Method and apparatus for producing polymer articles having different properties in different regions of the articles

The method of sequentially forming a plurality of shots of polymer which together comprise the charge to a mold cavity wherein each shot comprises a mixture of hardenable precursor constituent different in ratio or in kind from the mixture of a contiguous shot for producing a molded article having different properties in different regions, comprising the steps of

• forming the first shot by
• (a) establishing for each constituent a demand count rate representing the desired rate of flow of the constituent to a mixing head,
• (b) initially producing demand count rates starting at a low value and simultaneously and gradually increasing the demand count rates to the established rates for the several constituents,
• (c) producing for each constituent a measured count rate representing the actual rate of flow of the constituent, and
• (d) controlling the flow of each constituent to cause the measured count rate to substantially follow the demand count rate;
• forming each subsequent shot by establishing new desired count rates as required for the mixture of such shot and changing the demand count rate simultaneously and gradually to the new desired count rates for the several constituents, and
• terminating the final shot by simultaneously and gradually decreasing the demand count rates to zero to stop the flow of the constituents.

Apparatus for issuing sequential shots of polymers of varying constituent content for forming articles of varying properties comprising containers of constituents, a mixing head, a nozzle for discharging shots of polymer mixture from the mixing head, controllable pumps for supplying the constituents from the respective containers to the mixing head, and control means comprising

• feedback means responsive to the flow of each constituent producing a pulsed signal for each constituent having a count rate proportional to actual flow rate,
• a general purpose computer for receiving and storing input data specifying the constituent amounts for each shot of polymer, the computer being programmed to:
• (a) establish for the first shot of a sequence a demand value for each constituent representing the desired rate of flow of the constituent to the mixing head,
• (b) produce initial demand values representing low flow rates and gradually change the demand values from the initial values to the established values for the several constituents,
• (c) establish for each subsequent shot new desired values as required for the mixture of such shot and change the demand values simultaneously and gradually to the new desired demand values for the several constituents, and
• (d) terminate the final shot of the sequence by simultaneously and gradually changing the demand values to values representing low flow rates and then generating a flow stop signal;
• and
• a controller coupled to the feedback means and to the computer and responsive to the pulsed signal and signals corresponding to the demand values for each constituent to control each pump to deliver the desired amount of constituents to the mixing head so that the mixture discharged from the nozzle is established by the computer and the mixture ratio and/or flow rate changes when the various demand values change.