In the previous posting of this series we finished with the statement "in the next posting we will discuss molding with the PIMT". Once again, I would like to remind you that Stratasys PIMTs are not production tools, however they provide a clear advantage, being both cheaper and faster when a limited quantity of production parts is needed".
Injection molding with a PolyJet mold requires adjustments to the molding process. It is important to start with conservative values to avoid damaging the tool. Using test shots, slowly adjust the process parameters until desired parts are produced. The following information provides guidelines as a starting point for process adjustment.
Mold protruding from tooling pocket and mold approximately .008" above pocket face
Use the standard clamping force (injection pressure × total projected area or total surface area × manufacturer's suggested clamping factor). You can adjust this value with at least a 10% safety factor.
Test the clamping force by slowly closing the mold and observing if the PolyJet mold is compressing as designed. Use a two-stage process: rapid travel until just before contact followed by a slow, gentle speed to fully close the mold.
Before each injection molding cycle, liberally apply a non-silicone mold release agent to the mold cavity.
The goal of the trial shots is to keep temperatures, pressures and flashing to a minimum since they can reduce the tool life. Also, because PolyJet molds are poor thermal conductors, molded parts will require additional time to solidify. The trial shot process will identify the appropriate amount of time for cooling.
To begin, use the following parameters:
• Injection molding time limit: 20 seconds
• Pack & hold phase: 0 psi and 0 seconds
• Shot size: 75% of estimated part volume
• Barrel temperatures: Low end of that recommended for the resin.
• Injection speed: Low end of that recommended for the resin (10% to
20% of the machine's maximum screw speed).
• Cooling cycle: Depends on the thermoplastic material being molded.
For materials with slower solidification rates, increase cooling cycle.
Allow ample time between shots to allow the mold to cool to a target temperature of 50°C (120°F). Accelerate cooling by blowing compressed air onto the core and cavity. With each subsequent trial shot, adjust the process parameters until part quality is satisfactory.
A progression of test parts (good shots on left) as injection molding parameters are dialed in.
Adjust shot size first, with a target of 90% of the cavity volume. Next, adjust the packing pressure to 30% – 50% of the injection pressure. Review the results and adjust as necessary. To avoid sink marks begin to increase the holding time. For fine tuning, make adjustments to the barrel temperature and injection speed. Avoid using elevated temperature and pressure to resolve molding issues, because these settings can decrease the number of injection molded parts the PolyJet mold can produce. Increase the cooling cycle duration to achieve full solidification, however, do not allow the tool to cool too much as this will increase part shrinkage, and potentially cause the part to grip the core. If the grip is too strong, the core could be damaged when the part is ejected. If there is excessive flash after dialing-in the parameters, disassemble the tool and add additional shim stock between the PolyJet inserts and mold base.
Example of shrinkage due to over cooling.
Cooling fixture for blowing air onto the PolyJet mold.
After dialing-in the process, mold the desired number of parts. Because the material used in the PolyJet mold will act like an insulator, the temperature of the mold will increase to the point that parts will not solidify. To maintain a target temperature of 50°C (120°F), keep the mold open after part extraction and blow compressed air onto the core and cavity. This may be done manually or with an automated cooling fixture.
Injection molded polyethylene part -core side view
Injection molded polyethylene part – cavity side view
I would like to take a moment to say, thank you, for following this brief introductory series "Stratasys Polyjet 3D Printed Injection Molds."
CATI will be conducting injection molding workshops at the Buffalo Grove, IL 8/18/15, Brookfield, WI 8/20/15, Indianapolis, IN 8/25/15, and St. Louis, MO 8/27/15 office locations. Registration required
Resources: Stratasys Technical Application Guide: Polyjet for Injection Molding
Sr. Application Engineer