Last updated: 2026-09-04 · 6 min read
Which Mold Parts Should You Keep as Spares? — Building a Spare-Parts Plan That Prevents Line-Down Weeks
A spare-parts plan is an inventory decision, not a wish list. Stock too little and a $2 pin shuts down a press for days; stock too much and capital sits on a shelf waiting for a mold that may never run again. The right list ranks every component by lead time, failure probability and the cost of the press standing idle — then funds the top of that list first.
Key Facts
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Shrinkage — The dimensional reduction from mold cavity to cooled part. Amorphous resins: 0.4–0.7% (uniform). Semi-crystalline: 1.0–2.5% (directional). Mold makers apply shrinkage factor when machining cavity. Actual shrinkage varies with wall thickness, gate location and packing pressure. The related guide covers adjacent topics.
How Do You Decide Which Parts to Stock?
Three dimensions decide every item:
| Dimension | What It Measures | How to Estimate It |
|---|---|---|
| Lead time | Days from order to press-ready install | Ask the supplier; distinguish stock items vs made-to-order |
| Failure probability | How often the part wears or breaks within the mold’s campaign | Use historical replacement data, or the mold maker’s wear estimate |
| Downtime cost | Press hourly rate × expected hours lost per failure | Machine rate plus labor plus missed delivery penalties |
Multiply the three and you get a risk-priority number for every component. A set of ejector pins that fails every 200,000 shots, costs $30, has a 6-week lead time from the supplier, and would idle a press at $150/hour for 8 hours while waiting, has a risk-priority that dwarfs its invoice price.
A cavity insert that might crack once in a million shots and can be re-cut in 10 days looks less urgent on paper — until the press it serves is a dedicated cell with no backup.
The practical rule: stock everything where lead time exceeds one maintenance cycle and failure probability is more than negligible. Anything with a 4-week or longer lead time and a known wear mode belongs on the shelf before the mold ships. Items with short lead times and low failure probability can wait for a purchase order.
How Do You Structure the Spare-Parts List So It Actually Works?
Four tiers, each with its own funding logic:
- Consumables — always stock. Ejector pins, springs, O-rings, wiper seals, guide pin wipers and small fasteners. These wear on every cycle, cost little individually, and a missing one stops the mold. Buy in bulk at mold delivery, keep a running min-qty on the shelf, and reorder when the bin hits the trigger level. The annual cost is trivial compared to a single press-down event.
- Wear components — stock by predicted life. Guide pins and bushings, wear plates, slide rails, gib strips, and heater bands for hot-runner molds. These have a predictable service life measured in shots or months. Order one or two replacement sets when the mold is commissioned, timed to arrive before the predicted first replacement. Track actual life against predicted; adjust the reorder trigger when real data arrives.
- Critical inserts — stock by consequence, not probability. Core pins, cavity inserts, and slide faces that are expensive, long-lead, and whose failure would idle a dedicated press. Even if the probability of failure is low, the downtime cost of waiting 6 to 10 weeks for a replacement makes stocking one insert economically rational. The rule is simple: if the press cannot run without it and the lead time exceeds your maintenance window, buy the spare before the mold enters production.
- Capital spares — insure, do not stock. Complete mold bases, manifolds and large structural components are too expensive to hold as shelf inventory. Instead, negotiate a written lead-time guarantee with the mold maker for emergency re-cut or replacement, and carry the risk in your business-continuity plan rather than in a crate. For critical programs, a shared spare at the mold maker’s shop — paid for but held there — splits the cost.
Gate vestige — The visible mark left on the part surface where the gate was trimmed. Edge gate: 0.5–2.0mm vestige. Pin-point gate: 0.2–0.5mm. Valve gate: 0.05–0.1mm (flush). Vestige size affects cosmetic appearance and may require secondary trimming. The related guide covers adjacent topics.
The cost of a missing spare is never the part price. It is the press standing still, the operator paid to wait, and the shipment delayed — all multiplied by the lead time nobody checked before the failure.
Frequently Asked Questions
How many spare ejector pins should I keep for a new mold?
For a standard mold, stock at least two full sets of the diameters and lengths used, plus a handful of the most common sizes that are not in the mold but could be needed for future modifications. Ejector pins are inexpensive, they wear on every cycle, and a broken pin inside the mold without a replacement on hand is a multi-day stoppage.
The exact number depends on the pin diameter and the resin — abrasive filled resins wear pins faster than unfilled ones — but the principle is the same: if a pin fails and the replacement is 4 weeks away, the press is down for 4 weeks. Stock accordingly.
Should the mold maker include spares with the mold at delivery?
Yes, and it should be written into the purchase agreement before the deposit. A standard spare-parts list at delivery includes one extra set of ejector pins, springs, O-rings, and any small wear items specific to that mold, plus one spare heater band or nozzle tip for hot-runner tools.
The cost at the time of mold build is marginal because the supplier already has the specifications and materials in hand. Ordering the same parts months later as a one-off shipment costs more per piece and takes longer to arrive. The right time to buy spares is when the mold is being built, not after it fails.
How do I know when to reorder spare parts from the shelf?
Every item needs a written min-qty on the bin card or in the maintenance system. When the count hits the min-qty, a purchase order goes out automatically — no waiting for a breakdown to reveal the shelf is empty. The min-qty should cover at least one lead-time cycle of expected consumption.
For consumables like O-rings and pins, a monthly visual check of the shelf is enough. For wear components, tie the reorder to the shot count: when the mold hits the predicted replacement interval for a guide pin set, the spare should already be on order or on the shelf. Tracking actual consumption against predicted is what makes the plan more accurate over time.
Key Takeaways
- Rank every spare by lead time × failure probability × downtime cost — the expensive press-down events come from inexpensive, long-lead, high-wear parts, not from costly components.
- Four tiers fund differently: consumables always stock, wear items stock by predicted life, critical inserts stock by consequence, and capital spares get a lead-time guarantee rather than shelf inventory.
- A spare-parts plan without a min-qty and reorder trigger is just a pile; write the trigger into the bin card or the maintenance system so replenishment happens before the shelf empties.
A spare-parts plan is an inventory discipline, not a guess.
Rank the components by the risk they create when missing, fund the top of the list before the mold ships, and set a reorder trigger so the shelf never empties silently. The $30 pin that idles a $150-per-hour press for six weeks is the lesson nobody wants to learn twice.
Sources
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Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet