PMMA – Dimension out of tolerance after “ANNEALING”

Problem:  The Poly(methyl methacrylate); PMMA (Transparent) injection molded part out of tolerance. The dimension in the parallel to flow shorter -0.3 mm than allowing tolerance after annealing.

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In the last week of September 2021 afternoon, our current customer came via the Chat App (LINE), the most favorite chat Application for Thais at this moment. In the chat, the customer came with their problem the 2-cavity mold of transparent Poly (methyl methacrylate); PMMA parts with 4 mm thickness and molded by a high-performance injection molding machine. The post molded design considered doing an "Annealing" process for 72 hours to release stress inside the part.

I started to investigate the information I received. The cold runner mold applies 6.40 mm width and 1.20 mm gate thick; Fan Gate. (Fig.1) Runner and sprue size looked suitable for PMMA. For me, the gate was too thin, it should be thicker than 1.20 mm and thicker than the other Amorphous material design, likely ABS, PS design. But it has some reason for the thin gate, the customer says “You cannot make a gate thicker than this” because they will have a problem with the gate cutting. It was terrible to the molders if the thin gate was applied because the lower injection speed limits the processing window and the thin gate is very secure to BLUSH near the gate, especially since this part is transparent PMMA. The experienced molders maybe think about a thick tab gate to prevent the blush and improve the holding pressure phase then cut the thick tab gate with a hot cutter. I checked the screw diameter and shot stroke. The injection molding machine size is suitable for this shot.

Drying was done correctly, as discussed. PMMA is an acrylics material, and it is Hygroscopic, which requires less moisture content (0.1%) like PC, Polycarbonate. The way to reach low moisture content needs to dry in the desiccant-bed type dryer with the correct drying time. The shot stroke and V-P switch-over point and cushion remaining were good from the machine screen picture. I feel the fill time is entirely too long, 4.9 sec for a 1.54D shot stroke (15 mm/sec). The gate thin 1.20 mm is the limitation of applying the faster injection speed, but it was not too bad because the slow injection speed is one of the recommended technics for PMMA. The melt temperature is necessary to increase a bit to lower the melt viscosity.

A digital vernier caliper applied the measurement after ejected from the mold and measured once again before annealing, and the dimension was GOOD since ejected from the mold until before annealing. Then the part was annealed for 72 hours following the process design. After 72 hours, measure the part again after the pieces cool down to room temperature. The result was NOT GOOD, and the dimension is shorter than before and out of the allowing Tolerance -0.30 mm.

Know PMMA before solving the problem

PMMA is one of the unmodified acrylic materials (There are many acrylics polymers; acrylonitrile, acrylates, methacrylate, and copolymer). PMMA in the market has a few different grades depending on MW; molecular weight, comonomer, and additives. At room temperature, PMMA is an amorphous thermoplastic, hard, stiff, and brittle. PMMA’s Glass transition temperature is about 112 °C can be seen in the slope change in the Specific Volume vs Temperature plot (At 0 MPa) Fig.2. From rigid, it will be going to soft, and strength decrease at above Tg. 

Figure 2. Specific Volume vs Temperature show the Tg of PMMA

PMMA is a polar plastic that can absorb moisture(C=O), the water content can affect the dimensions. One of the most important that the designer and molders should know when designing with PMMA. It is the thermal properties concerning to CTE, Coefficient Thermal Expansion. PMMA has higher CTE than metals. When assembly PMMA with metal, it should not be fixed rigidly if the use is under fluctuating temperature. The plastic part can be under stress and crack. 

Fig.3 Viscosity plot of PMMA

Fig.3 shows the melt viscosity plot of PMMA shows the viscosity change when the Shear Rate increase. The viscosity gets lower when the Shear Rate increase. The viscosity of PMMA in the injection molding processing is higher than the general thermoplastic like Polyethylene, Polypropylene, and Polystyrene. PMMA viscosity is very sensitive to shear rate(injection speed) and temperature. The injection molding machine must be able to provide enough injection pressure (should have at least 100-140 MPa) to maintain the injection speed to push the molten into the cavity. For this case, the machine is quite good. An injection machine that had a vented barrel can be used well with PMMA processing.

A medium to high injection pressure is required due to the higher viscosity than the most above kind of plastics. Mold steel is another thing the mold designer and the molders need to consider. The steel should be strong enough to resist the high pressure during high-pressure filling. The steel with hardening is strongly recommended if high volume production.

 Polymers have a consistency index (m) and a power-law index (n) that describe their general viscosity behavior with respect to changing temperature and shear rate. The consistency index is primarily the relationship between the polymer's viscosity and temperature. PMMA has low both 2 indexes. The low number of indexes indicate that PMMA is very sensitive to viscosity change when shear rate and temperature change. The example of one grade PMMA has a power-law index, n=0.1435, PP+Talc20% (the information from Moldflow Cross-WLF viscosity model). Personally, the close loop temperature controller injection machine should consider used when working with PMMA. The newly developed injection machine mostly has a PID Control or the Proportional Integral Derivative Control; it would be best if the machine had a viscosity measuring and monitoring system. The machine without a good maintenance condition should be avoided.

ANNEALING

Most of the injection molding of PMMA part designed for stress released after molding call ANNEALING. It improves the part will not crack in the condition of use. Annealing is the process of putting the parts after molding into the oven and controlling the temperature and time. The anneal temperature depends on each grade of PMMA normally ranges between 60 °C – 90 °C; the annealing time considers based on the thickness of the part. If the thickness increase considers the annealing time is longer, and if the thickness decreases, the shorter time can be applied. The plastic manufacturer usually has a suggestion in the datasheets and recommendations. In this case, the part thickness is 4.00 mm, then apply 95 °C in the oven for 72 hours.

 When molders talk about the annealing process, they mostly think about the part's internal stress, which will make the part crack or break when the assembly or using condition. They mostly understand that it is the same thing as “Residual Stress.” Yes, It is the same thing they are not wrong, but the very important when we talk about stress in plastic. Molders should have the correct understanding of residual stress that it is the stress in polymer molecules. In the injection molding process, the stress in the molded part is the shear stress. The shear stress at the mold wall is the highest level, and secondary is the stress between molecules that entangle each other, see fig.4

Figure 4. Shear Stress Profile (Yellow)

Fast injection speed, High Residual Stress

From our discussion about residual stress, last time you said that the injection speed is very related. High speed means high residual stress. Is it so or reverse? The customer repeated his remembering. I started to explain to him when Injection speed pushes the molten plastic to flow into the cavity. During the plastic flow, molecules align and orient in the direction of flow. The faster injection speed made the molecules orient in a high degree of straightness (High orientation). From molten state to solid-state, the molecules require enough time to re-coil or re-arrange themselves to the natural orientation, amorphous try to return to the random morphology, crystalline try to return to crystallize morphology. Both the amorphous and semi-crystalline plastic; their molecules in nature do not love to transform to a solid-state while their molecules are still in high orientation.

The fast injection speed drives the molecules straight and aligns in the direction of flow. Under the part thickness inside the cavity, the molecules straight align and have high orientation much near the mold wall. The cold or warm mold surface temperature can stop the high orientation of the plastic molecules. A Colder mold surface stops the high orient molecules and develops the thicker frozen layers than the warmer mold surface temperature. The high-oriented molecules of polymer are frozen inside the frozen layers.

Polymer’s molecules that are frozen inside the frozen layer cannot return to the relaxed state as mentioned before (in nature, polymer’s molecules do not love to transform to a solid-state, while their molecules are still in high orientation). The molecules which are still under high orientation inside the frozen layers are under stress. Fast injection speed results in high orientation, cold or warmer mold surface temperature develops frozen layers and blocks the stress polymer molecules inside.

Quality Control Plan

The quality control of this part uses visual inspection and dimension inspection following the quality control plan. How many cycles are the biggest lot of production? I asked the factory. The production will take about 4 days or 8 shifts: day and night, if calculated by the cycle time and the number of shot requirements. The maintain of stability of the production over 4 days is very important, such as mold preventive in daily plan, mold cleaning, air venting cleaning, mold inspection, material drying, process control, and quality control plan. PMMA is a high viscosity, the correct air venting design, daily inspection to prevent the air venting obstructed by contamination can maintain the stability of the plastic flow inside the mold. The depth of air venting for PMMA suggests 0.025 – 0.050 mm. The factory confirmed that the air venting design was correct, and the QC process was the strict procedure.

How to identify - Residual stress

This is the general information I would like to share with the molders who may be interesting, especially the molders who are now reading and working with PMMA transparent part. The Residual stress can be identified and display the concentration of stress which will occur on the part before cutting the steel in mold making project by using CAE Software like “Moldflow” to analyze the "Birefringence". During production, PMMA transparent part can detect Residual stress by "Polarize Light", the change of stress region-to-region compared to the good part of the Golden sample can tell us the processing has changed. The Polarize light can help you to know the process change before the annealing process in quality control. 

The experiment - Problem Solving

Before any experiment started, the gate decided to modify thicker and a little wider. The gate width has been modified from 6.40 mm to 6.80 mm increase 0.40 mm, gate depth has also made it thicker 0.10 mm from 1.20 mm to 1.30 mm (still worry with the cutting). I started to study the current injection condition I received from the factory precisely. It was the latest injection molding condition which used in the production of the problem part, the injection speed was very compromised due to the gate being quite thin. The actual fill time was 4.99 sec, for the 75 mm shot stroke and 2 mm decompression(suck back). The filling control setting using 3-speed profiles, the slowest injection speed from the machine panel was 5 mm/sec the molders intended to maintain the blush at the gate, the mid-range 10 mm/sec and maximum speed at 20 mm/sec, the almost 60% of the shot stroke was controlling the part filled with speed 20 mm/sec. It was a bit too slow injection speed, but it can be understood that it cannot increase due to the shear heat and blush around the gate and effect on the transparent property. The holding pressure influenced the compensation phase 275 kgf/cm^2 while the Peak fill pressure was 458 kgf/cm^2. The weight increase after the V-P switch over was not recorded. The real plastic part from that condition was Good from the visual inspection.

According to my research, the main factor for the residual stress is below, the customer told me. 1) Melt temperature   2) Mold temperature  3) Holding time 4) Injection speed And some small effects are 5) Holding pressure and 6) Cooling time. The factory did the experiment plan by choosing the first 3 variables: Melt Temperature, Mold Temperature, and Holding time. Apply Max. – Mid. – Min. range of injection condition of those 3 variables. From the experiment, the parts after annealed the dimension result were GOOD for all samples part of the group that condition to :

– increase melt temperature for +10 °C from the middle value.

– decrease mold temperature -5 °C from the middle value.

– fixed the holding time at the middle value.

When the injection speed and holding time didn’t change, the mold temperature decreased -5 °C, and the higher melt temperature +10 °C, the melt viscosity became lower much if you see Fig. 3(a). The lower viscosity molten plastic flow inside the cavity better, the whole part has packed out and shrink less. The mold temperature has a very low influence refer to the result of all samples. The gate thicker and wider help a lot.

Summary

The modification of the gate improves the molding window wider, plastic flow easier during the filling, less stress to the molecules, and better packed-out during packing/Holding. The residual stress of the injection molding part is not the only stress that happens during plastic flow in the filling phase and packing phase, but stress occurs since the screw is rotating and shear the plastic pellets inside the barrel during plasticizing—the molten PMMA within the stress history since the plasticizing was injected into the mold. The molten plastic flow and the molecules align with high orientation in the direction of flow, then cool down and freeze inside the frozen layers before the molecules return to their comfortable state influenced by mold wall temperature. The temperature and time during annealing have a significant effect on the molecules start moving again after they freeze and re-arrange inside the part. The re-arrange of molecules happened with the whole part, especially the molecules which have high orientation; this is the “Relaxation” behavior of the polymer.

The molecules relaxation happened with the whole part in the solid-state of the polymer at the specific temperature range, especially the temperature near Tg; this is the behavior of the polymer. The shorter dimension -0.30 mm of this part after annealing the main reason is molecules re-arrangement or stress relaxation.

Solutions:  Modify gate (thicker and wider), apply the experiment plan with Melt temperature, Mold Temperature, and Holding pressure