Processes encountered in plastic industries:

Injection Moulding:

injection moulding is a manufacturing  process for producing parts by injecting material into a mould. Material for the part is fed into a heated barrel, mixed, and forced into a mould cavity, where it cools and hardens to the configuration of the cavity mould are made by a mould maker (or toolmaker) from metal, usually either chromium steel and precision-machined to form the features of the desired part.

Injection moulding machines can be categorised on basis of the mechanism used in the process of injection moulding. They are as follows:

1. Toggle injection moulding machine - the pressure that can be applied by the toggle clamp mechanism is much higher than its counterpart, however releasing the pressure In case of sudden shut down is much more troublesome due to locking of the joints.

Trouble shooting: During such situations, hitting the elbow joint in the opposite direction and releasing oil pressure is advisable. special care must be taken while doing this as this may result in a sudden jerk at the end. (info source : mondal da)

2. Ram injection moulding machine - here there is direct axial pressure due to movement of a screw.

Process Description

1. the feed is first allowed to enter the hopper which may be mechanically or vacuum fed. The hopper is often attached to a drier which gets rid of any excess moisture.

2. Then the feed is allowed to enter the injection barrel which has a thermal jacket surrounding it. The injection barrel is divided into three temperature zones .

3. Inside the barrel is a reciprocating screw/ archimedi’s screw.4. Forward motion of the screw by rotation of the hydraulic

motor through the injection barrel compresses the polymer and subsequent heating leads to reduction in viscosity and it starts to flow

5. Channel depth along the length of the barrel decreases forcing the plastic through a smaller area causing increase in frictional forces generating heat.

6. The tip of the reciprocating screw has a unidirectional valve. Once the portion in front of the valve fills up, it releases the pressure and subsequent forward motion injects the feed into the mould

7. Cooling lines within the mould allow for hardening of the product and faster cycle times.

8. Runners and flush on the product is manually removed before the product is sent for printing and packing. White lines on products can be removed by using heat guns, and bending can be taken care of by the use of custom jigs.

Insert moulding

Pre-moulded or machined components can be inserted into the cavity while the mould is open, allowing the material injected in the next cycle to form and solidify around them. This process is known as Insert moulding and allows single parts to contain multiple materials. This process is often used to create plastic parts with protruding metal screws, allowing them to be fastened and unfastened repeatedly.

Eg – lead inserts in battery containers which are pre heated and insert moulded after lacquering.

Extrusion Blow moulding

It is a molding process in which air pressure is used to inflate soft plastic into a mold cavity. Blow molding is accomplished in two steps:

(a) fabrication of a starting tube of molten plastic, called a parison and

(b) inflation of the tube to the desired final shape. After the parison is formed, the two halves of the mold come together and pinch one end of it while the other end is sealed around a blow pin. When air is blown into the mold, the tube gets inflated and takes the shape of the mold cavity. Once it cools, the mold is opened and the solidified part is removed.

Extrusion

The principle of extrusion moulding is similar to that of injection moulding.

The only difference is that the end of the injection barrel comprises of a steel die which casts the molten plastic into the required shape from which it may be periodically cut off.

The product is then cooled and sent for finishing.

This method is popularly used for making pipes.

PRODUCTS:

Preform moulding

This process is extensively used for the production of PET (Poly Ethelene Terepthalate) bottles.

This process mainly comprises of two stages:

1. Formation of small plastic preforms.-the process of injection moulding is used for formation of the preforms which are then ejected from the mould after cooling.

PREFORM:

2. Subsequent heating of the plastic preforms and inflating it by aid of air pressure within the mould to form the finished product.

Main products manufactured

s.no Product name Material used Why is it used?

1 buckets PPCP(poly propelene co polymer)

To ensure flexibility of buckets so that it sustains drop tests since PPCP has relatively higher MFI values.

2 Battery containers

PPCP/SAN(Styrene acrylo nitryl)

Corrosion resistant ( from acids)

3 Blow moulded bottles

HDPE( MFI 18) Lower MFI values ensure ease of extrusive blow moulding and flexibility.In some cases, acid resistant polymers must be used to prevent leakage.

4 Railway dowels

HDPE(MFI 0.1)

5 handles HDP (High density polethene)

High strength and low MFI values ensure lesser chance of failure.

6 dhama Recycled plastics Used in food or construction industry for transportation of materials, thus use of recycled plastic material is profitable

7 caps PPCP/HDP

Masterbatch of different colours are added to the feed in a specific percentage by weight in order to obtain different colours.

Products:

Railway Dowel Testing

Where is the product used: In a rail fastening system, the plastic dowel can be considered as the fastening foundation. It needs to be pre-inserted into the concrete sleeper separately before assembling the rail fastening system parts. It is used to guarantee the good connection and linkage of the rail fastening system with the concrete sleeper, with screw spikes or bolts.

About the material used: The material used in the manufacture of railway dowels is HDP (High Density Polyethylene). This material is used as it provides high strength and resistance to atmospheric corrosion and rusting.

Tests performed on each batch of dowels: Each batch of railway dowels consisted of 5000 dowels out of which 25 undergo physical testing and 3 undergo chemical testing. The tensile strength of the material, viz. HDP is also tested using simple tensile test. Given below, is a description of the tests performed:

Figure 1. Railway Dowels

Figure 2. Gauge box

(1) Physical tests: From each batch, 25 dowels were made to undergo physical tests where each dowel was tested with standard go and no-go gauges. The gauges are shown in Figure 8. The hardness of the material of the dowel was also tested using a Shore Durometer Hardness Tester and the value was required to be greater than or equal to 65(according to the ASTM D2240 standard) for the dowel to pass.

(2) Simple Tension Test: To check the tensile strength of the material used, a sheet of thickness about 1.9 to 2 mm was produced using the process of compression molding. From this sheet a dumbbell of standard dimensions was cut using a cutting die. The dumbbell was stretched as in the simple tension test and the maximum elongation (when it breaks) is noted. The maximum force required is also noted. For the material to pass this test, using a dumbbell of the standard dimensions, the maximum tensile stress must be above 35MPa and the elongation must be more than 400%.

(3) Chemical tests: From 3 selected dowels, material was scraped to perform the following chemical tests –

a) Melt Flow Index (MFI) – Material was fed into the melt flow apparatus and heated to a temperature of about 190⁰ C, which is well above the melting point of HDP. Pressure was applied with the help of a weight measuring 5 kgs. At first, the molten material was allowed to flow for some time. Then after removing this initial amount of material, we start a stopwatch and collect whatever material flows in 10 minutes. The collected material was weighed in a single pan balance. The HDP used was expected to have an MFI ≤ 0.1g/10min.

b) Specific Gravity Testing – The specific gravity of the material was calculated using a single pan balance by measuring the weights of a fixed amount of material in air and in respectively. The specific gravity of is known to be. So, the specific gravity of the HDP used can be calculated. For the material to pass, it must have a minimum specific gravity of at least 0.945.

c) Melting Point Testing (Hot Plate Method) – A small piece of the material

Figure 3. Apparatus required for MFI test and Specific Gravity test

Figure 4. Apparatus for Simple Tension Test

is kept on a flat surface, a drop of silicon oil is put on top of it and then it is covered by a glass slide. The flat surface (hot plate) is heated till the material melts and the oil begins to flow uniformly. The temperature at which this happens is noted as the melting point of the material which must lie between 135⁰C and 137.5⁰C.

Ash Percentage Testing – The material used had a maximum ash percentage of 0.5 %.

Printing

Melting Point Apparatus

(1) Silk screen printing: using this process only one color can be printed at a time, so the process is time consuming and is used for printing labels with only one or two colors. Surface heat treatment of the products is required before printing in order to ensure better adhesion of colors to the surface.

(2) Shrink sleeving: the labels are put on the container and passed through a machine where they get heated and the label shrinks and adheres to the container surface.

(3) Heat transfer labelling (HTL): The colors to be printed are transferred from a plastic surface to the container surface when the two come in contact with a heated rubber surface.