CRITICALITY OF FLOW PATH ASPECTS FOR RIGID PVC INJECTION MOULDING

Introduction:

For injection moulding, thermoplastic has to be sufficiently fluid to be injectable into the mould.

As compared to other thermoplastics plastics, PVC has two contradicting effects:

1.      Being amorphous thermoplastic, PVC becomes softer; fluid and processable with increasing temperature. However, with increasing temperature PVC also undergoes fusion that increases its viscosity during processing. It is reported that when temperature rises from 160 to 210 ° C, Tan δ which is ratio between viscous and elastic stresses increases by a factor of 4, indicating increase in viscous contribution. This is unique to PVC (Michel A. Huneault et al Jr. of Vinyl Technology, Dec. 1992, Vo. 14, No.4, PG 175).

2.      All other thermoplastics have less die swell with increase in temperature, however for PVC die swell increases with increase in temperature, helping to transfer shine from mould to product.

PVC derives its strength on the basis of optimum fusion (65-70%) that takes place only during processing on the injection side, at temperature between 180 -190° C.

PVC needs fluidity during injection moulding. This comes from selecting lower K- value resin, adding polar lubricants and increasing processing temperature. On the other hand, increasing temperature beyond optimum level leads to over fusion, degradation and more cooling time in the mould, affecting the cost and quality. Role of antidegradents is very important.

PVC is a shear sensitive polymer. While fusion adds strength, degradation reduces strength. Thus, temperature of the fluxed PVC exiting the nozzle and filling the mould path needs to be controlled.

Generally, fast injection speed results in burning and splay marks – (Marks or droplet type imperfections on the surface of the finished product).

Therefore, the rise in temperature due to shear in the flow path from nozzle through sprue, runner and gates has to be controlled as PVC degrades in just few seconds at higher temperature.

Actions taken to reduce shear heating allow –

1.      Faster screw RPM without degradation,

2.      Faster injection speed without shear burning,

3.      Lower injection pressure and therefore,

4.      Lower clamp force allowing the use of smaller injection molding machine.

The point to be understood here is that the screw design and the mould flow path design with reference to shear has to be based on PVC technology and not the other way round.

Shot weight:

Ideally volumetric amount of PVC needed for each shot should be approximately 50 -75% of the machine capacity.

More than 75% will have unfused PVC particles (especially when dry blend is used), and less than 50% will cause degradation.                                                

Because PVC melt is somewhat compressible, increased back pressure results in greater shot weight at the same volume.

Screw tip:

Screw tip should be smooth and pointed, having an included angle of 20 - 30 degrees. Sharp tip configuration is suited for PVC pipe fitting compounds.

The screw tip configuration must match the configuration of barrel head.

When the screw is fully moved forward, the clearance between screw and barrel should not be more than 1.60 mm. No melt should remain in the reservoir (bottom out).

To avoid material hang-up at the screw tip, one trick is to grind off the tip at an angle, so that the extreme end of the tip lies off the centre of rotation (smear tip).

Nozzle:

For PVC pipe fitting, the nozzle can be as short as possible.

Reverse taper at the nozzle orifice is desirable to prevent a cold slug formation between shots.

A spherical radius should be used at the transition to the nozzle orifice.

The nozzle orifice should be as large as possible, and perfectly aligned with the sprue bush.

A separate nozzle heater with separate temperature controller should be used. The temperature of PVC exiting from nozzle for PVC fittings should be in the range of 198 - 210° C

Sprue bush:

Plasticated and optimally fused PVC from the injection side is injected by forwarding screw through the nozzle to the mould, via sprue bush.

Sprue bush is a conical channel through which the melted material from the nozzle passes into the mould (See figure at the top).

The alignment of nozzle and sprue bush should be perfect and their radius should be matching.

Dimensions of the sprue should be such that –

1.      The pressure drop is minimal, and

2.      Its ability to deliver material to difficult mould should not be impaired.

Injection pressure of 15.8 MPa (2000 psi) seem to provide a good balance between residual stress and strength in the part, thermal degradation, wear & tear on the machine & economical cycle time.

For multiple cavities, sprue diameter should be 125% of the runner at principle diameter.

Runner system:

The runner system receives material from the sprue bushing and leads to the cavity via gate.

The pressure drop should be kept at minimum. Runner shapes will defer in size & geometry depending on the type of material used.

For PVC, full round or Elliptical runners are preferred. Diameter should be between 6 to 16 mm depending on the shot size to prevent pressure loss. Half round or sharp corner runners are avoided.

Where sharp turns are necessary, it is advisable to incorporate large radii in the runner system.

The surface finish of the runner system should be as good as that of cavity.

To trap the initial cold material from the nozzle, cold slug well is provided at the end of sprue or  main runner, about 1-1.5 D in depth.

Gates:

This is a section at the end of the runner at which point, the PVC melt is delivered to the cavity.

The type of gate to be used depends upon –

1.      The part geometry

2.      Material flow, and

3.      Product end use

As a thumb rule, gates should be as short as possible. Lengths of 0.6 – 1.3 mm are recommended.

Large gates can cause longer cycles, marred surface, sink marks & stresses.

While, small gates can cause splay marks, difficulty in filling the part, freezing.

However, to facilitate rapid filling without frictional decomposition and to reduce jetting effect of the initial surge of the material in the mold, flow channels and gates for rigid PVC are 20-40% larger than other polymers and have minimum land length.

Vents:

While mould is being filled, the air in the mould must be displaced. Trapped air can be heated to very high temperatures resulting in scorching or discolouration of PVC.

Air escape is achieved through vents.

Thus, depending on the mould filling time, to avoid air being trapped in the weld line- location, numbers as well as dimensions of vent matters. Due to highly viscous nature of PVC, vent dimensions are larger as compared to other thermoplastics.