Glossary of  Moulding Terminology


Cooling time: This is normally considered to be the time in seconds from the moment injection time (including injection hold time) ends, until the moment the mould starts to open. In reality, however, the cooling time is usually the same time as the cycle time since the coolant flows through the mould continuously, regardless of whether the mould is moving, stopped open or closed. However the statement is true if any form of pulsed cooling is used.

Cycle time: The time in seconds from the start of one shot to the start of the next shot including the mould open time. This is usually measured with a stopwatch at the point when the mould reaches the mould closed position.

Dry cycle (time): This is the time in seconds from the moment the mould starts closing from its fully open position, to the point it arrives again in the fully open position after having closed completely. The times for injection, hold and cooling periods arc not included.

The dry cycle includes the mould closing time, the time to clamp and unclamp the mould and the mould opening time.

The dry cyclc is an important measure of the clamping performance of a moulding machine. The shorter the dry cycle, the less time is wasted. This is of particular importance in fast cycling applications such as when moulding packaging products.

The dry cycle time depends significantly on the distance the moving platen has to move. The shorter the stroke, the shorter the dry cycle is.

Ejection time: This is the time in seconds required to eject all the mouldings from the mould after the mould starts opening. It includes the time to clear the mould of all mouldings and runners, multi-stroke ejection times and the use of any robots.

The ejection time is part of the overall cycle time. Long ejection times can affect the cycle time seriously because they may require a slow opening speed of the clamp (the dry cycle), or it may require additional mould open time, or the lengthening of any mould open time already in the cycle to accommodate special ejection designs.

Ideally, the ejection time should take place during the mould opening time, and be completed by the time the mould starts closing again, without requiring any mould open time or pause time.

Occasionally, the ejection phase extends into the mould closing time (during the mould closing phase); providing all obstructions are cleared before the mould closes.

Injection hold time: The time in seconds from the moment the first stage (high) injection pressure ends to the end of all subsequent secondary (lower) injection pressure phases. These lower pressure phases are necessary to compensate for the volumetric shrinkage of the material as it cools in the mould cavities.

In very thin walled parts the hold time is frequently not used.

Injection time: The time in seconds during which material is injected into the mould during the first high pressure phase,hut not including second stage or holding pressure phases.

Mould closed time: The time in seconds from the moment the mould is fully clamped up, to the moment when it starts opening.

Mould open time: The time in seconds from the moment the mould arrives in the fully open position to the moment when it starts closing for the next cycle. Ideally, a mould should run without any open time but it is often necessary to ensure the mould is clear of all mouldings, runners or robotic removal devices.

Passive cycle: In certain hydro-mechanical machines (using shutters) this occurs at the point the machine is ready to open. After the cooling cycle ends, the clamp force is first relaxed, which takes a fraction of a second, followed by the shutters being withdrawn, which takes another fraction of a second. The total of these time elements, before the mould actually starts opening, is called the passive cycle.

The mould will remain closed at this time and the actual set cooling time includes the time for the passive cycle. This has to be taken into account when setting the cooling period.

However, on fully hydraulic machines and on toggle machines, there is no passive cycle. As soon as the cooling time is ended, the mould starts to open.

Profiled injection: With modern machines controlled by microprocessors, injection and hold pressures can be programmed to change relative to the injection time and/or stroke. The injection pressures or speeds can be increased or decreased in steps, or at a variable rate, to “profile” the injection rate depending on the machine.

With older machines, injection and hold times are distinct times, which are individually set for each individual pressure and time.

Amorphous: Having no pattern or structure, molecular chains are oriented at random. Amorphous materials have low shrinkage values compared to crystalline materials.

Barrel: This has the same meaning as “Cylinder”,see below.

Bolster: A tool usually consists of a set of impressions containing the form to produce the mouldings and a set of plates screwed together into which they are located. This set of containing plates is termed a bolster.

Burned: When the melt is injected into a tool, the advancing front of material has to displace the air from the cavities in order to fill them and for the melt to solidify into the moulding. If the air gets trapped and cannot escape, the incoming melt compresses it, causing the air temperature to rise. This temperature can reach a value that degrades or decomposes the material which gives the moulding a characteristic black sooty appearance.

Cartridge heater: A cylindrical heating element available in different diameters, lengths and wattage ratings. Most often used for heating hot runner manifolds.

Cavity: The portion of the tool containing most of the female form of the moulding. A 4 cavity mould means 4 mouldings are produced each cycle — a 6 cavity, 6 mouldings produced, and so on. The term “impression” is also used in this context, i.e., a 6 impression mould would produce 6 mouldings per cycle.

Clamp force and preload: To ensure that the mould will not open during the injection phase, it must be held closed with a force, or “preload,,,greater than the minimum clamping force required to just hold the mould closed. This preload is generated by the clamping mechanism of the moulding machine.

As the mould closes, the two mould halves meet at the parting line. At this moment, there is no force on either of the mould halves.

As the clamping mechanism starts to exert its full force, the mould halves start to become compressed together. As the locking force increases, the fixed platen is forced away from the clamping mechanism until the full clamp is achieved. Counteracting this force are the tie bars (see below), which connect the stationary platen to the machine clamping mcchanism.

Clamping force: The force that holds the mould together from mould closing to the start of mould opening so that the injection pressure of the material inside the cavity space cannot force the mould open and cause it to flash at the split line. The required clamping force F can be determined by multiplying the total projected area of all cavities (plus any additional area due to cold runners) by the injection pressure.

The term “Locking force” has the same meaning.

Clamping force is measured in kN (kilo Newtons) or MN (Mega Newtons). See “Newton” below.

The clamping force should only be sufficient to ensure the mould will not open while the material is being injected. If excessively high clamping forces are used, the stress on the split line surfaces of the mould may he high enough to damage them. It is also undesirable to mount a small mould on a relatively large machine due to potential damage to the machine platens through the tendency of the mould to “hob” into them.

Clamping pressure: The clamping force divided by the total projected area of the tool.

Closed loop: This is a control system that monitors and adjusts melt temperatures, pressures and other moulding parameters automatically through feedback obtained via transducers in the mould.

Cold runner: A cold runner tool produces a solidified runner and sprue as well as the mouldings each cycle. The runner and sprue are usually re-processed and re-used.

Crystalline: Having a regular or ordered structure termed crystals.

Cushion: When adjusting the length of the injection stroke of a machine, the stroke should be about 10 mm more than the actual shot volume required to fill the mould. This means that when the mould is filled after the end of the injection stroke, there will be still a small amount of plastic left in the cylinder to provide a “cushion,,,of material at front of the machine screw.

With amorphous plastics (e.g., acrylics, PC, SAN, and PS), there is less or no need for a cushion since their shrinkage is quite small compared to crystalline materials.

The cushion provides a reservoir of material for the holding pressure, to make up for shrinkage. It also overcomes the difficulty of stopping the screw in precisely the right position every time as it retracts (screws back) during plasticising when preparing the shot for the next injection cycle.

A cushion also ensures that in the case of some plastic leaking back through the check valve at the tip of the screw during injection, there will always be sufficient material to fill the mould.

When moulding crystalline materials and/or thick walled products a cushion is essential to provide a reserve of material for adequate filling during the holding pressure phase. However, with fast cycling thin walled products there is often no cushion required.

Cylinder: The tubular container that holds the rotating screw and material. It has heater bands on the outside to help melt the plastic material.

Down time: A term used to describe the time a machine does not have a mould tool on it. This is frequently due to a mould suffering damage and having to be removed from the machine for immediate repair.

Ejected: A part has been ejected when it has been forced out of the mould by an ejector.

Ejector: A device for forcing the moulding components out of the mould tool after the mouldings have solidified and the mould opened (or during mould opening). These are available as pins and blades, or may be specially shaped.

End user: The company that markets the moulding either directly or as part of some other piece of equipment.

Feed: Another term used for “Gate”,see below.

Flash: This is surplus material (usually in the form of a thin film) which is attached to the moulding,resulting in the moulding being rejected. There may be several causes of flash occurring, as follows:

  • Material being forced into the cavities at too high a pressure resulting in material penetrating areas of the cavity construction it is not supposed to, for example, down the sides of cjectors
  • As the tool wears,gaps in mating sliding parts become larger (making flash more likely)
  • As the result of the locking force of the machine being insufficient to keep the tool closed against the incoming injection pressure, which causes the tool to open slightly and allows the material to spread across the faces of the split line of the tool
  • Using low viscosity materials that tend flash more easily than high viscosity materials Force: See “Newton”.

Freeze, frozen: Terms used to describe the condition when the melt has solidified.

Gate: A feature machined into the cavity to connect the runner to the moulding. It is smaller than the runner so that it may be detached easily from the moulding. The gate also performs other functions to control the way in which the melt enters the cavity.

A variety of different gates are used which vary in shape, size and form depending on the size of the moulding and the material being used. A gate is sometimes called a feed.

Guard: A safety device preventing access to the mould tool or dangerous parts of the machine. Opening the guard door of a machine renders the machine (and hence the tool) inoperable and stops all functions.

Guide pillar: Circular steel shafts fixed in one side of the tool and located into a bush (guide bush) on the other half of the tool. The purpose is to make sure that both halves of the tool align together properly as the tool closes.

“Heavy wall” and “thin wall” mouldings: These are subjective descriptions but one common method uses the length to thickness (L:T) ratio, which applies mainly to containers with a more or less uniform wall thickness. Depending on the ratio, a part is deemed either thin walled or heavy/thick walled.

Hobbing: A process where one piece of steel (A) is forced into another (B), thereby leaving a female impression of it in B. This is sometimes used in cavity construction.

Holding pressure: The reduced pressure exerted by the screw after the main injection pressure phase to compensate for volumetric shrinkage. See also “Holding time”.

Hopper: The container into which the raw material is poured to feed the material into the cylinder for processing.

Hot runner: A hot runner mould tool is one in which the runner remains at melt temperature. Only the mouldings are produced thus saving wastage or re-processing costs.

Impression: This is a term used to describe all the parts of the tool required for forming the moulding,usually (but not always) in separate insert sets.

Injection unit: The complete injection assembly comprising the cylinder, the screw, heater bands, hopper, metering devices, limit switches and the hydraulic cylinders and motors necessary to actuate injection.

Injection pressure: The first stage pressure exerted by the screw on the molten material to inject the material into the mould tool.

Injection stroke: The distance the screw moves forward to displace the melt into the mould tool. The longer the stroke, the more the material is forcccl into the mould tool.

Insert: A separate unit located in the mould tool distinct from the plates.

Lock, locking force: These have the same meaning as “Clamping force”.

Manifold: Used in hot runner tools. It consists of a block of steel into which a runner system is machined. It contains cartridge heaters and thermocouples to maintain the temperature required to keep the material at melt temperature. It is insulated from the rest of the mould tool to prevent loss of heat.

Mass: The inertia of a body can he described as being its reluctance to start moving,or to stop moving once it has started. A body of large mass requires a large force to change its speed or direction by a noticeable amount, i.e” the body has a large inertia. Thus the mass of a body is a measure of its inertia.

Mass is measured in grams (g) or kilogrammes (kg).

Material: The term used to describe the raw material or “plastic”. Polymer is also used in the same context.

Melt: A term used to describe a material in its molten form at its melt temperature.

Melt Flow Index (MFI): A measure of how easily a molten material will flow. The lower the MFI, the more difficult the material it is to inject.

Micron: 0.001 millimetres.

Mould: The total mechanical assembly that provides the means by which the mouldings are produced. The terms “Tool” and “Mould tool” have the same meaning.

Mould designer: The person responsible for designing the mould tool to a standard whereby satisfactory production and quality is achieved.

Moulder: A commonly used term to describe the injection moulding company or department carrying out the moulding operation.

Newton: The force required to give a mass of 1 kg an acceleration of 1 metre per second every second. Prefixes are used by international agreement (SI) to describe quantity. E.g.,

  • kN (Kilo Newtons) = 1,000 Newtons
  • MN (Mega Newtons) = 1,000,000 Newtons

There are many other prefixes but these (kN and MN) are most frequently used in the injection moulding industry.

Very roughly 10,000 N (or 10 kN) = 1 tonne = 1000 kg force. Machine locking or clamping forces are usually expressed in kN. Hence a 400 kN machine has a locking or clamping force of 40 tonnes (force).

Force is also sometimes expressed as 10 kgf, 100 gf,etc, to distinguish it from mass.

Open loop: A method of moulding where no automatic adjustments are made to the processing conditions during moulding, as opposed to a closed loop system (see above).

Operating window: A term used to describe the range of operating settings or conditions of the moulding machine and mould,such as melt temperatures, cooling periods, pressures and other cycle times within which mouldings of satisfactory quality can he produced.

The operating window is heavily dependent on the mould design and mould quality, and in particular the gate size and location and satisfactory ejection. The larger the operating window, the easier it is for the moulder to achieve satisfactory mouldings. By contrast, narrow operating windows require frequent “fiddling” of the moulding conditions incurring higher reject and production costs.

In effect, the wider the operating window, the easier it is for the moulder to produce mouldings of the required quality with low rejections levels.

Plasticised: A material is plasticised when it reaches the melt temperature.

Platen: The part of the machine on which the mould tool is mounted. The fixed platen is at the injection end of the machine and does not move. The moving platen is moved by the machine locking system, via toggles or links, to open and close the mould.

Plates: The basic structure of a mould tool is made up of steel (sometimes aluminium) plates. They may be circular solids or rectangular solids that are available in various diameters, widths, lengths and thicknesses.

The plates have to be machined to accept the cavities or impressions and all other operating mechanisms required by the design. Plates are supplied in different grades of steel or aluminium depending on where they used in the tool construction.

Polymer: The material used for the moulding operation. 丁he terms “Material” and “Plastic” are also used.

Projected area: The total area of the mouldings seen in the direction of the clamping force. In cold runner moulds the area of the runners must also be included. In hot runner moulds, only the projected area of the cavities need be considered.

The forces due to the projected area are the product of the projected area and the injection pressure being used. This force is resisted by the clamping force to ensure the mould is kept closed during the injection phase.

Reprocessed material: Material that is reclaimed by a process in which redundant material, like cold runners and rejects, generated during production may be reused. A granulator is used to chop up the material into small pieces which may be recycled in a controlled proportion in with the “virgin” material.

Runner: A channel machined into the mould tool to direct the flow of the melt to the gates and into the cavities.

Screw: A bar of steel machined with continuous spiral channels in the form of a helix. It is similar to an achimedian screw. The flights of the screw get progressively shallower towards the tip of the screw. The purpose of the screw is to transport the material to the front of the cylinder ready for injection and to provide frictional heat.

Seized, seizing, seized up: When two sliding components start to abrade each other, their surfaces become damaged and ultimately “weld” or “lock” themselves together.

Shear heat: Frictional heat developed in the material by the “shearing” action of the rotating screw on the material and by the material passing through the gates.

Short, shorts: A term used to describe an incomplete moulding.

Shot: The total number of items ejected from the mould tool each time it opens, i.e., all the mouldings and associated runner and sprue (if present).

Shot capacity: Used for defining the maximum amount of plastic that can he injected by the machine. This depends on the screw and cylinder diameter and the stroke of the screw. Shot capacity is usually specified as the maximum mass in grams that the machine can inject in polystyrene, having a specific gravity of 1.05. When moulding other materials, with a different specific gravity, this mass must then be divided by 1.05 to get the corresponding shot volume, and then he multiplied by the value of the specific gravity for such material to obtain its shot mass.

Shot weight: The weight of the total mass of all components moulded during one complete moulding cycle. The shot weight includes all the mouldings and the runner and sprue in cold runner moulds. To maintain adequate control the shot weight should not be less than 25% and not more than 80% of the rated shot capacity of the machine on which the mould is going to be run.

Shrinkage: All materials undergo a volumetric reduction when injected into a cavity. The final size of the moulding is less than that of the cavity in which it is formed. It is necessary to make the cavities large enough so that after shrinkage has taken place, the moulding will be the correct size. Crystalline materials have a higher shrinkage than amorphous ones.

Snatch, snatches, snatch pin: Features designed to ensure that runners are held on the correct half of the tool so that they may be successfully ejcctcd.

Split line: All mould tools must separate into two parts (sometimes more) to allow the part to be ejected from it. The plane where these two halves meet is called the split line.

Split line moulding: Most moulding operations are carried out on a horizontal machine. That is,all the movements of the machine and the tool take place in a horizontal plane. In split line moulding, the injection takes place along the split line of the tool. The tool movements are horizontal and the machine injection carriage vertical.

Sprue: A cylindrical tapered part of the runner system leading from the sprue bush to the runner and thus to the cavities.

Sprue bush: A shaped circular piece of steel that contains the sprue. It is hardened to resist the impact from the nozzle of the cylinder.

Starve feeding: A method of adjusting the injection system so that the injected volume of material is exactly the same as the volume of the cavity space (plus any runners). This method is usually used for moulding thin wall components where there is no need for holding pressure. This is because the material enters the cavity so quickly that it almost immediately fills it,with the result that the moulding instantly freezes along with the gate.

Until the time the cavity is filled, the material exerts relatively little pressure on the mould walls. However, in this case,by the time the mould is full, no more material is being injected due to the lack of cushion and hold pressure. Hence there is only a relatively small force trying to open the mould against the clamping force.

The advantage of this method is that a thin walled moulding with a large projected area can be moulded in a smaller machine with a lower locking force. Another advantage is that moulds with long, slender cores can be filled with minimal core deflection since the pressures inside the cavity are significantly less than with general moulding techniques.

Sticking back: A term used to describe a moulding staying in the wrong side of the tool (usually the fixed half) as opposed to the ejection side of the tool. Parts that have stuck back cannot be ejected from the tool and frequently have to he removed manually.

Stripper plate: Either a full plate or insert that supports the periphery of the moulding whilst ejecting it from the mould.

Temperature controller: A piece of mould coolant at a given value. A temperature of hot runner manifolds.quipment used to control the temperature of the temperature controller is also used to control the

Thermal degradation: If material is subjected to elevated temperatures for prolonged periods of time, it can decompose. This can take the form of burned particles being produced or in more serious cases, decomposition into completely carbonised material accompanied by gases.

Thermocouple: A sensor that sends information to a temperature controller enabling the temperature to be controlled within preset limits.

Thermoplastic: A material that can be melted, allowed to solidify and then rc-melted again.

Thermosetting: A material that may be melted hut once solidified cannot be re-melted.

Tie bar: 1 he high strength steel bars along which the moving platen slides. The tie bars of a machine are stretched to provide the clamping force necessary to keep the mould tool closed during the moulding process. There are usually four tie bars on a machine.

Tie bar stretch: The tie bar stretch is proportional to the clamping force. Knowing the tie bar stretch and the tie bar diameter and length,the actual clamp force can be calculated easily. The tie bar diameter determines the maximum possible clamp force of a machine. Machine manufacturers ensure that the maximum tie bar stretch is not greater than 10% of the yield stress of the tie bar material to maximise their operating fatigue life.

Tool: See “Mould”.

Toolmaker: This can either mean an individual that works on the manufacture of a mould tool or a company that manufactures mould tools.

Twin barrel: A machine having two cylinders or barrels; used for increasing the nominal capacity of the shot weight of the machine. It is also used for moulding in two colours on the same moulding.

Virgin: Describes material that has not previously been used. It is in the original untouched state as supplied by the material manufacturer.

Viscosity: The degree of fluidity of the melted polymer. The lower the viscosity the easier it flows and vice-versa. Low viscosity materials are more like water while high viscosity materials are more “treacle like”.

Weight: This is defined as the force acting on the mass due the local gravitational attraction of the earth, or:

W = mg

It is a consequence of Newtons Second Law of Motion (Force = Mass x Acceleration). In the UK, the value of g is usually taken as 9.81 metres per second per second. For less critical calculations a more approximate value of 10 metres per second per second is used.