COOLING

COOLING

During the final stage of cooling, mould temperature directly affects product dimensions and stability. Therefore, moulds must have an even distribution of water lines to accurately control the process and part-to-part uniformity.

A hot mould will...
· Produce parts with less stress and higher gloss.
· Usually requires less clamp tonnage to mould the part.
· Has a longer cycle time.
Engineering resins require hot moulds that run anywhere from 180 to 220 degrees Fahrenheit. Specialty resins require hot moulds in excess of 300°F.

A cold mould will...

· Produce parts with a dull surface appearance and more moulded in stress
· Requires more clamp tonnage to mould the part
· Has a shorter cycle time
Commodity type resins use a cold mould that typically runs at about 70°F or less.

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INJECTION

INJECTION

Several moulding parameters directly influence the injection of plastics into the mould, including the following:

· Injection speed
· Melt cushion
· Injection pressure
· Injection time

3.4.1 Injection Speed

Injection speed is a key processing parameter.

· For older machines

Older machines have only a simple flow control valve that regulates the amount of hydraulic oil going to the injection cylinder piston. Opening the valve will allow more oil to enter the piston at a greater rate and thus the plastics is injected faster.

With materials that flow with some difficulty it is recommended to use full injection velocity- (75% to 100% of the available injection velocity.) For moulding plastics which flow more readily it is recommended to start at 50% of the potential injection velocity and slowly work up as needed.

· For newer machines

More advanced machines control multiple stages of the injection velocity to more accurately control the process. Later model injection moulding machines allow you to better control your process and, as a result, will give you less part-to-part variation and better part performance.

3.4.2 Melt Cushion

The melt cushion is the material at the front of the screw when the screw is in the forward position. Always injection mould with a melt cushion of 1/8” to 1/4” to allow the part to pack out evenly. A pressure loss can result if the cushion is too high, and the parts will not mould consistently.

3.4.3 Injection Pressure

Pressure is created by a resistance to flow. As hydraulics are controlled by this property, injection pressure settings can be developed.

To establish first stage injection pressure, raise the pressure to a point where the part fills out without any packing. The screw moves forward and stops as it reaches the melt cushion.

At this point, second stage pressure is implemented to allow the cushion to pack out the part. Older machines usually have a combination hydraulic pump that includes a high-volume, low-pressure pump and a low-volume, high-pressure pump.

The high-volume pump allows for a fast, steady, forward movement of the screw, and the low-volume pump handles the packing of the part. As moulding machines are designed around their hydraulics, the high-volume pump consumes more energy than the low-volume pump. Therefore, it is more efficient to switch to the low-volume pump as soon as possible to reduce the amount of energy consumption. As a result, more energy is conserved by reducing the first stage time than by reducing the cooling time.


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The more advanced machines control multiple pressure settings and therefore more accurately control the moulding process and final moulded part quality.

3.4.4 Injection Time

Injection time is the amount of time that the screw remains forward. By controlling the rate of hydraulic oil to the injection cylinder piston, injection time is controlled.

Assuming a machine has only a first and second stage:

· The first stage is the amount of time it takes to fill the part.
· The second stage is the time required for the gate to freeze off.

In a multiple stage injection, the last stage is usually the packing stage because the sprue, runner, and parts are full. At this point, you will notice that the part weight remains constant from shot to shot.

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PHASES OF OPERATION

PHASES OF OPERATION

In injection moulding, we can describe the following three phases of operation:

· Plastication
· Injection
· Cooling

We will discuss how to determine the optimum moulding parameters for each phase of operation. As always, it is a good idea to refer to machine manufacturer information when setting up the machine parameters on the machine that you will be using.

3.3.1 Plastication

We begin with the first stage of melting the plastics material.

3.3.2 Barrel Temperature Settings

When selecting barrel temperature settings, refer to the lowest processing temperature that the material supplier has recommended. Moulding at a low temperature results in a shorter cycle time and reduces the chances for material degradation.

This lower processing temperature requires a higher injection pressure to fill the mould. However, the efficiency of the operation increases with a slightly faster cycle time, and the final product will have better moulded-in properties.

3.3.3 Heat Profile Settings

The heat profile settings along the barrel will determine how the plastics will melt. Plastics such as polyethylene, polypropylene, ABS, and PVC can be set up so that the lowest temperature is in the feed zone and the highest temperature is in the metering zone. This type of profile is referred to as a forward profile and is the most common profile used in injection moulding.

For nylon, acetal, PET, and PBT, the zone temperatures are fairly constant, creating what is called a straight profile. For materials that have a tendency to drool, such as nylon, a reverse profile may be used- with the lowest temperature in the metering zone and the highest in the feed zone.

3.3.4 Screw Speed

The actual melt temperature of the plastics will be higher than the barrel temperatures, due to the shearing action between the barrel and the screw. As a result, screw speed is a critical injection moulding parameter to control melt temperature because 70 to 90% of the heat required to melt the plastics
comes from this shearing action. Screw speed determines the rate at which the plastics pellets melt. It is necessary to maintain a slow, steady, consistent speed to evenly melt the material.




To determine the correct screw speed, refer to the processing information supplied by the material manufacturer. It is important to use the lowest possible setting that will allow for uniform melting. A high screw speed may overheat and degrade the plastics material. This will cause dimensional problems as well as a reduction in the physical properties of the material.

3.3.5 Screw Back Pressure

Screw back pressure causes the screw to make more revolutions, and the additional revolutions of the screw create more shear heat, which can cause material degradation.

You should use little or no back pressure with glass or mica filled materials because it breaks up the filler and as a result, reduces moulded part quality. This increased back pressure also accelerates the wear on the barrel and screw.

For material using a color concentrate, extra back pressure can be used to assist in dispersing and mixing the colorant more evenly. Extra back pressure can also be used for screws with a short L/D ratio of 15:1. However, screws with higher L/D ratios require very little back pressure as more mixing takes place.

3.3.6 Suck Back

After screw recovery, suck back prevents the nozzle from drooling. Too much suck back may cause splay or bubbles on clear parts or black parts.

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MATERIAL PREPARATION

MATERIAL PREPARATION

We begin with some initial material preparation before processing the plastics resin.

3.2.1 Inspecting Material

Check material carefully for consistency and for foreign materials such as dirt, paper, other plastics, etc.

SPECIAL NOTE:
In all cases, it is extremely important to properly identify and mark plastics containers to ensure correct usage.

3.2.2 Removing Moisture

In some cases, dryers are used to reduce the moisture content of the plastics resin to ensure proper processing. Moisture has an adverse effect on plastics, and if the resin is processed without removing it, cosmetic defects such as splay (bubbles) may occur. Moisture can also cause a reduction of physical characteristics of the moulded part, such as strength and impact resistance.




To remove the moisture in a plastics, a desiccant type dryer is used. A desiccant dryer works by passing hot air through a desiccant bed that will trap the moisture that is in the air.

To ensure that a plastics material has been properly dried, a dew meter is used to check the dew point of the circulating air. Resin manufacturers supply the recommended drying time, temperature, and dew point for each type of resin.

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PLASTICS PROCESSING

PLASTICS PROCESSING

Injection moulding is a process by which the plastics is melted and injected into a mould cavity. Once the melted material is in the mould, it cools to a shape that reflects the form of the cavity.

The resulting form is a finished part requiring no additional work before
assembly and, in most cases, is the finished product. Many details, such as bosses, ribs, and screw threads can be formed during the one-step injection moulding process.

3.1.1 Thermoplastics

Thermoplastics only undergo a physical change when exposed to heat. As these plastics are heated, they melt.

Plastics like polyethylene, polypropylene, ABS, Styrene, and Polycarbonate soften as they are heated. There is a wide temperature range in which these materials will melt without degrading.

Nylon, PBT, PET, and Acetal do not soften as they are heated and experience little or no change as they reach their melting temperature. However, once they reach their melting point, they melt very quickly. Therefore they are said to have a narrow melting range, and degradation may occur faster. As any plastics reaches the high end of its melting range it degrades.

The greatest benefit of thermoplastics is that they can be re-melted many times without much degradation.


3.1.2 Thermosets

Another category of plastics called thermosets will change both physically and chemically when heated. As a chemical change has also taken place, a thermoset cannot be re-softened by reheating and therefore cannot be reused. Thermosets are used for applications requiring high heat and are usually long lived products such as pot handles, ash trays and electrical fuse box holders.

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Temperature

Temperature

The key temperature parameters include:

(i) Barrel temperature
(ii) Nozzle temperature
(iii) Mould temperature

Most temperature controllers will maintain a set temperature to within a few degrees. It is important to accurately control these parameters to develop a consistent process. Reducing part-to-part variation and improving the efficiency of the moulding operation is critical to the success of your company.

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Time

Time

Injection time controls the transfer of hydraulic oil during the different stages of injection speed. Other time-controlled elements include:

(i) Mould open and close time
(ii) Screw rotation time
(iii) Overall cycle time

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Speed

Speed

Injection velocity or speed determines the rate at which the mould is filled. The amount of hydraulic oil pumped into the screw injection piston controls the injection velocity. Some advanced machines can precisely control as many as 12 stages of injection speed.

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Pressure

Pressure

Typically, the following types of pressure are controlled in the process:

(i) Injection pressures

· The first stages of injection will fill the mould. Once the mould is filled, an additional amount of packing pressure is used until the gate freezes off.



(ii) Screw back pressure

· Screw back pressure is pressure put on the back of the screw. Typically this is done to increase mixing; such as when color concentrate is added. In some cases, to break up glass filler. (This is not recommended) When the pressure at the front of the screw is greater than the screw back pressure, the screw moves back.

(iii) Clamp pressure

· The clamp pressure is the pressure required to keep the mould closed during the injection phase. Maximum clamp pressure is achieved when the parts are packing.

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MACHINE CONTROL PARAMETERS

MACHINE CONTROL PARAMETERS

Next, we will explain the parameters that an injection moulding machine controls. All injection moulding machines are designed to perform the same basic functions. These include:

· Clamping
· Injecting
· Rotating the screw
· Heating the screw barrel

The machine control parameters used to monitor these functions are:

· Pressure
· Speed
· Time
· Temperature

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Shut-Down

Shut-Down

To shut down the process:

1) Shut the feed throat to prevent any more plastics pellets from entering the screw and barrel assembly. After the machine cycles a few times, put it in semi-automatic mode.

2) After the completion of the one cycle, place the machine in the manual mode. Move the injection carriage away from the mould.

3) With the purge guard down, reduce the injection velocity and slowly jog the screw forward, repeating this procedure until all of the material has been purged. Leave the screw in the forward position.

4) Coat the mould with a mist of mould saver and close it, but do not put it under clamp tonnage.

5) Shut down the hydraulic pump and barrel heaters.

6) Be sure to shut down the mould heater or chiller and de-pressurise the water lines.






· Most mould heaters and chillers have a vent button that will de-pressurise the water lines and displace water to either the drain or back to the chiller return line.

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Machine Start-Up Procedure

Machine Start-Up Procedure

We will begin the start-up procedure with the assumption that the machine has been shut down for a weekend. This type of start-up is referred to as a cold start. Initial preparation is required for the injection moulding machine to warm up and reach its operating temperature and to fully stabilize machine components.

(i) Start-Up

To start the process:

1) Allow the hydraulic pump motor to run for one to two hours before the machine cycle begins.

2) Start any auxiliary equipment such as heaters and chillers. Also turn on the barrel and nozzle heaters. Be sure that the mould is closed all but 1/4 of an inch and that no tonnage is on the mould.

· Most injection moulding machines have a protection device that prevents the screw from turning until at least one of the heater zones reaches the set point temperature. Other machines require that the heats be on for some minimum amount of time (usually 15 minutes) before they allow the screw to turn. This protects the screw from any damage that could be caused from the initial starting torque.


3) To begin the moulding process, spray the mould with a small mist of mould release.

· Mould release may not be needed for medical or food applications or when the cavity has a high-surface finish.

4) While in the manual mode, close the mould and check the clamp tonnage and mechanical settings.

5) Open the feed throat to allow plastics pellets to enter the screw and barrel assembly.

· Jogging the screw motor on and off allows for a slow start-up of the motor. The screw moves backward until it reaches its predetermined setting.

6) With the purge guard, take several air shots and wipe the nozzle drool with a brass screwdriver or spatula.

Bring the injection carriage forward, and slowly jog the tip until it butts up against the sprue bushing.

Special Note:
Be careful not to slam the carriage into the sprue bushing because damage to the nozzle tip or sprue bushing may result.

7) With the machine in semi-automatic mode, allow the mould to complete one full cycle. Repeat this procedure until an acceptable process is developed.

8) At this point, place the machine in automatic mode and carefully monitor the process to be sure that the parts do not hang up. Allow the machine to cycle for 10 to 15 minutes.

9) Visually check the parts to be sure they are filling completely and that there is no sink or flash on the parts. Weigh the parts and be sure that the weight is consistent from shot to shot.

10) Once the parts are visually acceptable, check them for dimensional accuracy.

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Complete Machine Cycle

Complete Machine Cycle

The following describes on full machine cycle:

1) The injection moulding cycle begins when the mould closes. The screw moves forward, injecting the material through the nozzle to the sprue. The material fills the runners, gates, and finally the mould cavities.

2) During the packing phase, additional material is packed into the mould cavities until the gate solidifies or freezes off and plastics can no longer be forced into the mould cavities. (At this point, additional packing only results in packing of the sprues and runners).

3) The material is allowed to cool or solidify in the mould. During the cooling time, the screw rotates counterclockwise, melting the next shot of plastics. It stops moving when it reaches its preset setting. Pellets from the hopper enter through the feed throat.

4) As the mould opens, the parts are ejected and the machine is ready to cycle once again.

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Modes of Operation

Modes of Operation

Injection moulding machines operate in the following modes:

(i) Manual
(ii) Semi-automatic
(iii) Automatic
(iv) Set-up

(i) Manual Mode

The manual mode is used for machine set-up. You can perform the following functions in this mode:

· Clamp open and close
· Ejector stroke
· Material and color changes
· Screw rotation

(ii) Semi-Automatic Mode

In the semi-automatic mode, the machine makes one complete cycle. This mode provides an indication as to how the machine will perform in the automatic mode. This mode is used when an operator is required to remove the parts from the mould. In this mode you can also add inserts so that they can be moulded into the part.



Adjustments can be performed to the following elements to improve processing:
· Clamp stroke
· Mould open time
· Injection pressure
· Injection speed

(iii) Automatic Mode

The automatic mode allows the machine to run continuously without interruptions. You make final adjustments to the mould and pressure controls once the machine begins the automatic cycle. Fine tuning of the process will result in producing high-quality parts with few rejects.

(iv) Set-Up Mode

Usually maintenance or set up personnel use this machine mode for mould set-up. During the set-up mode, the machine operates at a very slow speed when the top gate is open. The clamp will operate from left to right when the front and back gates are closed. Some machines allow you to keep the front gate open and at the same time open the mould. You can make fine adjustments to the clamp positioning in the set-up mode.

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Mould Cavity and Core

Mould Cavity and Core

The mould cavity and core define the shape of the moulded part. The cavity defines the outside of the part, while the core defines the inside shape of the part.

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Sprue, Cold Slug Weld, Runner, and Gates

Sprue, Cold Slug Weld, Runner, and Gates

The sprue connects the machine nozzle to the mould. It provides a passageway for the plastics to travel.


Opposite the sprue is a circular depression, or a cold slug weld, that captures the first portion of the material injected into the mould. The runners then provide a passageway to deliver the molten plastics material to the vicinity of the mould cavity.

The gate is an opening at the end of the runner through which material enters the cavity and moulded parts are produced.

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Check Valve

Check Valve

Check valves are used on all machines to prevent material from flowing over the screw as it is moving forward during injection.


The most common check valve used is the slip ring check valve. When the slider moves to the right, it seats itself against the screw and blocks the melted plastics from flowing back over the screw. During injection, if the screw drifts forward through the melt cushion, the check valve is not functioning properly.

Material tends to collect and degrade in the check valve, causing a poor seal-off. The check valve specifically designed for PVC does not block the back flow of material as well as the slip ring check valve. However, due to its high viscosity, PVC does not tend to flow back over the screw during injection.

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Nozzle

Nozzle

The nozzle connects the machine to the mould.

Different types of nozzles exist for moulding materials with special characteristics. The tip screws into the nozzle and can be changed to accommodate moulding different materials. Typically, a heater band is located around the nozzle to control the temperature of the plastics melt.

In some moulding operations, a mechanical shut-off nozzle prevents material from drooling. This provides a positive shut-off because the valve will mechanically stop the flow of material. These types of nozzles are activated with a hydraulic cylinder.

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Screw Function

Screw Function

The screw functions in the following way:

1) The feed zone conveys the plastics to the transition zone where most of the shearing action takes place.
· Sometimes the transition zone is called the compression zone. In this zone, the flights of the screw are not deep so that the material can be compressed between the screw and barrel.

2) The metering zone will melt any plastics that was not fully melted in the transition zone.
· The material in the metering zone is of even consistency and in a molten state as it builds up in front of the screw.

3) This build-up of material creates pressure at the front of the screw which forces the screw back toward the rear of the injection unit until it reaches a pre-determined distance called the shot size.



4) The screw now moves forward like a piston, and the resulting ram action forces the hot melted material into the closed cool mould.

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Screw Specification

Screw Specification

The general purpose screw is the most common screw found in our industry because it can be used to process many different plastics materials. Other, more specialised screw designs exist to run special materials such as vinyl or thermosetting materials.

Figure 6. General Purpose Screw

A term used to specify screws is the length over the diameter ratio or “L over D”(L/D). This ratio will give you an idea of the amount of mixing that will take place. The higher the ratio- the more mixing.

The compression ratio is the ratio of the depth or volume of material in the metering section to the depth or volume of material in the feed section.

This gives an indication of how much the material is compressing. The more compression- the more sheer (heat).

General purpose screws typically have:
· L/D ratios of 16:1 to 25:1
· Compression ratios of 2:1 to 3.5:1

Plastics pellets enter the screw and barrel assembly through the hopper and enter the feed zone where the flights of the screw are the deepest.

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