Continuing with Part 1 of this article, by Dr.S.N.Chakravarty, President – Elastomer Technology Development Society and Ex-Chairman, Indian Rubber Institute (IRI).

Now, let us look  more deeply into Injection moulding process.

Types of Injection Moulding Machines

The following distinctions may be made between the machines used:

• The toggle or hydraulic mould closing mechanism
• The plunger or screw type plasticating and injection unit
• The plunger screw type plastication unit may be further subdivided into those with and those without reciprocating movement.

The fully hydraulic mould clamping mechanism is preferred today.

Operation Principles: Three types of injection moulding operations can be distinguished according to the differences in tool design and process sequence:

• The conventional
• The injection forming and
• The transfer-injection moulding methods

The essential advantage of screw plasticating machines is the very short cycle time which is very important for high volume production, e.g. using rotary table mould arrangements.

One of the requirements for automated rubber injection moulding is to produce a rubber compound of the highest possible uniformity, which is still quite a difficult task even today.

Other important factors depending on controlled mixing are those of optimum and uniform reheological properties, the dispersion of the ingredients  in the compounds, and computer-aided optimization of the vulcanization process, especially for thick-walled moulding. Compounds with consistent processing properties can now be produced using automatically controlled internal mixers.

Another problem is that of automatically feeding the raw material into the injection moulding machine. The strip feeding method in its original form is too imprecise and needs constant manual attention. Recently, however, the strip feeding method has been refined by using strips of precise dimensions wound on drums, allowing limited periods of automated production. Another method used is to feed the compound into the injection moulding machine in granulated form. This may prove to be the best method for fully automated production in the future.

The manufacturer who has installed injection moulding equipment (or who is considering so doing) must consider four general questions:

• What rubber to use ?
• How to adjust machine controls ?
• Which formulations ?
• What end properties to expect ?

Injection moulding is specially suitable for the mass production of high quality precision components.

For a given barrel temperature, the higher the heat build-up the shorter the cure time. Alternatively, for a fixed injection temperature, the higher the heat build-up the lower the temperature at which the barrel of the machine may be set thus minimizing the danger of premature vulcanization (scorch) in the barrel.

Injection moulding process for elastomeric compound have certain specific advantages over the moulding process.

In Compression and Transfer moulding,  elastomer is pushed into the mould cavity gradually as the mould closes.  Therefore  in the process, though the elastomer gets in contact with the hot surface of the mould , it is not subjected to any pressure, which is  created when the mould is at closing face.  For vulcanization, both temperature & pressure are required.

In injection moulding, mould first closes fully and elastomer is pushed into the mould with high speed.  The cavity of the mould is very quickly filled and a pressure is developed within a very short time, thereby, triggering the process of vulcanization.

In compression moulding process or in transfer moulding process, the flow of material stops immediately when the mould is completely closed. Therefore, typical problems like generation of small gases and vapour, shrinkage of elastomer after vulcanization cannot really be rectified in the above process.

In injection moulding machine there is always a special feature which is known as Post Injection Pressure. Even when the cavity is completely filled and the process of vulcanization has started, the injection unit still continue to push in the elastomer compound for a reasonable length of time ensuring completely porosity free moulding.

Advantages of Injection Moulding

• Allow automation to be introduced into the moulding operation.

• Simplifies preparation of rubber: cutting, shaping and weighing of blanks eliminated.

• Cure times reduced to one-tenth or less compared with compression moulding

• Materials, trimming and inspection costs reduced.

• Product quality improved: better dimensional control and more uniform physical properties.

Cure time is considered as a measure of productivity obtainable. This is the time elapsed between injection and opening the mould. For a given set of operating conditions, cure time depends on the thickness of the product.

Early work showed that barrel temperature was one of the most important factors affecting injection temperature.

As Screw speed increases, rubber in the barrel of the injection machine becomes hotter and it is possible to obtain higher injection temperatures and shorter cure times. Cure time, assessed by the hardness of the top, center and bottom of the moulding, closely follows the injection temperature and illustrates that under the prevailing back pressure and barrel temperature conditions there is an optimum screw speed.

Injection temperature increases with elevation of barrel temperature. When there is no screw delay in operation, temperatures above optimum level causes premature vulcanization of rubber in the barrel.

Screw-back pressure is the adjustable hydraulic pressure which acts to pressurize the plasticized rubber in the injection chamber during screw rotation. Injection temperature increases with screw-back pressure.

As the Nozzle orifice diameter is reduced, the injection temperature rises but, below a diameter of 2mm (0.075 in), it levels off .

With a relatively wide nozzle orifice (3.2mm; 0.125 in), an increase in the injection pressure gives a steady rise in injection temperature and a useful reduction in injection time.

Machine manufacturers advise use of 80-90 percent of the maximum available pressure to save wear and tear on the machine.

Compounding

The principles of successful compounding for injection moulding follow the normal rules. The choice of curing system determines properties such as scorch rate, cure rate, reversion resistance, modulus, strength, resilience and ageing resistance. Control of mix viscosity can be used with caution to optimize mould filling time and hence cure time. Required stiffness, strength and cost can be obtained by selection of filler at an appropriate volume loading.

Physical properties (tensile strength, modulus, etc.) of injection moulded components can be expected to be very similar to those made by compression moulding although the modulus of injection moulded samples may be equal to  that of compression moulded samples, the tensile strength tends to be higher. There is also published evidence that injection moulded components are similar to or better than compression moulded ones in respect of elongation at break, tear strength, hardness, resilience and oven ageing resistance. There have also been claims that in some circumstances certain properties (e.g. Dynamic properties ) of injection moulded products are superior.

Particular interest attaches to the injection moulding of thick  components such as engine mounting and shock absorbers – traditional high-quality natural rubber products.  The greatest benefits of injection moulding are obtained when moulding thick sections since injection at  high temperature saves the time required to heat rubber by the relatively slow transfer of heat into a mix of poor thermal conductivity.

Compound with higher Rubber Hydrocarbon content (RHC) is preferable for injection moulding process.

Advantages and Disadvantages of Injection Moulding Process

A comparison of injection moulding with compression moulding shows the following advantages and disadvantages:

Advantages are for example:

• Reduced preparatory labour

• Transport and storing of the cut reforms is eliminated.

• Considerably shorter vulcanization times because of homogeneously preheated material.

• More homogeneous degree of vulcanisaiton of the finished part at high injection temperature.

• No bumping.

• Faster removal of the finished parts

• Simplification of de-flashing

• Lowering of the waste and scrap rate

• Possibility of automation

These advantages are contrasted by the disadvantages of considerably higher investment costs for moulds and machinery. The injection moulding process can not also handle as high volumes as compression moulding.

Economic Efficiency

Which method is most economical for the production of moulded parts, compression moulding with low investment costs but with higher production costs or injection moulding with higher investment costs, depends to a high degree on the number of parts to be produced. Below certain limits it is advantageous to work with low investments but higher wave costs while for high volume production runs  higher investments may be preferable.

However, considering improvement in product quality leading to higher performance  and longer life made by injection moulding process, it is preferable to go for the same.

Taking into account of all factors associated with injection moulding, namely – Capital Investment, higher precision of working, higher (improved) compound cost, Qualified & experienced Technologist needed etc. would lead to a higher cost of the finished product.

Besides choice of Machine & Process one has to keep in mind that performance of a rubber product depends on many other factors which need equal attention. These are, for example, for certain metal to rubber bonded items used by Indian Railway.

1. Application condition
2. Environmental condition – Oxygen, Ozone, Fatigue, Displacement Amplitude etc.
3. Compression under continuous load
4. Geometry / design of the product – FEA Analysis results

 

Simultaneously to consider which aspects can improve product performance.  For example,

Compound Quality – Homogeneous Mixing / Mix quality (Kneader is not a suitable substitute of internal mixer)

Rheometer provides cure characteristics information but does not indicate mixing quality .

Homogeneity of the compound sheet

Lower Heat Built-up (Resilience /Hysteresis)

Ageing resistance over longer period

Summarizing, it is quite clear that only particular machine or process usage will not lead to improved product performance but combination of all related points development is essential to achieve the desired goal. Any specification should be drawn only after ascertaining the results of these studies.

References:

1. Injection moulding of natural rubber – MRPRA Booklet
2. Rubber Technology Handbook – W. Hofmann
3. A.I. Isayev  & M. Wan – Rubber Chemistry & Technol. 69, No. 2, p. 277, & 294, 1996
4. K. Yakemoto, T. Shiroganeya, H. ITO & T. Sakai. International Conference on Rubber, Calcutta, Dec. 12 -14, 1997, p.73

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