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newSchoolWorks.com › VoTechWorks › Manufacturing & Marine › Fiberglass / Composites (boatbuilding) › Capstone Project: Integrated Composite Boat Component Build › Lesson 2
MFG-COMP18-07 · Lesson 2 · 48 clock hours · lab

Mold Preparation, Layup, and Infusion Fabrication

Objective. Following the approved build plan, the student will prepare a mold, execute the laminate using at least two processes (wet hand layup and vacuum bag consolidation or resin infusion), and produce a cured part with no dry spots, no bridging at corners, and a fiber-to-resin ratio within the range specified in the build plan.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed engineer or certified journey-level tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Mold Preparation, Layup, and Infusion Fabrication

This lab session is the heart of your capstone project. You will combine skills from across the program—surface preparation, hand layup, vacuum consolidation or resin infusion, and quality documentation—into a single, inspected part. Every step follows the sequence a professional shop uses, and every hold point in your project binder has to be signed off before you move forward.

Mold Preparation

A mold that is not properly prepared will either stick to the part or produce surface defects that cannot be corrected after demolding. Begin by cleaning the mold surface with a clean, lint-free cloth and an appropriate solvent recommended by your release agent manufacturer. Inspect every square inch for chips, scratches, or porosity. Any defect in the mold telegraphs directly to the finished part surface.

Apply mold release wax or semi-permanent release agent (SPRA) strictly according to the manufacturer's technical data sheet (TDS). Wax-based releases require multiple coats with full cure time between each; SPRAs require a specified number of coats and a dwell period before they are wiped. Do not guess—follow the TDS. Insufficient release is one of the most common causes of parts sticking in student shops.

Gelcoat is applied by spray or brush in at least two cross-directed passes. Cross-directing means the second pass runs perpendicular to the first, ensuring even coverage and eliminating holidays (thin spots). Verify wet-film thickness with a wet-film gauge immediately after each pass; the TDS specifies the target range, typically in the range of 15–20 mils for sprayed gelcoat, but always defer to your product's TDS. Photograph every step and complete hold-point sign-offs in your project binder before proceeding.

Cutting and Organizing Reinforcement

Cut all reinforcement fabric before mixing any resin. Use your approved fiber-orientation schedule to label each ply with its layer number and orientation using painter's tape flags. Keeping plies organized prevents layup errors that cause structural weak spots. Stack plies in reverse order so the first ply to go into the mold is on top of your stack.

Hand Layup

Once gelcoat has reached a firm, tack-free state (confirm with the TDS—do not rely on guesswork), mix catalyzed resin according to the batch size and ambient temperature guidance in the resin TDS. MEKP catalyst percentage affects both pot life and exotherm; follow the manufacturer's recommended range. Higher shop temperatures shorten pot life significantly.

Apply resin to the mold or first ply, lay the reinforcement, then wet it out thoroughly with a chip brush using stippling (vertical tapping) strokes rather than painting strokes, which can shift fibers. Follow immediately with a consolidation roller to remove entrapped air and ensure full wet-out. Every ply must be fully saturated with no dry white areas and no air pockets before the next ply goes down. Work within the resin's pot life—discard any batch that begins to gel in the cup.

Vacuum Bag and Infusion

After the wet layup plies have reached a firm tack (green cure), apply peel ply over the entire laminate surface. Peel ply is a tightly woven release fabric that leaves a bondable surface texture and is removed after cure. If you are infusing additional plies, place flow media (distribution mesh) on top of the peel ply to help resin travel across the part. Dry reinforcement plies for infusion go on next.

Drape bag film over the layup and seal all edges with tacky tape, pressing firmly to eliminate leaks. Connect the vacuum line to a calibrated vacuum gauge and the resin inlet line. Draw the bag down to the vacuum level specified in your build plan—typically 25–29 inches of mercury for most infusion setups—and hold it for several minutes before opening the resin inlet. Inspect the bag for leaks by watching the gauge; a dropping gauge reading indicates a leak that must be found and sealed. Monitor resin flow until full wet-out is visible across the entire part.

Cure, Demolding, and Trimming

Allow the part to cure fully at shop ambient conditions or perform a post-cure per the resin TDS. Do not rush demolding; a part removed too early can deform permanently. Use wooden wedges or plastic shims to release the part—never metal tools that could gouge the mold.

Trim edges with a cutoff wheel or die grinder. Composite dust contains glass fibers and, with polyester or vinyl ester resin, residual styrene. Wear a half-face respirator fitted with organic-vapor/P100 combination cartridges, safety glasses, hearing protection, nitrile gloves, and a Tyvek suit. Work in a ventilated area. Dust generated during trimming is a serious inhalation hazard.

Tap Testing and Quality Documentation

Tap-test the entire cured surface by lightly tapping with a coin in a grid pattern. A solid, consistent ring indicates good laminate consolidation. A dull thud or hollow sound indicates delamination or a void. Mark suspect areas with painter's tape and record them on the part outline sketch in your QC checklist. Any dry spot larger than one square inch or any confirmed delamination must be remediated or documented as a repair task before you proceed to finishing.

Key terms10

Wet-film gaugeA small comb-type tool with teeth of graduated heights; pressed into a freshly applied coating to measure its thickness in mils before it cures.
Pot lifeThe usable working time of a catalyzed resin batch after mixing, before viscosity increases make application impractical; shortened by higher temperatures and larger batch volumes.
Semi-permanent release agent (SPRA)A chemically bonding release coating applied to a mold that provides multiple releases per application, as opposed to wax-based releases that must be reapplied more frequently.
Peel plyA tightly woven, resin-compatible fabric placed as the last laminate layer; peeled off after cure to leave a clean, textured surface ready for secondary bonding or painting.
Flow mediaAn open-mesh distribution material placed between peel ply and bag film in a resin infusion setup to allow resin to travel laterally and wet out the reinforcement plies below.
Tap testA non-destructive inspection method using a coin or small hammer tapped in a grid pattern across a cured laminate; a hollow or dull sound indicates voids or delamination beneath the surface.
Dry spotAn area of reinforcement fabric that has not been fully saturated with resin during layup or infusion, resulting in a structurally weak zone visible as a whitish, opaque area in a cured laminate.
BridgingA defect in which reinforcement fabric spans across a concave corner or detail rather than conforming to it, creating a void between the laminate and mold surface.
Cross-directed passesA gelcoat or paint application technique in which successive coats are applied perpendicular to each other to ensure uniform coverage and eliminate thin spots (holidays).
MEKP (methyl ethyl ketone peroxide)The organic peroxide catalyst used to initiate the cure of polyester and vinyl ester resins; a corrosive liquid that requires careful handling and must never be mixed with accelerators directly.

Worked examples4

1. Your build plan specifies a gelcoat wet-film thickness of 18 mils. After the first pass you measure readings of 10, 9, and 11 mils at three locations. How many additional mils do you need to add, on average, to reach the target?
  1. Calculate the average measured thickness: (10 + 9 + 11) ÷ 3 = 10 mils.
  2. Subtract the average from the target: 18 − 10 = 8 mils still needed.
  3. Plan the second cross-directed pass to add approximately 8 mils uniformly, then re-check with the wet-film gauge immediately after application.
Answer: You need to add approximately 8 mils of gelcoat in the second pass to reach the 18-mil target on average.
2. Your resin TDS calls for 1.5% MEKP by weight. You need to mix 800 grams of resin. How many grams of MEKP should you add?
  1. Convert the percentage to a decimal: 1.5% = 0.015.
  2. Multiply resin mass by the decimal: 800 g × 0.015 = 12 g.
  3. Measure 12 grams of MEKP on a calibrated scale and add it to the resin, mixing thoroughly for the time specified in the TDS.
Answer: Add 12 grams of MEKP to 800 grams of resin.
3. Your build plan specifies a cured laminate thickness of 6.0 mm with a tolerance of ±10%. After demolding you measure three locations and get 5.5 mm, 6.3 mm, and 6.6 mm. Which measurements, if any, are out of tolerance?
  1. Calculate the tolerance band: 10% of 6.0 mm = 0.6 mm.
  2. Determine the acceptable range: 6.0 − 0.6 = 5.4 mm (lower limit); 6.0 + 0.6 = 6.6 mm (upper limit). Acceptable range: 5.4 mm to 6.6 mm.
  3. Check each reading: 5.5 mm — within range (5.4–6.6). 6.3 mm — within range. 6.6 mm — exactly at the upper limit, therefore within tolerance.
  4. All three measurements fall within the acceptable range of 5.4 mm to 6.6 mm.
Answer: All three measurements (5.5 mm, 6.3 mm, and 6.6 mm) are within the ±10% tolerance band of 5.4 mm to 6.6 mm. No remediation is required for thickness at these points.
4. You are checking bag integrity before opening the resin line. The gauge reads 27 inches of mercury (inHg). You close the resin and vacuum lines and watch for two minutes. The gauge drops to 24 inHg. Is this acceptable, and what should you do?
  1. A drop of 3 inHg in two minutes indicates a leak in the bag or fittings; a properly sealed bag should hold vacuum with little to no drop over a similar period.
  2. Do not open the resin line—introducing resin into a leaking bag will cause air ingestion, voids, and dry spots.
  3. Locate the leak by running a hand slowly along all seams and fittings to feel for airflow, or apply a small amount of soapy water to seams and watch for bubbles.
  4. Re-seal or re-tape the leak, draw the bag back to the target vacuum, and repeat the hold test until the gauge is stable before proceeding.
Answer: The 3 inHg drop indicates an unacceptable leak. Find and seal the leak, restore target vacuum, confirm the bag holds steady, and only then open the resin inlet.

The math in this lesson2

Learners compute MEKP grams for 800 g of resin and a ±10 percent thickness tolerance band.
7.RP.3S-ID.2
Brush up in the math halls at newSchoolWorks.com:
In class: Students prepare the mold surface over two class sessions: clean and inspect for defects, apply mold release wax or semi-permanent release agent per the manufacturer's technical data sheet, and spray or brush gelcoat in two cross-directed passes to the target wet-film thickness verified with a wet-film gauge; the instructor and student record a photographic log entry at each step and complete the first three quality hold-point sign-offs in the project binder. · Students cut reinforcement fabric to pattern according to the approved fiber-orientation schedule, wet out successive plies using a chip brush and consolidation roller for the hand layup section of the laminate, monitoring resin pot life and ambient temperature; after the wet layup plies are tacked, students apply peel ply, flow media (if infusing), and bag film—sealing with tacky tape, connecting infusion or vacuum lines, and drawing the bag to the target vacuum level verified on a calibrated gauge—then monitor bag integrity and resin flow until full wet-out is confirmed. · After a full cure at shop ambient conditions (or post-cure per the resin manufacturer's data sheet), students demold the part, trim edges with a cutoff wheel or die grinder wearing a half-face respirator with organic-vapor/P100 combination cartridges, safety glasses, and hearing protection, then tap-test the entire surface with a coin and mark any suspect areas with painter's tape, recording findings in the quality-control checklist before proceeding to finish.
Materials: Plug or production mold (tool matched to project brief), mold release wax or semi-permanent release agent, gelcoat, 1.5-oz chopped-strand mat and woven roving or biaxial fabric per schedule, vinyl ester or polyester resin, MEKP catalyst, infusion resin if applicable, vacuum bag film, tacky tape, peel ply, flow media, infusion lines and fittings, vacuum pump, calibrated vacuum gauge, wet-film gauge, consolidation rollers, chip brushes, peel-ply burnishing tool, cutoff wheel, die grinder, half-face respirator with OV/P100 cartridges, nitrile gloves, Tyvek suit, safety glasses, hearing protection, tap-test coin, painter's tape, photographic log, project binder
Check: Instructor performs a structured part inspection at demolding using a standardized inspection sheet: visual check for dry areas, bridging, and print-through; tap-test map recorded on a part outline sketch; cross-section measurement at three locations compared to the target thickness in the build plan (within ±10%); bag-log review confirming vacuum held within specification throughout infusion. Passing standard: zero untreated dry spots greater than 1 square inch, zero delaminated areas confirmed by tap test, thickness within tolerance at all three measurement points, and all hold points in the project binder signed off. Any failing item must be remediated or documented as a repair task in Lesson 3.

Practice check7

Answer each question, then check it. Nothing is recorded; this is your own practice.

Fabrication processes and quality control
1. Why are gelcoat passes applied in cross-directed patterns rather than all in the same direction?
Resin systems and chemistry
2. A student mixes a batch of catalyzed polyester resin and notices it is beginning to gel in the mixing cup before all plies are wet out. What is the correct action?
Quality control and inspection
3. During a tap test after demolding, you hear a dull, hollow sound over a 2-inch area near a corner. What does this indicate and how should it be recorded?
Safety, PPE, and hazard control
4. When trimming cured fiberglass with a cutoff wheel, which respiratory protection is required?
Vacuum infusion and bagging processes
5. A vacuum bag is drawn to 27 inHg and the resin line is clamped. Over five minutes the gauge drops to 26 inHg. Which conclusion is most appropriate?
Defect identification and quality control
6. Which defect occurs when reinforcement fabric spans across a concave corner without conforming to the mold surface, leaving a gap behind it?
Measurement, quality control, and math
7. Your build plan specifies a target laminate thickness of 8.0 mm ±10%. At demolding, one measurement reads 7.1 mm. Is this measurement within tolerance?
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