They are angled slightly; examination of most household objects made from plastic will show this aspect of design, known as draft. Parts that are "bucket-like" tend to shrink onto the core while cooling and, after the cavity is pulled away, are typically ejected using pins. Parts can be easily welded together after moulding to allow for a hollow part (like a water jug or doll's head) that couldn't physically be designed as one mould.
More complex parts are formed using more complex moulds, which may require moveable sections, called slides, which are inserted into the mould to form particular features that cannot be formed using only a core and a cavity, but are then withdrawn to allow the part to be released. Some moulds even allow previously moulded parts to be re-inserted to allow a new plastic layer to form around the first part. This system can allow for production of fully tyred wheels.
Traditionally, moulds have been very expensive to manufacture; therefore, they were usually only used in mass production where thousands of parts are being produced.
Molds require: Engineering and design, special materials, machinery and highly skilled personnel to manufacture, assemble and test them.
Types of molds
The type of mold depends of the molding machine used.
Tools used to form a product using the injection molding process. A standard injection mold is made of a stationary or injection side containing one or more cavities and a moving or ejection side.
Tool used to form hollow plastic products, such as bottles and cans. Generally made of aluminum, molds can have either water jackets, cast in tubing, or drilled cooling lines.
Tools that form pre-extruded sheets by means of temperature and vacuum and/or pressure. Generally made from aluminum, either cast or machined.
Molds can be hand operated, semi-automatic or fully automatic.
Hand operated molds require an operator to physically remove the mold from the press and disassemble it to remove the molded part. They are used for very small quantities (1 - 500 parts) or parts that have undercuts, threads or other complicated features.
- They have long operating cycles (2-5 minutes or more) and by nature are labor intensive to operate.
- They are typically single cavities, inexpensive and fast to produce.
- Parts may required multiple post operations to meet design requirements
Semi-automatic molds are typically mounted in the press and require an operator only to perform a specific operation of the cycle (i.e., remove a core, place an insert, remove the part from an undercut).
- They can be very cost effective for insert molding or when quantities do not allow for the expense of core pulls, cams or unscrewing devices.
- Single or multi-cavity designs can be cost effective.
- Machine cycle times can approach that of automatic molds, but so can their cost.
Fully Automatic molds run with no operator. They may utilize a variety of mechanical devices to facilitate part removal such as mold sweeps, air blasts, sprue pickers or robots. They may contain core pulls, cams to remove undercuts, multiple plates, hot runner or insulated runner systems, unscrewing devices or any number of devices and techniques to improve efficiency.
- Depending on part design and material they can cycle extremely fast (as little as 4 seconds).
- Parts are normally finished as molded with no post operations, beyond value added operations, required.
Moulds are typically constructed from hardened steel or aluminium. The choice of material to build a mould is primarily one of economics. Steel moulds generally cost more to construct, but their longer lifespan will offset the higher initial cost over a higher number of parts made in the mould before wearing out.
Aluminium moulds can cost substantially less, and when designed and machined with modern computerized equipment, can be economical for moulding hundreds or even tens of thousands of parts.
Materials commonly used are beryllium copper, and kirksite.