Design for Freeform Injection Molding

Maximizing Printability

Designing your mold step by step

01 Adjust part shrinkage Part shrinkage is the dimensional change that occurs in a molded part as it cools off after injection. Shrinkage can vary a lot, and is mainly dependent on material type and temperature during injection.
02 Subtract part from mold Almost all 3D CAD systems have multiple tools that can assist users throughout the mold design process. These tools can be used to subtract the part from the mold insert.
03 Check build orientation Orientation of a part must reduce the presence of overhangs as much as possible. Z height print sequence on the Z-axis.
04 Select mold & frame size Choose the optimal mold size, based on part size and matching standard mold frame. The wall thickness between the cavity and outer surface should be at least 5mm.
05 Place inlet Inlet runner placement must secure that the feedstock material reaches all areas of the mold before hardening. The deepest cross-section is ideal to provide the best flow and minimize voids and sinking.
06 Place outlets Outlet runners should preferably be placed opposite of the inlet, and at or close to points at the end of fill. The outlets also capture any potential cold slug and make it possible for the operator to check if the mold has been properly filled.
07 Check flow Main considerations that influence the flow in a mold are. material flow rate, part volume, part design, material type and grade, and processing conditions.
08 Place gates Place gates where the runners connect to the part. Gates are a narrowing of the runners, which diminish the size of injection marks on the part surface, and which can help guide the feedstock material into the part cavity at the right angle and pressure.
09 Save mold as STL After designing the mold insert, it mujst be saved in the STL file format. The 3d printer software is then able to slice the model and create the program for the 3D printer.

Print orientation

When orienting the part in the printer, it is necessary to keep in mind that each individual layer is very thin. This is related to the concepts of overhangs and bridging. In this section, simple ways are shown which allow the printing of a hexagonal part.

Rotation A simple rotation of the part, can prevent large overhangs.
Splitting the mold Splitting the mold in two allows production of the part without printing a large overhang. Splitting the mold is only used to increase printability. Demolding will still be done through dissolving the mold resin, thus demolding considerations associated with conventional split molds, can be disregarded.
Support inside part If the part cannot be rotated or split, it may also be a solution to print pillars of support material which can be removed manually.
Vacuum relief tracks If a mold design involves a large enclosed cavity, it may act as a suction cup on the membrane in the printer vat during printing, deforming the membrane with each added layer. This will increase the membrane wear and may even lead to delamination of the part. Expanding outlet exits hole with a track is a good way to mitigate this.
What is an overhang A 3D printing overhang is any part of a print that extends outward, beyond the previous layer, without any direct support. As any layer in a 45 degree overhang is 50% supported by the layer beneath. Overhangs up to 45 degrees, and sometimes even more, can be printed without loss of quality. The potential side effects of tilting a print include altering the surface roughness and part tolerances.
Chamfer edges towards buildplane Adding a chamfer on the edges towards the buildplane makes it easier to remove the printed mold from the buildplane. Place a spatula in the groove created between the chamfered edge and the buildplane to separate the mold from the buildplane.

Split molds

Splitting molds into two or more pieces and assembling them with nuts and bolts has

several advantages.

It allows the designer to take advantage of the fact that the last layer printed always has a mirror finish.
It potentially solves problems with bridging or overhang during printing.
Cleaning of molds after printing becomes easier as leftover resin has fewer places to be trapped.
Inspection of molds for printing flaws such as closed vents or lost features becomes easier.
The strength of mold features increases as they are all available for the UV post-curing process.
The parting line can act as venting, just as in conventional injection molding.
Inspection of the molded result becomes easier and faster.
Some designs may even allow for a full manual demolding directly after molding, skipping the dissolving step entirely.

This guide is intended for 3D-printed tooling using Nexa3D's dissolvable resin, specifically. Thus, designing for demolding is not necessary. In cases where fast results are desired, you may choose to use manual demolding, as demolding through dissolving takes 24-72 hours. The printed molds differ from metal molds, as their surface is not smooth, but lightly rippled from the layer fusing. Therefore, manual release of a component from printed molds can be harder.

Grooves around split lines

Add grooves in split lines to ease separation. The groove makes it easier to place a spatula or similar separation tool in between the two mold sections.

Limiting dissolving

In cases where aspects of the component design may only be freed from the mold through dissolving, or certain areas are too fragile for manual demolding, a mixed method may be applied. The mold then must be designed so that sections of the mold can be manually removed. The less resin to dissolve, the quicker the dissolving.

Manual demolding

Complete manual demolding can sometimes be successful with softer plastics, provided the mold design allows for this. For feedstock materials which tolerate +80°C, part of the mold may be removed, followed by a heating of the part and remaining mold sections. This will make the mold more pliable and ease full manual demolding.

Drafts and angles

In a conventional mold drafts are used to ease demolding. In printed molds this does not always apply. As the angle of the draft is built up by layers, the drafts have staircasing, which depending on placement and angle can result in catching making the demolding more difficult.

Using slip

Using slip in printed resin molds can be helpful to release the component in some cases. However, some slip solutions may react with the resin or compromise the tolerances of the injected component. The slip can also get trapped like air, in small pockets, causing unfilled parts.

Testing your options

Splitting printed molds can present some challenges, as the rippled surface can cause material to get in the mold and overall increases the components adherence to the mold. Therefore, it is a good idea to print and fill 4-8 test molds, when making a new part, to safely determine the optimal demolding approach with regards to time, tolerances and quality.

Maximizing moldability

Debossed features If a part contains a debossed feature such as a blind hole, the strength of the printed material should be taken into consideration. At most a printed core can be expected to hold if the length does not exceed the core diameter.
Thin-walled hollow features When injecting molding thin-walled hollow features, best results are achieved with steel cores. Printed cores are applicable when they are well supported and have enough bulk to withstand the thermal impact of the injection molding process.
Minimizing core displacement When applying printed cores, the flow of the polymer melt should be carefully considered to prevent any dislocation of the core during the injection molding process. This is generally achieved by creating a uniform flow along the length of the core.
Supporting cores Printed as well as steel cores must be supported in both ends, ideally reaching from end to end of the mold.
Multi-mold assemblies Some cores may have holes or other features which must be precisely located in a mold. If the core is printed separately, a lock-and-key design will aid the operator assembling the mold parts. Using a size difference of 0.03 mm will give a tight fit. When designing a mold which consists of multiple splits and/or cores, it is advised to use lock-and-key design, to ensure precise assembly. This constitutes creating interfaces, which can only connect one way.
Multi-mold: keeping it all together Liberal application of pins and bolts will ensure that all mold parts stay where they should during injection molding. The most effective fixation agent however remains the closing pressure of the molding machine, so orienting the mold assembly to make best use of this is a good idea.
If in doubt, make an outlet Filling a cavity during injection molding can be likened to blowing up a balloon. To make sure that it reaches all corners without leaving air gaps, an outlet may be needed. It may also not be needed , but there is no harm in adding an extra outlet.
Printed mold sleeve By reducing the wall thickness of the oriented mold and letting it follow the contours of the part, it is possible to reduce the demolding time and the resin consumption.
Outer wall thickness While designing the mold, it is important to keep an eye on the wall thickness. The mold may rupture during injection molding if the walls are not strong enough. Generally speaking, the walls should be as thick as the cavity inside them.
Mold fit During injection molding, the printed mold is supported by the metal frame surrounding it. Design your mold with a 0.05mm gap to start. Over time you may need to adjust your gap size based on wear and tear of your frames. Note, too large a gap will cause your mold to rupture during injection molding.
Cylindric molds are easiest Your molds must be supported by a tightly fitting metal frame. Often, it will be possible to design the mold for both square and cylindrical frames. In this case, it is a good idea to choose a cylindrical mold, as it makes for a simpler fit.
Weld lines A weld line occurs where two melts fronts reach each other at a temperature where they do not merge completely. On the resulting part a crack or a weak line will be visible. Avoid this by moving the outlet or adding more inlet channels.
Air pockets The air inside the printed mold must have vents to escape during the molding action. Lack of vents may cause the part to have rounded corners or lack edge features. Solve this by adding vents.
A fixture for milling After injection molding, the part is fully encased and supported by the printed mold. This allows for easy machining or grinding to reach crucial surfaces finishes or dimensions.