9 Shrinkage

9 S h r i n k a g e 9.1 Introduction If plastics are processed by injection molding, deviations of the dimensions of the molding from the dimensions of the cavity cannot be avoided These deviations from the nominal size are summarized under the term shrinkage 9.2 Definition of Shrinkage In the injection-molding technique, shrinkage is the difference between an arbitrary dimension in the cavity and the corresponding dimension in the molding with reference to the cavity dimension S=1C~1M 100% (9.1) ^c Of course, this definition is not unambiguous (Figure 9.1) [9.1] On one side, the dimensions of the cavity change from thermal expansion (0 —> 1) and mechanical loading during operation (1 —> 2), on the other side, the effect of time on the dimensions of the molding has to be taken into consideration (2 -^ 5) One distinguishes the demolding shrinkage (point 3), which is measured immediately after the molding has been ejected, and the processing shrinkage (point 4) The processing shrinkage is measured after storing the molding in a standard climate for 16 hours [9.2] In this context the cavity dimension has to be determined at an ambient temperature of 23 0C ± C Shrinkage after demolding (SD) Processing shrinkage (PS) Total shrinkage (TS) Dimension of molding Figure 9.1 Dimensional changes as a function of time [9.1] Dimension in cold mold, Dimension in hot mold, Dimension in mold under clamping force and holding pressure, Dimension of molding after demolding, Measurement of processing shrinkage (DIN 16901), Dimension after storage After-shrinkage (AS) Timet After extended storage another dimensional change may occur from the effect of temperature changes and especially from post-conditioning It is called post-shrinkage (4 —> 5) This change is caused by relaxation of residual stresses, re-orientation and post-crystallization in crystalline materials Except in crystalline materials, it is negligibly small, though The sum of processing shrinkage and post-shrinkage is called total shrinkage If additional dimensional deviations from moisture absorption or higher temperatures of use have to be taken into account at the time of acceptance, post-treatment and conditions of measurement have to be negotiated between molder and customer In addition, one can distinguish shrinkage in dependence on the direction of flow (Figure 9.2) Radial processing shrinkage is shrinkage in the direction of flow, tangential shrinkage is that perpendicular to the direction of flow Contour shrunk Mold contour Plastic core Bound length Figure 9.2 Frozen frame Frozen model [9.7] The difference in processing shrinkage is the difference between radial and tangential shrinkage and is a measurement of the anisotropy of the shrinkage The shrinkage in thickness is measured as section thickness, but it is usually not of interest in practice For measuring, any kind of mechanical or optical instrument can be used, but a possible error from the measuring force should be taken into account for soft materials If the dimensions in Equation (9.1) are replaced by the volumes of cavity and molding, one talks about volume shrinkage [9.3] Figure 9.3 Magnitude of shrinkage depending on direction of flow SR Radial shrinkage, ST Tangential shrinkage, S Shrinkage difference (9-2) V = Specific volume of the material (Figure 9.7) Longitudinal and volume shrinkage are related to one another but because of anisotropy (dependency of shrinkage on direction), linear shrinkage cannot be calculated from volume shrinkage Another problem is the impossibility to measure volume shrinkage It is possible to make an assumption of shrinkage from volume shrinkage of thermoplastics in the following methode: The shrinkage in direction of thickness H of an injection molded part SH = (0.9 - 0.95) S v The shrinkage in the direction of length L SL = (0.05 - ) S v For crosslinking polymers exists a special standard for shrinkage (see source [9.4]) 9.3 Tolerances Direction of mold clamping The question of attainable tolerances is a cause of many complaints and, in extreme cases, has to be the subject of negotiations between molder and customer Tolerances should never be closer than required for the perfect functioning of the part in use Figure 9.4 Mold-related (top) and not-mold-related (bottom) dimensions [9.2] Direction of slide movement Tolerances are closely related to shrinkage but also to the nature of the particular plastic Close tolerances can only be expected with precise machine and mold control Therefore they are beyond any action the mold maker can take He has to try, however, to meet the required dimensions assuming normal processing conditions If they can only be met under extreme conditions, the results are not in the center of the tolerance range Then this range can easily be exceeded In addition, one has to differentiate between dimensions connected to the mold and those not connected to the mold Dimensions connected to the mold are those which are determined by duplicating one mold part (Figure 9.4 top) Dimensions not connected to the mold are generated by the interaction of parts movable towards each other (stationary STANDARDS AND PRACTICES OP PLASTICS CUSTOM MOLDERS Engineering and Technical Standards ABS NOTE: The Commercial values shown below represent common production tolerances at the most economical level The Fine values represent closer tolerances that can be held but at a greater cost Drawing Code Dimensions (Inches) Plus or Minus in Thousands of an Inch A = Diameter (see Note #1) B = Depth (see Note #3) C = Height (sec Note #3) 6.000 to 12.000 Comm ± for each additional 003 inch add (inches) D=BottomWall (see Note #3) E = Side Wall (see Note #4) F = Hole Size Diameter (see Note #1) G = Hole Size Depth (seeNote#5) 0.000 to 0.125 0.125 to 0.250 0.250 to 0.500 0.500 & Over 0.000 to 0.250 0.250 to 0.500 0.500 to 1.000 Draft Allowance per side (see Note #5) 0.000 to 3.000 Flatness (see Note #4) 3.000 to 6.000 Internal Thread Size (class) External Concentricity (sec Note #4) "Fillets, Ribs, Corners (see Note #6) Surface Finish Color Stability (T.I.R.) Finc± 002 004 002 003 002 002 002 003 004 003 004 005 001 001 002 002 002 002 003 2° 1° 015 030 1 010 020 2 009 005 025 015 REFERENCE NOTES — These tolerances not include allowance for aging characteristics of material - Tolerances based on W wall section — Parting line must be taken into consideration — Part design should maintain a wall thickness as nearly constant as possible Complete uniformity in this dimension is impossible to achieve — Care must be taken that the ratio of the depth of a cored hole to its diameter docs not reach a point that will result in excessive pin damage —These values should be increased whenever compatible with desired design and good molding technique — Customer-Molder understanding necessary prior to tooling (see Note #7) (see Note #7) Figure 9.5 Practical tolerances on dimensions of articles molded from ABS (Courtesy of the Society of the Plastics Industry) and movable mold half, slides) (Figure 9.4 bottom) This differentiation takes into account the lower accuracy, which results from movable mold components; they not have exactly reproducible end positions Standards for tolerances are given in the form of tables in the "Plastics Engineering Handbook" [9.11] These tables (an example is shown in Figure 9.5) were developed by the Society of the Plastics Industry, Inc and are based on data obtained from representative material suppliers and molders Table 9.1 Coordination of tolerance groups with molding materials [9.2] Moldings made of: Tolerance groups For For dimensions common with allowances tolerances entered in the drawing Grade Grade 130 120 140 130 Acetal (polyoxyrnethylene) (unfilled), part length: < 150 mm 140 Acetal (polyoxymethylene) (unfilled), part length: >150 mm 150 Acetal (polyoxymethylene) (filled) 130 120 110 Acrylic 130 120 110 Diallyl phthalate compounds (with inorganic filler) 130 120 110 Polyethersulfone (unfilled) 130 120 110 Polyethylene (unfilled) 150 140 130 Polyethylene terephthalate (amorphous) 130 120 110 Polyethylene terephthalate (crystalline) 140 130 120 Polyethylene terephthalate (filled) 130 120 110 Polyphenylene oxide 130 120 110 Polyphenylene oxide-styrene mixture (filled, unfilled) 130 120 110 Polyphenylene sulfide (filled) 130 120 110 Polypropylene (unfilled) 150 140 130 Polypropylene1 (filled with glass ribers or talc) 140 130 120 Polypropylene impact copolymer (unfilled) 140 130 120 Polystyrene 130 120 110 Polysulfone (filled, unfilled) 130 120 110 Polyvinyl chloride (without plasticizer) 130 120 110 Poly vinyl chloride (with plasticizer) No information at present Styrene-acrylonitrile (filled, unfilled) 130 120 110 Styrene-butadiene copolymers 130 120 110 Fluorinated ethylene propylene 150 140 130 Thermoplastic polyurethanes (hardness 70-90 Shore A) 150 140 130 Thermoplastic polyurethanes (hardness > 50 Shore D) 140 130 120 Take next higher tolerance group for unfilled, crystalline thermoplastics and wall thicknesses more than mm Pertinent information concerning tolerances may also be received from the British Standard BS 4042 The following data are provided with reference to the German Standard DIN 16 901 Depending on the molding material one can determine several tolerance groups (Table 9.1) Within each group a distinction is made between two grades of accuracy for entered allowances above and below nominal sizes For each tolerance group allowances depending on nominal sizes can be taken from a second table (Table 9.2) In this table a Table 9.2 Coordination of tolerances with tolerance groups [9.2] Tolerance Code letter1 group from Table Range of nominal dimensions (more than/until) 1 A ±0.28 ±0.30 B ±0.18 A 10 10 15 15 22 22 30 30 40 40 53 ±0.33 ±0.37 ±0.42 ±0.49 ±0.57 ±0.66 ±0.78 ±0.20 ±0.23 ±0.27 ±0.32 ±0.39 ±0.47 ±0.56 ±0.68 ±0.23 ±0.25 ±0.27 ±0.30 ±0.34 ±0.38 ±0.43 ±0.49 ±0.57 B ±0.13 ±0.15 ±0.17 ±0.20 ±0.24 ±0.28 ±0.33 ±0.39 ±0.47 A ±0.20 ±0.21 ±0.22 ±0.24 ±0.27 ±0.30 ±0.34 ±0.38 ±0.43 B ±0.10 ±0.11 ±0.12 ±0.14 ±0.17 ±0.20 ±0.24 ±0.28 ±0.33 A ±0.18 ±0.19 ±0.20 ±0.21 ±0.23 ±0.25 ±0.27 ±0.30 ±0.34 B ±0.08 ±0.09 ±0.10 ±0.11 ±0.13 ±0.15 ±0.17 ±0.20 ±0.24 Common tolerances 160 150 140 130 Tolerances for dimensions with entered allowances 160 150 140 130 120 110 Precision molding A 0.56 0.60 0.66 0.74 0.84 0.98 1.14 1.32 1.56 B 0.36 0.40 0.46 0.54 0.64 0.78 0.94 1.12 1.36 A 0.46 0.50 0.54 0.60 0.68 0.76 0.86 0.98 1.14 B 0.26 0.30 0.34 0.40 0.48 0.56 0.66 0.78 0.94 A 0.40 0.42 0.44 0.48 0.54 0.60 0.68 0.76 0.86 B 0.20 0.22 0.24 0.28 0.34 0.40 0.48 0.56 0.66 A 0.36 0.38 0.40 0.42 0.46 0.50 0.54 0.60 0.68 B 0.16 0.18 0.20 0.22 0.26 0.30 0.34 0.40 0.48 A 0.32 0.34 0.36 0.38 0.40 0.42 0.46 0.50 0.54 B 0.12 0.14 •0.16 0.18 0.20 0.22 0.26 0.30 0.34 A 0.18 0.20 0.22 0.24 0.26 0.28 0.30 0.32 0.36 B 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.26 A 0.10 0.12 0.14 0.16 0.20 0.22 0.24 0.26 0.28 B 0.05 0.06 0.07 0.08 0.10 0.12 0.14 0.16 0.18 A for dimensions not connected to the mold, B for dimensions connected to the mold Range of nominal dimensions (more than/until) 53 70 70 90 90 120 120 160 160 200 200 250 250 315 315 400 400 500 500 630 630 800 800 1000 Common tolerances ±0.94 ±1.15 ±1.40 ±1.80 ±2.20 ±2.70 ±3.30 ±4.10 ±5.10 ±0.84 ±1.05 ±1.30 ±1.70 ±2.10 ±2.60 ±3.20 ±4.00 ±5.00 ±6.20 ±7.80 ±9.90 ±0.68 ±0.81 ±0.97 ±1.20 ±1.50 ±1.80 ±2.20 ±2.80 ±3.40 ±4.30 ±5.30 ±6.60 ±0.58 ±0.71 ±0.87 ±1.10 ±1.40 ±1.70 ±2.10 ±2.70 ±3.30 ±4.20 ±5.20 ±6.50 ±0.50 ±0.60 ±0.70 ±0.85 ±1.05 ±1.25 ±1.55 ±1.90 ±2.30 ±2.90 ±3.60 ±4.50 ±0.40 ±0.50 ±0.60 ±0.75 ±0.95 ±1.15 ±1.45 ±1.80 ±2.20 ±2.80 ±3.50 ±4.40 ±0.38 ±0.44 ±0.51 ±0.60 ±0.70 ±0.90 ±1.10 ±1.30 ±1.60 ±2.00 ±2.50 ±3.00 ±0.28 ±0.34 ±0.41 ±0.50 ±0.60 ±0.80 ±1.00 ±1.20 ±1.50 ±1.90 ±2.40 ±2.90 ±6.30 ±7.90 ±10.00 Tolerances for dimensions with entered allowances 1.88 2.30 2.80 3.60 4.40 5.40 6.60 8.20 10.20 12.50 15.80 20.00 1.68 2.10 2.60 3.40 4.20 5.20 6.40 8.00 10.00 12.30 15.60 19.80 1.36 1.62 1.94 2.40 3.00 3.60 4.40 5.60 6.80 8.60 10.60 13.20 1.16 1.42 1.74 2.20 2.80 3.40 4.20 5.40 6.60 8.40 10.40 13.00 1.00 1.20 1.40 1.70 2.10 2.50 3.10 3.80 4.60 5.80 7.20 9.00 0.80 1.00 1.20 1.50 1.90 2.30 2.90 3.60 4.40 5.60 7.00 8.80 0.76 0.88 1.02 1.20 1.50 1.80 2.20 2.60 3.20 3.90 4.90 6.00 0.56 0.68 0.82 1.00 1.30 1.60 2.00 2.40 3.00 3.70 4.70 5.80 0.60 0.68 0.78 0.90 1.06 1.24 1.50 1.80 2.20 2.60 3.20 4.00 0.40 0.48 0.58 0.70 0.86 1.04 1.30 1.60 2.00 2.40 3.00 3.80 0.40 0.44 0.50 0.58 0.68 0.80 0.96 1.16 1.40 1.70 2.10 2.60 0.30 0.34 0.40 0.48 0.58 0.70 0.86 1.06 1.30 1.60 2.00 2.50 0.31 0.35 0.40 0.50 0.21 0.25 0.30 0.40 difference is also made between dimensions connected to the mold and those not connected to the mold 9.4 C a u s e s of S h r i n k a g e The intrinsic cause for shrinkage of injection-molded parts is the thermodynamic behavior of the material (Figure 9.6) It is also called p-v-T (pressure-volume-temperature) behavior and characterizes the compressibility and thermal expansion of plastics [9.5] There is a basically different p-v-T behavior between two classes of materials (amorphous and crystalline) As a melt, both classes show a linear dependency of the specific volume on the temperature For the solid phase, however, there are considerable differences On the basis of crystallization the specific volume decreases exponentially with decreasing temperature while amorphous materials also have a linear dependency in the solid phase This difference is the reason for the greater shrinkage of crystalline thermoplastics To assess the process with respect to shrinkage, the change in state in a p-v-T diagram is very helpful Pressure and temperature during the process are recorded isochronously in a p-v-T diagram (Figure 9.7) Following the volume filling of the cavity (0 —» 1) the material is compacted in the compression phase without substantial change in temperature (1 —» 2) The magnitude of the locally attainable pressure in the molding depends on the magnitude of the holding pressure exerted by the machine and on the resistance to flow in the cavity Subsequently, the molding steadily cools down (2 —» 3) Related to this is a volume contraction, which can be partly compensated by the holding pressure, which supplies additional melt to the cavity through the liquid core of the solidifying molding If no more melt can be fed into the cavity, e.g., by a solidified gate, the change in state is isochoric (3 —» 4) Specific volume amorphous cm3/g C Temperature Specific volume crystalline cmfyg C Temperature Figure 9.6 p-v-T diagram of an amorphous (top) and a crystalline (bottom) thermoplastic material Temperature T Pressure p Timet Timet Pressure Specific volume v Figure 9.7 Change of state in the p-v-T diagram [9.3] —> Volumetric filling, —> Compression, —> Effect of holding pressure, —> Isochoric pressure drop down to temperature T100 kPa —> Cooling to demolding temperature TE —> Cooling to room temperature TR -* Volume shrinkage Temperature The point where the 100-kPa line is met (point 4) establishes the local volume shrinkage A higher volume shrinkage occurs if this point is in the range of larger volumes Since the volume shrinkage is equivalent to the shrinkage potential, a larger volume shrinkage also results in a higher longitudinal shrinkage After the 100-kPa line has been reached, any further change of state is isobaric At the time the molding is ejected (point 5) and constraints from the surrounding cavity cease 9.5 C a u s e s of Anisotropic Shrinkage Dimensional changes of a molding in the mold are restricted or prevented by a forcelocking clamping of the mold halves [9.6] and shrinkage is non-uniform (anisotropic) A distinction must be drawn between internal and external constraints of contraction External restriction of shrinkage is a mechanical restriction against a change of shape by the surrounding mold The restriction of shrinkage and the related stress relaxation result in a lower level of shrinkage The shrinkage of a restricted part is less than that of a restricted one and there is, moreover, less dependence on the process parameters (Figure 9.8) The mechanical restriction, of course, is effective only as long as the molding is still in the mold After ejection, restricted dimensions can also shrink freely Therefore the temperature of demolding is a characteristic for the change in mechanical boundary conditions and for the shrinkage and distortion behavior Internal restriction of shrinkage is due to both cooling-related internal stresses and to orientation Molecule orientations affect shrinkage in two ways On the one hand, the coefficients of linear expansion dependent on orientation cause a difference in shrinkage On the other, re-orientation (contraction) in the direction of orientation contributes to an increase in shrinkage The molecular orientation is determined by the process parameters, and primarily by the type and location of gating, but exerts much less of an influence than does, e.g., fiber orientation in fiber-filled molding compounds near gate in center far trom gate Shrinkage S| Shrinkage Si near gate in center far from gate MPa MPa Holding pressure PH Figure 9.8 Holding pressure PH Shrinkage of a free (left) and a confined (right) circular plate Longitudinal shrinkage S| Oriented by the flow processes, the fibers hinder shrinkage primarily in the direction of orientation because they have a lower thermal expansion and greater stiffness relative to the matrix material (Figure 9.9) [9.8] Through the use of fibers, shrinkage may be reduced by up to 80% However, no further reductions in shrinkage behavior are observed at additions of more than 20% fiberglass Incorporation of fillers such as glass beads and mineral powder leads to isotropic shrinkage The reduction in overall shrinkage that occurs is due to the lower compressibility of the material as a whole Aside from fiber orientation, molecular orientation in the direction of flow leads to anisotropic shrinkage Figure 9.9 Effect of glass fibers and spheres on shrinkage Part: bushing, Material: PBPT unreinforced (X), C Temperature of cavity wall % 30% glass spheres (O), 30% glass fibers (D), TM = 251°C, P w = 33 MPa The low thermal conductivity of plastic results in the temperature profile shown in Figure 9.10 Different cooling conditions exist for the different layers and so volume contraction also varies Due to mechanical coupling between the layers, thermal contraction in the longitudinal and transverse directions is restricted This restriction does not exist in the direction of thickness, with the result that most of the volume shrinkage takes the form of shrinkage of the cross-section Restrictions of shrinkage of the same kind in longitudinal and transverse direction result in the same shrinkage, provided no warpage orientation of molecules or fibers occurs b) No mechanical coupling of layers o) Molded part thickness s c) Real change (mechanically coupled) Figure 9.10 9.6 Model of stress buildup [9.12] Thermal contraction C a u s e s of Distortion Distortion is one result of anisotropic shrinkage Frequently, it is caused by asymmetric cooling relative to the part thickness A higher wall temperature on the top side as shown in Figure 9.10 leads, for example, to higher temperatures in the upper layers and, via greater volume contraction, to deflection towards the warmer side This differential cooling may also be caused by inserts, such as decorative material in in-mold decoration Corner distortion (Figure 9.11; see also Figure 8.46) is due to poorer heat dissipation towards the inside of a corner This has the effect of reducing the corner angles Similarly, differences in the thickness of ribs will displace the temperature profile from its symmetrical center position and result in distortion of the moldings (Figure 9.12) This cooling-induced distortion can be prevented by altering the mold temperature, where necessary, by relocating the cooling channels or using mold inserts of different material Inner corners and thick ribs need to be cooled better than other part sections (see also Section 8.6) To avoid distortion caused by orientation, the gate should be repositioned or the flow path modified by changing the wall thickness Distortion in flat parts can be counteracted by applying thin bracing ribs Figure 9.11 Corner distortion through poorer heat dissipation in internal corners 9.7 Figure 9.12 Distortion caused by differences in wall thickness Effect of Processing o n S h r i n k a g e Gate size Shrinkage Shrinkage Mold temperature Holding pressure Shrinkage Shrinkage Flow path/wall thickness ratio Shrinkage Perpendicular to direction of flow Holding pressure time Runner length Shrinkage In direction of flow Runner profile cross-section Shrinkage Flow restriction Shrinkage Wall thickness Figure 9.13 Shrinkage Shrinkage Shrinkage Shrinkage Other than by modifications to the mold and a change of material, the molder can only influence shrinkage and distortion by making changes to the process From the p-v-T diagram, it can be seen that pressure and temperature are the main factors affecting shrinkage Design changes will affect these parameters and thus also the shrinkage A survey of the influences exerted by various parameters is shown in Figure 9.13 [9.11] With amorphous as well as with crystalline thermoplastics, the holding pressure exerts the greatest effect on shrinkage (Figure 9.8) Under holding pressure, the material in the cavity is compressed and the volume contraction from cooling is compensated by additional melt supply The influence of the holding pressure is shown in the p-v-T diagram charting the progress of the process (see Figure 9.7) If the holding pressure is increased, the process is shifted to lower specific volumes, reaching at lower specific volumes the 100-kPa line at which the part undergoes lower shrinkage The influence of the holding pressure is degressive, however; in other words, the reduction in shrinkage decreases with increase in holding pressure Melt temperature Relationships between shrinkage and characteristic parameters [9.11] Injection speed With an increase in holding pressure, a reduction in shrinkage of up to 0.5% can be obtained in crystalline materials With amorphous materials, a reduction of just 0.2% max is feasible because of their overall lower level of shrinkage The second major influence on shrinkage is the temperature of the material Theoretically, a higher injection temperature has two opposing effects on shrinkage: on the one hand, a higher temperature results in a higher thermal contraction potential of the material (see also Figure 9.7 and [9.7]) and, on the other, the decrease in melt viscosity causes a better transfer of pressure and with this a reduction in shrinkage Given sufficiently long holding-pressure stages, the effect of improved cavity pressure predominates in the case of crystalline materials (Figure 9.14) Shrinkage S| near gate in center far from gate~ C Melt temperature TM Shrinkage S ( near gate :' in center : far from gate" C Melt temperature TM Figure 9.14 material) Effect of melt temperature on shrinkage (top: Crystalline, bottom: Amorphous With crystalline materials, a reduction in shrinkage can be obtained of up to 0.5%; with amorphous plastics, the figure is up to 0.15% All other parameters determine the shrinkage behavior via the pressure and temperature While a greater wall thickness leads to better pressure transfer, the poor thermal conductivity of plastics makes volume contraction more noticeable at high temperatures and increases the shrinkage Restrictions to flow impair pressure transfer and so increase shrinkage In contrast, larger runner profile cross-sections and thicker gates make for better pressure transfer A large runner length leads, just as does a large flow path to wall thickness ratio, to a drop in pressure and thus to greater shrinkage Hot runners, however, reduce shrinkage The influences of the holding pressure time can be used to again illustrate the most important criterion concerning shrinkage As the holding pressure time increases, the forcing of additional material into the cavity reduces shrinkage This can only happen, however, as long as the melt, particularly the gate and sprue, has not frozen Prolonging the holding pressure time beyond that has no further effect For this reason, a part for homogeneous molding materials should always be gated at the thickest point and the wall thicknesses should be such that holding pressure can take effect even in those areas furthest away from the sprue With glass-reinforced materials, there are some particularities (Figure 9.15) In the direction of fiber orientation, it is not possible to affect shrinkage by modifying processing parameters, as the rigidity of the fibers exerts an extremely strong influence The effect perpendicular to the direction of fibers is approximately the same as it is with the matrix material only 9.8 Supplementary M e a n s for Predicting Shrinkage Longitudinal shrinkage The simplest w a y to estimate shrinkage for dimensioning a mold is to consult tables (Table 9.3) They are provided by the raw-material suppliers in the data sheets for their respective materials However, the partly wide range of listed data is problematic because it does not allow a sufficiently accurate prediction of shrinkage; nor are pertinent process parameters known or configurations of moldings from which the shrinkage was obtained Transfer to other configurations is, therefore, difficult Molding: plate Material: Nylon 6, 30% glass reinforced Thickness: mm Relation to direction of flow: 10OkPa Cavity pressure pc max Figure 9.15 Effect on shrinkage of glass fiber reinforced materials Table 9.3 Shrinkage of some thermoplastics [9.10] Material Shrinkage % Nylon 1-1.5 Nylon 6-GR 0.5 Nylon 6/6 1-2 Nylon 6/6-GR 0.5 Low-density polyethylene 1.5-3 High-density polyethylene 2-3 Polystyrene 0.5-0.7 Styrene-acrylonitrile 0.4-0.6 Polymethyl methacrylate (Acrylic) 0.3-0.6 Material Shrinkage % Polycarbonate Polyoxymethylene (Acetal) Polyvinyl chloride, rigid Polyvinyl chloride, soft Acrylonitrile-butadiene-styrene Polypropylene Cellulose acetate Cellulose acetate butyrate Cellulose propionate 0.8 0.5-0.7 1-3 0.4-0.6 1.2-2 0.5 0.5 0.5 A more accurate prediction can be made on the basis of a collection of data gained through experience This is the most reliable method for predicting linear shrinkage so far In such a collection the shrinkage data of all parts are listed which have been produced in the past as well as their processing conditions Because of different restrictions to shrinkage and geometry elements, families of dimensions are formed If the mold for a similar part has to be designed, these data can be used for dimensioning Another and increasingly more accurate estimate of shrinkage behavior is provided by FEA simulation [9.12] (see also Chapter 14) Exact dimensioning of parts with the aid of this method is not yet possible However, the process and the part can be optimized in respect to shrinkage and distortion behavior The ability to predict part dimensions and volume shrinkage, but more so the temperature and pressure behavior, are important tools for accomplishing this Since several physical parameters such as crystallization behavior cannot as yet be determined, some material data cannot be determined with sufficient accuracy batch fluctuations cannot be allowed for, and it is still not possible to make an exact predictive simulation of part dimensions References [9.1] [9.2] [9.3] [9.4] [9.5] [9.6] [9.7] [9.8] Hoven-Nievelstein, W B.: Die Verarbeitungsschwindung thermoplastischer Formmassen Dissertation, Tech University, Aachen, 1984 German Standard: DIN 16901: Kunststoff-Formteile Toleranzen und Abnahmebedingungen fiir LangenmaBe Schmidt, Th W.: Zur Abschatzung der Schwindung Dissertation, Tech University, Aachen, 1986 German Standard: DIN 53464: Prufung von Kunststoffen Bestimmung der Schwindungseigenschaften von PreBstoffen aus warmhartbaren PreBmassen Geisbiisch, P.: Ansatze zur Schwindungsberechnung ungefiillter und mineralisch gefullter Thermoplaste Dissertation, Tech University, Aachen, 1980 Zipp, Th.: Erfahrungsanalyse zur Ermittlung des notwendigen WerkzeugiibermaBes beim SpritzgieBen Unpublished report, IKV, Aachen, 1985 Stitz, S.: Analyse der Forrnteilbildung beim SpritzgieBen von Plastomeren als Grundlage fiir die ProzeBsteuerung Dissertation, Tech University, Aachen, 1973 Menges, G.; Hoven-Nievelstein, W B.; Zipp, Th.: Erfahrungskatalog zur Verarbeitungsschwindung thermoplastischer Formmassen beim SpritzgieBen Unpublished report, IKV, Aachen, 1984/85 [9.9] Baur, E.; Schleede, K.; Lessenich, V.; Ort, St.; FiIz, P.; Potsch, G.; Groth, S.; Greif, H.: Formteil- und Werkzeugkonstruktion aus einer Hand - Die modernen Hilfsmittel fur den Konstrukteur Contribution to 14th Technical Conference on Plastics, Aachen, 1988 [9.10] Strack Normalien fur Formwerkzeuge Handbook, Strack-Norma GmbH, Wuppertal [9.11] Frados, J.: Plastics Engineering Handbook Van Nostrand Reinhold, New York, 1976 [9.12] Potsch, M G.: Prozessimulation zur Abschatzung von Schwindung und Verzug thermoplastischer Spritzgussteile Dissertation, RWTM, Aachen, 1991