Blog Stratasys Polyjet 3D Printed Injection Molds….a series, part 2
In this session, we will discuss printed mold tool material, types of molds, molded material and expected number of parts, along with hints to design your printed mold. Once again, I want to state 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".
Material:
The Polyjet Injection Mold Tooling (PIMT) preferred material of choice is the Stratasys Digital ABS and Digital ABS2 material. The Digital ABS material is a strong and heat resistant polymer. It is recommended to use the PIMT as an insert installed on a metal mold base including the sprue, and the first part of the main runner. To assure a good contact between insert sides, they should protrude from the mold base by about 0.1- 0.2 mm (.003"-.004")
The two charts below are great indicators comparing PIMT tools to other bridge and production tooling in regard to molding material and number of shots expected.
Stand-Alone Mold:
By now I'm sure you're asking yourself if a stand-alone mold can be used instead of using the tool as an insert. The answer is yes. Full stand-alone molds can be printed however it is recommended that a sprue bushing is made as a metal insert. Another tip would be to offset the part and use a sprue gate and runner system designed and built into the tool. Shooting directly into the center of the part will work but tool life is compromised.
Tool Printing Direction:
PIMTs show some level of surface printing lines. These lines may affect the polymer melt flow during the injection molding process resulting in flow unbalance, short shots, hesitation and flow marks. It is highly recommended to print the tools in a manner that the printing lines are oriented in the main flow direction and not transverse to it. Being a rigid molding tool, PIMT is designed without significant undercuts by itself. This fact makes PIMTs great models to be printed in gloss mode. Better mechanical properties like surface quality and strength are additional reasons for printing PIMTs in gloss mode.
Radii and Features:
Stratasys 3D printing systems allow for the printing of very small radii and features. However, very small, thin-walled or too-tall features may compromise the mechanical integrity of the tool. Too small radii act like notches and can become a starting crack point that will reduce the tool life. It is recommended to use the biggest radii as possible and to avoid sharp corners.
Pressures and Temperatures:
High pressures or temperatures will reduce tool life. It is recommended to design the tool in a way that pressures and temperatures are minimized as much as possible, for example by increasing gate size and reducing flow lengths by using multiple gates. Injecting against the wall cavity is not recommended in the case of PIMTs. A high pressure developed on the wall cavity when injecting against the wall may reduce tool life. When injecting against the wall cannot be avoided, use generous radii in locations where flow changes direction.
Venting and Draft Angles:
Design parts with good venting and if possible design a gate/vent off the backside of the part. Higher draft angles are recommended to reduce stress on the tool during ejection. It is recommended to have draft angles of at least 1.5o. It is possible to use a draft angle of a lesser degree but remember you could be compromising tool life. However, the trade-off for low volume runs could be worth the risk.
In the next posting we will discuss molding with the PIMT.
Resources: Stratasys 3D Printed Injection Molding Tool ("PIMT") Guide
Derek Ellis
Sr. Application Engineer