17.0 COMPRESSION MOULDING
17.1 Introduction:
Compression moulding is a process of heating and compressing the powdered moulding material pellet or chopped resin - impregnated fabric{; in the cavity provided in the moulding, to flow and fill in the cavity and allow to harden and cure to take the shape of the cavity. Usually only thermostat plastics are compression moulded. The cavity provided with enough space to accommodate the moulding compound. The type of mould design mainly depends on the characteristics of moulding compound ad bulk factor.
17.2 Bulk factor
Bulk factor is defined as the ratio of the volume of the loose powder to the volume of the moulded part.
Volume of loose powder
Bulk factor =--------------------------------
Volume of moulded part
Bulk factor can vary from as low as 1: 1 to as high as 18. So the cavity would have to be much deeper to contain the material of a high factor. To minimize the effect of varying bulk factors with respect to the size of the mould cavity, performs are used.
Preforms are small pellets or tablets of moulding material that are formed to shape in a special preforms mould at room temperature, no curing takes place only densification. In addition to this other advantages of preforms are:
- The mould charges are of uniform weight and size.
- Mould cavities are filled more easily without the waste of loose powder
- Preforms heat faster, thus reducing the mould cycle.
- Because the mould cavity can be made smaller, the mould cost is reduced.
Preforms are usually made in automatic presses at a very rapid rate. The shapes of preforms are usually round, flat face, and disc, rectangular or spherical. Preforms are preheated before placing in the cavity. The purpose of preheating is to remove the moisture content from the pellets and to reduce the cycle time by pre warming it
17.3 Compression moulding procedure:
For optimum result in compression moulding the following factors are to be considered.
- Mould temperature: The mould must be heated to the correct temperature for
the particular resin and mould. These temperature ranges from 130-350°C.
- Mould pressure: The mould pressure to be used must be adjusted for each resin and mould.
- Press closing speed: Depending on the particular resin or mould configuration, the press closing speed is to be adjusted to result in the fastest closing commensurate with good resin flow and no damage.
- Degassing or breathing: Before final closing of the mould, it is usually degassed or allowed to breath. This procedure consists of bringing the mould almost to full closure and then opening slightly one or more times to allow the entrapped air, moisture and liberated volatile of escape. This action helps to ensure a sound part, free of internal voids, blisters or entrapped air pockets.
- Final cure time: This final cure time may be as low as 20 seconds for a very small thin walled part, 1- 3 min, for larger parts and as long as 24th for some massive, thick walled aerospace rocket nozzle.
17.4 Types of compression moulds
There are basically four different types of compression moulds. They differ only in the amount of compression, which can be applied to the moulding powder, and in the methods used to 'land' the mare portion of the mould to obtain an exact size.
1) Flash mould:
The simplest mould type is the flash mould. It is fairly inexpensive to make and produce uniformly dimensioned parts. The main draw back of this mould is the fact that the parts produced are not of the highest density, since the moulding material is never put under very high compression. Material wastage is more in these moulds. So flash moulds are usually used to produce small thin parts which are not of high quality.
A Contrasting mould formation is the open flash type, in which class many thousands of articles have been successfully moulded. The alignment of core and cavity can be accomplished by means of guide members i.e. pillars and bushes.
Open flash moulds are suitable for the production of articles where:
- Variation in wall thickness, or eccentricity, is permissible,
- The radius is comparatively small or, preferably, the corner is square on the outer edge of the open end of the moulding,
- The material is not full fabric.
- The displacement of the moulding cavity is sufficiently large to contain the required moulding material in powder form, or in some case preforms.
Open flash moulds have been wrongly employed to mould articles which do not fulfill these conditions. Of course, some loss of material cannot be avoided. To keep the flash thickness to a minimum, pressure must be concentrated near the cut-off faces. This can be accomplished by reducing the land area, which usually made not more than 5 mm wide irrespective of the size of moulding.
2) Fully positive moulds:
Fully positive mould is the opposite of flash moulds. It produces parts of highest density, since the full compression load can be applied on the moulding and not on the mould. The disadvantage of this mould is that unless the amount of moulding material is weighted very accurately, the part will not be the same each time in the direction of the applied force.
Positive moulds consist in the simplest form of two parts- the punch (plunger or force) and die (cavity).
The moulding takes all the applied pressure, and there is insufficient clearance between punch and die to allow any material to escape. The closing of the mould is limited only by the amount of material in the cavity. Any variation in the weight (charge) of material put into the cavity will result in a variation either in thickness of the part or part density. In multi-cavity moulds, any cavities that have more or less material than others will receive proportionally more or less pressure and seriously influence part thickness and density. Because of this basic problem in controlling the weight (charge) of material, the design quickly lost favour, except in very special cases.
3.0 semi-positive moulds
The semi positive mould combines some of the compression moulding possible with fully positive moulds, with at the last moment, a flash cut off and a land to assure good dimensional accuracy. A minor disadvantage of this type is that, if the land is made very narrow so as cause a quick, thin cut - off of the flash, the land will wear very quickly, thus resulting in dimensional instability.
They are similar to positive moulds, but the punch does not come down entirely onto the moulding, but to a stop formed by an annular part of the mould surrounding the moulding. The pressure is therefore taken partly by the annular ring, or rather by the flash forced onto it by displacement of the slight excess of powder that has been deposited into mould cavity.
This development was a great step forward, since it was now no longer necessary
accurately to weigh powder, which is normally weighed in volume. Mouldings of consistent thickness and weight were quite easily produced. The excess material that escapes as flash is normally only about 2-5 % of material used. Depending on size and complexity of flow, flash thickness can be controlled to less than 0.15 mm. Multi-cavity moulds were now a much more viable proposition, and this type of mould is universally used for all types of thermosetting materials. The only adjustment is that clearances between punch and die: 0.15 mm for soft flow materials and a little more -up to 0.25 mm-for filled materials.
An alternative to the semi-positive horizontal flash type of mould is the semi positive vertical flash type. As the description implies, the first type produces flash horizontally and the second type produces vertical flash.
If the part being produced has a height tolerance, then the horizontal flash type is preferable. The surface of the moulding remains untouched when excess flash is trimmed off, thereby maintaining the tolerance. This type is the most popular and most deflashing being done by barreling.
If, for other reasons, finishing is not objected to as the means by which flash removed, then a vertical-type flash mould may be preferable.
Design of semi-positive mouldsSemi-positive moulds are designed for the three purposes:
1. To provide a powder cavity for bulky material;
2. To provide a powder cavity for bulky mouldings in any material;
3. To provide a means of escape for surplus material.
With vertical flash tools the moulding pressure may be applied directly onto the material until pressure pads are contacted, but in the case of horizontal flash tools some pressure is in effect wasted on the area of the cavity itself. Obviously,
therefore the cavity area must be arranged at minimum consistent with strength of punch and should, if possible, follow the general profile of the moulding.
4. Landed positive moulds:
Landed positive moulds are similar to semi positive moulds except that the telescoping portion is deeper to allow more compression on moulding prior to landing. Usually this type of mould incorporates substantial 'lands that will not wear out quickly. There are two types .of landed positive mould:
- Internally landed positive mould or horizontal flash mould.
- Externally landed positive mould or vertical flash mould.
Despite the deeper telescoping of the mould portion, the final compression is not as great as in the fully positive mould. However the dimensional accuracy is better in landed positive moulds.
It is observed from the figure that the loss of material before the tools are closed makes the open flash type of mould unsuitable for certain articles. Consequently a departure from open flash is necessary to overcome the difficulty, and this third mould classification is known as the semi-positive vertical flash type or semi-positive horizontal flash type. Both forms are widely used in modern moulding practice.
There is a tendency to use this tool design method when the open flash type would be satisfactory but this is seldom a fault. There are reasons for this, and open flash mould is usually faster, than a semi-positive mould as it enables air to escape freely but location of punch to die is not so positive and this can be disadvantage in some cases.
Vertical Flash Horizontal Flash
17.5 Why clearance is important?
This is a usual question and the answer is twofold, firstly to ensure the production of a moulding of correct thickness and secondly to ensure that if a multi-impression mould is used, the complete force of the press is not applied on to one impression which may have been overloaded with powder.
17.6 Process difference - Injection & Compression moulds
The main difference between injection moulding and compression moulding is that the mould is cooled in case of injection moulding and the mould is heated to the required temperature for curing to take place in case of compression moulds.The polymerization of thermoset plastic takes place in the mould and necessary heat for the chemical change is to be supplied by the mould. Thermoplastic have scored over thermoset because of the availability of the machine which can control the process precisely, addition of filler materials which improves the material properties and the reduced cycle time.
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