Development of devices for self-injection: using tribological analysis to optimize injection force

Figures & data

Figure 1 The UnoPen™ disposable pen injector.

Figure 2 Theoretical relationship between CoF at the dose sleeve/threaded sleeve interface and dimensionless overall pen injector force transmission efficiency.

Notes: The graph depicts the force efficiency as a function of CoF between the dose sleeve and threaded sleeve, assuming constant, individually assigned realistic CoFs between the other parts in the mechanism. The mechanism is geared with a ratio of 4:1, meaning that the nominal maximum force efficiency in the absence of any friction losses would be 4.
Abbreviation: CoF, coefficient of friction.

Figure 3 Threaded sleeve (A) and dose sleeve (B) parts.

Table 1 Polymers and lubricants

Material	Composition	Code/source
Neat POM	POM	POM Hostaform C27021/MT24U01 from Celanese, Sulzbach, Germany
POM silicone	POM with silicone additive	POM Kepital TS-25H from Korea Engineering Plastics Co., Seoul, Republic of Korea
POM glass beads	POM with 30% glass beads added	Proprietary glass bead-filled POM
POM PTFE	POM with 13% PTFE additive	Proprietary developmental PTFE-modified POM
PBT	PBT	PBT Valox HX312C/260HPR from Sabic, Düsseldorf, Germany
PC	PC	PC Lexan HPX8R from Sabic
PA12	PA12	PA 12 Trogamid CX7323 from Evonik Degussa Corp., Parsippany, NJ, USA
Masterbatch POM	Silicone oil-based masterbatch/POM concentrate	Hostaform C 9021 S OEK from Celanese
Masterbatch PBT	Proprietary	CESA-Slip NBA0176001 from Clariant International, Muttenz, Switzerland
PTFE lubricant	PTFE solid film lubricant	Gleitmo 980 from Fuchs Lubritech, Kaiserslautern, Germany
Silicone lubricant	Silicone oil/spray	Unisilkon M2000 from Klüber Lubrication, Munich, Germany
Combination lubricant	PTFE–dimethyl silicone combination grease	Proprietary developmental combination lubricant

Notes: The two different POM Hostaform grades used for the neat POM material have identical material data sheets and are considered virtually identical in terms of properties. The two different PBT Valox grades show only minor differences between the material data sheets and are considered functionally equivalent for the purposes of this study.

Abbreviations: POM, polyoxymethylene; PTFE, polytetrafluoroethylene; PBT, polybutylene terephthalate; PC, polycarbonate; PA, polyamide.

Table 2 Combinations of materials tested

Number	Combination	Material dose sleeve	Material threaded sleeve	External lubricant
1	POM–PBT	Neat POM	PBT	–
2	POM–PC	Neat POM	PC	–
3	POM–PA12	Neat POM	PA12	–
4	Masterbatch internal lubricant	Neat POM with masterbatch POM	PBT	–
5	PTFE internal lubricant	POM PTFE	PBT	–
6	Glass beads internal lubricant	POM glass beads	PBT	–
7	Silicone internal lubricant	POM silicone	PBT	–
8	PTFE external lubricant	Neat POM	PBT	PTFE lubricant
9	Silicone external lubricant	Neat POM	PBT	Silicone lubricant
10	Combination external lubricant	Neat POM	PBT	Combination lubricant
11	Masterbatch internal + combination external lubricant	Neat POM with masterbatch POM	PBT with masterbatch PBT	Combination lubricant

Abbreviations: POM, polyoxymethylene; PBT, polybutylene terephthalate; PC, polycarbonate; PA, polyamide; PTFE, polytetrafluoroethylene.

Figure 4 An experimental test set-up for the measurement of CoF.

Abbreviation: CoF, coefficient of friction.

Figure 5 Two-dimensional model of the interface between the threaded sleeve and dose sleeve parts.

Abbreviations: ν, overall velocity; ν_ax, axial velocity; ν_circ, circumferential velocity; α, plane angle; R, thread radius.

Figure 6 Force equilibrium under constant velocity movement of the threaded sleeve.

Abbreviations: N, reaction force; α, plane angle; _μN, friction force; F_M, circumferential force; F_ax, axial force.

Figure 7 CoF measured for combination 1.

Notes: For each combination, ten part pairs were subjected to each 15 consecutive up and down cycles of 0°–450° rotation. The plot shows all data obtained for combination 1, ie, a total of 150 complete up and down cycles. For further analysis, each combined up and down cycle was condensed into a single kinetic CoF value by calculating the numerical average between 40° and 400° for the cycle (angle limits indicated by the vertical red lines).
Abbreviation: CoF, coefficient of friction.

Figure 8 Evolution of kinetic CoF over repeat cycles (combinations 1–3, different polymers).

Notes: Combination 1 is POM–PBT, combination 2 is POM–PC, and combination 3 is POM–PA12. Individual data points represent the average value over one complete up and down cycle. A set of data points (joined by a line) shows the values obtained for the 15 consecutive cycles performed on a pair of parts. A total of ten part pairs were tested per material combination.
Abbreviations: CoF, coefficient of friction; POM, polyoxymethylene; PBT, polybutylene terephthalate; PC, polycarbonate; PA, polyamide; no, number.

Figure 9 Evolution of kinetic CoF over repeat cycles (combinations 4–7, internal lubrication).

Notes: Combination 4 is masterbatch internal lubricant, combination 5 is PTFE internal lubricant, combination 6 is glass beads internal lubricant, and combination 7 is silicone internal lubricant. Individual data points represent the average value over one complete up and down cycle. A set of data points (joined by a line) shows the values obtained for the 15 consecutive cycles performed on a pair of parts. A total of ten part pairs were tested per material combination.
Abbreviations: CoF, coefficient of friction; PTFE, polytetrafluoroethylene; no, number.

Figure 10 Evolution of kinetic CoF over repeat cycles (combinations 8–11, external lubrication).

Notes: Combination 8 is PTFE external lubricant, combination 9 is silicone external lubricant, combination 10 is combination (PTFE + silicone) external lubricant, and combination 11 is masterbatch internal + combination (PTFE + silicone) external lubricant. Individual data points represent the average value over one complete up and down cycle. A set of data points (joined by a line) shows the values obtained for the 15 consecutive cycles performed on a pair of parts. A total of ten part pairs were tested per material combination.
Abbreviations: CoF, coefficient of friction; PTFE, polytetrafluoroethylene; no, number.

Figure 11 Dimensionless system efficiency calculated from the measured kinetic CoF for all combinations.

Notes: Box plots of all individual values (n=10×15 per combination). Combination 1 is POM–PBT, combination 2 is POM–PC, combination 3 is POM–PA12, combination 4 is masterbatch internal lubricant, combination 5 is PTFE internal lubricant, combination 6 is glass beads internal lubricant, combination 7 is silicone internal lubricant, combination 8 is PTFE external lubricant, combination 9 is silicone external lubricant, combination 10 is combination (PTFE + silicone) external lubricant, and combination 11 is masterbatch internal + combination (PTFE + silicone) external lubricant.
Abbreviations: CoF, coefficient of friction; POM, polyoxymethylene; PBT, polybutylene terephthalate; PC, polycarbonate; PA, polyamide; PTFE, polytetrafluoroethylene.