Complete Guide to Mold Parts Maintenance: SG MOLD's 19 Years of Practical Experience

Anyone in the mold industry knows: the mold base is the “skeleton” of the mold, and the parts are the “joints” and “nerves.” No matter how strong the skeleton, if the joints rust and the nerves fail, the entire mold is useless.

A good mold base can achieve micron-level precision — that's not unusual. What is unusual is maintaining that precision after three or five years. The difference? It comes down to maintenance.

SG MOLD has been in the industry for 19 years. We have seen too many mold bases scrapped not from normal wear, but from inadequate maintenance and improper operations that caused parts to “retire” early. Today, we’re combining our practical experience to give you a complete guide to mold parts maintenance.

I. Six Basic Principles of Daily Maintenance

These are tasks that must be done every day. They seem simple, but few factories truly execute every one properly.

1. Cleaning tools must be selected by material type

For high-gloss parts like mirror-finish cores and optical inserts, use only lint-free paper or ultra-fine soft cotton cloth. These parts have extremely high surface finishes, and any rough wiping will leave scratches.

Never use steel brushes, utility knives, regular sandpaper, or files directly on molding surfaces. These tools will immediately create scratches and pits — at best affecting 3C product appearance yield, at worst scrapping the entire insert.

2. Lubricants and greases must never be mixed

Many factories use one type of grease for everything to save trouble. This is a major mistake.

• High-temperature injection molds (above 200°C): Must use dedicated high-temperature white grease. Ordinary grease will carbonize at these temperatures.

• Ejector pins: Use specialized anti-seize ejector pin oil, designed for high-frequency reciprocating motion.

• Optical molds and light guide plate molds: Use only dry-film lubricants. Liquid lubricants are strictly forbidden, as oil bleed-out will contaminate plastic products, causing mass defects.

• Ordinary grease and low-quality machine oil: Absolutely prohibited for precision standard parts. Carbon buildup at high temperatures will seize the parts.

3. Rust preventives should be selected by scenario

Rust prevention is not one-size-fits-all; choose based on shutdown duration:

• Short-term shutdown (1-3 days): Use light volatile rust preventive oil, forming a thin protective film.

• Long-term storage (7+ days): Use hard-film rust preventive oil for a durable protective layer.

• Mirror-finished parts: Do not use heavy rust preventive oil; the residual film is extremely difficult to clean and directly affects light transmission and appearance.

4. Never apply rust preventive while parts are hot

This detail is often overlooked. Never spray rust preventive oil until mold and parts temperatures have dropped below 50°C.

The principle is simple: hot surfaces meeting cold oil create condensation moisture, which gets trapped under the oil layer and causes internal corrosion. The maintenance ends up worse than no maintenance at all.

5. Handle with care during disassembly and assembly

High-precision parts like precision ejector pins, sleeves, mirror inserts, and ball guide bushings must be handled individually.

A seemingly minor bump or casual stacking is fatal for micron-precision parts. Once precision is compromised, they’re useless even if installed.

6. Always run a test cycle after maintenance

After adding lubricants and spraying rust preventive, before production resumes, run several dry open-close cycles and wipe off excess oil.

The goal is to distribute lubricant evenly and purge excess oil to prevent oil stains on the next product — if you cause a batch of rejects right after maintenance, the maintenance becomes a joke.

II. Seven Key Parts: Specialized Maintenance Points

Different parts have different working environments and stress conditions, so maintenance methods vary accordingly.

1. Guide Parts (guide pillars, ball guide bushings, dowel pins, interlocks)

Guide parts determine the mold base's clamping accuracy. Once worn, the entire mold alignment shifts.

Ball guide bushing gaps easily trap plastic dust; blow them clean with high-pressure air every shift. Dust mixed with balls accelerates wear, increasing clearance and causing alignment failure.

Scratches on guide pillar surfaces must be repaired immediately. Minor scratches can be polished with fine sandpaper — do not continue production, as the scratched surface will worsen.

Never operate the mold for extended periods without lubrication. Ball guide bushings running dry wear at an alarming rate.

2. Ejector Parts (ejector pins, flat ejectors, sleeves, return pins)

Ejector pins are the critical parts that push products out of the mold — and one of the most problem-prone parts.

Carbon buildup on ejector pins must be cleaned regularly. If ejection feels rough, do not forcefully increase ejection force — especially for 3C micro-molds where ejector pins are only 0.3–1mm in diameter; forced ejection will snap them.

Do not over-grease ejector pin clearances; excess oil will cause oil stains at the ejector pin locations, directly affecting appearance yield.

Bent, deformed, or head-worn ejector pins must be replaced directly, not ground and reused. Grinding changes dimensional tolerances, and inaccurate ejection positions will throw off product dimensions.

3. Slide Wear Parts (wear plates, angle pins, wedge blocks)

The slide is the core mechanism for side core pulling. Wear plate wear directly affects slide precision.

Wear plates must be replaced once wear exceeds 0.005mm — that's finer than 1/20 of a human hair. If not replaced, slide misalignment will throw side-core dimensions out of tolerance.

For optical molds, slides can only use dry-film lubrication — liquid oil will flow along the parting line to the core, contaminating the molding surface.

Slide interiors tend to accumulate plastic residue; blow them completely clean, or residue friction will create scratches.

4. Cooling System Parts (water fittings, baffles, O-rings)

The cooling system controls mold temperature. Leaks of water or oil have serious consequences.

O-rings are consumables; high heat and oil accelerate aging. Do not reuse them long-term; replace all during each major overhaul.

When descaling water lines, strictly follow chemical concentration instructions. Over-soaking in strong acids will corrode metal fittings and baffles — a costly mistake.

Always pressure-test after maintenance to confirm no leaks before production. One leak can rust an entire set of mold parts.

5. Mold Springs

Small as they are, springs affect every mold open-close cycle.

Never exceed maximum stroke limits; over-stroking causes rapid fatigue fracture. Replacements must match original load ratings — yellow, blue, red, and brown represent different load classes; do not substitute arbitrarily.

Rusted, deformed, or cracked springs should be scrapped immediately. Do not grind off rust and reuse. A broken spring means at best a production stoppage for replacement, at worst damage to the entire mold.

6. Hot Runner Parts (hot tips, heaters, thermocouples, manifolds)

The hot runner system is the mold's “vascular system,” operating under high heat and pressure — maintenance requires extra caution.

When cleaning residue from hot tip gates, use only copper scrapers. Steel tools will scratch mirror-finished gate surfaces.

Heater wire connectors must never contact water or rust preventive oil, which can cause shorts and burn out heaters — an expensive replacement.

Allow the hot runner to cool completely before maintenance. Disassembling while hot risks burns and damaging insulating washers.

7. Mirror Inserts and Cores (3C/Optical Core Precision Parts)

These are the most precise and most fragile parts of the entire mold — and the key to product appearance.

Never touch the mirror molding surface with bare hands. Skin oils and fingerprints will corrode the steel, leaving permanent marks.

Use only diamond polishing compound and soft lambskin for polishing; never use coarse abrasives near these surfaces.

Store in individual foam shockproof cases, isolated from each other. Two mirror inserts rubbing together can cause tens of thousands in damage.

III. Warehouse Spare Parts Management Standards

Mold bases need maintenance, and spare parts need maintenance too. Many factories discover rust only when they need the part — that's a failure of this step.

Warehousing requires a constant dry environment, with humidity controlled at 40%-60%. In a damp workshop, parts start rusting within a month.

Rubber parts (seals, insulating washers, etc.) must be stored away from light; shelf life is typically 6 months. Strictly follow FIFO to avoid aging and cracking.

Do not unpack the original anti-rust packaging of precision non-standard inserts and fine ejector pins. If not fully used after opening, reapply oil-seal treatment.

Inspect spare parts every 3 months for rust condition. Parts stored over 6 months must be re-cleaned and re-coated with rust preventive.

Store different specifications separately with clear labels. Mixed storage means grabbing the wrong part during assembly — wasting time is one thing, but installing the wrong part and damaging the mold is a disaster.

IV. Safety Precautions for Production and Major Overhauls

Maintenance is important, but safety always comes first.

Before mold maintenance, always shut off the injection molding machine power, completely relieve pressure, and fully open the clamping mechanism to prevent accidental clamping injuries.

Cleaning agents and rust preventive oils are flammable; keep them away from hot mold bodies and heating bands.

When cleaning internal mold dust, use high-pressure air guns first. Never use bare hands to wipe hot parts — mold temperatures can still exceed 100°C immediately after shutdown.

Avoid prolonged skin contact with chemical descalers and polishing compounds; wear protective gloves at all times.

V. Management Layer: Assign Maintenance Responsibilities to Individuals

Good techniques mean nothing without management follow-through. Based on SG MOLD's 19 years of experience, here are four must-do management practices:

First, tiered maintenance with clear accountability.

• Daily: Operators perform basic cleaning and lubrication.

• Weekly: Mold technicians perform comprehensive inspection, tightening, and adjustment.

• Annual overhaul: Engineers perform full teardown inspection and replacement of worn parts.

All maintenance activities must be documented, signed off, and traceable.

Second, inspect wear during maintenance. Add worn parts to the procurement BOM for advance stocking. Waiting until failure means production line downtime waiting for parts — a major loss.

Third, maintain separate records for high-precision parts. Track wear data and replacement cycles for each part to build a “health file.” Data tells you when to stock and when to replace — no more guessing.

Fourth, never cut corners on maintenance to save costs. Skipping lubrication and rust prevention seems to save time and materials, but it drastically shortens part life and doubles replacement frequency, driving up total procurement costs.

Final Words

The value of a mold base isn’t just its precision at delivery — it's how much precision remains after three or five years.

SG MOLD has been in this industry for 19 years. We take responsibility not only for the products we ship, but also for sharing the maintenance knowledge we’ve accumulated. Because once a mold base reaches the customer, the quality of maintenance directly determines how long it lasts and how many good parts it produces.

Small parts, big impact. Fine maintenance, decisive outcome.

If you have any questions about mold maintenance or need a specific plan for your mold model, the SG MOLD team is always here to discuss — after all, helping every mold base achieve its full lifespan and value is our shared goal.