Injection Molding Processing Guides

Successful processing of flexible vinyl compounds by injection molding is dependent upon a wide range of variables such as mold design, screw geometry, machine size and shot size. For optimum processing, exact machine conditions will need to be determined by the processor. The following information will aid the processor in attaining the best possible results.

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Processing Guide

Successful processing of flexible PVC compounds by injection molding is dependent upon a wide range of variables such as, mold design, screw geometry, machine size and shot size. Exact machine conditions for optimum processing have to be determined by the processor. The following will help the processor in attaining optimum conditions.

Mold Temperature
Inlet water temperature should be between 70 and 100 degrees Fahrenheit. The ejector side should be 10 to 20 degrees colder to help in part removal.

Stock Temperature
This will be controlled by a combination of heater band settings, screw rpm, and screw back pressure. Consult technical data sheet for recommended temperature.

Heater Band Settings
For starting purposes, set barrel temperatures 20 degrees less than recommended stock temperature. Set nozzle temperature 10 degrees less than recommended stock temperature.

Screw RPM
A rotation speed of 35 to 50 rpm's should be satisfactory for most applications.

Screw Back Pressure
This will vary from machine to machine, but generally 60 to 100 PSIG is recommended.

Injection Speed
Slow to moderate for start up, then increase to moderate - fast for production run.

Injection Pressure
20% to 40% of the maximum available will yield the best consistency.

Holding Pressure
Usually 50% of the injection pressure. Use just enough to maintain a full part.

Equipment Guide

Machine Type	Reciprocating screw injection molding machine.
Barrel Capacity Shot Weight	A shot weight of 60% to 80% of the barrel capacity (rated in PVC) is recommended.
Clamp Capacity	2 to 3 tons per square inch of total part area including runners.
Screw	Multiplated with hard chrome
Screw Compression Ratio	1.5 to 1 or 2.0 to 1
Screw L/D	16 to 1 or 24 to 1
Screw Tip	Smearhead type recommended but check ring types also work.
Molds	414 or 420 type stainless steel
Vents	Locate near the last area of the cavity to be filled. Typical vent size is 0.20 to 0.5 in. wide by 0.001 in. deep.
Runners	Dependent on part size and weight. Recommend full round runners with diameters of 0.25 to 0.5 in.
Gates	Gates should have a large cross sectional area with no sharp corners or rough edges and be polished smooth.

Troubleshooting Guide

Defect	Possible Cause
Short Shots	Injection pressure too low Barrel temperatures too low Injection speed too slow Sprues, runners, gates too small Mold temperatures too low
Dull Streaks or Flow Lines	Stock temperature too low Mold temperature too low Runners too small Inadequate gate size Inadequate cold slug wells
Sink Marks	Injection pressure too low Cooling time too short Hold time too short Stock temperature too high Mold temperature too high
Flashing	Insufficient clamping pressure Injection pressure too high Injection speed too high Stock temperature too high Mold faces are not parallel and plane
Part Sticking in Cavity	Injection pressure too high Hold pressure too high Core side of mold too hot Hold time too long Rough surface on mold
Burned Areas in Part	Barrel temperatures too high Screw speed too fast Back pressure too high Gates too small Check temperature controllers

Mold Shrinkage Guide

Compound Type	Durometer Range
-	55 A to 79 A	80 A to 95 A	C & D Scale
Clear Compounds	0.019 - 0.024	0.015 - 0.018	0.010 - 0.014
Filled Compounds	0.017 - 0.022	0.013 - 0.016	0.010 - 0.012

Machine Clean-Up Procedure

Purging:
Purge the barrel until all of the material is out. You may use a commercial grade purge compound or general purpose polystyrene or ABS.

Mold:
Remove any residual plastic and clean the mold with a commercial metal polish or with a 2% solution of baking soda in water. Wipe clean, then add a coating of commercial oil or grease for storage.

For a complete listing of Sylvin's injection molding compounds, click here.

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