How to Pair Gas Vent Units with Spacers for MSTV/MSTVS/MSTVM
Key Takeaway: Each gas vent unit series (MSTV, MSTVS, MSTVM) requires its matching spacer series. Mixing series causes seating errors and gas bypass. Use the compatibility table below to verify your pairing before ordering.
Why Spacers Matter
Gas vent units are designed to a specific installed height. The spacer fills the gap between the vent unit base and the bottom of the mold pocket. Without the correct spacer, the vent unit either protrudes above the cavity surface (causing witness marks) or sits too deep (reducing vent effectiveness).
Compatibility Table
| Vent Unit Series | Compatible Spacer Series | Spacer OD Range | Height Increments |
|---|---|---|---|
| MSTV | MSTV-SP | Ø4–Ø12 mm | 0.5 mm steps |
| MSTVS | MSTVS-SP | Ø6–Ø16 mm | 0.5 mm steps |
| MSTVM | MSTVM-SP | Ø8–Ø20 mm | 1.0 mm steps |
Installation Sequence
- Step 1: Bore the mold pocket to the specified diameter and depth for the vent unit + spacer stack.
- Step 2: Insert the spacer first. It sits at the bottom of the pocket.
- Step 3: Place the vent unit on top of the spacer. Verify that the vent face is flush with (or 0.01–0.02 mm below) the cavity surface.
- Step 4: Secure with a set screw if the design uses lateral retention.
Common Mistakes
- Wrong spacer series: MSTV and MSTVS spacers look similar but have different ODs. Mixing causes gas bypass around the spacer perimeter.
- Missing spacer: Without a spacer, the vent unit sits too deep. Gas must travel around the insert body, reducing vent efficiency by 50–70%.
- Incorrect height: A spacer that is too tall pushes the vent face above the cavity surface, creating a witness mark. Always verify total stack height before final assembly.
For vent unit specifications and selection, see the Cavity Inserts for Gas Release product page. For general venting principles, review injection moulding references.
Ordering Checklist
| Item | What to Specify |
|---|---|
| Vent unit | Series (MSTV/MSTVS/MSTVM) + OD + installed height |
| Spacer | Matching series + OD + height (= pocket depth − vent height) |
| Pocket specification | Diameter tolerance: H7; depth tolerance: ±0.02 mm |
Spacer dimensions follow precision tool steel standards.