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PVC Injection Molding Process | Vinyl Injection Molding Process

Injection Molding Processing Guides

Processing Information

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.

Click on the panels below for helpful processing information.

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.

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.

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



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

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.

pvc injection molding