Vacuum Impregnation for Aluminum Castings: RFQ Policy

# Vacuum Impregnation Policy for Pressure-Tight Aluminum Casting RFQs

Vacuum impregnation can be useful in pressure-tight aluminum casting programs, but it can also create confusion if the buyer does not define the rule before quotation.

One supplier may quote a casting as accepted only if it passes leak test without impregnation. Another may assume impregnation is allowed as a normal production step. A third may treat impregnation as repair only after first leak-test failure. Those three prices are not the same quality plan.

This guide is for OEM buyers, SQE teams, quality managers, and procurement engineers preparing RFQs for pump housings, hydraulic valve bodies, compressor housings, EV drive housings, coolant housings, and other pressure-sensitive aluminum castings.

For related Bohua paths, review the leak-tight aluminum casting hub, pump housing leak-test RFQ checklist, hydraulic valve body leak-test RFQ checklist, and formal RFQ form.

What vacuum impregnation does and does not decide

Vacuum impregnation is generally used to seal interconnected porosity leak paths in castings. It should not be treated as a substitute for a stable casting process, correct gating, correct machining allowance, or a defined leak-test specification.

Foundry Management & Technology explains the practical distinction: the question is not simply the size of an individual pore, but whether there is a connected leak path that can be sealed. Springer research on pressure-tight die castings also highlights that interconnected porosity can create leakage paths, especially after machining opens cavity systems.

For RFQ work, the buyer should define policy before supplier comparison.

Four policy choices buyers can use

Use one of these four options in the RFQ.

Option 1: Impregnation not allowed

Use this when the part is safety-critical, fatigue-critical, structurally sensitive, or the buyer's internal standard does not permit porosity sealing.

RFQ language:

> Vacuum impregnation is not allowed for production acceptance unless buyer gives written deviation approval. Supplier must quote casting process, machining, and inspection plan based on passing leak test without impregnation.

Ask the supplier to define:

• •leak-test stage
• •sampling or 100% test requirement
• •porosity prevention approach
• •X-ray, CT, sectioning, or first-sample evidence
• •scrap disposition for leak failures

Option 2: Impregnation allowed as a controlled production process

Use this when the part is pressure-sensitive but the buyer accepts impregnation as part of the approved manufacturing route.

RFQ language:

> Vacuum impregnation is allowed only as an approved production process. Supplier must identify process stage, resin/sealant family, record format, retest method, traceability, and whether the operation is applied to all parts or only selected parts.

Ask the supplier to quote:

• •pre-impregnation leak test if required
• •impregnation process stage
• •post-impregnation cleaning
• •post-impregnation leak test
• •record retention
• •marking or traceability method

Option 3: Impregnation allowed only as repair

Use this when the buyer wants first-pass leak performance monitored but may allow limited salvage under controlled conditions.

RFQ language:

> Vacuum impregnation is repair, not normal acceptance. Supplier must report first-pass leak-test results before impregnation, identify repaired quantity, retest repaired parts, and ship repaired parts only with buyer approval.

This policy supports quality visibility. It prevents the buyer from receiving parts that technically pass after sealing while hiding a process shift in casting quality.

Option 4: Policy pending buyer approval

Use this when the buyer is still evaluating risk and needs supplier input.

RFQ language:

> Vacuum impregnation policy is pending buyer approval. Supplier must quote assumptions separately: no impregnation, impregnation as controlled process, and impregnation as repair after leak-test failure. State unit-price, lead-time, record, and yield assumptions for each route.

This is useful before design freeze or before supplier nomination.

Define leak-test before defining impregnation

A vacuum impregnation policy is incomplete without a leak-test policy.

State:

• •test medium
• •test pressure
• •hold time
• •allowed leak rate or pressure-drop criterion
• •temperature condition if relevant
• •whether test is before machining, after machining, after impregnation, after coating, or after assembly
• •sampling plan or 100% test requirement
• •record format and traceability

If the buyer does not define leak criteria, the supplier cannot define whether impregnation is acceptable.

Decide what must be recorded

For a pressure-tight RFQ, ask for record visibility. The record set may include:

• •first-pass leak-test result before impregnation
• •part serial number, lot number, or batch ID
• •impregnation date and process batch
• •sealant/resin family or approved process reference
• •post-impregnation leak-test result
• •cleaning and drying confirmation if required
• •quantity impregnated per batch
• •buyer deviation approval if impregnation is repair

The goal is not paperwork for its own sake. The goal is to keep supplier quotes comparable and to prevent hidden quality assumptions.

Which parts need the strictest policy?

Use a stricter policy for:

• •hydraulic valve bodies with port-to-port isolation risk
• •pump housings with machined sealing faces
• •compressor housings with pressure or vibration risk
• •EV motor housings or inverter housings with coolant paths
• •safety-critical structural castings
• •parts where machining can open subsurface porosity
• •parts with customer-specific quality documentation requirements

Use a more flexible policy for:

• •non-pressure covers
• •low-risk brackets
• •cosmetic or nonsealed housings
• •prototype parts where the buyer explicitly wants process-learning data

Supplier questions worth asking

Ask these before comparing price:

• •Is impregnation included in the quoted process?
• •Is it applied to all parts or only leak-test failures?
• •Is first-pass leak rate tracked before impregnation?
• •What retest is performed after impregnation?
• •What happens to parts that still fail after impregnation?
• •Is impregnation allowed by the buyer's specification?
• •Does coating, plating, or cleaning happen before or after impregnation?
• •Can the supplier separate no-impregnation price from impregnation-allowed price?
• •Which document proves the final part met the leak requirement?

RFQ checklist for impregnation policy

Send these inputs with the drawing package:

• •part function and pressure-sensitive zones
• •leak-test method, pressure, hold time, and acceptance criterion
• •test stage and sampling plan
• •impregnation policy: not allowed, allowed process, repair only, or pending
• •record requirements before and after impregnation
• •traceability field required by buyer
• •coating, machining, and cleaning sequence
• •approval rule for repaired parts
• •first-sample evidence requirement
• •production-lot evidence requirement

Example RFQ note

Use this copy in a supplier RFQ:

> This casting has pressure-tight requirements. Please quote leak-test scope separately from casting and machining. Vacuum impregnation policy is repair-only unless approved otherwise. Report first-pass leak-test result before impregnation, repaired quantity, post-impregnation retest result, and record format. If you recommend impregnation as a controlled production process, quote that assumption separately and state process stage, cleaning, retest, and traceability method.

When this RFQ should move to a formal quote

Move to formal RFQ when the buyer can define leak-test criterion, pressure boundary, machining stage, and the impregnation policy. If those points are not ready, ask suppliers to quote assumptions separately.

For drawing-ready pressure-tight parts, use Bohua's formal RFQ form. Attach PDF and STEP files, mark sealed cavities and machined sealing faces, and state whether impregnation is not allowed, allowed, repair-only, or pending buyer approval.

Sources for RFQ planning

• •Springer: Modelling the Impregnation of a Pressure-Tight Casting
• •Foundry Management & Technology: Can Vacuum Impregnation Seal This?
• •Anoplate vacuum impregnation overview
• •Ultraseal casting impregnation FAQ