Injection molding machines are workhorses of the plastics industry. Every machine has tie bars, plasticizing barrels, and platens that require deep hole drilling. These components operate under high pressure and cyclic loading, so the bore quality directly affects machine reliability.
I have drilled injection molding machine components for several OEMs over the years. The parts range from 2-meter tie bars to 4-meter plasticizing barrels with complex internal geometries. Tie bars in particular share similarities with general long shaft deep hole drilling but with specific requirements for hydraulic oil passage bores.
Tie Bar Drilling
Tie bars are the structural columns that hold the clamping unit together. A typical tie bar on a 500-ton injection molding machine is 2.5 meters long and 120mm in diameter with a 40-50mm through-bore.
The tie bar bore serves two purposes: weight reduction and hydraulic oil passage. Some machines route hydraulic fluid through the tie bar bore to actuate the moving platen. This means the bore surface must be smooth enough to avoid turbulence in the oil flow.
For tie bars in 4140 alloy steel:
| Parameter | Value |
|---|---|
| Bore diameter | 35-55mm |
| Length | 2-4 meters |
| Material | 4140 or 4340 steel |
| Cutting speed | 70-90 m/min |
| Feed rate | 0.10-0.18 mm/rev |
| Coolant pressure | 500-800 psi |
| Straightness spec | 0.5mm per meter |
I have found that tie bars need consistent material hardness along the full length. If the heat treatment varies, the drill will steer toward the softer section. I check hardness at both ends and mid-length before drilling. A variation of more than 3 HRC across the length means I adjust the parameters or reject the bar.
Plasticizing Barrel Bores
The plasticizing barrel in an injection molding machine is the heated cylinder where plastic pellets melt and pressurize. These barrels need a smooth, straight bore for the reciprocating screw to travel inside.
Plasticizing barrels are different from tie bars in two ways: the bore diameter is larger relative to the OD, and the material is usually nitrided or bimetallic.
For a nitrided barrel with a 60mm bore:
| Section | Length | Bore Diameter | Notes |
|---|---|---|---|
| Feed section | 800mm | 60mm | Coolest section, lowest pressure |
| Compression section | 600mm | Tapered | Plastic melts here |
| Metering section | 600mm | 60mm | Hottest section, highest pressure |
The barrel bore must be concentric with the outer diameter within 0.2mm. If the bore drifts, the screw rubs against one side of the barrel wall and causes uneven wear.
I use BTA drilling for the initial bore, then follow with a skiving or roller burnishing pass for the final surface. Roller burnishing produces a surface finish of Ra 0.4um or better, which is ideal for the screw-to-barrel interface.
Platen Drilling
Platens are the large steel blocks that hold the mold halves. They need drilled holes for clamping bolt passages and hydraulic cylinder mounting. The holes are usually shorter than tie bar bores but larger in diameter.
Typical platen holes I have drilled:
- Tie bar passage holes: 120-180mm diameter through 300-500mm thickness
- Hydraulic cylinder mounting holes: 60-100mm diameter through full platen
- Ejector pin passage holes: 20-40mm diameter
The challenge with platen drilling is the interrupted cut when the drill breaks through into an existing cavity. I reduce the feed by 40% in the last 20mm before breakthrough to prevent the drill from grabbing.
Barrel Screw Drilling
The reciprocating screw inside the barrel also needs a through-bore for cooling or heating fluid circulation. Screw drilling is similar to standard long-shaft work but with the added complexity of the screw flight geometry.
I drill the screw blank before the flights are machined. A typical screw is 1.5-3 meters long with a 10-20mm bore. The straightness requirement is tight because the screw rotates at high speed inside the barrel.
For screw drilling, I use counter-rotation gun drilling. The counter-rotation produces straightness within 0.1mm per meter, which keeps the screw centered in the barrel during operation.
Maintenance and Repair Work
A significant portion of injection molding machine drilling work is repair and reconditioning. Machines that have been in service for 10-20 years come in for barrel replacement or tie bar refurbishment.
The most common repair I see is re-boring a worn barrel to accept a new liner. The worn barrel gets bored out 2-3mm oversize, then a new bimetallic liner is installed. The liner material is typically a wear-resistant alloy like Xaloy or similar.
Re-boring a worn barrel is straightforward BTA work. The challenge is that the barrel has already been in service and might be warped. I check the straightness of the existing OD before setting up and adjust the alignment accordingly.
Key Takeaways
- Tie bars need hardness variation under 3 HRC across the length to prevent drill steering
- Plasticizing barrels benefit from roller burnishing after BTA drilling for Ra 0.4um finish
- Platen drilling requires a 40% feed reduction in the last 20mm before breakthrough
- Screw blanks should be drilled before flight machining to avoid distortion
- Re-boring worn barrels for liner installation is a common repair application in injection molding