Linear guide rail and carriage of the injection unit

The linear guide of the injection unit is a hardened steel guide rail bolted to the machine base and one or more carriage blocks that slide along it on recirculating steel balls, carrying the injection unit carriage. When the carriage cylinders move the injection unit forward to press the nozzle against the mold, and back for purging or sprue break, the linear guide keeps it moving in a straight line with little friction, so the nozzle stays centered on the sprue bushing. Older and smaller machines may use plain slideways or round guide rods instead.

Where the linear guide sits

Under the injection unit, usually two rails run parallel along the machine base, with blocks bolted to the underside of the injection unit carriage. The drawing shows a rail and block in side view and end view, with the guide rail (1), the carriage block (2), the balls (3), the end caps and seals (4), the grease nipple (5) and the mounting bolts (6). How the carriage fits in the cycle is explained in the injection unit guide.

How it works

• Guide rail. A hardened steel rail with ground ball grooves (raceways) along both sides. It is bolted to the base, often against a reference edge so it is straight.
• Carriage block. Sits over the rail and carries the load. Rows of steel balls roll between the block's grooves and the rail's grooves.
• Balls. At the end of the loaded zone, each ball enters a return channel in the block and rolls back to start again, so the block can travel the whole length of the rail.
• End caps and seals. Plastic end caps turn the balls around; rubber seals wipe dirt off the rail as the block moves.
• Grease nipple. The lubrication point on the end cap. Grease keeps a film between the balls and the raceways.
• Mounting bolts. Fix the rail to the base and the block to the carriage.

Linear guides are made in accuracy classes and preload classes. Preload is a slight interference between the balls and the grooves that removes play and stiffens the guide. The linear guide rail and slider we supply is hardened alloy steel in H or P accuracy grade.

Signs of wear or damage

• Rough, notchy or noisy movement of the carriage, felt when it is moved slowly.
• Play: the injection unit moves sideways or lifts when pushed with the carriage cylinders depressurized.
• Rust, pitting or flaked spots on the rail raceways.
• Damaged or missing end seals, and dirt or plastic dust packed against the block.
• Dry raceways or discolored, dark grease.
• Nozzle misalignment: the nozzle no longer meets the sprue bushing centrally, and melt leaks at the joint. See nozzle problems.
• Loose bolts, which let the rail or block shift out of line.

Causes

• Lack of grease. The most common cause. The balls and raceways wear and pit without a film between them.
• Contamination. Pellets, plastic dust and purged material falling onto the rail and getting past worn seals.
• Water and corrosion. Cooling water leaks and condensation rust the raceways, especially when the machine stands idle.
• Impact and overload. A carriage driven hard against its end stop, or heavy loads during maintenance, dent the raceways.
• Misalignment. Rails not parallel, or a carriage that twists, load the blocks unevenly.

Inspecting the linear guide

1. Lock off the machine and depressurize the carriage cylinders, following the machine manual.
2. Clean the rails and look along their full length for rust, pitting and dents, especially in the zone where the carriage normally stops.
3. Check the end seals and grease nipples, and grease the blocks.
4. Move the carriage slowly over its stroke and feel for rough spots.
5. Check for play with a dial indicator on the base and its tip on the carriage, pushing the carriage sideways and lifting it.
6. Check that the rail and block bolts are tight.

Replacing a rail and blocks

1. Support the injection unit securely before releasing the blocks.
2. Do not slide a block off its rail without a dummy rail or the retaining strip supplied with it: the balls can fall out.
3. Clean the mounting surfaces and the reference edge on the base. Burrs or dirt under a rail bend it.
4. Fit the new rail against its reference edge and tighten its bolts in the sequence and to the torque given by the guide maker or the machine manual.
5. Align the second rail parallel to the first, as the machine manual describes.
6. Fit the blocks, bolt the carriage down, grease the blocks and move the carriage through its full stroke by hand or at low speed.
7. Check that the nozzle meets the sprue bushing centrally.

Rails and blocks are usually replaced as a set. Mixing a new block with a worn rail, or blocks of different preload, gives uneven load and short life.

What to send for a replacement quote

1. The markings on the rail and block: maker, size and model code.
2. The rail width and height, its length, and the hole pitch.
3. The number of blocks per rail, the block type (flanged or square, standard or long), and the preload and accuracy class if marked.
4. The machine brand and model, and photos of the rail, a block and its markings.

Request a quote with these details. For the other moving parts of the injection unit, see guide columns and bushings and the injection cylinder.