1. Injection Speed
What is Injection Speed: Injection speed we usually set refers to speed at which screw advances. However, what's truly important is speed at which melt moves through mold cavity, which is related to cross-sectional area of flow direction.
How to Determine Injection Speed: As a general rule, the faster the injection speed, the better. Its determination depends on melt's cooling rate and viscosity: melts with fast cooling rates or high viscosity require higher injection speeds.
Note: Cooling rate depends on material's properties, wall thickness, and mold temperature.

 

Injection Speed Too Fast/Too Slow
Too fast: Prone to burnt spots, flash, internal bubbles, or melt jetting.
Too slow: Prone to flow marks, weld lines, and a rough, dull surface.
2. Switching Point
How to Determine Switching Point: Generally, switching point is point at which product is filled to 95% capacity under zero holding pressure, where injection transitions to holding pressure. For thin-walled products (e.g., cable ties): generally 98% capacity. For unbalanced runners: generally 70%-80%, depending on specific situation. A slow-fast-slow multi-stage injection technique is recommended.
Transfer Point Too High/Too Low
Too high a transfer point: Insufficient mold filling, weld lines, dents, undersized products, etc.
Too low a transfer point: Flash, difficulty in demolding, oversized products, etc.
3. Holding Pressure
How to Determine Holding Pressure: Optimized holding pressure is generally midpoint between minimum and maximum holding pressure.
Minimum Holding Pressure: Based on accurate transfer point, apply a certain holding pressure when product just begins to show signs of insufficient mold filling.
Maximum Holding Pressure: Based on accurate transfer point, apply a certain holding pressure when product just begins to show signs of burrs. (From minimum and maximum holding pressure, we can see safe range of product process fluctuations.)
Generally speaking: PA holding pressure = 50% injection pressure; POM holding pressure = 80% injection pressure, for products with high dimensional requirements, it can reach 100% injection pressure; PP/PE holding pressure = 30-50% injection pressure
4. Holding Time
How to determine holding time: Holding time is determined based on gate solidification. It is determined by weighing product.
Holding time too long/too short
Too long holding time: affects cycle time
Too short holding time: insufficient weight, internal voids in product, smaller dimensions
Note: Holding pressure affects length of holding time. The higher holding pressure, the longer holding time.
5. Screw Speed
Goal of pre-plasticizing is to obtain a uniform and stable melt (i.e., uniform plasticization, no cold material, no degradation, and no excessive gas).
How to Determine Screw Speed: As a principle, the screw speed must be determined so that sum of screw's pre-plasticizing time, suction time, and injection unit retraction time is slightly shorter than cooling time.
Screw Speed Too Fast/Too Slow
Too fast a screw speed: uneven plasticization (resulting in cold material, insufficient mold filling, and breakage), material decomposition (resulting in scorch marks, color difference, breakage).
Too slow a screw speed: affects cycle time.
6. Cooling Time
As a principle, cooling time should be set as short as possible, with basic requirements being that product does not deform, does not stick to mold, and has no excessively deep ejection marks.
Furthermore: Screw speed: PA < 1.0 m/s; POM < 0.7 m/s; PP/PE/PS < 1.3 m/s; ABS/PC/PMMA < 0.6 m/s.

 

7 Back Pressure
What is Back Pressure: Back pressure refers to force exerted by hydraulic cylinder to prevent screw from retracting during pre-plasticizing. Its magnitude is equal to reaction force of melt at screw tip on screw.
How to Determine Back Pressure: Determination of back pressure depends on properties of different materials and is usually provided by material supplier. Generally: PA: 20-80 Bar; POM: 50-100 Bar; PP/PE: 50-200 Bar
How to determine back pressure
Too high back pressure: material decomposition; drooling; longer pre-plasticizing time required.
Too low back pressure: uneven plasticization (especially for color masterbatches), incomplete plasticization (resulting in product bubbles, scorch marks, etc.)
8. Back suction
How to determine back suction: Determination of back suction (in conjunction with back pressure) should be based on principle of preventing drooling.
Too high/too low back suction
Too high back suction: bubbles, scorch marks, unstable material pad
Too low back suction: drooling, Unstable material pad (due to check valve not closing)
9. Clamping force
Determining clamping force: Clamping force depends on projected area of cavity and injection pressure.
Clamping force too high/too low:
Too high clamping force: poor venting (burnt spots, insufficient filling), mold deformation
Too low clamping force: flash
10. Melt temperature
How to determine melt temperature: Melt temperature is usually determined by properties of different materials and is provided by material supplier.
Barrel Temperature Setting

 

Melt Temperature Too High/Too Low
Too High Melt Temperature: Material decomposition (resulting in product bubbles, color difference, scorch marks, cracking, etc.)
Too Low Melt Temperature: Uneven plasticization of material, cold material in melt (resulting in insufficient mold filling, cold material, product cracking, etc.)
11. Mold Temperature
Why is Mold Temperature Necessary: Regardless of mold temperature, its function is always to maintain a certain temperature in mold during stable production, serving a cooling function.
Truly important mold temperature refers to temperature of mold cavity, not temperature displayed on mold temperature controller. Typically, during stable production, cavity temperature will reach a stable dynamic equilibrium and be about 10 degrees Celsius higher than displayed temperature. (For large molds, mold must be fully heated before production, especially for thin-walled products with a large flow length ratio.)
What Does Mold Temperature Affect: It affects flowability and cooling rate of melt. Because it affects flowability, it impacts product appearance (surface quality, burrs) and injection pressure; Because it affects cooling rate, it affects product crystallinity, which in turn affects product shrinkage and mechanical strength properties.
Mold temperature too high/too low
High mold temperature: Good flowability; high crystallinity; large shrinkage (resulting in smaller dimensions); deformation; requires longer cooling time.
Low mold temperature: Poor flowability (resulting in flow lines, weld lines); low crystallinity; small shrinkage (resulting in larger dimensions).