Injection mold design, an art in itself, requires years of experience and a profound understanding of the injection molding process. Although the design considerations for creating and using a PolyJet mold are fundamentally the same as traditionally crafted molds, there are some variations. Tool designers should consider the following changes when creating a PolyJet mold as opposed to a conventional steel mold design.
1. DESIGNING THE MOLD
• Increase draft angles as much as the part design allows. This will facilitate ejection and reduce stress on the tool as the part is ejected.
• Increase gate size to reduce shear stress.
• The gate should be located so that the melt entering the cavity will not impinge on small/thin features in the mold.
• Avoid using tunnel gates and point gates. Instead, use gates that reduce shear such as a sprue gate or edge gate.
2. PRINTING THE MOLD
To maximize the opportunities created by PolyJet 3D printing, the following guidelines
• Print in glossy mode to ensure smoothness.
• Orient the part in Objet Studio™ software so that the glossy surfaces are maximized.
• Orient the mold so that the flow of polymer is in the same direction as the print lines.
3. FINISHING THE MOLD
A key benefit of PolyJet molds is that they can be designed, built and used within hours.
Most will require little or no post-processing work, however further finishing may be
• The mold will be fitted to an ejection system. To ensure a tight fit between the ejector pins and the ejector pin holes, program the holes into the STL file but reduce their diameter by 0.2 – 0.3mm. Then, when the mold is cured, ream the holes to the exact final size.
• Inserts are being fitted onto a base.
• Extra smoothing of surfaces is needed.
Occasionally, light sanding of surfaces transverse to the mold opening is recommended.
For example. prior to using a mold with a tall core, some light smoothing can facilitate part
• Stand-alone molds — those that are not constrained to a base frame — can be mounted directly onto standard or steel machine back-plates using screws or doublesided tape.
• Figure 8 mold inserts are fitted onto a base mold using bolts. With any chosen mounting option, it is critical to avoid direct contact between the nozzle and the printed mold by using standard sprue bushing. An alternative option would be to center the mold’s runner with the sprue located on a regular steel plate.
5. INJECTION MOLDING PROCESS
When using the PolyJet mold for the first time, the best practice procedure is:
• Start with a short shot and a slow injection speed. The fill time can be high as the melt does not freeze off as it enters the mold. Increase shot size until the cavity is 90-95% full.
• In the holding process, use 50-80% of actual injection pressure and adjust the holding time as needed to avoid sink marks.
• Apply normal calculated clamping force value (injection pressure x projected part area) as initial value.
Information for this article can be found in the Precision Prototyping: The Role of 3D Printed Molds in the Injection Molding Industry Whitepaper published by Stratasys, Ltd. Download the full Whitepaper Here.
Computer Aided Technology, Inc.