MACHINE SELECTION of injection moulding

Selecting an Injection molding machine with proper design is critical to molding a quality part and ensuring economic success.

Below are 3 important molding machine points to consider when moulding operation with Polypropylene. These issues are discussed in detail here.

• Proper Screw and Barrel selection.
• Proper nozzle tip type.
• Clamp requirement.
  

Screw and Barrel Selection

The screw performs the following function in the injection molding process.

1. Conveys the material through the barrel.

2. Mixes the material to the proper molten state.

3. Compresses the material to maximum density.

4. Forces the material into the mold.

Adjustments to the injection process often involve the screw to some degree. The screw, when perform its function provides important contribution to the overall process.

The screw speed for the polypropylene material ranges 50-125 rpm.

Below are several process parameters that are affected by screw design.

• Material melting profile.
• Melt Temperature.
• Material Mixing.
• Shearing of the resin.
  

Therefore it is important to be aware of the type of screw that is being used for each applied specific Barrel and screw designs are recommended for maximum machine wear resistance and proper material plasticization.

While moulding Polypropylene products, should be aware of two critical parameters are L/D ratio and compression Ratio.

(L/D)= (Flight Length of screw)/ (Outside diameter of screw)

Compression Ratio= (Depth of feed section)/ (Depth of metering section)

The recommended L/D ratio and compression ratio for Polypropylene are tested. This will ensure proper melt dispersion.

Polypropylene material type L/D Compression ratio

Unreinforced homopolymers mm, 20:1 3:1

Reinforced material mm, 20:1 2.5:1

Barrel Sizing:

When choosing a press in which to run a mold, it is important to check the shot size as a % of the total barrel capacity. Most barrels are rated ounces of general-purpose system (GPPS) to know the shot capacity of any material other than GPPS, the GPPS once rating must be converted to the density of the other material.

For polypropylene resins one must insert the specific gravity (S.G. - a measure of density) as shown in formula.

Barrel Size X S.G (PP) = Barrel size

(Tons GPPS) S.G (GPP) (Tons PP)

Nozzle Tip Type

Reverse Taper "Steel" tip nozzles recommended when molding Poly propylene products. The reverse taper will minimize material drool and stringing, which can be encountered when molding crystalline resins. The reverse taper design is suggested when molding unreinforced polypropylene, and may be used with reinforced grades.

However the nozzle bore diameter are recommended to be approximately 25% larger for reinforced Poly propylene products than for unreinforced polypropylene due to the higher material viscosities encountered.

Typically, general-purpose nozzles are recommended when using reinforced grades. Temperature control on the nozzle should be controlled separately from other barrel zones. Extended nozzles may require two or more zones of control.

Clamp Requirement:

The injection moulding machine clamp is used to close the mold, hold it closed during the injection and curing of the plastic material, and open the mold for removal of the formed part. The requirements in terms of clamp area, tonnage, clamp opening force, open Day light closed daylight. There are three different types of clamp designs; any one of them can be used for clamping purpose during moulding.

1. Straight Hydraulic Clamp.

2. Toggle Clamp.

3. Hydro- mechanical Clamp.

INJECTION MOULDING MACHINE PARAMETERS

Injection Moulding Machines have become highly sophisticated, complex, computerized machines with many features that need to be considered when deciding which machine will be able to make a particular product.

In the design of injection mould, the following machine specifications are to be considered.

1. Injection pressure

This is the pressure in the barrel at the point of injection, expressed in kg/cm2 or kbar. A higher L/D ratio produces greater Injection Pressure. The greater the Injection Pressure the better the quality of product produced.

2. Shot weight

The shot weight is the weight of the component including the feed system. Calculated shot weight should be less than the machine shot weight.

The shot weight of a machine does not tell us the maximum volume of part that that machine can produce, because the pressures for moulding are higher than for extruding. Typically, the maximum volume of a low quality part will be 85% of the Shot Weight, or around 75% for a high quality part.

Parts need to weigh between 35% and 85% of the Shot Weight of a machine, any lower and the machine may be damaged during production, any higher and the mould cavity or cavities will not be filled.

3. Clamp force

The melt is injected into the mould under very great pressure, which the mould halves, and any other components or cores, must be able to resist in order to maintain their shape and to avoid any seepage. The larger a component is, the greater the pressure and so, the greater the clamp force.

The clamping force required to keep the mould closed during injection must exceed the force given by the product of the opening pressure in the cavity and the total projected area of all impressions and runners.
4. Distance between Tie bars

It is the minimum distance between the edges of the tie bars that guides the moving platens. This measurement limits the size of moulds that can be placed between the tie-bars and into the moulding machine. Tie bars are used in moulding machine to hold the platens. It also guides the moving platen during closing and opening of the mould.

5. Minimum and Maximum Mould Height

It is the minimum and maximum distance that can be adjusted between the stationary and moving platens. The mould shut height should always lie in between the maximum and minimum mould height.

6. Mould opening stroke

This is the distance that the moving mould half moves from mould closed to mould open. Because the injection moulded part has to clear the mould and have room to be removed from the machine, the Opening Stroke must be greater than:

7. Ejection force

The force required to eject the component after cooling of the plastic material in the impression. It is about 3-5% of the locking force.

8. Maximum Ejection Stroke

This is the maximum distance through which the component can be ejected during the open condition. The ejection stroke to be adjusted on the machine depends on the component depth.

CLASSIFICATION OF INJECTION MOULDING MACHINE

Injection Moulding machines are broadly classified into two types,

1) Plunger type Injection Moulding Machine.

2) Screw type Injection Moulding Machine.

1) Plunger type Injection Moulding Machine

The earliest moulding machines were of plunger type as illustrated in fig.and still many of these types of machines are used. In this type of injection moulding machine the resin is fed from hopper into the barrel and heated through the input of thermal energy from heaters around the barrel. The molten resin collects in a pool in the barrel called the injection chamber.

The molten resin is then pushed forward by action of a plunger driven by a hydraulic system at the head of the machine. To facilitate the melting of any residual solid material and to give better mixing of the melt, the molten resin is pushed past a torpedo or spreader that, along with a back pressure plate, imparts shear to the melt. Then the resin flows through a nozzle into the mould.

2) Screw type Injection Moulding Machine

Figure shows a schematic drawing of the injection end of a reciprocating screw machine. The extruder screw, which is contained in the barrel, is turned most often by a hydraulic motor (as contrasted to an electric motor attached to a gear system) point, at which time the screw stops. The rotary shutoff valve is rotated so that when the injection plunger advances, the material is injected into the mold. The main advantages of a two-stage screw are that the material passes over the whole

As the screw turns, it picks up material from the hopper. As it progresses down the screw, the resin is compacted, degassed, melted, and pumped past the nonreturn flow valve assembly at the injection side of the screw. This, in essence, is a check valve, which allows material to flow only from the back of the screw forward.

As the material is pumped in front of the screw, it forces back the screw, hydraulic motor, and screw drive system. In so doing, it also moves the piston and rod of the hydraulic cylinder(s) used for injection. Oil from behind the piston(s) goes into a tank through a variable resistance valve, called the back pressure valve. Increasing this resistance requires higher pressures from the pumping section of the screw, and results in better mixing, a slower cycle, and greater energy consumption.

The screw will continue to turn, forcing the carriage back until a predetermined location is reached. Then the rotation is stopped, and an exact amount of melted material is in front of the screw and will be injected into the mold at the appropriate time in the cycle. This is accomplished by using the hydraulic injection cylinder(s).

INJECTION MOULDING MACHINE

The injection molding process is used to turn plastic stock into finished products. Injection Molding employs sophisticated technology to process plastic stock into finished products. It requires precise motion, pressure, and temperature control.

The process involves many steps:

1) Plasticize the raw material: Heat the plastic stock hot enough to melt it, but not hot enough to damage it.

2) Fill the mold: Inject the plasticized material into the mold. This “shot” step requires precise motion and pressure control.

3) Pack the mold: Reduce the pressure to the pack value and maintain it for a specified time to assure the mold is full.

4) Hold time: Reduce and maintain the pressure at the hold value while the plastic cools.

5) Pressure release: Release the pressure and wait for the part to finish curing while moving the injector into position for the next part.

6) Open the mold: Both motion control and pressure controls are needed to operate the mold.

7) Eject the part: Ejection of moulded part by suitable ejection system and closing the mold so it is ready for the next cycle.

INJECTION MOULDING PROCESS

Injection molding is a cyclic process of forming plastic into a desired shape by forcing the material under pressure into a cavity. The shaping is achieved by cooling (thermoplastics) or by a chemical reaction (thermosets). It is one of the most common and versatile operations for mass production of complex plastics parts with excellent dimensional tolerance. It requires minimal or no finishing or assembly operations. In addition to thermoplastics and thermosets, the process is being extended to such materials as fibers, ceramics, and powdered metals, with polymers as binders.

Injection moulding of thermoplastics was patented by John and Isaiah Hyatt in 1872 to mold camphor-plasticized cellulose nitrate (celluloid). The first multicavity mold was introduced by John Hyatt in 1878. Modem technology began to develop in the late 1930s and was accelerated by the demands of World War II. A similar surge in the technology of materials and equipment took place in the late sixties and early seventies.

The injection molding process is used to turn plastic stock into finished products. The process involves many steps:

1. Feeding raw material.

2. Plasticize the raw material.

3. Fill the mould.

4. Pack the mould.

5. Hold pressure.

6. Cooling of mould.

7. Opening of mould and Part ejection.

The main factors in the injection moulding are the temperature and pressure history during the process, the orientation of flowing material and the shrinkage of the material. This means that the structure and the properties of injection moulded parts are inhomogeneous and the products have always internal stresses.

Feeding raw material:

This is a primary step of injection moulding process. The moulding machine operator feeds the resin in the form of pellets into the hopper of injection moulding machine either with hands (distributed feeding) or through under pressure pipes (centralized feeding) . Drying and mixing of raw material can be done at the same time. where they fall into an augur-type screw channel, which feeds the pellets forward inside the heated barrel.

Plasticize the raw material:

The resin entered in the barrel through feed throat is plasticized by screw extruder and accumulating it in the forward section of the barrel. The heater bands surrounded on barrel maintains the melt temperature of the resin. This plasticized resin is then injected into the mould after mould closing.

Fill the mould:

After the mould closes, the entire screw moves forward (usually driven by a hydraulic mechanism at the drive end of the machine) and pushes the molten resin out through the end of the barrel and inject melt into the cooled mould. The air inside the mould will be pushed out through small vents at the furthest extremities of the melt flow path. This phase takes time from tenth of seconds to few seconds depending on the plastics grade, wall thickness and the shape of the part. This phase fills about 95% of the mould cavity.

Pack the mould:

After filling the mould with resin reduce the pressure to the pack value and maintain it for a specified time to assure the mould is full. Normally the screw will stay in the forward position until the resin begins to harden in the mould. This creates the pressure in the mould, thus ensuring that the mould fills completely.

Hold pressure:

The screw is held in the forward position for a set period of time, usually with a molten 'cushion' of thermoplastic material in front of the screw tip such that a 'holding' pressure may be maintained on the solidifying material within the mould, thus allowing compensating material to enter the mould as the moulded part solidifies and shrinks. Reduce and maintain the pressure at the hold value while the plastic cools. This holding pressure is only effective as long as the gate(s) remain open.

Cooling:

The cooling process starts immediately upon the injection of the molten polymer, but the cooling time is referred to as the time from the solidification of the gate, when the holding step is finished, to the ejection of the part, when it has reached a temperature low enough to withstand the forces during ejection.

Critical dimensions, surface finish, cycle time, etc., are all affected by mold cooling. Hence mold cooling is a decisive factor if a product will be manufactured with good quality and at a competitive cost.

Opening of mould and Part ejection:

When the cooling phase is complete the mould is opened and the moulding is ejected. This is usually carried out with ejector pins in the tool, which are coupled via an ejector plate to a hydraulic actuator, or by an air operated ejector valve on the face of the mould tool. Ejection is the removal of the cooled molded part from the mold cavity and from any cores or inserts. Once the moulding is clear from the mould tool, the complete moulding cycle can be repeated.

POLYMERIC MATERIALS FOR MOULDING

Polymers are substances containing a large number of structural units joined by the same type of linkage. These substances often form into a chain-like structure. Polymers in the natural world have been around since the beginning of time. Starch, cellulose, and rubber all possess polymeric properties. Man-made polymers have been studied since 1832. Today, the polymer industry has grown to be larger than the aluminum, copper and steel industries combined.

Polymers already have a range of applications that far exceeds that of any other class of material available to man. Current applications extend from adhesives, coatings, foams, and packaging materials to textile and industrial fibers, composites, electronic devices, biomedical devices, optical devices, and precursors for many newly developed high-tech ceramics.

Plastics are synthetic materials called polymers, which are long-chain molecules made up of repeating units joined together. These units contain various combinations of oxygen, hydrogen, nitrogen, carbon, silicon, chlorine, fluorine, and sulfur. Although plastics are soft and moldable and approach a liquid condition during manufacture, they are solid in their finished state. As more repeating units are added, the plastic’s molecular weight increases. Addition of more repeating units to the chain makes the molecule heavier.

The mechanical and physical properties of plastics are directly related to the bonds between molecular chains, as well as to the chain length and composition. Plastic properties can also be modified both by alloying and blending with various substances and reinforcements.

Electron Beam Fabrication of Injection Mold Tooling With Conformal Cooling

Electron Beam Fabrication of Injection Mold Tooling With Conformal Cooling
Channels by Ada V. Villalon.

Fast mold cooling is necessary in order to achieve high production rates, as cooling time represents a significant portion of the total cycle time - sometimes up to 80%. However, an inappropriate cooling rate can result in deformations if the part is ejected when it is still soft. Poorly designed cooling channels can also result in uneven cooling that produces warpage or other defects.

Some of the most important factors that affect the cooling of a mold are listed:

• Flow of coolant.

• Temperature increase of the coolant.

• Chemical composition of the coolant.

• Layout and sizing of cooling channels.

• Thermal conductivity of mold parts.

The heat contained in the molten plastic is transferred to the mold cavity, and it has to be removed so that the plastic can return to its solid form prior to ejection. The heat removal takes place mostly through the cooling medium. Some factors such as high flow pressures (in the order of 0.4-0.5 Mpa), large cross sections of channels and turbulent flow are highly desired in order to remove the heat from the mold efficiently.