Good low pressure molding mold design comes down to three things: gates that let molten polyamide flow evenly around the insert without hitting sensitive components, shallow vents at the last-to-fill areas, and fixtures that hold the PCB and cable firmly during injection. Low pressure molding (LPM) encapsulates electronics at far lower pressure than conventional injection molding, so tools can be lighter and simpler. But because the insert is a finished electronic assembly, a design mistake quickly turns into a damaged board, exposed components or a failed sealing test. This guide covers the decisions to make when designing an LPM mold and the basic rules for gates, venting and insert fixing.
Key takeaways
- Place gates at thick sections and steer flow away from chips, solder joints and sensor elements.
- Vent the last-to-fill areas, but keep vents shallow because polyamide melts are thin enough to flash.
- Stable insert fixing and a correct cable shut-off are what keep parts free of exposure, flash and leaks.
How LPM molds differ from injection molds
Conventional injection molding pushes resin in at hundreds of bar or more. Low pressure molding typically works in the range of a few bar to a few tens of bar, because polyamide hot melts have a low melt viscosity and can fill narrow gaps without high force. Lower pressure means many LPM molds are machined from aluminum, which shortens lead time and reduces tooling cost compared with hardened steel.
The trade-off is that the insert is the product itself: a populated board, a sensor or a wire harness. If the mold does not locate the insert accurately, the board can tilt or the cable can slide during injection, and the melt will pile up on one side. Melt temperatures are generally around 180-240°C, so the flow path should also avoid letting hot material dwell next to heat-sensitive components.
In short, an LPM mold is less about withstanding pressure and more about locating the insert, managing flow and air, and removing heat quickly. For process settings, see our guide to low pressure molding process parameters.
A step-by-step low pressure molding mold design sequence
The design work starts with the molded shape and ends with trial shots. Working through the steps below helps avoid missed details.
Low pressure molding mold design sequence
- Review the insert
Mark board size, heat-sensitive parts, connectors and any areas that must stay exposed. - Set shape and wall thickness
Keep walls as even as possible and round off sharp corners. - Parting line and insert fixing
Decide where the board and cable are supported so injection pressure cannot shift them. - Gate location and size
Fill from thick to thin sections and steer melt away from sensitive components. - Vents and cooling
Vent the last-to-fill areas and keep mold temperature even with cooling channels. - Trial shots and tuning
Adjust gates and vents based on short shots, voids and flash.
A typical sequence for illustration; details depend on the part and equipment.
Wall thickness should be as uniform as possible. Polyamide shrinks as it cools, so large thickness changes tend to cause sink marks or internal voids in thick areas, while thin areas freeze first and can block the flow. Walls of roughly 1-5 mm are generally easy to fill; very thin sections are worth checking with a fill trial first. These figures are examples only and depend on grade and geometry.
Rounded corners help the melt flow smoothly and reduce stress concentration. A small draft in the opening direction lowers the force on the board when the part is ejected.
Choosing gate location and size
The gate is where the melt enters the cavity. A few rules help when placing it:
- Fill from thick to thin. Injecting into the thickest section keeps the melt fluid until the far end of the cavity.
- Do not aim at sensitive parts. Turn the flow so it does not hit small chip components, solder joints or sensor elements head-on.
- Keep flow lengths short and balanced. Where two flow fronts meet, a weld line forms; make sure that happens away from critical sealing surfaces.
- Think about finishing. Choose a spot where the gate mark will not sit on a sealing face or a cosmetic surface.
A gate that is too small causes high pressure loss and short shots. A gate that is too large keeps the gate area molten longer and stretches the cycle. In practice, teams adjust gate position and size after trial shots, based on where short shots and weld lines appear.
Venting: letting the air out
When the mold closes, the air inside needs somewhere to go. Missing or undersized vents trap air and lead to voids, short shots and burn marks. Vents belong at the last areas to fill, at points where flow fronts meet and at the ends of deep pockets.
Because LPM polyamides are thin when molten, deep vents simply let material escape as flash. Vents are therefore generally kept to a few tens of microns (for example 0.02-0.05 mm) and connected to deeper relief channels that lead outside the cavity. The right depth depends on the grade viscosity and processing temperature and should be confirmed in trials.
| Item | Design point | Typical defect if missed |
|---|---|---|
| Gate location | Thick section, away from sensitive parts | Short shot, damaged parts |
| Gate size | Balance flow and freeze time | Short shot, long cycle |
| Vent location | Last-to-fill areas, weld lines | Voids, burn marks |
| Vent depth | Shallow, linked to relief channel | Flash, trapped air |
| Cooling | Even mold temperature | Sink marks, warpage |
Cooling deserves the same attention. If mold temperature drifts, fill behavior and shrinkage drift with it. Cooling channels placed close to and evenly around the cavity shorten the cycle and reduce variation between shots.
Fixing the insert and sealing cable exits
LPM quality depends heavily on how accurately the insert is held. Boards are usually located with pins, support blocks or the connector housing, with supports placed opposite the direction of injection pressure so the board cannot bend or shift. Wherever a support touches the board, that spot will be exposed after molding, so plan those locations in advance.
Where a cable or wire harness leaves the mold, melt can escape. A common approach is a groove slightly smaller than the cable diameter that grips the jacket lightly. If cable diameter tolerance is wide, the result is either flash or a crushed jacket. Jacket material, such as PVC or PA, also affects how well the polyamide bonds, so check it for any part that must be watertight. Our sensor overmolding guide shows how these points apply to automotive sensors.
During trials, cut sample parts open to check internal voids, insert position and contact between board and polyamide. A part that looks fine from the outside can still fail water ingress or thermal cycling tests if it hides internal voids.
LPM polyamides from SILVERSTAR CHEMICAL
SILVERSTAR CHEMICAL Co., Ltd. manufactures solvent-free ERAMID polyamides for low pressure molding of PCBs, sensors, connectors and wire harnesses. The range includes UL 94 V-0 flame-retardant grades and grades with thermal stability up to 200°C, and LPM offers shorter cycles than potting. Because we handle raw material selection, formulation and quality inspection in-house, we can tailor viscosity and formulation to your mold and equipment. See our LPM polyamide solutions or the SILVERSTAR CHEMICAL company FAQ for more.
Frequently Asked Questions
Q. What should be decided first in low pressure molding mold design?
Start with the insert: its size, the location of heat-sensitive parts and any areas that must stay exposed. Then set the molded shape and wall thickness, followed by insert fixing, gates and vents.
Q. Why are low pressure molding molds often made of aluminum?
LPM runs at much lower pressure than conventional injection molding, so the strength of hardened steel is often not needed. Aluminum machines quickly and conducts heat well, which helps cooling.
Q. Can vents be made deeper to remove air faster?
Molten LPM polyamides are thin enough to escape through deep vents as flash. Vents are generally kept shallow, connected to deeper relief channels, and their depth is confirmed in trials.
Q. How do I stop flash where the cable leaves the mold?
Check that the groove matches the cable diameter and whether the cable tolerance is wide. Adjusting the groove size or lowering processing temperature and pressure often solves it.
Q. Can SILVERSTAR CHEMICAL suggest a grade for my mold?
Yes. Send a drawing or photo of the part, your equipment, processing temperature and performance requirements, and we will suggest a grade and, if needed, tailor viscosity and formulation.
If you would like to review material conditions while your mold is still on the drawing board, send us a drawing or photo of the part. Contact our technical team · +82-55-337-8841 · snschem@snschem.kr
