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The Molding Machine Chapter 2 Professor Joe Greene CSU, CHICO MFGT 144 September 8, 1999
Chapter 1: Introduction to Plastics • Objectives – Main Components of the injection molding machine – Operations of an injection molding machine – Injection requirements – Sizing the clamp unit 2
• Background Introduction – Concept is simple • Melt plastic, flow into mold and take part shape, cool, demold – Injection molding makes parts in discrete (discontinuous) process – More injection molding machines used for plastic processing than any other equipment – Almost all thermoplastic and some thermosets materials can be injection molded – Process is automated and highly repeatable parts – Injection molding parts are finished with little post molding operations – Very complex parts can be made – Machines are expensive – Molds are expensive, usually P-20 steel 3
Injection Molding Materials • Thermoplastic Materials – Most thermoplastic materials are injection molded – A few thermoset materials are injection molded, silicone rubbers 4
Injection Molding Process 5
Injection Molding Operations • Cycle Time ·Injection Pressure 6
Main Components • The Injection Unit – Sized to contain two full cycles’ worth of material, or – 50 % of the capacity of the injection cylinder should be emptied each cycle. – Shot size should be between 20% (min) and 80% (max) – Heat sensitivity affects shot size. Some material are very heat sensitive, burn easily and shouldn’t be in the barrel very long. Degradation could occur. – Example, • If a part weighs 2 -oz (56. 7 g), the ideal injection unit should have 4 oz shot capacity, • The smallest machine is 2. 5 -oz machine (80% capacity) and the 7 biggest machine is 10 -oz machine (20% capacity).
Main Components • The Injection Unit – Capacity of injection molding machine based upon weight of polystyrene it can hold. – Conversion to actual plastic injection by multiplying by density of plastic and dividing by density of polystyrene. – Example, • Capacity of machine is 10 oz. How much HDPE can the machine hold? Density of PS = 1. 04 g/cc, PC =1. 2 g/cc • Answer. Ratio of densities. Density of new resin/density of PS – Machine capable of injection 10 oz of PS, can also inject 8 11. 5 oz of PC
Clamp Size and Shot Size • Table II-1. – Clamp size, tons (k. N) 10 (89) 25 50 100 250 450 1000 4000 Shot size, oz. (g) 0. 5 (14. 2) 2 4 8 16 20 60 200 9
• Purpose Injection Unit – Melt solid pellets to liquid form and then inject into mold • Steps – Hopper- manual or pneumatic loaded. Can have a mixer, volumetric or gravimetric units to meter material (2 hrs worth) – Screw • Reciprocating screw – – – – – most common with clearance of 0. 003 to 0. 005 in. similar to general purpose extrusion screw much shorter than extrusion screws, L/D of 12: 1 to 20: 1 (E: 20: 1 to 30: 1) compression ratios (diameter of feed to diameter of metering) are often 2: 1 to 5: 1 which is lower than for extrusion. lower compression ratio means less mechanical action and heating Step 1: turns of the screw melts resin and collects it at end of screw Step 2: the screw moves forward via a hydraulic mechanism Step 3: retraction of screw 10 Step 4: part cooling and removal
Injection Designs • Injection Unit Components • Injection Screw 11
Injection Screw Designs • Different types of screw designs – various shapes of flight, distances between flights, amounts of shearing action, screw tip geometries, and methods of shutoff. – Metering screw • Feed zone in rear section has smaller screw diameter • Melt zone in middle of screw • Metering zone in front section has larger screw diameter • Screw Tip and Check Ring 12
Injection Screw Tip and Check Ring – The screw pushes forward and pulls backward acting as a plunger to inject the molten plastic. – The tip is inserted through a check ring and seat designed to keep molten plastic from flowing back over the screw flights during injection. – The tip fits into the face, usually with a left-hand thread. – The screw tip angle and length are determined by the viscosity of the plastic being molded. – Fig 2. 5 13
Non-return Valves and Ball Shutoffs – Action of check ring allows that material to move in front of the screw tip. The sequence of non-return is: • Screw pushes forward, injecting a charge of molten material into a mold. • Check ring is forced back against the screw tip seat and seals against it, preventing material from passing back over screw. • Screw stops pushing and begins to turn (bringing new material forward) • Check ring slips forward under the influence of pressure buildup. • Molten plastic flows into the space in front of the screw tip. – Many different non-return mechanisms. • Check ring is most common, but ball-type device is also popular • Fig 2 -6 14
Non-return Valves and Ball Shutoffs • Action of ball-type shutoff device • Shutoff ball travels back and forth between the stop pin and the flow hole. • When screw moves forward (plunger), the ball moves backward, plugging the hole and keeping material from flowing back over the screw flights. • When screw stops injecting and turns to auger fresh material forward, the ball is pushed forward, allowing material to flow and fill the space in front of the screw tip and into the machine nozzle. • Plastic material is restricted in both check-ring and ball-shutoff cases, even when the non-return devices are in the open flow position. This can cause degradation in some materials. • Non-return mechanisms are usually not used when molding shear 15 sensitive and high viscosity materials.
Injection Molding Nozzle • The nozzle is the final unit for the complete injection unit. • The Nozzle is a two-piece, tube-shaped component that bolts to the face of the injection barrel. • Fig 2 -7 and Fig 2 -8. 16
Injection Molding Nozzle • Nozzle cap has an internal taper that matches that of the screw tip and a tapered hole through the nozzle tip itself. • Nozzle heater is controlled via a heater band. • Some nozzle designs incorporate shutoff devices in the forms of needles, springs, or sliding balls. (No drool) – Shut-off flow of plastic for low viscosity materials, e. g. , nylon 17
Injection Requirements • Shear rate – Defined as the surface velocity of the plastic at the wall of the heating barrel, divided by the depth of the screw flight. (feet/min) – Formula: SR = (D x N)/h, • where, SR = shear rate, D = Diameter of screw, N = rate of screw rotation (rpm), h = depth of channel – An average shear rate would be approx. 150 ft/min for injection molding – Each plastic has a shear rate, beyond which it will degrade. • Example, PVC is sensitive has max shear rate of 100 ft/min; whereas other non-sensitive materials have shear rate of 175 ft/min and higher. – Example, • For standard 2 in diameter screw, the maximum rotational speed is 230 RPM to achieve an average shear rate of 150 ft/min. 18 • Any faster speeds could degrade the material.
Injection Requirements • Screw Output – – Amount of material an injection machine can inject in lb/hr Function of horsepower in the screw Range of HP = 2” D screw = 15 HP; 4. 5”D screw = 150 HP Range = 5 to 15 lb/h for each HP. • Example, 15 -HP system and 2” screw produces 75 to 225 lb/hr of plastic • Injection Pressure – Average machine has 20, 000 psi. Best to use highest pressure. – 20, 000 psi on plastic and 2, 000 psi on hyd fluid. Multiplier needed. • L/D Ratio of screw – – Critical factor to create injection pressure. Length/Diameter Length is entire length of screw. Diameter is the largest diameter. Range from 12: 1 to 20: 1 (Extrusion: 20: 1 to 30: 1) 19 The greater the ratio, the more gentle the shearing action.
Clamping Unit • Clamping Force – Clamping unit holds the molds together while the resin is injected, packed, and cooled, and ejected. – Clamping force is the rating of the injection molder, e. g. , 150 tons clamping force. • Clamping force = Injection Pressure x Total Cavity Projected Area – Projected area is the area projected into a single plane, that is, the widest area of the part. – Examples • The force necessary to mold a part that has 100 in 2 projected area and has 3, 000 psi is 3, 000 * 100 = 300, 000 lbs force = 150 tons (note 1 ton = 2000 lbs) • The maximum projected surface area of a part on a 200 ton machine with a maximum injection pressure of 2, 000 psi is: 400, 000 lbs force / 2, 000 psi = 20 200 in 2
Clamping Unit • How much force required? – Function of projected area. • Higher projected area, higher clamping force required. • Higher viscosity of resin, higher clamping force required. – Example, PC requires 15, 000 psi due to high viscosity, versus acetal with low viscosity required 5, 000 psi. Thus, PC will require 3 times clamp force on mold. – Projected area example. 21