Lubomír Zeman: Plastic injection molding and surface quality, part 4

Surface Treatments for Plastic Products

So far, we have discussed surface treatments used in thermoplastic injection moulding – creating a surface finish on the shaping components of the injection mould and transferring its negative impression to the visible surface of the moulded part. In this chapter, I will briefly mention other technologies that can be used to create gloss and textures directly on the surface of thermoplastic parts and, of course, evaluate the result, such as gloss or roughness.

Laser Polishing of Plastics

In imaging technology and automotive headlights, aspherical – non-circular – surfaces and free-form surfaces are increasingly used, which are difficult to polish using conventional material-removal polishing techniques. 

Here, laser polishing technology based on surface melting without material removal can be used. The process is based on the absorption of CO2 laser radiation in a thin surface layer of the part being polished, where a temperature just below the material's vaporisation temperature is generated. As a result of this temperature, the viscosity of the material in the thin surface layer is significantly reduced – the material flows – and surface roughness is smoothed out by surface tension – remelted – achieving a reduction in microroughness.

By selecting appropriate parameters, such as the preheating temperature, the laser beam traverse speed and its intensity distribution, users of plastics laser polishing technology can flexibly use this process to polish virtually any surface, including surfaces inside holes, where the beam is directed to the relevant location using deflectors (beam-steering devices).

The polishing speed is up to 10 cm2.s-1, which is approximately 2 x faster than conventional polishing, making it advantageous, for example, for polishing PC lenses.

Mechanical Texturing of Plastics 

This involves creating three-dimensional patterns on the surface of plastic objects. The principle of mechanical texturing consists of pressing a heated texturing roller or stamp into the cold surface of a thermoplastic object.

The reverse procedure involves heating the surface of the thermoplastic object and pressing a cold texturing tool into it.

A relief surface can also be created chemically, for example by printing with activators that promote decomposition of the polymer concerned, creating a local change in thickness on the surface of the part.

Polishing of Plastics 

Polishing is the opposite of texturing, as it involves removing surface irregularities. The surface of plastic parts can be polished mechanically, with solvents or with a flame – by briefly passing a non-smoking flame, for example a hydrogen flame, over the surface. 

Metallisation of Plastics 

In addition to an attractive appearance, metallisation technology also improves physical and mechanical properties: it increases elongation, impact toughness, flexural strength and wear resistance, reduces moisture absorption and permeability to liquids and gases, increases resistance to solvents and oils, limits ageing of the plastic and increases resistance to heat deformation. The coating on a metallised part acts as a Faraday cage and can therefore provide electrical or magnetic shielding (Faraday cage = a principle based on the understanding that electric charge is concentrated only on the surface of a conductor, not within its volume, meaning that no electric field acts inside the conductor).

Suitable part design and manufacturing technology have a decisive influence on the successful metallisation of plastic parts – the process parameters used in their manufacture and the surface treatment before metallisation are decisive for the adhesion of the metal coating to the plastic.

A whole range of processes is used for metallisation:

• electroless plating - metal is deposited onto the surface of objects from a solution of a salt of the coating metal through the action of a reducing agent either permanently present in the plating bath or applied to the prepared surface of the plastic parts; the method is advantageous for plating even complex products with copper, nickel, as well as silver or gold 
• electroplating - metal is deposited from a solution by the action of direct electric current - the electroplating process consists of several plating operations: activation - etching the base thermoplastic material and incorporating colloidal metal particles into the plastic surface; the first metal layer forms on these particles as electroless nickel; some processes use electroless copper, which is coated in the next step with a thin layer of immersion copper (copper, sulphuric acid, inhibitors), thereby producing a continuous metal layer onto which further metal layers are applied by conventional electrolytic methods - acid copper plating, semi-bright nickel plating, bright nickel plating, satin nickel plating, a microporous nickel layer and a final decorative chromium layer - drying the plated part and removing it from the plating rack; the thickness of the individual layers ranges from 0.0001 mm to approximately 0.025 mm. Among thermoplastics, the most suitable and most commonly used for plating are amorphous plastics - ABS and PC/ABS, and, to a lesser extent, semi-crystalline PA
• vacuum metallisation - deposition and evaporation of thin metal layers onto the surface of plastic parts by vacuum sputtering – vacuum of 10 E-3 Pa to 1 Pa; Cu, Al, Ni and their alloys are usually sputtered 
• thermal spraying of metals - molten metal is applied to the surface of the part by a stream of air
• a metal coating can also be formed from the gas phase by decomposing vapours of volatile metal compounds through the action of heat, or by thermal spraying of metals whose melting temperature is lower than the degradation temperature of the plastic being treated

Surface Protection by Coating with Plastics or Powdered Plastics

This makes use of the resistance of certain types of plastics to chemical and weathering effects.

Surface protection, particularly of metal structures, machinery and equipment, buildings and facilities, is primarily associated with coating materials and coating application technologies.

Coating materials include low-viscosity polymer solutions (solvent-based), dispersions (systems in which particles of one substance are uniformly dispersed in another substance), pastes, or solutions of thermosets (two-component types), prepolymers, etc.
The most widely used materials are cellulose derivatives, PVC, acrylic polymers, styrene polymers and copolymers, polyvinyl acetate, polyurethanes, phenol-formaldehyde, alkylphenol, silicone and coumarone resins.

For poorly soluble plastics - PTFE and other fluoroplastics, PVC and its copolymers, etc. - dispersions are used for application.

Powdered plastics can also be used for surface finishing, primarily polyolefins, particularly PE, and polyamides, most commonly PA 11.

Plastics are applied in coating layers ranging from several tens of microns to several millimetres :

• flame spraying - powdered polymer is applied using a spray gun; the powder carried by a transport gas is melted in the gun in an oxygen - acetylene flame and propelled by the stream of combustion gases onto the heated surface of the part being treated
• fluidised-bed coating - the preheated part is immersed in a layer of fluidised polymer powder; a modification of this process involves sprinkling the heated object with polymer powder and subsequently tumbling it
• electrostatic powder spraying - polymer powder is sprayed by a stream of air from a spray gun connected to a high voltage - 50 kV to 90 kV - the object being coated forms the other pole
• another option is lamination, used to manufacture multilayer products - similar to laminating, where plastic films are laminated onto a substrate material
• cladding - plastic films are bonded to metal sheets using adhesives

In addition to their protective functions, the applied coatings also serve aesthetic functions.

Velvet Finishing – Flocking

This is a technological process in which a velvet-like structure is created on the surface of a product by first applying a layer of adhesive - dispersion or polyurethane adhesives - to the substrate material – textiles, paper, plastic films, plastic semi-finished products, plastic parts - and then depositing flock consisting of short fibres – cotton, polyamide, etc. - onto the prepared surface in an electrostatic field. The sieve - applicator, through which the fibres are deposited, forms one pole - the positive pole, and the object being treated is earthed. The fibres land in the direction of the electric field lines, i.e. perpendicular to the surface, where they become embedded in the adhesive layer. The fibres – cotton, viscose, acetate, polyamide - can range in length from 0.3 mm to approximately 12 mm.

Influence of Selected Parameters on Roughness – Surface Texture

At the beginning of this chapter, let us recall the indicative surface roughness values for injection moulds that can be achieved using selected machining technologies – Table 7.

Table 7 : Indicative surface roughness values for injection moulds depending on the machining technology used

The issue of the surface roughness of injection mould components in relation to thermoplastic injection moulding technology was studied fairly intensively at the time by staff and students at UTB in Zlín. Allow me to present several general conclusions here.

Injection Moulding Parameters – Flowability of Polymer Melts – Surface Quality

Flowability was evaluated by injecting a polymer melt into a mould whose cavity formed an Archimedean spiral of a specified size. The length of the spiral was used as the measure of flowability. The results are shown in Figures 28, 29 and 30.

Figure 28 : Influence of gate size and surface quality resulting from the respective machining technology on the flowability of a polymer melt, evaluated by the length of the Archimedean spiral ; left – mould cavity with the moulded part ; right – Archimedean spiral moulding

Figure 29 : Effect of injection speed and surface finish produced by the respective machining technology on polymer melt flowability assessed by the length of an Archimedean spiral

Figure 30 : Effect of injection pressure and surface finish produced by the respective machining technology on polymer melt flowability assessed by the length of an Archimedean spiral

Commentary - all three parameters, at their highest values, exhibit the longest flow lengths on the surface with the greatest roughness. If we look at the differences in spiral length between polished surfaces and surfaces with a coarse texture when using the highest parameter values (the situation is similar at the other levels), the average difference is approximately 6 %, which is not a significant value, and from my perspective it is therefore not necessary to devote any substantial attention to the issue of the surface finish of runner channels and cavity components of injection moulds in relation to polymer melt flow. 

Further Selected Results – Surface Roughness - Conditions

The further results discussed were obtained under the following conditions:

• injection-moulded materials – amorphous: ABS (three grades with melt flow indices of 7,9 / 16,1 / 29,5 g/10 min. at 220 °C and a load of 10,0 kg), PC, PMMA ; semi-crystalline: PP , PA 66
• mould cavity material – steel ČSN 19552.4 (1.2343), thermal conductivity at 20 °C 25 W.m-1.K-1, duralumin ČSN 424222 (AW-7075), thermal conductivity at 20 °C 150 W.m-1.K-1, copper ČSN 423001 (CW0004A), thermal conductivity at 20 °C 388 W.m-1.K-1
• mould cavity surface finish – 10 mould cavities were produced from each of the three cavity component materials using the same manufacturing process for each cavity, for example: 
  • cavity 1 – all three mould insert materials – turned using cutting fluid, depth of cut 0,10 mm, feed 0,01 mm/ot., rotational speed 250 1/min.; manual grinding – grit size 1200, 900, 600 ; lapping paste with a grain size of 45 and 25 µm ; polishing with cotton wool and paste with a grain size of 5 µm, 1 µm and 0,5 µm at a rotational speed of 1200 1/min. ; mould cavities 1 to 6 – the finishing operations were grinding or lapping ; inserts for cavities 7 to 10 – their functional surfaces were produced solely by turning using different feed rates and rotational speeds

Surface roughness, among other things, also affects the visual perception of roughness, i.e. how we perceive the surface – surface gloss or mattness across their range defined by the Ra and Rz values.   

The Ra and Rz roughness values, including standard deviations for the individual cavities – cavities 1 to 10 – and the given material are listed in Tables 8 a,b,c.

Table 8 : Ra and Rz roughness values, including standard deviations for the individual cavities – cavities 1 to 10 and the given mould cavity material

A selection of the findings established at UTB in Zlín is further documented in the relevant figures and in the following commentary. These concern the effect of selected parameters on surface roughness.

Injection Pressure

Figure 31 : Effect of injection pressure on surface finish defined by the Ra and Rz roughness values for a cavity insert made of 1.2343 steel and the injection-moulded amorphous material ABS Magnum 3453 , 3D roughness = cavity insert No.1

Commentary – injection pressure 30 MPa – the surface consists of high peaks and shallow depressions – the polymer melt is unable to replicate the surface of the lapped and polished areas of the cavity insert; the melt did not replicate the surface of any of the cavity inserts and formed its functional surface with the given roughness independently of it ; pressure 60 MPa – low and uniformly distributed peaks and depressions – here it is also necessary to recognise the relationship between injection pressure and injection speed, or any injection speed profile throughout the injected melt shot, whereby the injection pressure must be high enough to overcome, at the required injection speed, all hydraulic resistance that impedes polymer melt flow in the mould's runner system and its cavities. The above also applies to semi-crystalline materials, with the crystalline fraction affecting roughness – see below – and applies to all subsequent comments.

Injection Speed

Figure 32 :Effect of injection speed on surface quality defined by the roughness values Ra and Rz for the mould insert material steel 1.2343 and the injection-moulded amorphous material ABS Magnum 3453, 3D roughness = mould insert No.2 – turned, ground, lapped

Comment – the 3D surface profiles are very similar for both injection speeds – in general : the surface roughness values of the mouldings yield very similar results for all mould inserts and injection speeds – they do not differ significantly – changing the injection speed has no substantial effect on the surface quality of the mouldings as defined by their roughness. The relationship between injection pressure and injection speed is given in the comment accompanying Figure 31.

Pressure Level in the Holding Phase

Figure 33 :Effect of the pressure level in the holding phase of the injection moulding process on surface quality defined by the roughness values Ra and Rz for the mould insert material steel 1.2343 and the injection-moulded amorphous material ABS Magnum 3453, 3D roughness = mould insert No.3 – electrical discharge machining, grinding, lapping

Comment – the 3D roughness profile is significantly higher at a low pressure in the holding phase than at a higher holding pressure ; the low pressure in the holding phase was unable to ensure replication of the desired mould insert surface even though the moulding was free of flash and sink marks – the pressure in the holding phase has a strong influence on good replication of the mould insert surface – it should be noted here that high pressures in the holding phase may affect mould rigidity and increase the level of residual stress in the mouldings.

Melt Flow Index - MFI

Figure 34 :Effect of the melt flow index of the injection-moulded material on surface quality defined by the roughness values Ra and Rz for the mould insert material steel 1.2343 and the injection-moulded amorphous material ABS Magnum 3616 / 3453 / 8391, 3D roughness = mould insert No.1

Comment – ABS 3616 with a low flow index (7,9 g/10 min. / 220 °C ; 10,0 kg) exhibits an uneven structure of high peaks and low valleys, ABS 8391 with a high MFI (29,5 g/10 min. / 220 °C ; 10,0 kg – with good flowability) has a regular structure with a low profile ; the MFI, or the viscosity of the injected polymer melt, has a significant influence on the surface quality of the moulding

Crystalline Content – Thermal Conductivity

The influence of the supramolecular structure (crystalline content) on surface quality for semi-crystalline materials, a comparison with amorphous materials, including the influence of the thermal conductivity of the mould insert material, is shown in Figures 35 and 36 and in Table 9a, b, c.

Table 9 : Effect of the thermal conductivity of polished mould insert materials on crystallinity – crystalline content – in the semi-crystalline materials PP and PA 66

Comment – the thermal conductivity of injection mould cavity materials affects the cooling rate gradient of the polymer melt and influences the crystalline content in semi-crystalline injection moulding materials. Slow cooling produces a small number of crystallisation nuclei and vice versa – an example is given in the table: melt temperature PA 66 280°C, PP 210°C , cooling time 30 s, total injection moulding cycle time 56 s – the cooling gradient for PA 66 is low, with slow cooling and thus also a relatively lower crystalline content compared with PP – with duralumin and Cu, their greater thermal conductivity resulted in a certain increase in the crystalline content of PA 66 – it must be borne in mind that a lower crystalline content in semi-crystalline materials allows larger spherulites to form and thus also increases the risk of internal defects in the mouldings

Amorphous Materials vs Semi-crystalline Materials

Figure 35 : Effect of the supramolecular structure on surface quality defined by the roughness values Ra and Rz for the mould insert materials steel 1.2343, duralumin 424222 and copper 423001 – comparison of amorphous materials ( PC, PMMA ) and semi-crystalline materials ( PP, PA 66 )

Commentary – amorphous materials – PC, PMMA – replicate polished surfaces (mould inserts 1 to 6) relatively accurately – see the detailed views in the inset boxes; semi-crystalline materials – PP, steel, 40.84 % crystalline content, PA 66, steel, 18.58 % crystalline content – exhibit up to 10 x greater roughness on polished surfaces than amorphous materials, with PP showing the greatest deviations due to its high crystalline content; on rougher, turned surfaces (mould inserts 7 to 10), the differences between amorphous and semi-crystalline materials more or less even out – the same applies to mould inserts made of duralumin and Cu – generalisation: the quality of surface replication by the moulded part depends both on the supramolecular structure of the polymer materials (amorphous x semi-crystalline) and on the crystalline content in semi-crystalline materials and the size of the spherulites.
The statements above, in the commentary on Figure 35, are supported by the 3D roughness measurements shown in Figure 36. 

Figure 36 : Effect of the supramolecular structure (amorphous x semi-crystalline) of PC and PP materials on 3D roughness for mould cavity 1 (turned, ground, lapped, polished surface) and mould cavity materials steel, duralumin, Cu

Commentary – the surface quality of moulded parts made of semi-crystalline PP and PA 66 is substantially worse, in terms of mould cavity surface quality, than that of amorphous PMMA – the roughness height for PP and PA 66 is substantially greater, which is caused by the formation of a crystalline structure as the polymer melt cools and the moulded part forms – unlike with amorphous materials, the surface is not replicated well.  

Mould Cavity Material

Figure 37 : Effect of the mould cavity material on surface quality defined by the roughness values Ra and Rz for injection moulding materials – amorphous PC, PMMA and semi-crystalline PP, PA 66

Commentary :

• PP – the crystalline content depending on the mould cavity material is within a relatively narrow range – Table 9, approximately from 37 % to 42 % ; in this case, the mould cavity material has no significant effect on crystallinity ; for mould cavities with a higher-quality surface – polishing – see the inset box, cavities 1 to 6 – the variation in roughness depending on the mould cavity material is more pronounced than for cavities 7 to 10 – surfaces turned with different turning parameters
• PA 66 – the difference in crystalline content is also not particularly significant – steel 18.6 %, Cu 22 % , duralumin 23.5 % - the effect of thermal conductivity was evident for Cu and duralumin ; in the detailed inset box, we can see that mould cavities with the highest surface quality values – gloss – cavities 1 to 3 – show pronounced variation in roughness, which becomes less pronounced as quality decreases – roughness increases – and we can therefore conclude that PA 66 is unable to replicate highly polished mould cavity surfaces well
• PC – relatively very good replication of mould cavity surfaces
• PMMA – the material replicates high-quality surfaces well – 1 to 6 ( 7 ) ; for cavities with greater surface roughness - 7 to 10 – dependence on the mould cavity material is already apparent

A more detailed view obtained by injecting PP into highly polished mould cavity 1, made of steel, duralumin and Cu, is documented in Figure 38.

Figure 38 : Effect of the mould cavity material on the surface quality of the moulded part – detail – PP, cavity 1, high gloss – differences in roughness profile heights are shown by 2D sections along the X axis – a section in the direction parallel to the flow of the polymer melt in the mould cavity

CONCLUSION – EFFECT OF SELECTED INJECTION MOULDING PROCESS PARAMETERS ON THE SURFACE ROUGHNESS OF THERMOPLASTIC MOULDED PARTS

Conclusions : the conclusions below relate to roughness and do not state or reflect other links and relationships between injection moulding process parameters 

• the surface roughness Ra and Rz of thermoplastic moulded parts can be influenced by changing the injection pressure
• the surface roughness Ra and Rz of thermoplastic moulded parts is more or less independent of the injection speed value
• the surface roughness Ra and Rz of thermoplastic moulded parts can be influenced by changing the pressure during the holding phase of the injection moulding cycle
• the surface roughness Ra and Rz of thermoplastic moulded parts depends on the viscosity (flowability) of the injected polymer melt  
• the surface roughness Ra and Rz of thermoplastic moulded parts is influenced by the supramolecular structure of the injected polymer material – amorphous x semi-crystalline
• the surface roughness Ra and Rz of thermoplastic moulded parts is influenced, for semi-crystalline materials, by the crystalline content   
• the surface roughness Ra and Rz of thermoplastic moulded parts is relatively strongly dependent on the material of the cavity-forming components of injection moulds
• for semi-crystalline materials, their crystallinity is influenced by the material used to manufacture the cavity-forming components of injection moulds
• some injection moulding materials are not suitable for injection into highly polished mould cavities because they are unable to replicate the high gloss 

End of the fourth and final part.

You can find the first part here.
You can find the second part here.
You can find the third part here. 

Cover photograph: R & D MOLD MACHINING s. r. o.