Compounds of VamacŪ can be satisfactorily processed using standard techniques of compression, transfer and injection molding. However, their inherently low viscosity and other factors need to be considered for best results.
All compounds of VamacŪ contain a three-part release system intended mostly for mill release. This also can be useful for the molding process. Assuming such a release system is present, then an ideal molding compound of VamacŪ will typically be based on a higher-viscosity grade (such as VamacŪ HVG) and feature the following:
- an amine cure system;
- a high cross-link density;
- a moderate loading of high structure carbon black; and
- no plasticizer.
Practical compounds of VamacŪ that differ markedly from the above can still give perfectly good moldings. Conversely, a combination of the following may prove less easy to mold, depending on the exact formulation:
- a lower viscosity VamacŪ grade;
- a low cross-link density; and
- a high loading of plasticizer/mineral filler.
Any compound cured solely or partly by peroxides may be undesirable for molding, because of poor hot tear strength and a tendency to stick to the mold.
Because compounds of VamacŪ tend to be soft and tacky, they are sensitive to minor variations in mold surface. This can translate into mold sticking, unless preventive steps are taken. It is important that mold surfaces be extremely smooth and either finished with hard chrome plate or made from a good quality tool steel, giving a similarly polished surface. The mold should close accurately, to avoid excessive flash.
Blisters in the finished part often can be traced to excessive porosity in the molding blank. It is suggested that accurate blanks are preformed in a vacuum extruder and that, ideally, a vacuum facility should be available on the mold. Good natural venting of non-vacuum molds is essential, as the normal compound viscosity is unhelpful in squeezing out air.
Some compounds will mold readily in clean, untreated mold surfaces. However, the majority requires that a particular procedure be followed for best results. It is essential that the mold be cleaned of residues from previous compounds or polymers. The mold should then be heated to the desired curing temperature and spray-coated with a dry bake polymerizing mold release. After baking for the prescribed time at cure temperature, the application and bake should be repeated as directed.
The baked-on lubricant should be sufficient to last several hours and will prevent the VamacŪ from effectively wetting the metal surface, which can cause sticking or the trapping of air blisters. If further improvement is necessary, then a secondary release agent can be applied sparingly between mold cycles.
Compression molding is comparable to that for other low viscosity compounds. In some cases, bumping of the press can help remove trapped air from the cavity; however, rapid or over-bumping can aggravate blister formation. An exact procedure may need to be developed for an individual mold.
Compounds of VamacŪ are especially suited to injection molding, because of their low viscosities. The scorch time of VamacŪ compounds is relatively short (10-15 min.) at 120°C (248°F), but increases considerably at 100°C (212°F). It is suggested, therefore, that nozzle temperatures be controlled to 95 to 100°C (203 to 212°F). Barrel temperatures should be cool at 50 to 75°C (122 to 167°), while mold temperatures are best kept in the range of 180 to 195°C (356 to 383°F). A cycle time of 1.5 to 3 minutes is common, depending on part size.
Amine-cured molding compounds normally have a cure curve with a gradual slope. This factor, along with the typical need for oven post-curing to obtain optimum properties (especially compression set), enables partly-cured vulcanizates to be demolded satisfactorily, with a consequent saving in press time.
Compounds of VamacŪ exhibit relatively high shrinkage. Typical linear values for good quality black compounds are shown in Table 1:
| |
| |
Unplasticized, Shore A 65-75 |
2.5 |
3.5 |
| |
Plasticized(1), Shore A 65-75 |
2.5 |
3.7 |
|
VamacŪ polymer contains very low levels of residual methyl acrylate. This can be released during the molding operation and will be apparent to personnel at levels well below statutory TLV limits. It is recommended to remove all elastomer molding fumes, regardless of whether or not they are noticeable. While some molding shops will have sufficient natural or general forced extraction, it is much more effective to ensure that a positive local extraction is available in the immediate vicinity of the mold opening operation.
|
