1. Shrinkage
Below are the factors affecting thermoplastic shrinkage:
1.1 Types of Plastics
Thermoplastics have higher shrinkage rate, wider range of shrinkage area and clearer shrinkage directions than thermosetting plastics because of volume change caused by crystallization during injection molding processing, high internal stresses, high frozen in-mold residual stresses and molecule orientation. Additionally thermoplastic still have higher shrinkage even after post-mold shrinkage, annealing and humidity treatment.
1.2 low density solid outer layer of an injection article formed when hot melt cooled immediately after contacting mold cavity surface. The low thermal conductivity of plastic makes the inner part of an article cool slow and form large shrinked high density solid layer. Therefore, thick wall, slow cooling and high density layer has shrinkage. Inserters, inserter layout and their quantity directly affect melt flow, density distribution and shrinkage force, so injection part geometry and its characteristics affect shrinkage and its directions.
1.3 Inlet shape, dimension and distribution has direct effects on melt flow direction and its density, holding pressure to offset shrinkage and holding pressure time. A direct inlet with a large cross-section area (specifically a thick cross section) leads to low shrinkage and low variation in directions. A wide short inlet leads to high variation in directions. Area close to inlet or parallel to flow direction has high shrinkage.
1.4 Mold conditions Effect. High mold temperature allows melt to cool slowly and to form high density and high shrinkage. Especially for crystalline plastics, high crystallinity and large volume change result in even higher shrinkage. Mold temperature distribution, cooling differential inside and outside injection part and density uniform directly affect shrinkage of every section of the part and its direction. Holding pressure and holding pressure time also affect shrinkage. The higher pressure and the longer time, the lower shrinkage and the more directions. High injection pressure results in low melt viscosity differential, low shear stress among layers and high ejection bounce impact force, hence, shrinkage can be reduced a bit. High melt temperature leads to high shrinkage and low variation in directions. Shrinkage can be reduced by adjusting mold temperature, pressure, injection speed and cooling time.
To design a mold, taking into consideration factors, such as plastic shrinkage rate, part wall thickness, geometry, and inlet shape and dimension as well its distribution, empirically determine shrinkage rate of all sections of a part, then calculate the cavity dimension. Below are the general rules to design a mold where high precision parts are required and shrinkage is difficult to control.
1)Design small shrinkage rate for outside diameter and large shrinkage for inside diameter of a part, leaving room for adjustment after mold tryout.
2)Perform mold tryout to determine gating system type, dimension and process conditions.
3)Determine dimension change (24 hours after post-mold treatment) to parts where post-mold treatment is required.
4)Modify mold based on actual shrinkage.
5)Perform mold tryout again to fine adjust process conditions to meet required shrinkage.
2.Melt Flowability
2.1 A plastic flowability can be analyzed by molecular weight, melt index,Archimedes spiral flow length, apparent viscosity and flow ratio (flow length/part wall thickness).A plastic has good flowability with low molecular weight and high MWD, poor molecule structure regularity, high melt index, high spiral flow length, low apparent viscosity and high flow ratio. To the same brand plastic, it is required to read the technical instructions to determine if it is suitable for injection mold processing. Plastic flowability can be classified in three groups according to mold design.
1)Excellent flowability: PA, PE, PS, PP, CA, Poly(4-methylpentene);
2)Good flowability: PS series resin (ABS, AS), PMMA, POW, polyphenylene ether;
3)Poor flowability: PC, rigid PC, polyphenylene ether, polysulfone, polyarylsulfone, fluorinated plastics.
2.2 A plastic flowability changes with different processing conditions. Below are major factors:
1) High melt temperature increases flowability, but different plastics have different behaviors. The flowability of PS, specifically impact resistant grade with high MFR, PP, PA, PMMA, modified PS (such as ABS, AS), PC an CA is significantly affected by melt temperature. But the change in melt temperature has very little effect on PE and POM flowability. Hence, adjusting temperature is good to control the flowability of the former plastics.
2) Pressure. The flowability of some plastics, such as PE and POM, is improved when melt undergoes high shear stress under injection pressure. Adjustment of injection pressure is used to control the flowability during process.
3) Mold Structure.
The actual melt flow in cavity is affected by gating system type, dimension, layout, cooling system design, resistance to melt flow (such as surface finish, sprue cross-section thickness, cavity geometry an exhaust system. Melt flow decreases by all factors reducing melt temperature and increasing melt flow resistance. The right structure should be selected to design a mold based on plastic flowability. It is also required to adjust melt filling during processing by control of melt temperature, mold temperature, injection pressure and injection speed.
3. Crystallinity
Thermoplastics can be classified into two groups depending on whether crystallinity occurs or not during cooling stage: crystalline plastics and amorphous plastics.
When plastic melt cools down, the independently free-moved, disordered molecules stop moving and move into somewhat fixed sites with tendency to organize into formal model. This phenomenon is so-called crystallinity.
The visual method to tell two types of plastics is based on the transparency of a thick wall of part. Generally a crystalline plastic is opaque or semitransparent like POM, while amorphous plastic is transparent such as PMMA. ButPoly(4-methylpentene)and ABS are exception.Poly(4-methylpentene)is a crystalline plastic with high transparency, ABS is an amorphous plastic which is opaque.
Below is the general guidance to design mold and to select injection molding machine when process crystalline plastics.
1)Much heat is needed to heat the materials to processing temperature. Equipment with high plasticizing capacity is required.
2)Much heat is released during cooling and solid ting stage, sufficient cooling is required.
3)There is big difference in specific gravity between melt and solid, high shrinkage, diminished voids and gas cavity occur.
4)Fast cooling leads to low crystallinity, low shrinkage and high transparency. Crystallinity degree depends on part wall thickness. Thick wall cools slowly, resulting high crystallinity, high shrinkage. Hence, mold temperature is required to be controlled for crystalline plastics.
5)Anisotropy is noticeable with high residual stresses. After released from mold, non-crystalized molecules tends to crystallize and to be under energy non-equilibrium, resulting in deformation and warpage.
6) Crystallinity temperature range is narrow, it easily happens for non-melt material not to move in mold or to block gate.
4. Heat sensitive plastics and hydrolysis sensitive plastics
4.1 Heat sensitivity is that some plastics are sensitive to heat. These plastics are prone to discolor, degrade and decompose when heated for long time or when melt temperature increases under high shear stresses in gate with very small cross-section area. These plastics, so-called heat sensitive ones, include PVC, PVDC, vinyl acetate copolymer, POM, PCTFE, etc. when decomposed, heat sensitive plastic releases monomers, gas and solid by-product, some of which is irritating, corrosive and toxic to human, machine and mold. Therefore, screw type injection machine, gate system with large cross-section area, chrome coated mold and barrel without dead points must be selected to design mold, select injection machine and process these materials. Processing temperature should be strictly controlled. Adding heat stabilizers in plastic helps minimize heat sensitivity.
4.2 Some plastics such as PC contain very low moisture, but they decompose at high temperature and high pressure. This phenomenon is called hydrolysis; Pre drying is required.
5. Stress Crack and Melt Rupture
5.1 Some plastics are sensitive to stress. When processed these plastics generate internal stresses, making them brittle and cracking. Part starts to crack under external stresses or in solution. Apart from adding anti-cracking additives, it is important to keep material dry and to select proper processing conditions to reduce internal stresses and to enhance crack resistance. Increase in ejection slope and proper inlet and ejection system should be taken into consideration to design mold. It is also important to adjust melt temperature, mold temperature, injection pressure and cooling time. Don’t eject parts when they are cold and brittle. Post-mold treatment is preferred to increase crack resistance and minimize internal stresses. Avoid parts contacting solutions.
5.2 When plastic melt at constant temperature flows through nozzle exceeding a certain point, transverse crack occurs on melt surface. This phenomenon is called melt rupture, which resulting in poor surface appearance and poor physicality. When processing plastic under high shear rate, it is recommended to enlarge nozzle, sprue, inlet cross section area to reduce injection speed and increase melt temperature
6Thermal Property and Cooling Speed
6.1 Different plastics have different specific heat, thermal conductivity and heat deformation temperature. Plastics with high specific heat need much heat to melt; injection machine with high plasticizing capacity should be selected. Plastics with high deformation temperature need short cooling time; ejection part can be at high temperature. But cooling distortion should be avoided after ejection. Plastics with low thermal conductivity cool slowly (such as ionic polymer), sufficient cooling and enhanced mold cooling are required. Hot runner mold is suitable for plastics with low specific heat and high thermal conductivity. High speed mold is adverse to plastics with high specific heat, low thermal conductivity, low deformation temperature and slow cooling. Right injection machine and enhanced cooling mold are required.
6.2 Different plastics require different proper cooling rate depending on their characteristics and part geometry. Mold requires heating and cooling system to maintain certain mold temperature. When plastic melt increases mold temperature, mold needs cooling to minimize part distortion after ejection, to reduce cycle time and to reduce crystallinity degree. When the residual heat in plastic material is insufficient to maintain the mold temperature at certain degree, a heating system needs to be set in mold to keep the mold at certain degree. As such, the cooling rate is controlled and plastic is still flowable to improve filling. The slow cooling prevents nonuniform cooling inside and outside of a thick wall part and also increase crystallinity degree. In a case of processing a plastic with good flowability, non-uniform melt temperature and needing to fill large area, sometimes mold heating and mold cooling can be alternately used or molding heating in partial area and mold cooling can be concurrently used. Thus, the mold should be set up with a cooling or a heating system.
7. Hygroscopy
Plastics contain all kinds of additives which have different water affinities. Plastics are classified into two groups: hygroscopic plastics absorbing moisture and non-hygroscopic plastics not absorbing moisture. The moisture content must be controlled within an allowable range. Otherwise, under high temperature and high pressure conditions, moisture absorbed in plastics will vapors or hydrolysis will occur, resulting in foaming, reduced flowability and poor appearance and poor mechanical properties. Hygroscopic plastics need proper drying or pre-drying.