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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