4.1 Bulk polymerised / cast sheet Acrylic
At Temperature within 130 and 150 degree C cast poly methyl methacrylate becomes soft and rubbery and it is this property which makes it possible to fabricate shapes by bending or stretching under low positive and negative pressure. (3)
Unpigmented poly methyl methacrylate sheet has remarkable clarity which can allow up to 92% light penetration. This clarity does not deteriorate with time due to PMMA virtually does not affected by prolonged outdoor exposure, even under tropical conditions. (3,5)
Chemically, PMMA sheet is resistance to inorganic chemicals and some organic compounds such as aliphatic hydrocarbons, hydrogenated aromatic compounds, fats, oil, etc., but is attacked and weakened by aromatic hydrocarbons, esters ketones and chlorinated hydrocarbons.
PMMA has a strong surface hardness that comparable to aluminium.
Low water absorbance, high coefficient of expansion, so dimensionally stable.
4.2 Granule polymerised Acrylic
Used in injection molding or extrusion has marginally different properties to bulk polymerized polymer, has marginally different properties with those of bulk polymerized polymer.
Bulk polymerization polymers contain molecules with wide range of molecular weights. This range leads to lack of homogeneity and causes the material to have poor melt flow characteristics. Granular polymer can be made with a much narrower range of molecular weights and free of the very high molecular weight fraction such found in the bulk molecular weight which means that the melting range is sharper and the melt viscosity predictable. (3,4)
Generally, examination of the properties of bulk polymerized versus granular polymerized indicate that the bulk polymerized polymer to have the better physical properties. Bulk polymerized cast sheet is harder than extruded sheet; it has higher tensile strength, lower elongation and better heart distortion properties. Chemical, electrical and weathering properties compare favorably with material from both types of polymerization.
5. Fabrication and Processing of Cast Sheet Acrylic
5.1 Forming
The bulk Polymerization process uses pure methyl methacrylate monomer into which benzoyl peroxide has been added to prevent coagulation. The monomer is then heated up until the polymerization has converted the free flowing liquid into viscous syrup which ten poured into mold. The mold consist of two pieces of glass plate held apart by required distance held by the gasket such that forming a compartment. This compartment is placed in a water bath, oven or autoclave, and heated until polymerization is complete. Oven is the most widely used. Prior to the process, the glass surface finish and high polish are restore, therefore no further polishing is required.
As mentioned briefly in the previous chapter, the ability of cast sheet to become soft and rubbery between the temperatures of 130 - 150 degrees C.
The precise oven temperature for shaping will depend upon the thickness of material used, the shaping cycle, the temperature and nature of shaping jigs, and the temperature of the workshop It is should lie in the range 150-170 degree C. (4)
Above 170 degree C degradation, as measured by a fall in mechanical strength and softening point, increases rapidly with temperatures and time, and above 180 degree C. it soon becomes severe and eventually becomes visible and bubbles on the surface. Uniform temperature should be maintained throughout the sheet. To obtain the best result, the oven performance should be checked by measuring the actual sheet temperature at several points by using several thermocouples. (4,6)
The time of heating will depend on the sheet thickness and on the type of the oven used, by general indication is given by the formula:
T = 10 + 3.15X
Where T = time in minutes
X = thickness in mm
By considering all of these factors, this method will produces an isotropic acrylic sheet that is uniform properties in all direction without any grain present. Furthermore, it has a very low internal strain which leads to good dimensional stability.
5.2 Shaping
The cast acrylic sheet is also can be formed into different shape. A practical guide can be used as indicator when a sheet is ready to be shaped that the sheet will spring back immediately when folded, and no sign of stiffness. There are two common method used for shaping an acrylic sheet, "free blowing" and "snap back" methods.
5.2.1 Free blowing
The method is practically applying pressure to the heated acrylics sheet which secure by the clamping frame or mold of the desired shape. Normally used for aircraft glazing production. Flat bottomed vessels can also be obtained by blowing the sheet against a retaining plate.
Blowing produce a thinning of the material in the area of greatest movement and this may result in reduction of strength of the article. Therefore care must be taken to prevent loss in strength of the article below acceptable limit. the pressure applied for this shaping method is normally less than one atmosphere.
5.2.2 Snap back
The snap back method is an adaptation of the vacuum technique which allows the sheet to be sucked into an oversize box to form an extended bubble. A former is then clamped inside the bubble and the vacuum broken. The PMMA sheet snaps back on the former which is free from mold marks on the convex side. Generally, blowing method is more favorable as it can apply to form more variety of shape.
6. Injection moulding granular polymers and compounds
One of the most essential aspects in injection molding processing is the granular, which exist in a number of varieties and in wide different range of grades. From the very easy flow grade 5, 6 and 7 and grade 8 and 9 for high heat distortion.
There are several things must be considered when using granule or moulding powder of PMMA, such as, cleanliness which is essential to preserve clarity and purity because like all plastics which are good electrical insulators, methyl methacrylate moulding powders readily pick up dust by electrostatic attraction. Another important factor must be taken into account is that Acrylic moulding powders are not compatible with any other molding materials and therefore rigorous precautions must be taken to prevent contamination.
Although moisture has no effect on moldings it does affect the molding behavior of the raw material. Therefore PMMA moulding powders should be molded as dry as possible, the drier the material the greater the latitude in molding conditions. Furthermore, polymers and compounds absorb moisture if exposed to the atmosphere so the storage condition of raw material must also prolong.
Redrying wet raw material due to incorrect storage handling is also possible by loading onto trays to a depth not exceeding 50 mm, and placing the trays in an air circulating oven. A drying period of about four hours at 75c is suggested for Grade 8 materials and a similar period at 65°C for Grade 6 Materials. Compounds can be dried using hopper driers. (4,5,6)
6.1 Type of injection molding machine
Single and twin screw incline preplasticising and "piggy-back" screw preplasticising can mould acrylics readily It the screw plasticising machines are always to be preferred. Plunger machines are not recommended.
6.2 Machine temperature control
By considering the viscosity and temperature relationship, there are two facts must be taken into consideration during processing.
i) The viscosity of poly methyl methacrylate is very temperature dependent.
ii) The viscosity of poly methyl methacrylate is considerably higher than that of polythene and is more like that of unplasticised PVC.
The implications are obvious: equipment capable of giving accurate temperature control of the mold and accurate temperature control of the heating cylinder are of paramount importance, and accurate temperature measurement is also necessary.
Open type nozzles and mechanical seal nozzles may be used successfully with acrylics, the open type being preferred because there are no potential dead spots where material may degrade. Close temperature control is required on open nozzles to prevent either nozzle dribble or freezing.(3)
6.3 Strain and molecular orientation
Besides mould shrinkage, further dimensional changes can be exhibited by molded parts when they are subjected to high temperature. These dimensional changes have two causes:
i) The release of strains introduced into the molding by differential contraction or mould adhesion on cooling ie. "Quenching strains"
ii) The release of molecular orientation introduced during the flow of the molten polymer into the mould and frozen during cooling.(3)
Annealing is carried out to ensure strain free moldings and involves heating the _ molded part to a high temperature followed by slow cooling. Grade 8 moldings should be annealed at 75 to 85°C then cooled at 30 to 45°C per hour. Grade 6 moldings should be annealed at 55 to 65°C then cooled at 25 to 35°C per hour. The time of annealing depends on the thickness of the part and must be found by trial but a section of 4 mm would require about four hours heating followed by cooling at a rate of 30° per hour.(3,4,5)
7. Applications
Because of its excellent weather resistance, particularly to UV light, cast acrylic sheet is used widely for the manufacture of outdoor signs. Many advertising signs still in current use have been exposed to outdoor conditions in Australia for more than twelve years with no noticeable deterioration of the material. It is used extensively for lighting fittings particularly in association with fluorescent lighting. One of its earliest uses was the glazing of aircraft where the ease of fabrication into complex shapes its high clarity and low weight was useful properties. It is now widely used for the roof glazing of large area buildings. Because of its chemical resistance and non-toxicity it is used in food handling equipment and food display trays. Baths and wash hand basins are two of the many
applications it finds in the sanitary ware field.(4)
The injection molding of automotive light lenses from poly methyl meth. acrylate is almost universal because of its resistance to weathering and petrol and its low cost. Other automotive applications include insignia and indicator panels. Included in other applications are drawing instruments, tap handles, magnifying lenses, tumblers. For many years PMMA was limited to small moldings but with modern screw preplasticising machines and new techniques moldings used for roof lighting up to 1.6 m in diameter and weighing many kilograms are being produced.(4)
The main application for extrusion until recently was light fittings but with improved extruder and die design the extrusion of sheet has become a major industry. Extruded sheet is improving all the time and is now being made to a quality approaching that of the more expensive cast sheet.(4)
Conclusion
This essay has been providing explanation of Acrylics, including the properties of cast sheet and granule polymerisation of acrylics, including their fabrication and processing.
It has also mention about fabrication and processing of Cast Sheet Acrylic including forming and shaping by free blowing and snap back technique, and brief explanation on injection moulding granular polymers and compounds including type of injection moulding machine and machine temperature control.
