Vamac® dipolymer is of special interest in molding applications because of the relatively good compression set values that are attainable - without the need for post-curing. However, for injection molding, the major criterion is a short cycle time and this requirement will clearly impact upon compression set resistance. The currently available Vamac® dipolymer, Vamac® DP, can be injection molded successfully within certain limitations and these are discussed in the following paragraphs.

The most common processing fault likely to be encountered is that of blistering. The main cause appears to be a level of peroxide volatiles beyond the compound tolerance and is best controlled by limiting the level of peroxide to around four parts (for DiCup 40 C). There are also benefits in increased demolding elongation from this approach. Other adverse influences are a necessarily high temperature for economic molding cycles and the use of low levels of carbon black filler. Low hardness compounds based upon Vamac® dipolymer will present the most difficult molding problems. Similarly, typical plasticizers for Vamac® retard the cure, as well as reducing modulus and hardness. Plasticizers are best avoided, particularly the ether/esters; trimellitate types should be evaluated, if essential.

The need to rapidly reach a high state of cure over the temperature range 170-190°C directs attention to combinations of dicumyl peroxide and HVA-2 as being the most suitable cure system. A medium reinforcing carbon black, like FEF N550, works well but whichever black or combination of blacks is selected, a minimum loading of 40 to 60 parts is advisable for the production of good parts. The resultant hardness will be approximately 70°A ASTM (65 DIN). Vamac® dipolymer contains sufficient alkyl phosphate release agent for good milling performance and it is normally only necessary to incorporate stearic acid and octadecylamine (Armeen 18D). Natural mold release of Vamac® dipolymer is much better than peroxide-cured terpolymer, although not quite as good as amine-cured terpolymer. Release can be enhanced by the addition of other additives not reactive with peroxides. A simple hydrocarbon oil has been used for the model formulation detailed in the table. External mould release sprays, suitable for peroxide curing, are effective with Vamac® dipolymer.

Low levels of silica and the addition of fibrilated Teflon® micropowder have been tried to reduce blistering in soft black formulations. No benefits were noted, although compound viscosity does increase. Similarly, calcium oxide is ineffective, indicating that included volatiles are other than water (many peroxides evolve ketone and alcohol byproducts).

The starting point formulation detailed on the attached table has been run on a medium sized molding machine at 190°C, using a moderately thick part. Lower temperatures and thickness should increase processing latitude. Vamac® DP has been reported to injection mold satisfactorily

Candidate Vamac® Dipolymer Compound for Injection Molding